induction motor protection system seminar report

INDUCTION MOTOR PROTECTION SYSTEM

ABSTRACT

The seminar topic is introducd to protect an induction motor from single phasing and

over temperature. Providing a protection system is very important in industries, using lot of

motors such that production is not hampered owing to failure of any motor.

The basic idea for the development of this topic is to provide safety to the industrial

motor/pump/lift Motor etc. If any of the phases, out of the 3 phases is missing or if the

temperature of the motor during operation exceeds the threshold value, motor stops immediately.

The system uses a 3-Phase power supply where three single phase transformers are connected to

it. If any of the phases is not available the corresponding transformer stops supplying power to

the circuit. This leads to one of the four relays getting switched OFF. The main relay which is

powered through a set of four relays gets disconnected because of one relay not being powered.

Thus the main relay that delivers 3 phase supply to the motor gets disconnected. A thermistor is

connected to the motor body to sense the temperature. If the temperature increases then supply to

the fourth relay is disconnected.

S.S.G.B.C.OE&T,BHUSAWAL Page 1


CHAPTER NO 1

INTRODUCTIONThree phase induction motor generally suffers from under voltage, overvoltage,

overheating, single phasing and phase reversal problems. When the three phase induction motor

supply with higher voltage than is rated then induction motor starts overheated. In our project a

variable resistance is used when supply voltage is lower than rated then voltage drop across the

resistance is higher than it protects the motor from this fault. When supply voltage is lower than

voltage drop across the resistance is lower than specified value and motor fails to start. When

supply is only one phase, this is single phasing problem and supply voltage fall the rated and

once again motor fails to start.

In the case of motor overheating a LM sensor is used which sense the temperature of

winding if it is exceed the specified limit then once again motor fails to start. It is highly desired

that 3 phase induction motor works freely from these all types’ of faults. Induction motor is the

most widely used motor in the industry due to its simple and rugged construction. It requires

least maintenance as compare to the other electrical motors. Induction motor speed control is

nowadays more easy and versatile due to the advancement in the field of power electronics and

hence is easy to replace other costly and controllable motors. The protection of induction motor

plays an important role in its long life service. Researchers have done costly and limited

protection for the stator windings protections, broken rotor bars protection, thermal protection

etc.

Mainly the induction motor needs protection from the variation of the input supply for

small motors which is in common use not only in big industry but also in small scale industries.

The small scale industries are not able to provide costly protection to the drives in use as it will

increase their capital cost. Hence a cheap and compact design has been done for protection of

induction motor against unbalance voltages, under voltage, over voltage, short circuit and

thermal protection. It has been also designed for critical loads which need to be run even under

single phasing condition. Due to the poor power quality the damage of induction motors in small

scale industries needs to be taken care of. The proposed design can be also used for speed

control, improvement of efficiency under poor power quality service manually by introduction of



a single two way switch. Many researches has been done in this area but they are costly and

unfeasible in our Indian condition. The overall cost of the protection equipment should not be

more than 15% of the total cost of the actual machine. Keeping this in mind

design has been proposed using 16- bit microcontroller, MOSFETs, relays, small CTs and PTs,

so that the overall cost is low. But the efficiency of the protection scheme should not be

compromised.

1.1 Objectives of the Thesis Work

The main objective of the work is to make a cheap and reliable protection system for

three phase induction motor. The protection system should protect the motor from voltage

unbalancing, single phasing, under voltage, over voltage and thermal protection. Further to

improve the technique to run the motor under single phasing.

Classical monitoring techniques for three-phase IMs are generally provided by some

combination of mechanical and electrical monitoring equipment. Mechanical forms of motor

sensing are also limited in ability to detect electrical faults, such as stator insulation failures. In

addition, the mechanical parts of the equipment can cause problems in the course of operation

and can reduce the life and efficiency of a system.

It is well known that IM monitoring has been studied by many researchers and reviewed

in a number of works [3]–[5]. Reviews about various stator faults and their causes, and detec-

tion techniques, latest trends, and diagnosis methods supported by the artificial intelligence, the

microprocessor, the computer, and other techniques in monitoring and protection technologies

have been presented.



CHAPTER NO 22 LITERATURE REVIEW:

William H. Kersting [1] stated that three phase induction motor can continue to run when

one phase of the supply gone out of service. This may be due to any fuse blowing or opening of

protective device of the motor, at step-down transformer or at feeder end. At this condition

the three-phase induction motor continue to run but the motor will heat up quickly and it

should be protected by removing it from the service at the instant of single phasing. When

phase opens at step down transformer or at feeder end, the stator and rotor losses increases to

ten times and the shaft output power decreases to negligible. But if the single phasing occurs

at motor terminals the losses increases twice as compare to steady state losses and the shaft

power reduces to nearly 70%. To protect the motor all the terminal should be open.

Sutherland P. E. and Short T.A. [2] described that the for single phase fault the three

phase reclosers are widely employed on distribution feeders. The majority faults are single

phase. Its negative effect occurs on the other two phase customers, because the distribution line

is mainly supplying the load to single phase customers. If three phase reclosers did not open

from the service, and the problem arises for three phase industry. On an average single phase

fault occurs at 70%, two phase fault occurs at 20% and three phase fault occurrence is 10%.

Sudha M. and Anbalagan [3] proposed a technique to save the three phase induction motor

from single phasing. In this technique, PIC16F877 microcontroller has been used to sample

the values of each phase and converted them to low voltage ac by means of transformer. The

signals are converted to digital value using ADC converter. The controller continuously

compare the digital value with the reference value and when the fault occurs, it opens the

normally close contactor and disconnects it from the power supply. Single phasing, under

voltage and over voltage protection is done practically on a 2kW motor and the motor is

isolated if any of these condition occurs.

Pragasen Pillay et.al. [4] examines the three phase induction motor under the influence of

under voltage and over voltage. The voltage at motor terminals may be higher than the

nominal value in a complex industrial system and can be well below from nominal value in a



heavily loaded industrial system. IEEE, NEMA and other power communities have different

defined the voltage unbalance. The complex algebra is avoided in these definitions. In this

paper calculation of the unbalance of voltage have been done on true basis with complex

algebra and compared with NEMA standards.

Faiz J. et.al. [5] has studied the negative impact of the unbalanced voltages on the

performance of three phase induction motor. In this paper the comparison of the voltage

unbalance definitions of NEMA, IEEE and IEC (International Electro technical Commission)

has been done. The studies showed that the definition given by the NEMA, IEEE are simple

to calculate as compared to IEC. But all the three give only an idea about the percentage

unbalance and needs to be modified.

Javed A. and Izhar T. [6] have proposed the protection of three phase induction motor

basedon voltage measurement and is not enough to protect the motor if the fault occurs at

distribution transformer or at substation feeder. If fault occurs at motor terminals then the

voltage measurement can protect the motor very well. The current measurement device

should be implemented within the protective device. They have also proposed a phase

measurement device which can measure the phase difference of the voltages because when

the fault occurs at any other location rather than the motor terminals, then the faulted phase

will draw negative sequence current and work as a voltage generator. The voltage developed

is close to line voltage but the measurement scheme is not able to detect the fault, however

the phasor difference of the faulted phase changes.

Chattopadhyay et.al. [7] analysed the stator current of three phase induction motor by

using different techniques. The single phasing can also be measured by the zero crossing

detection method and has proposed to use 8085 microprocessor for doing this work. The

accuracy can be increased by increasing the sampling time. The phase shift can also be measured

by the use of microprocessor. The phase shift helps to protect the motor from any increased or

decreased phase difference



CHAPTER: 3IMPLEMENTATION OF PROTECTION SYSTEM3.1 Objective of protection

The objective of dissertation is to protect the three phase induction motor under single phasing

fault. Block diagram of the protection technique is given below in fig. 3.1 :

FIG. 3.1 BLOCK DIAGRAM OF PROTECTION TECHNIQUE FOR 3 PHASE INDUCTION MOTOR

In this protection technique we have :

i. 3 Current transformers

ii. 1 Atmega32 Microcontroller

iii. 3 Normally Closed Relay

iv. 3 MOSFET

v. 1 9V Battery 5V Supply for microcontroller

The micro controller based motor protection system combines control, monitoring and protection

function of induction motor from incipient faults in one assembly. The overall block diagram of

the motor protection system . The system does not require special sensors. Only conventional



Current Transformers (CT) and Potential Transformer (PT) are used for monitoring line current

and line voltage under running condition. The data gathered from Current Transformer (CT) and

Potential Transformer (PT) is transferred to the micro controller digitally by passing through the

current and voltage measuring circuits.

The needed comparisons are made in micro controller according to limit values, which

are earlier entered and when an unexpected situation is encountered, the motor is being stopped

by means of the control signal. The system provides protection schemes for unbalanced supply

voltage, over current/overload, phase reversing, single phasing, under/over voltage and ground

fault. . The input data (limit values) to the system is given through the keypad. LED Seven

Segment display unit is used as an output device to display the output data, warning message and

fault condition. The system works with any motor design with high degree of accuracy. The

method is very sensitive, fast and detects faults while running and before start. The prototype

model is developed and tested on a 3 phase induction motor with rated current of 5 A and the test

results re satisfying the design criteria. The motor parameters like the full load current in

amperes, service factor and class of motor, etc., are needed to be entered into the relay

programming unit to automatically calculate the correct motor protection curve.

3.2 Specification of Induction MachineThe three-phase squirrel cage induction machine under test has the following specifications:

Power rating =2.2 kw

Lin voltage = 415v

Rated current = 5amp

Frequency = 5ohz

No. of. Poles =4

Rated speed = 1440 rpm

Connection = delta

Class type =k



FI

G :

3.2

OVERALL BLOCK

DIAGRAM OF

PROTECTION

SYSTEM FOR 3

PHASE INDUCTION

MACHINE



CHAPTER: 4

4.1 OVERVOLTAGE PROTECTION In overvoltage protection system of 3 phase induction motor, protects the motor from

overvoltage, the voltage which is higher than the rated voltage. In circuit diagram of overvoltage

protection it consists the comparator which compare two voltages one is supply and another one

is drop across the variable resistance. When the voltage drop across the variable résistance is

higher than specified value then comparator generates signals. This signal is fed to

microcontroller and microcontroller takes the appropriate action as shown in fig.1.

FIG. 4.1 CIRCUIT DIAGRAM OF OVER VOLTAGE PROTECTION

4.2UNDER VOLTAGE PROTECTION In under voltage protection of 3 phase induction motor provides the protection from the

under voltage. When supply system has low voltage than the rated of induction motor then under

voltage protection section of protection supply is provided to motor. Single phasing works. It has

same concept as overvoltage it also has comparator which compare two voltage one form supply



and another from the voltage drop across the variable résistance. When voltage drop across the

variable resistance is lower than specified value, this signal sends to microcontroller and

microcontroller stop the operation of motor in the case of running and fails to operate in case of

starting. Preset is used to set the specified value as shown in fig.3. This circuit works in same

manner as overvoltage protection works only the different is that value set by preset. In this Case

set value is minimum but in overvoltage case set values by preset resister. When appropriate

voltage drop across the Resister exceeds from the set values of preset the signal sends to

microcontrollers.

FIG.43.2 PRESET TO SET VALVE

4.3 SINGLE PHASING

In single phasing protection to 3 phase induction motor, if other two phases is faulted and only

one protection of motor section starts functioning. Generally in single phase supply voltage is

lower value than specified value. On this value of voltage motor is unable to start. Comparator

which compares single phasing supply voltage and rated specified voltage, and single sends to



FIG 4.3. CIRCUIT DIAGRAM OF SINGLE PHASING

microcontroller and microcontroller generates single which stop the motor if motor is running

and does not allow to motor start in case of standstill. Sometimes single phasing protection

looking much motor important when the motor is tight which important function like furnishing,

pump driving and crane driving etc.

This fig.4.3 show the typical single phasing condition in three phase induction motor where one

phase break down and motor is only supplied by remaining phases which is equivalent to single

phasing condition. Single phasing occurs as a result of several possibilities. A loose wire, a bad

connection, bad starter contacts, overload relay problems, a bad breaker, a blown fuse, and other

things can cause this destructive condition. Obvious signs are a louder than normal humming

from the motor and/or a shaft that vibrates rather than rotating.

4.4 PHASE REVERSAL

Phase reversal problem occurs in motor when the supply phase is reversed due to

wrong connection (except than RYB) due to phase reversal motor starts running in anticlockwise

(opposite direction from normal) would cause operation and safety problem. Most of three

phases motor run opposite phases. This type of protection is used in application like elevators

where it would be damaging or dangerous for the motor to run in reverse. Generally when motor



is connected with the important application then type of protection being much more

important .When the load is connected with motor then reversal of phase means Direction of

rotation is changed. It could cause serious problem therefore much more care is required to

protect the motor form such type of fault. The overheating protection system is placed to turn the

motor off when the exceeds heat is generated within the motor. This protection system rested the

motor cools to safe operating temperature. Direction by switching the connection of any two of

three although the motor having shut down because it tripped the thermal limit in inconvenient.

4.5 OVERHEATING PROTECTION

Overheating protection of motor means protect the motor from overheating of its

winding. This overheating in motor is generally caused by overloading of motor, bearing seizes

up something locked the motor shaft from turning. Motor simply fails to starts properly, a failure

to start of motor may cause by faulty start in winding in motor. For sensing the heat LM 35

sensor is used for this purpose. This sensor is connected to comparator inputs. With the help of

sensor which sense the temperature of winding & its temperature exceeds to some particular

level then comparator sends this signal to microcontroller as shown in fig3.5.

4.5 CIRCUIT DIAGRAM OVERHEATING PROTECTION



CHAPTER NO 55.1 TYPES OF PROTECTIONS NEEDED FOR INDUCTION MOTOR

FIG.5.1 STARTING OF MOTOR OF INDUCTION MOTOR

Three-phase induction motors are accountable for 85 percent of the installed capacity of

the industrial driving systems. Therefore, the protection of these motors is necessary for reliable

operation of loads. Motor failures are mainly divided into three groups: electrical, mechanical

and environmental. Mechanical stresses cause overheating resulting in the rotor bearings’ wear

and tear, whereas the over mechanical load causes heavy currents to draw, and thus results in

increasing temperatures. Electrical failures are caused by various faults like Phase-to-phase and

phase-to-ground faults, single phasing, over and under voltage, voltage and current unbalance,

under frequency, etc

In addition to the motor protection systems for the above mentioned faults, it is also

necessary to use three-phase motor starter to limit the staring current of the induction motor. As

we know – in every electrical machine, when supply is provided, there is opposition to this

supply by an induced EMF – which is called back EMF. This limits the current drawing by the

machine, but at the beginning, the EMF is zero because it is directly proportional to the speed of

the motor. And therefore, the zero back EMF’s huge current will be drawn by the motor at the

start, and this will be 8-12 times the full-load current as shown in the figure. To protect the motor


https://www.elprocus.com/wp-content/uploads/2014/05/31.jpg


from the high-staring current, there are different staring methods available like reduced voltage,

rotor resistance, DOL,star-delta starter, auto transformer, soft starter, etc. And, for protecting the

motor from the above discussed faults; various protection equipments like relays, circuit

breakers, contractors and various drives are implemented.

These are some of the protection systems for three-phase induction motors against starting inrush

currents,

5.2 INDUCTION MOTOR PROTECTION SYSTEM

This system protects the 3-phase AC motor from single phasing and overheating. When

any of the phases is out, then this system recognizes it and immediately turns off the motor,

which is powered by the mains. All the three phases are rectified, filtered and regulated and

given to operational amplifier where this supply voltage is compared with certain voltage. If any

of the phases is missed, then it gives zero voltage at the Op-amp input, and therefore, it gives low

logic to the transistor which further de-energizes the relay. Hence, the main relay gets turned off

and the power to the motor is interrupted.

FIG.5.2 INDUCTION MOTOR PROTECTION


https://www.edgefxkits.com/induction-motor-protection-system

https://www.elprocus.com/industrial-star-delta-starter-3-phase-induction-motor/

https://www.elprocus.com/wp-content/uploads/2014/05/2-2.jpg


Similarly, when the temperature of the motor exceeds certain limit, the operational amplifier

output de-energizes the appropriate relay; even then also the main relay gets turned off. In this

way, the single phasing faults and over-temperature conditions can be overcome in the induction

motor.

5.3 MAIN COMPONENTS USED FOR PROTECTION

5.3.1 THERMISTORA thermistor is a type of resistor whose resistance varies significantly (more than in standard

resistors) with temperature.

The word is a portmanteau of thermal and resistor.

Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting

over current protectors, and self-regulating heating elements.

FIG 5.3. THERMISTOR

5.3.2 RELAY A relay is an electrically operated switch.

Current flowing through the coil of the relay creates a magnetic field which attracts a

lever and changes the switch contacts.

The coil current can be on or off so relays have two switch positions and have double

throw (changeover) switch contacts as shown in the diagram.


https://www.elprocus.com/operational-amplifiers/

https://www.elprocus.com/operational-amplifiers/


FIG.5.3.2 RELAY

5.3.3 COMPARETOR op amps & comparators look very similar

But a comparator gives a logic output indicating the relative potentials on its two inputs

An op amp amplifies the differential voltage between its two inputs – and is designed

always to be used in closed-loop applications

FIG 5.3.3 COMPARETER



5.3.4 RECTIFIRE

A rectifier is an electrical device, which converts alternating current to direct

current, a process known as rectification. Rectifiers are used as components of

power supplies and as detectors of radio signals. Rectifiers may be made of solid

state diodes, vacuum tube diodes, mercury arc valves, and other technologies.

When just one diode is used to rectify AC (by blocking the negative or

positive portion of the waveform) the difference between the term diode and the

term rectifier is merely one of usage, e.g., the term rectifier describes a diode that

is being used to convert AC to DC. Almost all rectifiers comprise a number of

diodes in a specific arrangement for more efficiently converting AC to DC than is

possible with just a single diode. Before the development of solid state rectifiers,

vacuum tube diodes and copper oxide or selenium rectifier stacks were used.

Early radio receivers called crystal sets, used a "cat's whisker" of fine wire

pressing on a crystal of galena (lead sulfide) to serve as a point contact rectifier or

"crystal detector". In gas heating systems "flame rectification" can be used to

detect a flame. Two metal electrodes in the outer layer of the flame provide a

current path and rectification of an applied alternating voltage, but only while the

flame is present.

5.3.5 WORKING In normal operations RV1, RV2, RV3 & RV4 are so set that the output of the

comparators is held low resulting in 4 relays in deactivated condition while the 3CO relay

is in active operation.



In the event of failure of any phase the corresponding comparator output goes high that

drives the relay the contact of which opens to discontinue the DC supply to the 3CO

relay.

The phase motor connected in series with the NO contacts thus open to stop the motor.

Similarly while the temperature goes high on the body of the motor the mounted

Thermistor resistance falls to develop logic high for Q4 to operate R4 & disconnect the

DC voltage to the 3CO relay.

5.3.6 THERMAL OVER LOADING OF ELECTRICAL MOTOR

Now we will discuss about thermal over loading of electrical motor or over heating

problem of electric motor and the necessity of motor thermal overload protection. Whenever

we think about the overheating of a motor, the first thing strikes in our mind is over loading. Due

to mechanical over loading of the motor draws higher current from the supply which leads to

excessive over heating of the motor. The motor can also be excessively over heated if the rotor is

mechanically locked i.e. becomes stationary by any external mechanical force. In this situation

the motor will draw excessively high current from the supply which also leads to thermal over

loading of electrical motor or excessive over heating problem. Another cause of overheating is

low supply voltage. As the power id drawn by the motor from the supply depends upon the

loading condition of the motor, for lower supply voltage, motor will draw higher current from

mains to maintain required torque. Single phasing also causes thermal over loading of motor.

When one phase of the supply is out of service, the remaining two phases draw higher current to

maintain required load torque and this leads to overheating of the motor. Unbalance condition

between three phases of supply also causes over heating of the motor winding, as because

unbalance system results to negative sequence current in the stator winding. Again, due to

sudden loss and reestablish of supply voltage may cause excessive heating of the motor. Since

due to sudden loss of supply voltage, the motor is de-accelerated and due to sudden

reestablishment of voltage the motor is accelerated to achieve its rated speed and hence for that

motor draws higher current form the supply. As the thermal over loading or over heating of the



motor may lead to insulation failure and damage of winding, hence for proper motor thermal

overload protection, the motor should be protected against the following conditions

1. Mechanical over loading,

2. Stalling of motor shaft,

3. Low supply voltage,

4. Single phasing of supply mains,

5. Unbalancing of supply mains,

6. Sudden Loss and rebuilding of supply voltage.

The most basic protection scheme of the motor is thermal over load protection which primarily

covers the protection of all the above mentioned condition. To understand the basic principle of

thermal over load protection let’s examine the schematic diagram of basic motor control scheme.

FIG.5.3.6[a] BASIC MOTOR CONTROL SCHEME

In the figure above, when START push is closed, the starter coil is energized through the

transformer. As the starter coil is energized, normally open (NO) contacts 5 are closed hence

motor gets supply voltage at its terminal and it starts rotating. This start coil also closes contact 4

which makes the starter coil energized even the START push button contact is released from its

close position. To stop the motor there are several normally closed (NC) contacts in series with

the starter coil as shown in the figure. One of them is STOP push button contact. If the STOP



push button is pressed, this button contact opens and breaks the continuity of the starter coil

circuit consequently makes the starter coil de-energized. Hence the contact 5 and 4 come back to

their normally open position. Then, in absence of voltage at motor terminals it will ultimately

stop running. Similarly any of the other NC contacts (1, 2 & 3) connected in series with starter

coil if open; it will also stop the motor. These NC contacts are electrically coupled with various

protection relays to stop operation of the motor in different abnormal conditions.

Let’s look at the thermal over load relay and its function in motor thermal overload protection.

The secondary of the CTs in series with motor supply circuit, are connected with a bimetallic

strip of the thermal over load relay (49). As shown in the figure below, when current through the

secondary of any of the CTs, crosses it’s predetermined values for a predetermined time, the bi-

metallic strip is over heated and it deforms which ultimately causes to operate the relay 49. As

soon as the relay 49 is operated, the NC contacts 1 and 2 are opened which de-energizes the

starter coil and hence stop the motor.

FIG.5.3.6 [B] THE THERMAL OVER LOAD RELAY

Another thing we have to remember during providing motor thermal overload protection.

Actually every motor does have some predetermined overload tolerance value. That means every



motor may run beyond its rated load for a specific allowable period depending on its loading

condition. How long a motor can run safely for a particular load is specified by the manufacturer.

The relation between different loads on motor and corresponding allowable periods for running

the same in safe condition is referred as thermal limit curve of the motor. Let’s look at the curve

of a particular motor, given below.

FIG.5.3.6 [C] THE CURVE OF A PARTICULAR MOTOR

Here Y axis or vertical axis represents the allowable time in seconds and X axis or horizontal

axis represents percentage of overload. Here it is clear from the curve that, motor can run safely

without any damage due to overheating for prolonged period at 100% of the rated load. It can run

safely 1000 seconds at 200% of normal rated load. It can run safely 100 seconds at 300% of

normal rated load. It can run safely 15 seconds at 600% of normal rated load. The upper portion

of the curve represents the normal running condition of the rotor and the lower most portion

represents the mechanical locked condition of the rotor.



Now the operating time Vs actuating current curve of the chosen thermal over load relay should

be situated below the thermal limit curve of the motor for satisfactory and safe operation. Let’s

have a discussion on more details-

FIG. 5.3.6 [D] TIME VS ACTUATING CURRENT CURVE

Remember the characteristics of starting current of the motor – During start up of the

induction motor, the stator current goes beyond 600% of normal rated current but it stays up to

10 to 12 seconds after that stator current suddenly falls to normal rated value. So if the thermal

overload relay is operated before that 10 to 12 seconds for the current 600% of normal rated then

the motor cannot be started. Hence it can be concluded that the operating time Vs actuating

current curve of the chosen thermal over load relay should be situated below the thermal limit

curve of the motor but above the starting current characteristics curve of the motor. Probable

position of the thermal current relay characteristics is bounded by these two said curves as shown

in the graph by highlighted area.



Another thing has to be remembered during choosing of thermal overload relay. This relay is not

an instantaneous relay. It has a minimum delay in operation as the bimetallic strip required a

minimum time to be heated up and deformed for maximum value of operating current. From the

graph it is found that the thermal relay will be operated after 25 to 30 seconds if either the rotor

is suddenly mechanically blocked or motor is fail to start. At this situation the motor will draw a

huge current from the supply. If the motor is not isolated sooner, severer damage may occur.

FIG. 5.3.6[E] COMPLETE MOTOR THERMAL OVERLOAD PROTECTION.

This problem is overcome by providing time over current relay with high pickup. The time

current characteristics of these over current relays are so chosen that for lower value of over

load, the relay will not operate since thermal overload relay will be actuated before it. But for

higher value of overload and for blocked rotor condition time over load relay will be operated

instead of thermal relay because former will actuate much before the latter.

Hence both the bimetallic over load relay and time over current relay are provided for complete

motor thermal overload protection.



There is one main disadvantage of bimetallic thermal over load relay, as the rate of heating

and cooling of bi-metal is affected by ambient temperature, the performance of the relay may

differ for different ambient temperatures. This problem can be overcome by using RTD or

resistance temperature detector. The bigger and more sophisticated motors are protected against

thermal over load more accurately by using RTD. In stator slots, RTDs are placed along with

stator winding. Resistance of the RTD changes with changing temperature and this changed

resistive value is sensed by a Wheatstone bridge circuit.

This motor thermal overload protection scheme is very simple. RTD of stator is used as one arm

of balanced Wheatstone bridge. The amount of current through the relay 49 depends upon the

degree of unbalancing of the bridge. As the temperature of the stator winding is increased, the

electrical resistance of the detector increases which disturbs the balanced condition of the bridge.

As a result current start flowing through the relay 49 and the relay will be actuated after a

predetermined value of this unbalanced current and ultimately starter contact will open to stop

the supply to the motor.

FIG. 5.3.6.[F] WHEATSTONE BRIDGE CIRCUIT.

CHAPTER – 6



FAULT DIAGNOSIS FOR THREE PHASE INDUCTION MOTOR

6.1 Different kinds of Fault

Induction motors are the workhorse for the industry because of its versatility, ruggedness

and low manufacturing cost. Induction machines are the reliable machines but their failure rate is

approximately 3% and it can be as high as 12% in pulp and paper industry. Downtime of the

machine in indusssstry may be expensive. The protection system may enhance the reliability,

personal safety and protect the motor from over heating.

The external motor troubles are described in four groups :

i) Single phasing effect

ii) Unbalanced voltages and frequency

iii) Overloading and Starting effect

iv) Maintenance, environmental and manufacturing effect

6.1.1 SINGLE PHASING CONDITION :If the condition of single phasing arises during the running of motor, the winding of

motor gets heated due to the negative sequence current in the faulted phase. Two phases of three

phase induction motor will get power supply in single phasing condition and they produce

negative sequence current in the faulted phase because the internal connection of three phase

motor are connected with each other. Single phasing fault may arise at three locations :

FIG.6.1.1.[A] FAULT LOCATION OF SINGLE PHASING



Fault Location 1 : Opening one phase of the primary side of substation transformer

Fault Location 2 : Opening one phase of the primary side of distribution transformer

Fault Location 3 : Opening one phase at the motor terminals. Phase opening at substation or distribution transformer Out of these three fault locations, the most severe condition is when phase opens at distribution step down transformer or at substation feeder end. The current goes to ten times higher. The shaft output power also approaches to negligible. If the condition of single phasing arises when the motor is in running condition, the motor continues to rotate but it is not capable of starting under single phasing condition. If we allow the run the motor in this condition, then the motor will heat up very fast and we should remove the motor from the service. If any overload protective device is provided to isolate the motor from the main supply during single phasing condition and if it later attempts to start the motor during single phasing then it will draw locked rotor current which is 6-8 times of normal running current. It will permanently damage the motor. Single phasing is worst than the unbalance voltages[3]. When the fault occur at the substation end or at distribution transformer, it raises one more problem for the voltage magnitude fault detection devices located at the motor terminals. As when the fault occur at any one phase at substation or at distribution transformer then the third phase of the motor will draw the negative sequence current and the torque will be produced by the remaining two phases. The winding of the faulted phases will behave like a generator and the generated voltage is nearly same of the line voltage[6]. The high current will damage the winding insulation and the motor will permanently damage. This makes a problem for the voltage sensing protective devices. A schematic diagram is shown if the Single phasing occurs at feeder end or distribution secondary winding :

FIG. 6.1.1 [B] SINGLE PHASING AT FEEDER END OR AT DISTRIBUTION SECONDARY WINDING



As shown in the figure 6.1.1[B] two three phase induction motors are connected with the three phase line and some single phase loads are also connected with the line. Whenever the single phasing fault occurs other then the motor terminal, then the faulted phase will receive the generated voltage from the other two phases because the three phase are connected with each other. Third phase will get negative sequence current and the voltage is generated nearly to the line voltage. The generated voltage is not in the phase and can be detected by the phase measurement device. Now, if the protection devices are based on voltage magnitude sensing then they will sense the magnitude of voltage and will not trip the circuit. Hence as shown in the figure, the other loads which are connected to the same phase will draw the current through the motor winding and a large current is drawn from the three windings of the motor. The large current will damage the motor windings. If any voltage and current protection device is placed at the single phase load, which is connected to the faulted phase, will not respond. Because the current required by that load is drawn from the motor and the voltage is also at nominal level. So no protection device with voltage and current sensing will work at single phase load, if it is connected to the faulted phase [6]. If any single phasing fault occurs at any of these three fault locations i.e. at motor terminals, at substation end, at distribution transformer, then the current profile will surely change. So with the voltage sensing protection device, the current sensing protection proves better protection. If fault occurs at primary of wye-wye transformer then the secondary winding line to ground voltages of one phase will show 0 p.u. and the rest two winding will show 1 p.u. but in case of delta-wye line to ground voltages will be 0.58 p.u., 1 p.u. and .58 p.u. This means that in case of delta-wye transformer, all the three phase will show voltages but the two phases will give lower voltages. In wye-wye transformer, the secondary of two phases will give rated voltage but the faulted phase will show zero voltage. If the distribution transformer of wye-wye winding then the single phasing condition can be easily measured because one phase will show 0 p.u. But in case of delta-wye transformer the single phasing condition cannot be measured efficiently because two of the three phases of the secondary winding will show .58 p.u. voltage. Current measuring device with voltage measuring device can protect the motor effectively. 11 2.2.2 Phase opening at motor terminals If fault at motor terminals is not much severe as compare to previous case. The current rises two to three times if the phase opens at motor terminal and shaft power output decreases to nearly 70 percent. The windings will heat up quickly and all the phases should be isolated from the power supply. 2.2.3 Basic protection from single phasing Motors are extremely sensitive to voltage unbalance. The negative sequence stator current sets up counter rotating flux field in the motor and causes the local heating in rotor iron. The motor heats up rapidly about 25-30% as compared to balanced condition, due to negative sequence currents. It is recommended that the protective relay time delay set under single phasing conditions should be 4 seconds. Multilin 469, a typical relay trips at a current imbalance of 40% or greater than nominal value with 2 seconds delay [22]. In three phase rectifier, during single phasing the current in the remaining two phases may increase to two to three times [2]. In DC drives, which uses controlled rectifier, the mis-firing of the SCR, commutation failure etc. occurs. Ferroresonance can occur due to single phasing in transformers due to capacitance in the cable fed line and results high voltage as much as 5 p.u.



can occur on the open terminals of the transformer which can damage metal-oxide surge arrestors. Ferroresonance also occurs at low core loss transformer because capacitance power is not fully dissipated in core resistance when single phase fault occurs. Ferroresonance in transformer can be avoided by grounded wye transformer. 2.2.4 Advantages and disadvantages of single phasing prevention recloser If any single phase fault occurs on the distribution transformer, we have one option to trip the single phase and allow the other two phase because all the three phases are connected to the different single phase customers and some three phase customers. The second option is to close all the three phases. Both of them have some advantages and disadvantages. If we close the one phase, the single phase consumer connected to other two phases will not be interrupted but the three phase customer will face adverse effect on its three phase machine. The three phase load will get damaged if it is not disconnected from main supply. On the other side, if all the three phases are closed, then the both single phase customer and three phase customer will face power outage till the fault is not cleared [2]. 12 2.3 Unbalanced voltages and frequency : When the three phase voltages have different magnitude and phase angle is not accurate with 120 degree difference, it is called unbalanced voltage. According to NEMA MG1-2009 standard the voltage variation should be in limit of +10% and +5% for frequency variation[12]. However more than 5% voltage is not recommended by NEMA (National Electrical Manufacturer Association Motor and Generator Standard) guidelines [4]. This effects the insulation life of winding, reduce efficiency, increase losses and increase temperature with in the motor.

6.2 DERATING OF MOTOR DURING VOLTAGE UNBALANCE

When there is any unbalance in the voltage, according to NEMA guide lines, there should be derating of the motor. The motor should be derated, because unbalance voltage introduce the negative sequence current and the current heats up the winding. If the motor is allowed to the rated torque, it will draw more current then the rated current. So to avoid te over heating, the motor should be derated. For example, if the unbalance voltage reaches the value of 5% then the motor should be derated to 77% of its original value. For a 90% undervoltage the derating should be 0.92 [4]. The derating chart is also drawn by NEMA for the under voltage condition and the over voltage condition.

The figure for derating of motor during unbalance voltages is given below



FIG. 5.2 DERATING OF MOTOR DURING UNBALANCE OF VOLTAGES



6.3 EFFECT OF VOLTAGE UNBALANCE ON POWER FACTOR AND EFFICIENCY OF MOTOR

When VUF occurs, Undervoltage and overvoltage also effects the power factor and efficiency of the induction motor. Power factor decreases as the voltage increases and the efficiency increases as the voltage increases. But the efficiency described on the name plate is always higher whether the case is under voltage or over voltage. For the customer‟s view if there is any efficiency reduction due to voltage unbalance, he has to pay more electricity bills for the same work and for the utility point of view, they have to generate more power. The efficiency decreases very fast in the case of 3-phase under voltage.

Table 6.1 Comparison for eight unbalance voltages in terms of efficiency and power factor

The efficiency decreases negligible in case of 3-phase over voltage. Under-voltage reduce the

efficiency and cost increases. However in some cases, lower VUF may result into lower

efficiency. The customer installs capacitors to improve the power factor in balanced condition

and in under voltage condition the power factor improves higher than required, it may result in

over voltage. In case of three phase under voltage the power factor increases and efficiency

decrease. This means that the customer have to run the motor for extra time for same kind of

work. This will cost him extra. For example, a survey in Taiwan on three phase induction motors

by ministry of economic administration shows that there were approximately 2 lakh induction

motors running with power capacity of 1-5 HP. The calculation is done by taking 3HP motor an



average in table 6.1. VUF is taken as 4% & cost is taken as 7 Rs./kWh. 15 Table 2.2 Cost

increase for same work due to decrease in efficiency Voltage unbalance cases Total installed

capacity(kW) Motor efficiency (%) Load increase rate (LdIR) Extra power consumption per year

(kW/yr) Average running time per year (Hour) Extra

6.4 OVERLOADING EFFECTS :

Overloading of the three phase induction motor can produce hot spot within the winding, which may exceed the thermal limits of motor. Time is a very important factor in case of over temperature. Induction motor has a relatively large heat storage capacity, so a short period overload cannot damage the motor windings because a large part of heat is stored in the core, conductor mass and in structural members [12]. But in the case of locked rotor condition the current rises very rapidly and a very little amount of heat transmits to the other parts of the motor. The winding insulation thermal level may reach to its limit within seconds. 2.4.1 Design of polyphase induction motor according to NEMA MG1 standard : According to NEMA : i) The stator current of the induction motor should be capable of withstanding 1.5 times of rated current for not less then 2 minutes[13]. ii) If motor is designed for different frequency system then it can be used to some another frequency system only if its horsepower and voltage ratings are set according to volts/hertz. iii) The locked rotor current should be withstanding capability upto 12 seconds by the motor. National Electric Code (NEC) (NPFA 70-2011) has defined the trip for the 125% of rated current for the continuous motors. There are several NEMA design depending upon the speed, voltage, horsepower rating, service factor etc.

6.5 MAINTENANCE, ENVIRONMENTAL AND MANUFACTURING EFFECTS :

6.5.1 VENTILATION EFFECTS :

Ventilation is necessary for the smooth operation of the motor because a clogged or partially clogged ventilation will cause increase in the temperature of the motor. A small motor with clogged ventilation can get damaged. Ventilation inadequacy detecting devices like airflow detector, temperature sensing device etc. may help to protect the motor. 2.5.2 Manufacturing effects : Manufacturing and selling of the machines are now a global factor. This has increase the competition between the manufacturer. This has put pressure on the designers to reduce the cost of the machine. Some of the methods which they applied are : 20 i) Reducing conductor cross section area ii) Reducing insulation thickness iii) Reducing amount of steel core material iv) Developing fast manufacturing techniques to reduce labour cost [10] The data of previous years shows that the machine have more problems which are manufactured in the last ten years as compared to previous fifty years. We see the manufacturing designs of electrical machine as W/kg. W/kg. can be defined as the wattage of machine divided by the weight of machine.



6.5.2. MANUFACTURING EFFECTS :

Manufacturing and selling of the machines are now a global factor. This has increase

the competition between the manufacturer. This has put pressure on the designers to reduce the

cost of the machine. Some of the methods which they applied are :

i) Reducing conductor cross section area

ii) Reducing insulation thickness

iii) Reducing amount of steel core material

iv) Developing fast manufacturing techniques to reduce labour cost.



LIST OF ABREVATION

1-UV Single Phase Under Voltage

2-UV Two Phase Under Voltage

3-UV Three Phase Under Voltage

1-OV Single Phase Over Voltage

2-OV Two Phase Over Voltage

3-OV Three Phase Over Voltage

1-A Single Phase angle displacement

2-A Two Phase angle displacement

RTD Resistance temperature detector


Engineering

induction motor protection system seminar report