Starting Current of an IM while second is running

[okivrak]

Hello everybody;

We want to use 2 squirrel cage induction motors connected parallel for a water pumping station. At normal operation, only one motor will be running, and if necessary the second one can be started.

The rating of each motor is 370 kVA, 3-phase 380 V with minimum 0.85 PF.

My question is; are there any cicumstances for the starting current of a squirrel cage motor while the other is running? In other words, are the conditions change for starting of an induction machine depending on the present parallel connections?

 

[Shortstub]

Not appreciably. Consider the impedance of the running motor in parallel with the starting one. The resultant impedance will be somewhat less than that of a motor starting alone!

As an example: If the starting current is 6 times that of the normal current, then the resultant impedance magnitude will be about 6/7 or about 86% of that of a starting motor. To be on the conservative side use the starting power factor of that of the starting motor!

 

[jraef]

Electrically speaking, the machine itself would not be appreciably different, but the application conditions probably would. The way I see it, the first motor must supply sufficient pressure to overcome the head pressure in the pipe. The second motor will start into a condition where that has already been done, so all it will do is increase flow from that point. Given the same starting method, motor 2 will conceivably draw the same starting current, but for a shorter time than motor one. I'm not a mechanical engineer however so this is probably something better answered by the folks in the Pump Engineering Forum. Quando Omni Flunkus Moritati

 

[electricpete]

As further illustration, look at fig 4.10-6 which plots all parameters vs time. Torque has initial oscillating component which by eyeball you can see is varying at the same frequency as the plotted curents.

 

[electricpete]

Sorry - my last post ended up in the wrong thread.

 

[DanDel]

okivrak, your main concern should be the voltage drop on starting the second motor while the first is running. This would be the worst-case situation for the system, and a Motor Starting Study may be necessary to determine if the second motor will successfully start and if other parts of the system won't experience excessive voltage drop.

 

[Shortstub]

What is the size of the upstream transformer feeding the bus that supplies the two motors?

 

[okivrak]

System is under project now. In fact, I wanted to decide the size of the transformer in effect of the maximum instananeous current of the system. System consists of two parallel 370 kVA motors. In normal conditions, total possibe current can be determinded. But, at starting conditions, current value could reach some high values. The considered transformer data: 1250 kVA 31500 / 400 V

 

[jbartos]

Suggestion: Also, the protective devices upstream should be checked for proper ratings. If there is the need for an increase in rating, the conductors downstream should be checked for their proper protection.

 

[Shortstub]

The transformer size you propose, 1250kVA, should not present a problem!