PROTECTION OF MOTOR                                 CHAPTER - 6

INTRODUCTION

 General Information 3-Phase Induction Motor

A.C. system is universally used for generation of electricity and so also for transmission and distribution on account of its various advantages. As a electrical drive, A.C. 3-phase and single phase motors therefore find the wide applications in various industries, out of so many types of  3-phase A.C. motors, the 3- phase Induction Motor is very popular in the industries due to the following properties:

1)    Compact, very simple and robust construction.

2)    Shape and weight is small in comparison with other types.

3)    Cheaper in cost.

4)    Very easy for repairs and re-winding.

5)    Maintenance cost is very low.

6)    In operation it requires less attention.

7)    Efficiency is better.

8)    Frictional losses are lesser.

9)    It is a self starting motor.

10) On higher loads it operates at a reasonably better power factor.

11) Starters required for controlling the initial current are of very simple construction and therefore cheaper in cost.

12) Used for various purposes universally under any climatic conditions.

13) Spark less operation.

14) Its speed slightly decreases on load about 4 to 5% only.

In spite of the above different qualities of the 3-phase induction motor following are the drawbacks due to which it is not used in few cases.

Drawbacks:

1)    Wide-range and smooth speed variation is difficult.

2)    Speed variation arrangements are not reliable and not cheaper.

3)    Its efficiency decreases if speed variations.

4)    Starting torque is comparatively low.

Due to the above drawbacks of the motor it is not used where higher starting torque is required and where torquent speed change is required. Otherwise these motors meet the requirements of the industries and find a large range of applications.

CONSTRUCTION DETAILS OF A 3-PHASE INDUCTION MOTORS

1.    Stator Winding: A 3-phase star of delta connected copper winding is put in the stator slots, high qualities insulations are used in the slots and over the windings.

2.    Outer Frame: It serves as a protecting cover and made of cast steel.

3.    Stator: As the name suggest it is a stationary part of the motor. This is of silicon steel stampings(i.e. thin sheets). The stampings are slotted when complete stator is assembled the through slots are formed on the inner side of the stator. The slots may be open type, semi-open type or closed type.

4.    Rotor Winding: It is made of good quality copper and insulated (enameled). In case of squirrel cage rotor it is in the form of round bar. In the second type of rotor it is in the form of Star connected 3-phase winding.

  5.    Rotor:

a)    Squirrel cage type: It is made of silicon steel stampings and have round slots to receive round bar rotor conductor. Conductors are shorted on both side by copper rings, thus rotor windings are shorted.

 6.    Wound rotor: This rotor is used for slip-ring type motor. It is cylindrical and made of silicon steel stampings and has slots to receive a 3-phase star connected rotor winding.ip Rings: Three lip rings are mounted on the shaft and are made of high quality phosphor-bronze.

7.    Carbon Brushes: These are kept in the brush holders and kept touching the slip ring under the tension of the spring.

8.    Other mechanical parts: Shaft, bearings, fans and covers, foundations etc.

MAIN TYPES OF MOTOR

1.    Squirrel Cage Type Motor: Construction includes all the pats explained above the rotor is squirrel cage type and supported on the shaft and bearings and rotates in the space of stator.

2.    Wound Rotor Type Motor: It is also called as slip ring type motor. The construction is similar but uses wound rotor and slip rings. The rotor winding three terminals are connected to three slip-rings so that external resistance can be connected.

It is important to know the various abnormalities which may take place in these motors so that proper protection scheme can be provided. Common faults in the motor are generally of two types: (i) Short circuit fault, (ii) Overload fault.

Short circuit fault is the most severe type fault; however it does not appear frequently. Short circuit fault may be phase to phase short, phase to earth fault, inter-turn fault. Current during short circuit fault may reach very high value (many times of full load current). These faults are very dangerous because rate of rise of temperature of windings and machine pats is very high. High temperature burns out the insulation of windings and other parts. For protection of small motors in this type of fault, fast acting protection should be provided to isolate the motor from the supply.

“Cartridge fuse” provides the protection as it is fast acting and it has a inverse time-current characteristic. This fuse is very compact, cheaper and has operating time very less for high amplitude currents. The drawback is that if out of the three fuses only one blows off, the motor works with single phasing and hence its use is limited.

For medium H.P. motors, instantaneous relays are provided for quick action in short circuit fault. For high HP motors “differential protection” scheme is provided.

Some times “time relays” are introduced to avail frequent tripping due to momentary over loading. Thermal relays using bimetallic strips are also used as protective systems provided in starters. For combined protection of short circuit and overload both fuse and thermal overload relays are provided. Study will continue in the further articles.

BEHAVIOUR OF 3-PHASE INDUCTION MOTOR UNDER ABNORMAL CONDITION.

Due to variation in voltage, change in supply frequency, over load, blowing off fuses, mechanical faults crawling/cogging, single phasing, reverse phasing, phase to phase faults, short circuit etc the motor shows abnormal behavior and may be damaged due to over heating and burning of insulation. Following paragraphs explain such behaviors’ and successive articles suggest the protective relaying for these motors.

Permissible Limits of Variation of Supply Voltage to Motor and Variation of Frequency

The performance of Electric Machines particularly working on A.C. supply depends on voltage and frequency variation. It is expected that the machine shall be capable of delivering rated output at the rated pow3er factor within the variation of voltage limit of +5% to -5% and frequency variation between +1% to -1% and in such variations the temperature shall not exceed more than 5⁰C.

The variation of voltage and frequency beyond these limits affect the performance of the machine.

Behavior of 3-Phase Induction Motor Under Abnormal Voltage

a.        Effect of change of supply voltage on starting torque.

                                                    K₁E₂² R₂

                               Starting torque T_st =  ----------- but E₂ ∞  V₂

                                                                        Z₂²

                                                                                         K₂V² R₂

                                                          T_st =  ----------- but R_2, X_2,, Z₂ Constant.

                                                                        Z₂²

     So T_st is proportional to V² hence starting torque is in the square proportion of supply voltage.                                 

b.            Effect of change of supply voltage on torque under running condition.

                K Φ_sE₂ R₂

 T_st =  -----------  but E_{2, is proportional to}   Φ  _{is proportional to} V _and R_2, X_2,……. Constant

        R₂² + (_sX₂)²            So T ∞ V²_s

Hence, if supply voltage is decreased then torque under running condition decreases. To maintain the same torque therefore slip increases i.e. speed falls.

c.      Effect of change of voltage on current: If the voltage is increased the starting current increases. Also no load current increases. In loaded condition, the current decreases.

d.      Effect of change of voltage on power factor: If the  motor is working on no-load and if the supply voltage increases the magnetizing current is increased to produce more flux. The power factor angle as shown in the figure decreases. On the other hand if the supply voltage decreases power factor increases. The same effect is observed in – on load condition also.

                           

 e.     Effect of change of voltage on efficiency: There is slight change in efficiency. As T is proportional to V, the increase in voltage will increase torque and speed will increase slightly.

Effect of Unbalance Voltage

It may cause to some extend a reversing rotating field.

The table shows the changes of the above discussed terms for 10% increase or 10% decrease of voltage.

Sr  No	Terms Affected	10% Increase in Voltage	10% decrease in Voltage
1	Current: a)     Staring current b)     Magnetizing current c)     Full load current	Increases 10-12% Increases Decreases  7-8%	Decreases 10-12% Decreases Increases 10-12%
2	Temperature rise	Decreases 3-4%	Increases 7%
3	Magnetic Noise	Increases slightly	Decreases slight
4	Starting torque	Increases 20%	Decreases 19%
5	Speed a)     Synchronous speed b)     Full load speed	No change Increases 1%	No change Decreases 1%
6	% slip	Decreases 17%	Increases 23%
7	Power factor at full load	Decreases 3%	Increases 1%
8	Efficiency at full load	Increases 1%	Decreases 2%

Behavior of 3-phase Induction Motor Under Abnormal Frequency :

Generally, the frequency on the large distribution system hardly changes to a large extend. But large change ;may be in the system where electricity is generated on small scale by diesel engines.

The main effect of change of frequency is on the synchronous speed and therefore on the actual speed also. Due to higher speeds the appliances may be damaged earlier and gears are to be used to adjust he normal speed.

If a 50 Hz motor to be operated on 60 Hz frequency, then it will give the normal performance provided the applied voltage to the motor is 6/5 times its name plate rated voltage.

Also reduction in frequency increases the starting current and vice-versa.

The following Table shows the effect of increases of 5% frequency or decrease of 5% frequency on the various terms.

Sn	Terms Affected	By 5% Increase in Frequency	By 5% Decrease in Frequency
1	Current a)     Starting current b)     Magnetizing current c)     Full load current	Decreases 5 to 6% Decreases Decreases slight	Increases 5-6% Increases Increases slightly
2	Temperature rise	Decreases slightly	Increases slightly
3	Starting torque	Decreases 105	Increases 10%
4	Speed a)     Synchronous speed b)     Full load speed	Increases 5% Increases 5%	Decreases 5% Decreases 5%
5	% slip	Small change	Small change
6	Power factor	Slight increase	Slight decrease
7	Efficiency	Slight increase	Slight decrease

Behaviour of 3-Phase Induction Motor Under Single Phasing

Single phasing means, one of the three phases is disconnected from the 3-phase supply so that motor’s two phases are only remain electrically connected. This happens due to burning out of one fuse out of three fuses. Generally, this happens in the operation of star-delta starter or Auto-transformer starter used to start the motor due to defective contact in ‘run’ side of starter.

When single phasing occurs, and the motor is running on load, the speed of the motor suddenly decreases and motor tries to stop or run-slow (stalling). If in such case the motor does not stall and if it is carrying more than half the full load, both stator and rotor will be seriously over heated. Because the load is shared by two phases of winding, instead of three phases and stator and rotor current increases the temperature. The temperature rise in the stator will be most marked in two phases if it is star connected, and in one phase if stator is delta connected.

    Crawling in Induction Motor

AC winding of induction motor produces a flux wave which is not a pure sine wave but it is a complex wave consisting of a fundamental wave (which rotates at synchronous speed) and odd harmonics like 3^rd, 5^th and 7^th  etc (which rotate at Ns/3, Ns/5, Ns/7). Thus in addition to the fundamental torque, harmonic torques are produce. Third harmonic is absent in balanced 3-phase system. 5the harmonic produces reverse braking torque which is negligible. Seventh harmonic torque reaches its positive maximum value just before 1/7^th on synchronous speed beyond which it becomes negative in value. Consequently, the resultant torque may fall below the load torque. Then motor may not accelerate up to its normal speed but may run at 1/7^th of the full speed. This is predominant particularly in squirrel cage type motor. The tendency to run at 1/7^th of the speed due to the harmonic effect is called as ‘crawling’.

Cogging in 3-Phase Induction Motor

When the number of stator slots are equal to number of rotor slots, then there is a possibility of minimum reluctance when stator teeth and rotor teeth face each other. In such case, the rotor teeth tends to remain fixed in front of stator teeth which is called as magnetic locking between the rotor and stator teeth. This magnetic locking due to which the rotor refuses to start is called as ‘cogging’. This effect is more marked at low voltages.

Noise Production of Induction Motor

There are two causes of producing noise.

1.    Electrical causes,   2. Mechanical causes.

1.    Electrical causes: This may be due to wrong connection of winding which makes reversed phase. Due to magnetic cause such as uneven air gap, parasitic magnetic fields are produced by harmonic currents. The leakage flux generate unbalanced magnetic pull encouraging vibration and noise. Asynchronous harmonic torque produce noise and vibration.

2.    Mechanical causes: Vibration of parts, scratching of stator and rotor. Worn out bearing, foreign matter inside the bearing. Fretting corrosion, etching of the races due to shaft currents, excess greases, improper play adjustments.

How to Reduce Noise

1.    Check the phase reversal and rectify the same. Mechanical noise be reduced after locating and rectifying mechanical fault.

2.    Check polarity by giving D.C. supply, check terminal connecting as per diagram.

3.    Check the air gap, rotate rotor through 120⁰, check air gap again, check up worn out bearing, true up rotor and replace the bearings.

4.    Check for balanced line voltage, measure insulation resistance, repair fault.

5.    Without connecting the motor to the supply check bearings, loose connections, loose contacts etc.

Under Voltage

Due to under voltage for the same load motor takes more current and it is over loaded and hence it is to be protected from over loaded by thermal over current relay.

Reverse Phase

Due to change in phase sequence the motor runs in reverse direction. In some cases, it is not desirable for which reverse phase protection is to be provided.

Prolonged Over Loading

Motors are generally designed for 10 to 15% over loading capacity for small period. But if over load persists for a long time then over heating increases the temperature and insulation is damaged. Over load protection is provided to protect the motor.

Short Circuiting

This may be due to stator winding fault or phase to phase short in the supply leads. In such case, the current becomes very high and if the protection like (i) HRC fuses, (ii) Short circuit release, (iii) Instantaneous relaying is provided, so that motor is protected from damaging.

  IN ABNORMAL CONDITION VARIOUS PROTECTIVE SCHEMES ARE ADOPTED AS GIVEN BELOW.

1	Single phasing	Single phasing preventer, thermal relay.
2	Unbalanced voltage of supply	Negative phase sequence relay.
3	Under voltage	Under voltage relays
4	Reverse phase	Phase reversal protection
5	Faults on stator side	HRC fuse, differential protection, instantaneous over current relay.
6	Rotor faults	Instantaneous over current relay.
7	Stalling	Instantaneous over current relay or thermal relay.
8	Over load	Over load release, thermal relay, MCB with trip-coils unibilt, electromagnetic relays.

The suggested relaying schemes are explained in Chapter 3.

Thermal relaying and single phase protections are explained in the following articles. Thermal relays are provided in he respective starters of the motors and hence function of these relays are explained in the starters circuit. In some big motors for over load protection static relays, electromagnetic relays are used to give tripping signal to switches or CBs to disconnected the motor from the supply.