50/60 Hz Motor Selection 8

[roydm]

I wonder if you motor gurus could give me a few pointers.
We have a project in China (50 Hz) where we are installing a 200 kW pump. We want to run the pump on a VFD somewhere between 3000 and 3600 rpm.
The pump will draw ~ 200 kW right at the point where the client specify changing from 380 V to 10 kV
Q1) Should we be selecting a 60 Hz motor and underspeeding it or a 50 Hz motor and overspeeding?
Q2) Would the 50 Hz option be the same HP motor as the 60 Hz option?
Q3) If you overspeed a 50 Hz motor does the Voltage go over as well or would we set the VFD for 380 Volts at the new top speed?
Q4)Would a 10 kV 200 kW motor be the same frame size as a 380 V motor?
Q5) What is the relevent cost of a 10 kV motor v/s a 380 Volt one?
Thanks in advance for the help.
Regards
Roy

 

[LionelHutz]

Since the torque has fallen off by the inverse ratio of overspeed, and hp = T x rpm/5250, the hp will remain constant.

I was looking for the explanation of this...

Someone want to explain why it takes more current to produce less torque as the motor goes over its base frequency? (Keep it basic enough for us ordinary folk, please!)

This one is easy now I think about it again, if the HP or output power is constant then the input power should be constant (assuming losses stay constant). So, above base speed the voltage and current remain constant which produces a constant output power.

Torque current rises as slip increases in the weak field area? Just thinking aloud, brain ain't fully awake yet.

I'd say Scotty hinted at the answer. The available torque when operating at twice the rated speed (at rated slip) has dropped to 1/4 of rated. In other words, if the motor was running at rated slip and 2x base frequency then the current and voltage would both be 50% of rated and the output torque would be 25% of rated. However, operate at about 2X rated slip and the current should go back up to 100% of rated with a voltage of 50% of rated giving an output torque that is 50% of rated.

So, it appears the constant HP theory relies on the motor having a breakdown torque that is at least 200% of rated torque.

Realize that the fan load could go up to 225% at 150% speed and 400% at 200% speed. I'm guessing the windage increases similarily and bearing lossing maybe increase linearly? Add these to the mix and the HP curve would drop drop off as the speed increases, not remain constant. The special VFD duty Baldor motors are either fanless or blower cooled which eliminates the fan load problem. I think the same applies to the Marathon BlackMAX motor line. So, eliminate the fan and the HP may drop but it wouldn't be a huge drop off.

In the end, the OP is looking at a fairly small overspeed with an oversized motor. I do't believe he will have any problems.

 

[LionelHutz]

This may be a silly question but if you know you want to run a motor slightly overspeed why would you enter the nameplate voltage so it maxes out at synchronous, why not enter nameplate + 10% if you want to run it 10% over.I know you will run out of voltage perhaps before you reach 10% over but why artificially limit the voltage at nameplate.

Yes, you could put in the motor data as 380V, 52.8Hz for a 360V, 50Hz motor. This would extend the operation using the correct V/Hz ratio to 52.8Hz.

Class 1 Div 2 - Are you using this in a classified area? I recall that a Class 1 Div 2 motor loses that rating when operated with a VFD so I'd recommend you check into this further.

 

[roydm]

Once again, thanks for the valuable input.
Lionel - yes we are aware that VFDs can cause overheating, thanks for the reminder though.
The versions of NEC (1999) and CEC (2002) that I have don't call for a motor to be explosion proof for Class 1 Div 2 (actually our area is Class 1 Zone 2 T1 (450°C)) hopefully the motor won't get that hot. The CEC points out the potential harmonic overheating problem.
Both codes say you can use open type motors for Class 1 Div 2, but of course we will use TEFC. The pump is pumping Sulphuric Acid.
I think its very important to specify that the fan and or the coupling gaurd is non sparking.
I recall reading somewhere that the VFD and motor should be matched but that's pretty hard to do since we are providing the motor, the client is providing the VFD.
Ninety % of our work is Class 1 Div 2 or Zone 2