VSDs and Motor Performance General Questions

[goldyvb]

Hi All,

A friend of mine asked me these questions, and I though seeing as I only have a general knowledge of the area I would throw it out to the Gurus:

with vsd's how is the inrush current reduced during startup at low speed/frequ?

Also, for a standard motor, can you exceed the absorbed power by increasing the speed. If not what happens to the torque. If so, by how much. From memory i think you can increase the speed using VSDs to 110%. What happens to the motor power?

and finally, how do u get a 6300rpm motor. 2 pole? 4 pole would mean 100Hz+ ????

Thanks in advance

Goldyvb

 

[itsmoked]

with vsd's how is the inrush current reduced during startup at low speed/frequ?

By lowering the average voltage.

Also, for a standard motor, can you exceed the absorbed power by increasing the speed.

Absorbed power? You can increase the power consumption yes.

what happens to the torque.

It is limited to the designed motor torque.

how do u get a 6300rpm motor. 2 pole?

3450RPM (2-pole, 60Hz) 60 x 6300/3450 = 110Hz



Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Well, it's a little more complex than that.

Inrush current (assuming you mean Starting current, not magnetization inrush) is controlled better with a VFD because by changing BOTH the voltage and the frequency, you can product full torque all the way though the speed range. In other words, you are controlling the slip at all times. When starting across the line (DOL), the current surges because there is essentially 100% slip at standstill and the slip remains very high for most of the starting process. By controlling the frequency with the voltage, you can start off with low slip and maintain that throughout acceleration.

Changing only the voltage is what a soft starter does. It will reduce the starting current, but it also reduces the torque by the square of the applied voltage. So you increase the acceleration time but the motor current is still higher than normal, meaning there are limits to doing that.

With regards to torque and power with VFDs, Keith is right up to and including the motor's rated speed, but I gather from the nature of your subsequent questions that you are considering over speeding a motor with a VFD. In that case it changes.

Motor torque is dependent upon applying a constant V/Hz ratio in any given design. The power output (which I believe is what you refer to as "absorbed power") cannot increase above what the motor is designed for, so torque actually begins to drop above base speed. For instance, a 400V 50Hz motor is designed to produce design torque at full speed and voltage, a V/Hz ratio of 400/50 or 8:1. Once you go over design speed, you have run out of voltage to apply with the frequency increase, so your V/Hz ratio begins to go down. For example, if you want to run that motor at 100Hz, but you still have only 400V, the ratio has changed to 400/100 or 4:1. The rate of torque decrease is non-linear, so although at 110% speed the torque loss is minimal, at 200% speed it is substantial.

So assuming again that you are not in N. America and use 50Hz power, a 2 pole motor will typically be 3400RPM. To get 6300RPM, you need to increase the frequency to roughly 93Hz. But again, you will have lost torque at that speed, so if your load still requires the same torque it did at 50Hz, you will need to probably double the size of the motor.

A pitfall to watch out for however. If you have a centrifugal load (a.k.a a quadratic load), the power it requires will increase with speed / flow, so your situation may actually be even worse than what I said above.

I know I am leaving out a lot of detail, but this is "Eng-Tips" not "Eng-Classroom".

 

[Skogsgurra]

And, as has been said so many times in these fora, if you can connect the motor in delta instead of star, then you can increase speed 73 percent with same torque. That would give you 73 percent extra power out of the motor.

I am sure that Jeff knows about 60 Hz in North America. Just wanted to correct that so the OP doesn't get confused.

Gunnar Englund

http://www.gke.org

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

 

[Skogsgurra]

Hmm.. Missed that important negation..

Gunnar Englund

http://www.gke.org

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

 

[Skogsgurra]

..and the 3400 RPM.. Now, I am confused.

Gunnar Englund

http://www.gke.org

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