Reduced torque on motors with VFD below base speed

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philly

Senior Member
I'm somewhat confused by the top attached torque vs. speed curve for a vfd operating a motor below base speed. Can someone explain to me why the maximum torque decreases for speeds below base speed. I was under the assumption that as long as you kept the V/Hz ratio constant below base speed then you would have the maximum torque avaliable for the motor throughout the entire speed range. Why does this plot appear to be showing the torque decreasing with speed?

I understand that the bottom plot shows torque for speeds above base speed and that it shows a reduced torque values above base speed since the voltage is fixed. This reduced torque above base speed makes sense to me, however I cant seem to justify the top curve for speeds below base speed.

Can someone help explain?
 

Jraef

Moderator, OTD
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San Francisco Bay Area, CA, USA
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Electrical Engineer
I think I see your concern with that and I think there is an error in those charts. They are showing the Y axis is Torque in N-m (Newton-meters) which is a hard number, not a percentage. Yet they have that horizontal line at "100" that is typically shown on torque-speed curves that display percentage as being 100%, but in this case, would mean 100N-m. That makes no sense to me either. Why show that? And it can't be percentages because there is no way you are going to get 650% BDT as it indicates on the 1800RPM curve!

But if it is hard torque values and not percentages, and the chart is referring to a single motor operating at different commanded speeds, then the curves to the left make no sense either. Assuming for example that the FLT point at the 1800RPM curve is somewhere around 250N-m, then at 1400RPM commanded speed the FLT should still be 250N-m, but at 190RPM you can't even get to 200N-m.

My interpretation is that this chart is not the same motor, but taken from multiple motors, in which case it is a nearly pointless chart...
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I was under the assumption that as long as you kept the V/Hz ratio constant below base speed then you would have the maximum torque avaliable for the motor throughout the entire speed range. Why does this plot appear to be showing the torque decreasing with speed?

The above is a good approximation, but not quite true. It would be true if the stator circuit were a _pure_ inductance, but the real stator circuit has some resistance.

To overcome the resistance, you need (approximately) 'constant V/Hz plus voltage boost'. The constant V/Hz is for the inductive part of the stator circuit and magnetizing branch, the voltage boost is for the resistive part of the stator circuit.

My gut is that the graph was calculated assuming a very high stator resistance, in order to make the resistance effect quite prominent. I don't believe that a practical motor would be built with such a large stator resistance.

-Jon
 
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