ROTOR CURRENT IN SLIP RING INDUCTION MOTORS An interesting Case Study

by K. Sivakumar, Manager – Training, Larsen & Toubro Limited, Switchgear Training Centre, Coonoor – 643 243. Tamilnadu.

(Published in the March 2008 issue of Electrical India – an Industry Magazine)

(Also published in the March 2008 issue of “HYDEL” – the technical journal of the Kerala State Electricity Board Engineers’ Association)

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Introduction: As most of us are aware, there are two broad classifications in Three Phase Asynchronous Induction Motors – viz. – Squirrel Cage Induction Motor and Wound Rotor or Slip Ring Induction Motor. The basic difference is that in squirrel cage induction motors, the stator is wound and the rotor is made up of solid bars shorted internally at both ends, where as in slip ring induction motors the stator is wound and also the rotor will have a winding for each phase, the ends of which will be shorted (star connected) at one end and the other ends of the three rotor windings brought to a slip ring for any external connection. Normally resistances are connected through carbon brushes to these slip rings, to effect an increase in starting torque - at the same time with a reduced starting current. Generally, slip ring motors are employed for high inertia loads, which need a very high starting torque, such as Fans & Blowers.

In a typical rotor-resistance starter, there will be three or four steps of external resistances connected to the rotor circuit, which will be cut or shorted in steps. In the final step, after the motor has accelerated to its rated speed, no external resistance will be in the rotor circuit and the rotor winding will be shorted through the final step contactor in the rotor circuit.

The author wants to share a strange experience that he faced while operating a slip ring induction motor.

Description: The project was a boiler plant consisting of three coal-fired boilers. The motor in case was a 110 kW (150HP), 3-phase Slip-Ring Induction Motor driving the boiler ID Fan. The motor name plate details are as below: (Make is intentionally omitted)

3-Phase TEFC Slip Ring Induction Motor

kW (HP) - 110 (150); Volts - 415 +/- 10%

Frequency - 50 +/- 5% Hz; Type - Slip Ring

Ambient - 45oC; Temp. Rise - 75oC;

Rating - Continuous; Insulation - Class ‘F’;

Protection - IP 55; Duty - S1

Frame Size - 315M; RPM - 1485;

FLC - 192 Amps. Efficiency - 92.5%;

Cos φ - 0.86; GD2 - 20.21 kg-m2;

OC Rotor Volts - 500; Rotor Current - 132A

*****The starter was an air-cooled rotor resistance starter, supplied by one of the leading names in electrical panel building. The switchgear used was also of a very reputed make. The cables used were 1R x 3c x 300 sq.mm AYFY for the stator side and 1R x 3c x 70 sq.mm AYFY for the rotor side.

In the starter, the stator contactor was a 3-pole, 250A, AC-2 Utilisation Category Contactor and there were 4 numbers of 3-pole, 150 A, AC-1 Utilisation Category Contactors for rotor side for the 4-step transition of rotor resistances.

During one of the days of its regular operation, immediately after commissioning, the author - who was then a shift engineer in the plant – during the regular plant round-up, observed that the rotor cable of the above motor was very hot. He immediately opened up the starter door to look for any loose-connections inside the starter. To his surprise he also observed that the rotor side final step contactor in the starter was running abnormally hot. All the connections were tight enough. The author was puzzled. He immediately brought the clamp on ammeter (tong-tester) – only of the analogue type, in those days – and measured the rotor current. He found that the pointer of the ammeter was hunting between zero and 12.5 amperes. This meant that the maximum current that flows in the rotor side contactor and the rotor cable was only 12.5 amperes, as indicated by the meter.

Now, what do you say? Then author could not believe his eyes. A 70 sq.mm Aluminium cable, whose standard current rating is about 130 amperes, heating up for just 12.5 amperes and …. the 150 ampere contactor also had heated up abnormally for just 12.5 amperes.

The author immediately took this to the notice of his superiors in the plant as well as to the panel builder and to the switchgear (contactor) manufacturer. His superiors in the plant could not explain the strange phenomenon as also the panel builder and the switchgear manufacturer. In fact, the switchgear manufacturer even snubbed the author that the author is a mere novice to the field – the author was in the early

days of his career – and thus, should not find fault with the reputed switchgear that is manufactured by them. The problem could be something related to the installation, they said. But, when the author told them that this was happening to all the 15 ID Fan motors in the 5 Project sites that were being handled by the author and all the motors, starters and switchgear were of the same make, the switchgear manufacturer had no reply to explain if the same installation mistake can happen in all the 15 Drives.

Now, that is the puzzle. Can you guess what was happening?

In fact, when shorted, the rotor was drawing the rated rotor current of 132 amperes. But, the frequency of the rotor current was only 0.5 Hz.

(Frequency of the Rotor Current, fr = S x fs

Where,

fr = Frequency of the rotor current in Hz.

S = Slip = [(1500-1485)/1500] = 0.01 in this case

fs = Stator Frequency in Hz. = 50 Hz.)

*****But, the analogue tong tester (Clamp-on ammeter) was designed to measure only the 50 Hz. current. Hence, it could not measure the 132 Amperes that was flowing in the rotor circuit at just 0.5 Hz. Frequency. When it was hunting, it was indicating the 50 Hz. (Ripple) component of the rotor current.

But, the thermal effect of electric current has nothing to do with frequency. The heat produced is I2rt, irrespective of the frequency of ‘I’. So, the entire 132 ampere was heating up the cable as well as the contactor. Even though the standard continuous current rating of the 70 sq. mm Aluminium Cable was about 130 amperes, that would only be under standard conditions of laying. In practice, a lot of deviations would be effected from standard conditions and thus the cable has to be de-rated. Even with a 25% de-rating, the continuous current capacity of the cable comes down to about 97.5 amperes. Hence, while carrying 132 amperes the cable was becoming abnormally hot. So also with the contactor. The contactor had an AC-1 or I th rating of 150 amperes. Ith rating is the Free Air Thermal Current rating and it is an 8-hour rating. But, in the case in study, the contactor was enclosed and it was carrying the current continuously beyond 8 hours. (Uninterrupted current). Now, that would explain the reasons for the contactor becoming abnormally hot.

Now, that’s a good lesson we have learnt. Haven’t we?********