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Induction Motor Theory.

Induction Motor Design has a major effect on the behaviour and performance of an induction

motor.

Name plate details

1. Manufacturer details

The manufacturer’s name identifies the manufacturer of the motor

The model number identifies the model of the motor produced by the specific

manufacturer.

The Serial number is specific for the manufacturer and identifies the specific

motor and is useful in establishing the age of the motor for replacement parts and

warranties.

2. Type of motor

• Generally describes the starting method including

– DC Motors - Shunt wound, Series wound, Compound wound, Permanent

Magnet, and Universal.

– Single Phase Motors – Split phase, Shaded pole, Permanent Split

Capacitor, Capacitor Start, Capacitor Start-Capacitor Run, Universal

– Three Phase Motors – Squirrel Cage Induction, Wound Rotor,

Synchronous, Reluctance.

3. Duty type

S1 Continuous duty The motor works at a constant load for enough time to

reach temperature equilibrium.

S2 Short-time duty The motor works at a constant load, but not long enough

to reach temperature equilibrium. The rest periods are

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long enough for the motor to reach ambient temperature.

S3 Intermittent periodic duty

Sequential, identical run and rest cycles with constant

load. Temperature equilibrium is never reached. Starting

current has little effect on temperature rise.

S4 Intermittent periodic duty

with starting

Sequential, identical start, run and rest cycles with

constant load. Temperature equilibrium is not reached,

but starting current affects temperature rise.

S5 Intermittent periodic duty

with electric braking

Sequential, identical cycles of starting, running at

constant load and running with no load. No rest periods.

S6 Continuous operation

with intermittent load

Sequential, identical cycles of running with constant load

and running with no load. No rest periods.

S7 Continuous operation

with electric braking

Sequential identical cycles of starting, running at

constant load and electric braking. No rest periods.

S8

Continuous operation

with periodic changes in

load and speed

Sequential, identical duty cycles run at constant load and

given speed, then run at other constant loads and speeds.

No rest periods.

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4. Enclosure type

5. Frame size

a. Two Digit Frame Size – Indicates a fractional horsepower motor of less than 1

horsepower.

i. This number is the distance from the center of the drive shaft to the

center of the bottom of the mount in sixteenths of an inch.

b. Three Digit Frame Size – Indicates an integral horsepower motor 1

horsepower or greater

i. Divide the first two digits by four to calculate the distance from the

center of the driveshaft to the center bottom of the mount in fourths of

an inch.

ii. Example 145 – 14 divided by 4 = 3.5 inches from the shaft to the

mount.

6. Insulation class

7. HP

8. RPM

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9. Volts

10. Amperes

11. Cycle

12. Type of bearing

13. Phase

14. Efficiency

15. Power factor

16. Ambient temperature

i) Stator design.

The stator is the outer body of the motor which houses the driven windings on an iron core.

In a single speed three phase motor design, the standard stator has three windings, while a

single phase motor typically has two windings. The stator core is made up of a stack of round

pre-punched laminations pressed into a frame which may be made of aluminium or cast iron.

The laminations are basically round with a round hole inside through which the rotor is

positioned. The inner surface of the stator is made up of a number of deep slots or grooves

right around the stator. It is into these slots that the windings are positioned. The arrangement

of the windings or coils within the stator determines the number of poles that the motor has.

A standard bar magnet has two poles, generally known as North and South. Likewise, an

electromagnet also has a North and a South pole. As the induction motor Stator is essentially

like one or more electromagnets depending on the stator windings, it also has poles in

multiples of two. i.e. 2 pole, 4 pole, 6 pole etc.

ii) Rotor Design.

The Rotor comprises a cylinder made up of round laminations pressed onto the motor shaft,

and a number of short-circuited windings.The rotor windings are made up of rotor bars

passed through the rotor, from one end to the other, around the surface of the rotor. The bars

protrude beyond the rotor and are connected together by a shorting ring at each end. The bars

are usually made of aluminium or copper, but sometimes made of brass. The position relative

to the surface of the rotor, shape, cross sectional area and material of the bars determine the

rotor characteristics. Essentially, the rotor windings exhibit inductance and resistance, and

these characteristics can effectively be dependant on the frequency of the current flowing in

the rotor.

A bar with a large cross sectional area will exhibit a low resistance, while a bar of a small

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cross sectional area will exhibit a high resistance. Likewise a copper bar will have a low

resistance compared to a brass bar of equal proportions.

NEMA design A

maximum 5% slip

high to medium starting current

normal locked rotor torque

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normal breakdown torque

suited for a broad variety of applications - as fans and pumps

NEMA design B

maximum 5% slip

low starting current

high locked rotor torque

normal breakdown torque

suited for a broad variety of applications, normal starting torque - common in HVAC

application with fans, blowers and pumps

NEMA design C

maximum 5% slip

low starting current

high locked rotor torque

normal breakdown torque

suited for equipment with high inertia starts - as positive displacement pumps

NEMA design D

maximum 5-13% slip

low starting current

very high locked rotor torque

suited for equipment with very high inertia starts - as cranes, hoists etc.

Selection

More than one type of motor may be used in many applications. Four key factors play a

prominent role in the process of selecting a motor that will be compatible with a particular

piece of equipment.

1. Mechanical requirements of the driven load.

2. Electrical system supply.

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3. Physical and environmental conditions.

4. Efficiency and economic considerations.

The use of electric motors involves high concentrations of electrical power, which is

potentially very dangerous. Hazardous operations diminish through the proper selection,

installation, and use of a motor. When selecting a motor, one should consider the overall

safety of the system, as well as the risks inherent to particular motor types.

Most of the information pertinent to the motor selection process is present on the original

motor's nameplate. Ultimately, motor selection is a compromise among these four

considerations and the availability of particular motor types.

Mechanical Requirements Of A Driven Load

A motor must produce enough torque to accelerate from a standstill to operating speed, and

supply enough power for all possible demands without exceeding its design limits. Selecting

the right motor for the application is not always a simple task.

External Speed Control Drives

The speed at which a load is driven can be adjusted using devices external to the motor

drives. These drives convert the rated speed of the motor to the compatible load speed. The

type of drive used is an important aspect of motor selection.

Direct Drives

With the use of direct drive systems, very few power losses occur. The direct drive offers the

most efficient transfer of power from motor to load of all drive types.

Direct drive motor and load shafts connect by a coupling. Where the motor and load shafts

are misaligned, or if a motor's speed is not controllable, direct drives function poorly. A

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flexible coupling, to correct this, allows slight misalignment while minimizing the

transmission of adverse thrust to motor bearings.

As an added advantage, direct drives require very little maintenance.

Belt & Pulley Drives

A belt drive has at least two pulleys. Connected to the motor shaft is the drive pulley. The

driven pulley connects to the load shaft. A belt joins these pulleys, transferring power from

the motor to the load. Belt and pulley drives are low in cost, and capable of speed variation

through alterations in pulley size. This type system is not as efficient as a direct drive, since

wear and loosening of the belts results in wavering efficiency. Because of this, these systems

require heavy and frequent maintenance.

Chain & Sprocket

Chain and sprocket drives resemble belt and pulley systems. They are, however, capable of

transmitting more power, since metal chains don't slip as pulley belts do. Chain and sprocket

systems cost more than belt and pulley systems, but are more efficient. Drive efficiency

diminishes rapidly as the chain and sprocket components wear-out, so these systems require a

significant amount of maintenance.

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Gear Drives

Certain drives have shafts placed closely together to transmit large amounts of power. These

drives use gears. Geared drives are more expensive than others but are nearly as efficient as

direct drives. As long as gears remain well lubricated, they do not wear out as quickly as belt-

pulley or chain-sprocket systems. Other than regular lubrication, gear drives require very

little maintenance.

Variable or Adjustable Speed Drives

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Variable or adjustable speed drives have been used for more than 20 years in a wide range of

applications. Ranging from single motor driven pumps, fans and compressors, to highly

sophisticated multi-drive machines, variable speed drives have the ability to change motor

speed to meet the needs of a driven load. Controlled speed allows more efficient processing

and reduces energy consumption.

Variable Speed Drives use solid state devices to vary supply voltage frequency. They allow

for a constant ratio of voltage to frequency.

Variable Speed Drives are relatively expensive. However, these devices provide an

exceptional degree of control over motor operations. Furthermore, the energy savings

resulting from variable speed drives more than offsets the higher first cost.

Rectifier section

Soft charge circuit

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On larger drives, 22 – 450kW (30 – 600Hp), a part of the rectifier section is known as the soft charge circuit, which is used to power up the drive. With this circuit, when power is applied, the inrush of current is restricted going to the large capacitors in the DC Link, so that they may charge up slowly (within a couple of seconds). If this circuit was absent, line fuses would be blown every time the VFD was started. The soft charge circuit on some of the VFDs has a resistor or two in line with the current to slowly allow charging of the capacitors. This current resistor even has its own safety, a thermal switch, which shorts out if the current rush is too high in the soft charge circuit. The shorted thermal switch blows fuses on the soft charge circuit preventing the drive from starting.

Intermediate circuit

Using a large bank of capacitors and DC reactors the rippling DC voltage becomes more stable

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Brake circuit

Inverter section

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MOTOR TROUBLESHOOTING

No Noise

A. Check the input power to the motor and motor starter.

B. Check the motor and starter overload devices.

C. Check the motor and starter wiring connections.

D. Check the driven load.

E. Check the size of the motor versus the load.

F. Check the motor windings.

Starts If Shaft Is Spun

A. This will occur with a split phase or capacitor start motor only.

B. Check to see if the start winding is burned out.

If so, replace the winding or rewind motor completely.

C. Centrifugal start switch may be bad or contacts dirty

Disassemble motor in order to make the appropriate repairs.

D. Start winding lead may be loose

Disassemble motor in order to make the appropriate repair.

Check the centrifugal starting switch. It may have dirty contacts. If the starting winding is

burned out, the motor may start if the shaft is turned. The starting winding should also be

checked to see if it is loose or burned out. If the motor has a starting capacitor, check to see

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that the capacitor is good and replace the capacitor if it is bad. Check the bearings to see if

they are locked and will not allow the shaft and rotor to turn.

Hums / Heats Up

A. Verify correct voltage

B. Check starting windings/switch

C. Check for single phasing

D. The rotor or bearings may be locked.

Shuts Off Before Full Speed

A. Motor also hums (single phase motor)

1. Run winding may have a loose connection - disassemble motor in order to make the

appropriate repair

2. Run winding is burned out - motor must be replaced or rewound

B. Overloaded

1. Reduce the load.

C. Excessively low voltage.

1. A reasonable overload or voltage drop of 10-15% will reduce speed only 1 to 2

percent.

2. A report of any greater drop would be questionable.

D. Inaccurate method of measuring RPM

1. Check meter using another device or method.

E. Wrong connections

F. Bad capacitor

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Shuts Off After Reaching Full Speed

A. Overload

B. Bearings

C. Defective overload protection

D. Fuses

Starts Slowly

10 or more seconds on small motors - 15 or more seconds on large motors

A. Bad capacitor

B. Overloaded

C. Low voltage

D. Wrong connections

E. Across the line start.

1. Excessive voltage drop

(5-10% voltage drop causes 10-20% or more drop in starting torque).

o Check connections between power supply and motor.

o Check conductor size between power supply and motor.

o Consult power company/check system.

2. High Inertia Load.

o Reduce starting load.

o Increase motor size.

F. Reduced voltage starter

1. Loss of starting torque

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a. Y-Delta - starting torque reduced to 33%.

o Reduce starting load

o Increase motor size.

o Choose type of starter with higher starting torque.

o Reduce time delay between 1st and 2nd step on starter

b. Part winding starter - starting torque reduced to 50%.

o Reduce starting load

o Increase motor size.

o Choose type of starter with higher starting torque.

o Reduce time delay between 1st and 2nd step on starter.

c. Auto transformer starter - starting torque reduced 25% to 64%.

o Reduce the starting load

o Increase the motor size.

o Choose a different model of starter with higher starting torque.

o Reduce the time delay between the 1st and 2nd starter steps.

o Get the motor across the line sooner.

Runs Excessively Hot

A. Overloaded

1. Reduce load or load peaks

2. Reduce number of load starts/cycles

3. Increase motor size

B. Blocked Ventilation:

1. Fan cooled motors

o Clean external ventilation system

o Check fan operation

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2. Open motors

o Blow out internal ventilation passages.

o Eliminate external interference to motor ventilation.

C. High ambient temperature over 40°C (104°F)

1. Reduce ambient temperature or provide outside source of cooler air.

D. Unbalanced input current.

1. Balance supply voltage.

2. Check motor leads for tightness.

E. Single phased

1. Eliminate the single phase condition.

F. Drive belt too tight

G. Bad capacitor

H. Worn bearings

I. Wrong connections

J. Dirt, dust, trash

Runs Noisily Under Load

A. Motor hums excessively

1. The motor may not be connected properly

2. Motor is single phased

3. Improper overload relays

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4. There may be a winding fault or a short circuit in the motor

5. Air gap between rotor and field stators may be uneven

6. The rotor is unbalanced or the PTO shaft is bent

B. Noisy bearings (listen to bearings)

1. Smooth mid range hum - OK.

2. High whine - internal fit of bearing too tight.

3. Low rumble - internal fit of bearing too loose.

4. Rough clatter

o Make sure mounting is solid level.

o The bearing may have been destroyed.

C. Mechanical noise

1. Driven machine or motor noise

2. Motor noise amplified by resonant mounting

3. Driven machine noise transmitted to motor through drive

4. Misalignment on close coupled application - improve alignment.

D. Clicking sound is regularly heard

1. Check for foreign matter in the air gap

2. The cooling fan may be hitting the end bell or housing

E. Rapid knocking sound heard on all types of motors - motor is out alignment causing

excessive end play.

F. Check for brush chatter.

Never Reaches Normal Operating Speed (RPM)

A. Motor also hums (single phase motor)

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1. Run winding may have loose connection

2. Run winding may have burned out

B. Possible overload

C. Excessively low voltage

D. Inaccurate method of measuring RPM

E. Wrong connections

F. Bad capacitor

Smells Funny / Smokes

A. Smell something burning or overheating or see smoke

1. The motor is overheating.

2. The cooling air flow may be restricted.

o Clean motor by wiping off excess dirt

o Use compressed air to blow dust particles out of passageways

3. Bearing problem may be causing an overload

o Bearings are overheated

o Too much lubrication of the bearing

o Bearings are bad

4. Short in the windings

B. Smell of ozone - burning windings

High Input Current

A. Accuracy of ammeter readings

B. Motor running idle

C. Motor running loaded

1. Motor overloaded

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2. Motor voltage rating does not match power system voltage.

Unbalanced Input Current

5% or more deviation from the average input current

A. Unbalanced line voltage due to:

1. Power supply

2. Unbalanced system loading

3. High resistance connection

4. Undersized supply lines

B. Defective motor

If there is doubt as to whether the trouble lies with the power supply or the motor,

perform the following test:

Rotate all three input power lines to the motor by one position

If the unbalanced current pattern follows the input power lines, the problem is in the power

supply.

If the unbalanced current pattern follows the motor leads, the problem is in the motor.

{NOTE: A small voltage unbalance will produce a large current unbalance.}

Excessive Voltage Drop

More than 2 or 3% of nominal supply voltage

A. Excessive starting or running load

Reduce load.

B. Inadequate power supply

Consult power company.

C. Undersized supply conductors

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Increase supply conductor line sizes.

D. High resistance connections

Check motor leads and eliminate poor connections.

E. Each phase lead run in separate conduits

All 3 phase leads shall be in a single conduit, per National Electrical Code. (This

applies only to metal conduit with magnetic properties.)

Excessive Vibration

A. Check alignment of motor and driven load - check and realign motor with the driven

machine.

B. Check for Unbalance in Motor and Driven Load

1. Check Motor Mounting

o Be sure motor mounting/shims are tight and solid.

o Check grouting for cracks

2. Check Driven Load

o Disconnect belt or coupling

o Restart motor

o If vibration stops, the unbalance is in the driven load.

3. Check Sheaves or Coupling

o Remove sheave or coupling

o Securely tape 1/2 key in shaft keyway

o Restart motor

o If vibration stops, the unbalance is in the sheave or coupling.

4. Check the Motor

o If the vibration does not stop after checking a, b and c above, the unbalance is

in the motor

o Replace the motor. Make sure the new mounting is proper.

C. Tightness of mounting

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D. Bearings?

E. Load and motor

F. Grounds or shorts?

Overload Relays Tripping

A. During motor starting

1. Slow starting (10 - 15 seconds or more) due to high inertia load.)

o Reduce starting load.

o Increase motor size if necessary.

2. Low voltage at motor terminals

o Improve power supply.

o Increase motor size if necessary.

B. When running loaded

1. Overloaded

2. Unbalanced input current

3. Single phasing

4. Excessive voltage drop

5. Starting or overloading too frequent.

6. Ambient starter temperatures too high.

7. Wrong size or type of relays

Sparks

A. Sparking and flashing coming from the brushes

1. Roughened commutator

B. The commutator is blackened

1. Commutator may have a low bar

2. Commutator may have a high bar

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3. May have a high mica

4. Worn or short brushes

5. Not enough tension on the brushes

6. Brush springs may be weak

7. Brushes stick in holders, do not fully surface on commutator

8. Commutator has dirt or oil on it

9. Water dripping onto the commutator

10. Short in the armature or field windings

Wound Rotor Motor Problems

A. Sparking and flashing from brushes

B. Brushes are worn, stuck, not set

C. Commutator is dirty, rough, has high bars, or is out of round.

D. Voltage is either too high or too low.

E. Governor is either stuck or out of adjustment