KSK Paper 6 - MCCBs for Motors

MOULDED CASE CIRCUIT BREAKERS FOR THREE PHASE SQUIRREL CAGE MOTOR STARTER APPLICATIONS – SOME SELECTION CONSIDERATIONS – by: K. Sivakumar, Manager-Training, Larsen & Toubro Limited, Switchgear Training

Centre, Coonoor. (Paper sent for publication in the July 2012 issue of Electrical India magazine)



Introduction: Motors – AC three-phase squirrel cage induction motors, in particular – consume about 75% of the total electric power consumed in any industry. And, they are the vital elements in any production process, being the prime movers of the various mechanical and process drive applications. As such, smooth starting and protection of such motors are of prime concern to any electrical system design engineer and/or switchgear application engineer. Motor Starting Current: Any electrical design engineer and/or switchgear application engineer is only too well aware of the starting current or the locked rotor current encountered while starting any induction motor. The magnitude of such starting current can vary from about 5 times to 8 times of the rated full load current of the motor, depending upon the motor design & construction. The magnitude of the starting current for any given motor is given by the motor manufacturer in the motor catalogue or data sheet. While the magnitude of the starting current is the phenomenon of the motor, the duration of the starting current is the phenomenon of the connected load or the driven load. For example, if for a given motor, the starting current is 6 times its rated full load current, then, if the motor is drivin g a centrifugal pump, this starting current of 6 times the rated full load current, might last only for about 10 seconds or less. Else, if the motor is driving a centrifugal blower, this starting current of 6


Centre, Coonoor. (Paper sent for publication in the July 2012 issue of Electrical India magazine) times the rated full load current, might last, for about 20 seconds or even more, depending upon the inertia of the blower’s rotor .

Careful consideration must be given to both the magnitude and the duration of the starting current, while selecting starter components like the short circuit protective device (SCPD), switching device like a contactor, overload protective device (OLPD), etc., so that there are no nuisance trippings of the OLPD and/or the SCPD and no damage to the starter components. And, many a times electrical designers and switchgear application engineers do pay due attention to this issue of starting current (magnitude & duration) and do select the starter components accordingly. Thus, nuisance tripping and/or damage to the starter components are avoided. But.... Motor Inrush Current: There is a much more severe and unrealised component of the motor starting current, which still causes nuisance tripping and/or damage to starter components. It is the magnetising inrush component of the motor starting current. Any electrical person is aware that a transformer draws a very high magnetising inrush current at the time of switching-on. This is due to the core excitation requirements. One must also have studied that an induction motor is nothing but a transformer, with a short-circuited, rotating secondary. Haven’t we? Thus, there would be inrush current while switching-on an induction motor too, as in the case of a transformer. Note that this inrush current can be experienced by the motor even during transition from star to delta in an open-transition type star-delta starter.

12

CURRENT 6 IrM

IrM

TIME

TYPICAL CURVE OF MOTOR CURRENT DURING A D.O.L. START



TYPICAL CURVE OF MOTOR CURRENT DURING AN OPEN TRANSITION STAR-DELTA START One can call this, the ‘transient component’ of the motor starting current. (This could be a misnomer, as motor starting current itself is transient in nature). May be, one can call the inrush current as a ‘sub-transient component’, the starting current as the ‘transient component’ and the running current as the ‘steady-state component’ of motor current. The magnitude of such

magnetising inrush current in motors would vary, depending upon the point of the voltage waveform at which the motor is energised, the amount of residual flux in the motor, etc. It could reach a value as high as 12 times the motor rated current. And, the duration could be a few cycles too. Under such conditions, the SCPD provided in the motor starter might sense this high magnitude current as a fault & could cause nuisance tripping and the motor could never be started. Due consideration must be given to the inrush current too, while selecting switchgear components for motor starter feeders. Various Starter Component Combinations: Today, an electrical system engineer has a wide choice of switchgear components, while designing a motor starter. One can choose between: i)

Switch + Fuse + Contactor + Overload Relay


Centre, Coonoor. (Paper sent for publication in the July 2012 issue of Electrical India magazine) ii) MCCB + Contactor + Overload Relay iii) MPCB (with ‘built-in’ overload & short circuit protection) + Contactor Among the above three combinations, we will consider the MCCB + Contactor + Overload Relay combination in this paper, because many a times, it is observed that there is ample scope for wrong selection of MCCB for this application. MCCBs for Motors: It is observed that when a customer wants to provide an MCCB for his motor feeder, either as a fresh application or as a replacement for an existing Switch-Fuse combination, he simply calculates (or refers to a switchgear selection chart) the required rating of such an MCCB. Then he simply procures the most economical type of MCCB available in the market place, without realising that it might not be suitable for the given motor application. Thus, he ends up having nuisance tripping of the motor and/or damage to the starter components, thus causing avoidable production downtimes. While selecting an MCCB for motor application, it MUST be ensured that the Instantaneous Release setting in the MCCB is set to a value higher than the highest anticipated magnetising inrush current, while switching-on the motor. How to calculate this value? Typical Magnitude of Staring Current (I st): 6 to 8 times the motor rated current (IrM) Tolerance for Locked Rotor Current as per IS 325 (Table -1): + 20%. So, the maximum anticipated starting current: 1.2 x 8 = 9.6 IrM Transient inrush current (peak): √2 x 9.6 IrM = 13.57IrM Also, vide IS/IEC 60947-2, Clause 7.2.1.2.4, there is a tolerance of +/- 20% in the values of current for tripping of the circuit breaker by short-circuit releases. This means that, even with the -20% tolerance, the MCCB shall not trip for the above magnitude of the magnetising inrush current. This also means that the effective setting of the instantaneous release setting in the MCCB should be above 13.57 / 0.8 = 16.96 I rM; But, this is peak value. Whereas the instantaneous setting in the MCCB is in RMS value. Hence, the above value has to be converted to RMS value; (i.e.) 16.96 / 1.414 = 11.99 I rM; Thus, the instantaneous setting in the MCCB has to be above 11.99 IrM or say, it should be 12 IrM. Assuming that the rated operational current (I n) of the MCCB is the same as that of the rated full load motor current (I rM), then, the instantaneous setting in the MCCB shall be set to 12 In. Selection of MCCB: Now, assuming that the user selects a conventional feeder protection MCCB – the instantaneous release setting in these MCCBs range from 3 In to 9 In. Even with MCCBs with modern Electronic / Numerical Protection Releases, the instantaneous setting range is 6 to 10 I n.


Centre, Coonoor. (Paper sent for publication in the July 2012 issue of Electrical India magazine) Thus, there is a chance that the MCCB might trip during switching ON of the motor, due to the inrush current, even if kept at the maximum possible instantaneous setting. It is to be noted that such spurious tripping might not happen all the times. Depending upon the instant of switching and the residual flux in the motor, if the inrush current is more than the instantaneous setting made in the MCCB, then only it would trip. Otherwise, it wouldn’t. Hence,

one might hear complaints from the user about this strange “arbitrary” tripping behaviour of the MCCB’s during motors starting (“sometime it trips, sometime it doesn’t”). To ensure that the MCCB wouldn’t trip at all for the above inrush conditions, one must select an

MCCB, whose instantaneous release setting is more than the worst case magnetising inrush current, as calculated above. One might opt for a special Motor Protection Type MCCB, whose instantaneous setting is fixed at 12 I n. Contactor & Relay selection with Motor Protection Range MCCBs: It is also observed many times, that whenever a customer wants to replace his Switch-Fuse Combination in a motor starter feeder with an MCCB, he simply removes the Switch-Fuse Combinations and replaces the same with an equally rated MCCB. Unfortunately, the user does not pay any attention to the adequacy of the short time withstand capabilities of the contactor & the overload relay, with the revised SCPD (i.e.) MCCB. It is to be noted that when a particular frame of contactor and relay were suggested for use with a 2

switch-fuse combination unit, they were sized based on the I t let through energy of the Fuse, which would clear a short circuit. As HRC fuses are fast acting (typically operating within 4 milliseconds for a heavy short circuit fault), the energy let through would also be less during a short circuit and thus the contactor and relay would be subjected to a lesser degree of short circuit stresses. Hence, they would be sized accordingly. But, when the switch-fuse combination is replaced with an MCCB of equal rating, now during a short circuit, the MCCB would clear the fault. Even with the most advanced current limiting type of MCCBs, the fault clearing time would be around 10 milli-seconds and thus the energy let through would be higher during a short circuit, than that with a HRC Fuse. Now, the smaller contactor and the relay – which were actually selected for the shorter let-thro with HRC fuses – could not handle the increased let-thro, now with MCCBs and thus would damage. Thus, what was Type-2 Coordination with Switch-Fuse combination gets converted into Type-1 Coordination with MCCB now. Thus, it leads to damage of starter components and increased production downtimes. To avoid this, the adequacy of the contactor and relay to tolerate the increased let-through energy - during a short circuit – with an MCCB as a back-up switching and protection device must be verified & established. This can be easily established by truly following the Type-2 Coordination based Switchgear Selection charts with MCCB + Contactor + Overload Relay combination, which is published by the switchgear manufacturer.


Centre, Coonoor. (Paper sent for publication in the July 2012 issue of Electrical India magazine) Conclusion: It is hoped that the reasons for the arbitrary spurious trippings of MCCBs during motor starting are well understood now. It is also hoped and wished that proper selection guidelines be followed by customers and end users, while opting for an MCCB as a back-up switching & protection device in motor starter feeder applications.

KSK Paper 6 - MCCBs for Motors

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