THE HONG KONG POLYTECHNIC UNIVERSITY D ELECTRICAL …

THE HONG KONG POLYTECHNIC UNIVERSITY

DEPARTMENT OF ELECTRICAL ENGINEERING

Project ID: PT_FYP_24

Induction Motor Protection System

by

Leung Ka Ho

14122373D

Final Report

Bachelor of Engineering (Honours)

in

Electrical Engineering

Of

The Hong Kong Polytechnic University

Supervisor: Edward Lo Date: 11th

November, 2017



Abstract

Three-phase induction motors are commonly used for industrial, commercial or even

residential proposes. To achieve optimal operation of 3-phase induction motor, the

voltage of each phase should be equal. When abnormal voltages are supplied to the 3-

phase induction motor, the efficiency of the motor may be greatly reduced and the motor

may even be damaged. Therefore, a protection system is required to stop the 3-phase

induction motor when the mentioned abnormal conditions occurred.

The protection system mainly contains two parts: relay circuit and electronic circuit. In

relay circuit, 12VDC is supplying 4 nos. of 12VDC NC relays and 1 no. of 3CO NO relay

in series. 3 of 12VDC NC relays are for under-voltage & over-voltage detection of the

three phases and 1 of them is for overheating detection. Normally, the 3CO NO relay is

energized by 12VDC which links the connection between the 220VAC power source to

the three-phase induction motor. When under-voltage, over-voltage, single phasing or

overheating occurred, the corresponding NC relay would be energized and, therefore,

opens the circuit. Power would be stopped supplying to the load due to the de-energized

of 3CO NO relay. Another part is electronic circuit. Considering one of the phases,

220VAC is first stepped down to 12VAC by using a step-down transformer. The 12VAC

would then be converted to 12V DC as an input voltage and also regulated to 5VDC as a

DC voltage source. Op-amp is used as a comparator which compares the input voltage

with a pre-set voltage. The output would go high when the input voltage is out of

acceptable range, which leads to the saturation of transistor and finally energized the

12VDC corresponding NC relay. As for overheating detection, an NTC thermistor is

powered by the 5VDC voltage. The surrounding temperature of the thermistor would

affect its resistance, causing changes in input voltage.

Three 12V lamps are used to replace the 3-phase induction motor for simulation. A

variable resistor of each phase is adjusted in order to simulate the conditions of over-

voltage, under-voltage, single-phasing and over-heating. 220V±20% and 31°C of over-

heating temperature are chosen as the requirement in the simulation. As the input voltage

of comparator under stable and healthy voltage supply is 2.5V, the fault LED lamp of the

corresponding phase will light up when the input voltage is higher than 3.0V or lower

than 2.0V (2.5V±20%) and hence stop the motor immediately. Moreover, the input

voltage for overheating detection is 2.8V, calculated by applying voltage divider. The

fault LED lamp of overheating will light up when the input voltage is higher than 2.8V.



Table of Content

1. Introduction --------------------------------------------------------------- P.1

2. Objective --------------------------------------------------------------- P.4

3. Background --------------------------------------------------------------- P.5

4. Methodology --------------------------------------------------------------- P.7

4a. Component used --------------------------------------------- P.9

Transformer --------------------------------------------- P.9

Relay ----------------------------------------------------- P.10

Full-wave Bridge Rectifier -------------------------- P.11

Voltage Regulator ----------------------------------- P.13

Op-amps -------------------------------------------- P.14

NOR Gate -------------------------------------------- P.17

Thermistor -------------------------------------------- P.18

Diode ----------------------------------------------------- P.19

Transistor (BJT) -------------------------------------------- P.20

Resistor ----------------------------------------------------- P.22

Variable Resistor -------------------------------------------- P.24

Capacitor ----------------------------------------------------- P.25

4b. Procedure ------------------------------------------------------ P.26

5. Simulation --------------------------------------------------------------- P.45

6. Discussion --------------------------------------------------------------- P.54

7. Conclusion --------------------------------------------------------------- P.55

8. Reference --------------------------------------------------------------- P.56

9. Appendix --------------------------------------------------------------- P.57



1

1. Introduction

This project aimed to design a protection system of an induction motor to protect a 3-phase

induction motor under conditions including under-voltage, over-voltage, single phasing and

over-temperature.

3-phase induction motors are commonly used for industrial, commercial or even

residential proposes. MCB (Miniature Circuit Breaker) is generally used to protect the

motor from over-current. However, it may not be enough to keep the motor operating in

good condition. When unbalanced voltage is supplied to the motor, the torque and speed

of the motor would be affected. The motor may also be overheated when the ambient

temperature is too high. All the above conditions may lead to reduced efficiency or even

damaged motor. Therefore, protection system is important for three-phase induction

motor.



2

The 3-phase induction motor protection system can be divided in 2 parts: The Electronics

Circuit and The Relay Circuit:

Fig.1 Block Diagram of Induction Motor Protection System



3

The basic operation of this protection system is to stop supplying power to the motor

immediately in the case of under-voltage, over-voltage, single phasing and overheating of

motor. The supply voltage is stepped down and rectified for an electronic circuit and a relay

circuit. The rectified voltage is further regulated to be a DC voltage source for electronics

circuit. The electronics circuit mainly compares the input voltages from the three phases

and thermistor with pre-set voltages. It would give signals to relay circuit if any of the input

voltages are out of expected ranges. The relay circuit directly controls the voltage supply of

the induction motor. It would cut of the supply to the induction motor when signal from the

electronic circuit received.



4

2. Objective

Cut off the power supply to the induction motor when:

- Under-voltage

- Over-voltage

- Single-phasing

- Overheating

Same voltage source used for supplying both the motor and the protection system

Customizable upper-limit and lower-limit of voltage supply

Customizable upper-limit of operating temperature



5

3. Background

As a three-phase induction motor, all the three phases have to be supplied by balanced

voltages to achieve optimal operation. When one of the phases is missing, the voltages will

be unbalanced. William H. Kersting has concluded that single-phasing may be happened

due to blown fuse or activated protective device on one of the phase of the motor [1]. The

motor will keep operating when it is without any protection system [1]. Overheat of motor

will be occurred when the voltages are unbalanced. Moreover, the efficiency of the motor

is greatly reduced when the terminal voltages are single phased because there is a lot of

energy converted into heat instead of power. The motor may also be damaged if the

operation of the motor is not stopped immediately. It may also lead to extra load and

further cause additional electricity charges [2]. In addition, there might be some abnormal

conditions of power supply which causing the voltage supplied to the motor is out of the

expected range while no phases are missing. It is found that different kinds of serious

problems may be occurred if protection system did not apply to an induction motor.

Fig. 2 A typical automatic circuit reclosers



6

Regarding single-phasing, automatic circuit reclosers are widely applied in distribution

system [3]. It recloses the circuit after instant transient voltage or stops the voltage supply

when the detected voltage keeps abnormal after a number of retries. The automatic circuit

recloser requires less maintenance and avoid single-phasing of the voltage supply.

Compare the recloser with the designed protection system of this report, the recloser is

more expensive and space-occupying. It is also dangerous when a person is touching the

circuit [4]. Moreover, automatic circuit reclosers are usually used at overhead lines which

focus on a distribution system. It may not be necessary to apply if only an induction motor

is focused on. Therefore, a simple, cost-effective, small-sized induction motor protection

system is required to protect an induction motor independently.



7

4. Methodology

The electrical circuit design of the induction motor protection system is shown below:

Fig.3 Electrical Circuit Design for Induction Motor Protection System



8

The protection system mainly contains two parts: relay circuit and electronics circuit. In

relay circuit, 12VDC is supplying 4 nos. of 12VDC NC relays and 1 no. of 3CO NO relay

in series. 3 of the 12VDC NC relays are for under-voltage & over-voltage detection of the

three phases and 1 of them is for overheating detection. In normal situation, the 3CO NO

relay is energized by 12VDC which links the connection between the 220VAC power

source to the three-phase induction motor. When under-voltage, over-voltage, single

phasing or overheating occurred, the corresponding NC relay would be energized and,

therefore, opens the circuit. Power would be stopped supplying to the load due to the de-

energized of 3CO NO relay.

Another part is electronic circuit. Considering one of the phases, 220VAC is first stepped

down to 12VAC by using a step-down transformer. The 12VAC would then be converted

to 12V DC by full-wave bridge rectifier as an input voltage and also regulated to 5VDC

by a voltage regulator as a DC voltage source. Op-amp is used as a comparator which

compares the input voltage with a pre-set voltage. Regarding L1, L2 and L3, the output of

the comparator would go low when the input voltage is out of acceptable range and then

inverted to high by a NOR gate. As for overheating detection, an NTC thermistor is

powered by the 5VDC voltage. The surrounding temperature of the thermistor would

affect its resistance, causing changes in input voltage. The output of the comparator

would go high directly when the input voltage is higher than the pre-set voltage. Any

outputs will lead to the saturation of the corresponding transistor and finally energized the

corresponding 12VDC NC relay.



9

4a. Component Used

Transformer

Transformer is an electrical device which uses electromagnetic induction for voltage

change. It normally consists of 2 windings. The voltage ratio is directly proportional to the

ratio of winding turns.

Fig.4 A general transformer Fig.5 Basic circuit of transformer



10

Relay

Relay is an electrical switch which closes or opens a circuit by using electro-magnet. The

figure below shows an NC relay which normally opened when the relay is not powered.

When enough voltage is supplied to coil terminals, the energized electro-magnet would

attract the armature and hence forcing the common contact switches its position and touch

the NO contact.

Fig.6 General relay Fig.7 Basic circuit of an NC relay

Fig.8 Structure of an NC relay Fig.9 Types of Relay

Normally, relays are described in XPYT, where X refers to number of poles and Y refers to

number of throws.



11

Full-wave Bridge Rectifier

Full-wave bridge rectifier consists of 4 diodes which build a bridge and convert full AC

input voltage into DC voltage.

Fig.10 W06M full-wave bridge rectifier Fig.11 Structure of a full-wave bridge rectifier

In the above full-wave bridge rectifier, 2 legs signed “~” are the AC input terminal while

another two legs with positive sign and negative sign are the DC output terminal.

Fig.12a One of the direction of current Fig. 12b Another direction of current

flow of a full-wave bridge rectifier flow of a full-wave bridge rectifier

No matter the direction of the input current flow, the output direction remains unchanged.



12

However, the output waveform is not smoothed after rectified. Capacitor is usually added

after a rectifier to smoothen the voltage.

Fig.13 The waveform of input voltage, output voltage and output voltage after a

smoothing capacitor of a full-wave bridge rectifier



13

Voltage Regulator

Voltage Regulator is used to turn an input voltage into a constant voltage.

It can be used as a voltage stabilizer and used for electronics devices which required 5VDC

source to activate.

Fig.15 General Circuit of a voltage regulator

Fig.14 7805 Voltage Regulator



14

Op-amp

Op-amp (Operational amplifier) normally used to output a voltage based on the differences

between two input terminals.

Fig.16 Symbol of an op-amp

Ideally, the two inputs are connected with an infinite resistance of resistor in series. It

means that no currents are flowing into the input terminal in ideal situation. Therefore, it

basically only takes the voltage differences between the two inputs to give corresponding

output.



15

The following circuit shows a simply circuit of comparator. The positive side is the

reference voltage while the negative side is the input voltage. When the voltage on positive

side is higher than the voltage on negative side, +V will be the output voltage. Oppositely,

when the voltage on positive side is lower than the voltage on negative side, -V will be the

output voltage.

Fig.17 Basic circuit of a comparator

There is a variable resistor on the positive input so the reference voltage can be adjusted by

applying voltage divider:

Fig.18 Concept of voltage divider



16

Voltage across R2 = Input voltage x R1/(R1+R2)

Normally, comparators are installed in ICs (Integrated Circuits) such as LM339N.

LM339N are commonly used IC which contains 4 nos. of comparators.

Fig.19 LM339N Quad Comparator Fig.20 Structure of a LM339N Quad Comparator

All the four comparators can be operated after voltage is supplied to the IC through

terminals 3 and 12.



17

NOR Gate

NOR gate is a digital component which outputs high only when both the inputs are high.

Fig. 21 Symbol and truth table of NOR gate

It can be used as an NOT gate if the same input goes to both terminal A and B. Similar to

comparators, NOR gates are generally installed in ICs such as 74HC02 Quad NOR gate.



18

Thermistor

Thermistor is a resistor which resistance changes while its temperature changes.

Fig.22 MF52-10310kΩ NTC thermistor

There are two types of thermistors: PTC (Positive Temperature Coefficient) and NTC

(Negative Temperature Coefficient). The resistance of PTC thermistor increases when

temperature rises. Oppositely, the resistance of NTC thermistor decreases when

temperature rises.

The above figure shows a 10kΩ NTC thermistor. Its resistance is 10kΩ at room

temperature (≈25°C)



19

Diode

Diode is an electronic element which current can only pass through it in one direction only.

When current are flowing in reversed direction, the diode will form a barrier and block the

current.

Fig.23 IN4007 Diode

Basically, the P-type material contains electron holes while the N-type material contains

electrons. The two materials touch together and form a depletion layer at PN-junction.

When positvie voltage is applied to the anode of the diode and negatvie voltage is applied

to the cathode of the diode, current can be passed through the diode if the voltage

difference is high enough(≈0.7v) to comeover the PN-junction. When the direction of the

applied voltage inversed, the depletion layer of PN-junction becomes thicker and block the

current.

Fig.24 Basic structure of a diode



20

Transistor (BJT)

Fig.25 BC547 Transistor Fig.26 Basic structure of a BJT transistor

Transistor is a semi-conductor which is normally used as an amplifier or a switching device.

The above figure shows an NPN type bipolar junction transistor (BJT) which has three

terminals: Base, Collector and Emitter.

Fig.27 Graph of Collector Current Ic versus Collector-emitter Voltage Vce of a BJT



21

Transistor can be used as a switch. When there is not enough current flowing into Base

terminal, it does not allow current flowing through from Collector to Emitter. However,

current can flow through from Collector to Emitter when there is enough current flowing

into Base terminal. Hence, the Base acts like a switch between Collector and Emitter.

Typically, the Base-Emitter saturation voltage of a BJT is around 0.7V.

Fig.28 Basic concept of a BJT as a switch



22

Resistor

When current passes through a resistor, power has been dissipated and voltage has been

dropped.

Fig.29 A general metal film resistor

Resistor is commonly used in electronics devices. It can be used for power dissipation,

voltage reduction or current limitation.



23

Generally, there is a color code printed on each resistor in order to identify its resistance

and tolerance. The first, second and third colors represent the first three digits of its

resistance. The forth colors represent the multiplier of the three digits. The fifth colors

represent the tolerance of the shown resistance.

Fig.30 Color code of a resistor



24

Variable Resistor

Variable resistor is similar to normal resistor while its resistance can be adjusted. For

example, the resistance of a 10k Ohm variable resistor can be adjusted between 0Ω to 10kΩ.

Fig.31 A 10kΩ variable resistor



25

Capacitor

Fig.32 A 470uF capacitor

When current flows into a capacitor, the capacitor stores the energy. Once the voltage of

the capacitor is higher than the connect circuit, the capacitor will discharge. It helps to

stabilize the voltage. It can also smooth the rectified voltage. Moreover, different sizes of

capacitor are able to filter different ranges of frequency so capacitors are commonly used

on electronic devices.



26

4b. Procedure

In order to simulate the abnormal conditions of voltage supply, the model of induction

Fig.33 Electrical Circuit of Induction Motor Protection System for simulation



27

motor protection system is built as follow:

There are some changes from the original design for simulation:-

1. 3 nos. of 12V lamp are used instead of a 3-phase induction motor.

2. 12VDC 3P NO Relay is used instead of a 3-phase C/O relay.

3. 220VAC voltage supply is used instead of 380VAC 3-phase voltage which the

220VAC is applying to each phase.

4. After the 220VAC voltage supply is stepped down to 12VAC, the 12VAC is applying

to the lamps in parallel.

5. A 1kΩ resistor is added in series to the lamps to limit the current.

6. The simulation of over-voltage, under-voltage and missing phase is performed by

adjusting the variable resistors VR1, VR4, VR7.



28

Component List

Component Model Quantity(nos.)

220/12VAC Transformer - 1

12VDC 2PDT Relay MY2-GS x1, MY2 x3 4

12VDC 4PDT Relay MY4N-J 1

Full-wave Bridge Rectifier W06M 3

Voltage Regulator LM7805 1

Quad Comparator LM339N 2

Quad NOR Gate 74HC02 2

10k NTC Thermistor 103 1

Diode 1N4007 11

Transistor BC547 4

10kΩ Resistor - 9

2.2kΩ Resistor - 8

1kΩ Resistor - 6

10kΩ Variable Resistor - 10

470uF Capacitor - 4

4700uF Capacitor - 1

LED Light Bulb 4

Table.1 Component List for Electrical Circuit of Induction Motor Protection System for

simulation



29

The following photo shows the model of induction motor protection system.

Fig.34 Model of Induction Motor Protection System



30

1. 220V AC is stepped down to 12V AC by transformer (see Fig.35).

Fig.35

2. 12V AC is then rectified to 12V DC by full-wave bridge rectifier. 470uF capacitor is

applied d after the rectifier to smooth the rectified voltage (see Fig.36).

Fig.36



31

3. All phases are combined together after the voltage is rectified and smoothed (see

Fig.37b).

Fig.37a Fig.37b

4. 12V DC is then regulated to 5V DC by voltage regulator (see Fig.38). The regulator

can keep operating if 1 or 2 phases are missing because all the phases are combined in

Fig.37.

Fig.38



32

5. 470uF and 4700uF capacitors are connected in parallel after the rectified in order to

stabilize the voltage and to filter the noise frequencies (see Fig.39).

Fig.39



33

6. 5VDC is applied to another input of the comparators through the middle variable

resistor VR2, VR5 and VR8 respectively. The pre-set upper-limit of the input voltage

can be set by adjusting those variable resistors. If the input voltage is higher than the

pre-set voltage, it should be regarded as over-voltage.

5VDC is applied to another input of the comparators through the right variable resistor

VR3, VR6 and VR9 respectively. The pre-set lower-limit of the input voltage can be

set by adjusting those variable resistors. If the input voltage is lower than the pre-set

voltage or the input voltage is even lost, it should be regarded as under-voltage.

The smoothed 12VDC of the three phases go to the inputs of the comparator through

the left variable resistor VR1, VR4 and VR7 respectively. Those variable resistors set

the initial input voltage when the supply voltage is stable and healthy. As both the pre-

set voltage for over-voltage and under-voltage are within the range of 0-5V, the initial

input voltage can be set to 2.5V. As a result, the maximum range of the input voltage

is ±100%.



34

Regarding one of the phases, the voltages passed through the left and middle

comparators are the inputs of the comparator for over-voltage (OV) while the voltages

passed through the left and right comparators are the inputs of another comparator for

under-voltage and single phasing (UV) (see Fig.41 & Fig.42).

Adjustment for

initial input voltage

Upper-limit

Adjustment for over-

voltage

Lower-limit

Adjustment for under-

voltage & single

phasing

Fig.40



35

Fig.41

Fig.42 Electrical circuit at Fig.41

OV UV

UV

OV



36

Both the comparators of each phase share the same output, the output would be

grounded and give 0V to the NOR gate when any outputs of comparators go low (see

Fig.43).

Fig.43

Therefore, the output state of comparator would be low when the over-voltage, under-

voltage or single phasing is detected. The output signals go to the BJT transistor after

inverted by NOR gates (see Fig.44).



37

Fig.44

The following table shows the results of the comparator output, let the acceptable

range of input voltage is be X±10%.

Input voltage

(V)

Output

from OV

Output

from UV

Final

Output

State

Output State

after NOR gate

0 High Low Low High

X-10% High Low Low High

X-5% High High High Low

X High High High Low

X+5% High High High Low

X+10% Low High Low High

Table.2 Final Output State of Comparator



38

In fact, the output state of the comparator would be as same as the output state after

NOR gate if the positive and negative inputs are switched:

Input voltage

(V)

Output

from OV

Output

from UV

Final

Output

State

0 Low High Low

X-10% Low High Low

X-5% Low Low Low

X Low Low Low

X+5% Low Low Low

X+10% High Low Low

Table.3 Table of Final Output State of comparator with switched input sides

Due to the fact that the output would be grounded when any outputs of comparators go

low, the output state always goes low. As a result, the two comparators cannot share

the same output in this situation.



39

7. As for overheating detection, a 10kΩ NTC thermistor is used to affect the input

voltage (see Fig.45). 5VDC is applied to the input of the comparator through the

thermistor. When ambient temperature rises, the resistance of the thermistor decreases

and thus increases the input voltage of the comparator.

Similarly, 5VDC is applied to another input of the comparators through the variable

resistor VR10. The pre-set upper-limit of the input voltage can be set by adjusting that

variable resistor. If the input voltage is higher than the pre-set voltage, it should be

regarded as overheating.

The output of the comparator will then go to the transistor without inverting the signal.

Fig.45



40

8. All the 4 outputs go to Base of the transistor. With the help of pull-up voltage (see

Fig.46), current can pass through the transistor from Collector to Emitter when the

output level is high.

Fig.46

Pull-up

Resistor



41

9. The combined 12VDC voltage at step3 also supplied to the relay circuit via the brown

and green wires indicated in Fig.47.

Fig.47

10. The corresponding LED lamp will light up after the transistor is powered for the

indication of fault(see Fig.48).

Fig.48



42

11. At the same time, the corresponding NC relay will also be powered due to the

saturation of transistor (see red arrows in Fig.49). The coil of the relay will be

energized and hence cut the current flowing to the 3-phase NO relay (which is 3C/O

relay in realistic system).

Fig.49

Current Flowing to

3-phase Relay



43

12. When there are no faults, current can pass through all the NC relays and energize the

coil of the 3-phase relay (see Fig.50). The corresponding yellow LED lamp will light

up for indication of no faults (see Fig.51).

Fig.50

Fig.51

Current Flowing to

3-phase Relay



44

Voltage source can then pass through the 3-phase relay and supply to the lamps (see

Fig.52).

Fig.52

Current flows from voltage source to the common terminal of 3-phase relay (Red arrow);

and current flows from NO terminal of 3-phase relay (Blue arrow)



45

5. Simulation

Procedure for Fault Simulation of Induction Motor Protection System

Requirement set:

-The acceptable range of voltage supplied to the load is 220V±20%.

-Regard as overheating when ambient temperature is higher than 31°C.

1) Check to see if the input voltage is stable and healthy.

2) Adjust the resistance of the left variable resistor so the voltage inputted to the

comparator is 2.5V (see Fig.53).

Fig.53

3) Adjust the resistance of the middle variable resistor so the voltage inputted to the

comparator is 2.5V x (1+20%) = 3.0V (see Fig.54).



46

Fig.54

4) Adjust the resistance of the right variable resistor so the voltage inputted to the

comparator is 2.5V x (1-20%) = 2.0V (see Fig.55).

Fig.55



47

5) After step4, the setting of the protection system is done. The system is currently

operating.

6) In order to simulate over-voltage, under-voltage, single phasing of the voltage supply,

the left variable resistor will be adjusted so that the voltage inputted to the comparator

is higher than 3.0V (Fig.56), lower than 2.0V (Fig.57) and 0V (Fig.58) respectively.

The red LED lamp lights up for the fault indication of L1.

Fig.56

Fig.57



48

Apply the simulation in step 6 to L2 and L3. The yellow and blue LED lamps light up

for the fault indication of L2 and L3 respectively:-

Fig.58

Fig.59



49

Fig.60

Fig.61



50

Fig.62

Fig.63



51

7) Try out different combination of faults to check whether the system is working well.

The following figure (Fig.65) shows one of the situations when under-voltage occurred

in L2 and over-voltage occurred in L3 at the same time.

Fig.64

Fig.65



52

8) As for overheating simulation, the expected input voltage is 2.5V which is the voltage

when the system is placed at room temperature (≈25°C). The actual input voltage is

2.57V which is so close to 2.5V (see Fig.66).

9) By checking the table of the using thermistor resistance related to its temperature (see

Table.4), the thermistor resistance is around 7.86kΩat 31°C.

Table.4 Thermistor Resistance versus Temperature

Fig.66



53

The theoretical value of the input voltage at 31°C can then be calculated by applying the

formula of voltage divider:

5 ×10

7.8608 + 10= 2.799 ≈ 2.8𝑉

As a result, the corresponding variable resistor is adjusted so the voltage inputted to the

comparator is 2.8V (see Fig.67)

10) As typical body temperature is higher than 31°C, it should be regarded as overheating

after the thermistor is touched with fingertips for a few second (see Fig.68).

End of Simulation

Fig.67

Fig.68



54

6. Discussion

This induction motor protection system is customizable, small in size, cost-effective,

electrically safe and voltage re-applying available. Users can adjust the pre-set voltage for

over-voltage, under-voltage and over-heating to fit their needs. The system size is relatively

small to an induction motor. The components are not expensive while the system can be

operated effectively. The system design is safe which the motor will be stopped when the

system is not operated due to faults. Also, the voltage supply can automatically re-apply to

the motor after faults disappeared.

However, it may have some disadvantages such as short lifetime, unstable and relatively

low accuracy. The lifetime of the electronic devices is short so maintenance is required.

Also, it may not be suitable for motors or equipment which required very high accuracy of

protection system. The relay would bounce rapidly when the inputs difference of a

comparator is too small. Therefore, it is recommended that a timer relay can be applied to

the induction motor to avoid the impacts to the motor due to the bouncing of relay.



55

7. Conclusion

To conclude, this is a customizable induction motor protection system which is able to

protect the motor from over-voltage, under-voltage, single-phasing and overheating

effectively. However, it may not be suitable for motors or equipment which required very

high accuracy of voltage monitoring. A timer relay may be required to be applied to the

motor to avoid the impacts to the motor due to the bouncing of relay when the inputs

difference of a comparator is too small.

End of Report



56

8. Reference

[1] W. H. Kersting, “Causes and effects of single-phasing induction motors,” IEEE

Transactions on Industry Applications, vol. 41, no. 6, pp. 1499-1505, Nov. 2005.

[2] C.Y. Lee, “Effects of unbalanced voltage on the operation performance of a three-

phase induction motor,” IEEE Transactions on Energy Conversion, vol. 14, no. 2, pp.

202-208, Jun. 1999.

[3] E. B. Agamloh, S. Peele and J. Grappe, “Induction motor single-phasing performance

under distribution feeder recloser operations,” IEEE Transactions on Industry

Applications, vol. 50, no. 2, pp. 1568-576, Mar. 2014.

[4] J. D. Glover, M. S. Sarma and T. Overbye, Power System Analysis & Design.

Cengage Learning, 2011



57

9. Appendix

Appendix1. Datasheet of MY2 Relay



58



59

Appendix2. Datasheet of MY2-GS Relay



60



61



62



63

Appendix3. Datasheet of W06M Full-wave Bridge Rectifier



64

Appendix4. Datasheet of 7805 Voltage Regulator



65

Appendix.5 Datasheet of LM339N Quad Comparator



66

Appendix.6 Datasheet of 74HC02 Quad 2-Input NOR Gate



67

Appendix.7 Table of MF52-103 Thermistor Resistance Rated to Temperature



68



69

Appendix.8 Datasheet of 1N4007 Diode



70



71

Appendix.9 Datasheet of BC547 Bipolar Junction Transistor

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THE HONG KONG POLYTECHNIC UNIVERSITY D ELECTRICAL …