Does anyone know if a motor's torque is dramatically reduced if it's controlled by a VFD and is set to run at a very low speed?
Here's the situation: We have a couple of 15 hp roof top process related exhaust fan motors that are VFD controlled and set to operate at about 30% of full speed (18hz). When the outside temps recently dropped to -10 degrees here these motors wouldn't start. After we warmed the fan bearings with a torch they started fine. We increased the fan speeds to 50% and haven't had a problem since.
In order to prevent saturation in the motor's iron core, VFDs will reduce the voltage proprtionally to the reduced frequency. This means that the availabel power will be reduced by aproximately the square of the reduction in speed. (P=V^2/R)
I am hopeing that some of you will call me on over simplifying the motor model, and well I did. It is true that the apparent resistance of a motor is affected by the loading, a higher loaded motor will have a lower apparant resistance becuase of the increased power needed to feed the load. This variability has limits, that is why a 1 HP motor wil not turn a 15 HP load.
VFDs typicaly work quite well on blowers since blower loads typicaly have very linear torque speed curves, asuming that a frozen bearing does not throw a wrench in the works.
If a motor is kept below rated speed than the torque available should be 100%. The horsepower however will be lower. Over rated speed, the torque begins to fall as horsepower stays constant.
We have had problems with using drives at factory settings with loads that require high starting torque. In the drives we work with there are parameters that can be changed to provide a boost in torque at startup. This may be all you need to do to get the fans turning. On our drives I believe it is under torque compensation in the manual.
A VFD, when _properly configured_, will permit full torque operation at low speed. The requirement for full torque is that the magnetic field be developed to full flux density.
The motor may not be capable of continuous operation at 100% torque and low speed without overheating, because cooling will be reduced, but it should be able to produce full torque.
One significant issue is the proper drive voltage at low speed. To a very good approximation, the proper voltage to achieve full saturation is proportional to the drive frequency. This is such a good approximation that some drives simply use 'constant V/Hz'. However this approximation ignores the resistance of the winding, and falls apart at low speed.
When you want to operate a motor at low speed, the proper drive voltage to achieve full saturation is greater than that predicted by 'constant V/Hz', and a drive using constant V/Hz control would end up with insufficient magnetic flux and thus reduced torque capability. I believe that even the simplest 'constant V/Hz' drives have a 'low voltage boost' setting that helps compensate for this.