High/Low Voltage Motor 5

[pootong]

Our power plant specification normally stated that motor 200 kW and larger shall be medium or high voltage type. We always have problems with the contractor when it comes to the decision whether medium voltage motor should be used when motor output is slightly higher than 200 kW (let say 220 kW. Is there any recommendation on the selection of motor in this range.

 

[ScottyUK]

In Europe 150 - 200kW is a typical breakpoint for the change from LV to MV. If MV switchgear is already present for other purposes it makes sense to take advantage of it, as it allows more reasonable size conductors to be used and avoids the problems of disturbance to other LV loads caused by starting a large drive. Conversely if there is no MV supply readily available and the motor is marginally over the accepted norm, and there are no sensitve loads sharing the LV transformer, it would make sense to use an LV motor.

In a power plant environment there must be a number of MV or HV drives, so I would suggest that there is no reason why you can't go to MV at a power threshold that you choose. The MV motor may or may not be slightly more expensive than its LV equivalent, but the cabling will be cheaper. The switchgear will likely be more expensive for MV than LV, but will almost certainly not require as much maintenance as an equivalent-rated LV contactor. Remember that the contractor works for you, to your specification.



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

 

[ScottyUK]

It is probably best to direct you toward something like the excellent Network Protection & Automation Guide available as a free download from Areva:

http://www.areva-td.com/static/html/TDE-AGF_Product-Product_Detail1_1089880316992.html?

Chapter 9 on Overcurrent Protection should be a good starting point.



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

 

[jraef]

Excellent response from ScottyUK.
I'll add my bit for those reading in North America. The reason why a number of specs will start calling for Medium Voltage above 250HP (roughly 187kW) is because that was the limit for NEMA frame motors (449T)and most motor manufacturers did not bother to make MV motors smaller than that.

That is no longer true, and a couple of MV motor mfgrs I just looked at now have offerings as low as 100HP @ 2300/4160V. I would imagine that IEC suppliers are probably similar. Mind you, I think a 100HP 4160V motor is a bit overkill, but if that is the smallest motor in a facility it might make sense to do that rather than install a transformer for 1 small motor.

"Venditori de oleum-vipera non vigere excordis populi"

 

[rbulsara]

Also it depends on how many large motors you have..

If only one or two in otherwise a group of small motors than going to MV for them may not make sense. But I whole heartedly agree with SocttyUk in advatages of MV motors..they require little maintenance (the starter that is)

 

[pootong]

Thank you all for recommendation, I would like to give you additional information, the 220 kW motor is needed to run reverse rotation sometimes because the driven machine requirement. Is this possible to design MV motor to have CW and CCW rotation? or it is cheaper to use low voltage motor for this application.

 

[jraef]

Any AC 3 phase motor can be made to run in either direction by simply switching 2 of the 3 legs. You would use what is called a reversing contactor. Not a problem at all.

"Venditori de oleum-vipera non vigere excordis populi"

 

[ScottyUK]

jraef,

I'm not an expert in this by any means, but aren't some white metal bearings designed for running in one direction only? Something to do with oil wedge formation perhaps? On the small motors being discussed the bearings would likely be ball races, so they would not have a preferred rotation direction and would be equally capable of operation in either direction as you say.



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

 

[RRaghunath]

Pootong,

For your application of 220kW motor for forward as well as reverse rotation, the LV contactor control is preferable than MV breaker / contactor control. You will need two nos power contactors with electromechanical interlocking etc. To achieve the same with MV switchgear could be more expensive.

On the other hand, you need to look at the starting current / available voltage at the motor terminals, after considering the source transformer rating and impedance. This could be a limiting criterion with large motors.

raghunath_n00@rediffmail.com

 

[jraef]

ScottyUK,
That is an issue for some DC motor designs, but all 3 phase AC motors are designed to run in either direction. In fact, nobody knows what direction it will spin until after it is manufactured and tested. That is why they don't lable the motor leads until after the test!

I also agree with rraghunath that at 220kW, a reversing vacuum contactor set is going to be very expensive compared to a LV air-gap contactor, and may sway the balance of benefits towards LV.

"Venditori de oleum-vipera non vigere excordis populi"

 

[aolalde]

Jraef, rraghunath :

It is not true that all 3 phase motors are designed to run in either direction. Lots of large motors have directional cooling fans. May be 220 kW size is bi-directional, but I will check it rather than guess.

 

[jraef]

aolalde,
That will be news to me! I have never seen that on any standard motor specification. Perhapse that is something done on definite purpose motors custom designed for some particular application, like high temperature or something, but certainly not off-the-shelf motors.

But having learned from this forum not to take even my own knowledge for granted, I'm going to check it out. ;-)

"Venditori de oleum-vipera non vigere excordis populi"

 

[jraef]

Aolalde,
OK, I stand corrected. Here is a statement out of the TECO Westinghouse instruction manual for motors above NEMA frame sizes:

"4.2.3 Rotating direction
(a) Most TWMC motors are bi-directional. However, when some special types, such as high speed 2-Pole, certain large capacity motors, those with a non-reversing ratchet etc.,
should rotate in one direction, please ensure the rotation is in conformity with the directional arrow-mark shown on the attached nameplate."

 

[SteveKW]

"That is an issue for some DC motor designs, but all 3 phase AC motors are designed to run in either direction. In fact, nobody knows what direction it will spin until after it is manufactured and tested. That is why they don't lable the motor leads until after the test!"


I would disagree with that, if your talking three phase. If you know the power supply phases (sequence), I can wind a stator knowing which way the rotor will turn!