200hp Motor Doesn't Have Enough Torque?

[bytebyte]

A customer has a 3-ph 200hp motor rated 440V. The customer is fed from 3-167 kVA transformers at 277/480V. Full Load Amps of motor is 257A and he does not have info on Locked Rotor Amps or Letter Code. The customer claimed the 200 hp motor did not have enough torque to get past the initial startup and he measured the intial current to peak at only 1400A. If the locked rotor amps is ~6 times the FLA then the inrush current should be at ~1542A. Does anyone know why this motor is not starting?

A potential factor is the operating tolerance(+-10% of 440V) of the motor is not completely within the tolerance of the operating limits of the utility (+-5% of 480V). If the supply voltage is above the voltage tolerance of the motor, wouldn't the motor still start but will overheat and burn? Thanks.

 

[ScottyUK]

You might have a problem with iron saturation in the core if the motor is rated for 440V and the source is 480V. That wouldn't cause a stall, but would result in excessive current draw and the thermal O/L relay tripping. So yes, you may have a problem. Isn't 460 the nominal motor voltage for a 480V system? Where did the 440V motor come from: it's not a European 50Hz type that you are using on a 60Hz system by any chance? That would not be helping matters. Check the rating plate. If the nominal speed is roughly 750, 1000, 1500 or 3000rpm then it's 50Hz; if it's roughly 900, 1200, 1800 or 3600rpm then it's 60Hz.

What is the impedance of the transformer bank? It is likely that the transformers are limiting the starting current by effectively inserting a reactance between the source and the motor. This will show up as reduced voltage at the transformer secondary side, and therefore at the motor. Less voltage equates to less torque. Have you got a fast multimeter or a 'scope with an isolation probe that you can use to monitor the voltage at the motor terminals during the start attempt? Anything more than 20% drop at the motor terminals should start ringing warning bells, and 15% drop is the generally accepted worst case drop at locked rotor conditions. Some loads are more forgiving than others. You need to find out where that drop is occuring - under-sized and / or excessively long cables to the motor, or a transformer bank with too high an impedance. There's a possibility that the MV network could itself be adding to the problem but probably not unless you are on the end of a long low capacity OHL.

Take some measurements and post the results, then see where we go from there.


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If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

I understand that the total transformer power is 3x167=500 kVA?

Assuming a standard Uk equal to 6%, that would give you a voltage drop around 70 V during start, which means 410 V instead of rated 440. That reduces available torque around 13%.

Normally, motors are rated 460 V, so your starting torque is probably down a little more than 20%. Even more if you take voltage drop in the MV grid plus motor cables into account.

If the rotor letter code is unsuitable and your static friction is high, you will have a problem.

The risk of burning the motor because voltage is in the upper tolerance range is very small. Your thermal protection should take care of that - if it ever gets nasty.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Kiribanda]

bytebyte,

"he measured the intial current to peak at only 1400A. If the locked rotor amps is ~6 times the FLA then the inrush current should be at ~1542A"

You must remember that 6x257=1542 A is valid only when the rated voltage of 440 is applied to the motor terminals. Since the torque is propotional to voltage square, when the voltage is dropped at the start, correspondingly the starting current is reduced.

Allowing 15% drop at 480 V bus during starting, if you calculate back, you will need a MINIMUM short circuit level of 8 MVA at 480 V.

It looks like the 277-480 V transformer may be offering a bigger impedance during starting.

Have you calculated the available minimum short circuit level at 480 V?

Hope this helps.