We have a 480 volt 40 hp 49.5flc motor driving a large blend tank. The motor is controlled by a VFD which is usually set around 10hz. The motor has been running at full load amps at the low frequency and often over amps and trips the drive out. I believe the blend tank load is simply too much for the motor, however I would like to know how the motor reacts to low frequency. My question is; does the motor rated hp decrease along with the decrease in frequency? Does the torque produced change with frequency? There is also about 150 feet distance between motor and drive cabinet. Would a load side reactor help? Any information is very much appreciated!
motor hp on vfd
- Thread starter mwuniseal
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
- Torque remains the same as speed changes, so HP is declining with lower speed. But most blenders work based on torque, not HP, so technically this should work. One thing that can happen though is that as the blender does its job, the material does its thing and may end up changing viscosity, which changes the torque requirements. But hopefully, someone who specified the blender understood this and sized it appropriately.
- Constant torque operation is true only in theory, your mileage may vary depending on the TYPE of drive you have. A basic V/Hz drive is OK for speeds down to about a 4:1 ratio. You are at 6:1 (assuming you are in the US based on 480V and the use of HP). If you have a V/Hz drive, it will not be able to properly respond at low speed operation to maintain full torque. You need a drive with at least Sensorless Vector Control. That will see the added speed / torque error and automatically tweak the VFD's output to get better performance out of it. If you are using an SVC drive, most likely you do not have it properly set up, the VFD must be tuned to the motor at setup time. If this was not performed, you are not getting the benefit.
- No, a load side reactor, even though it is a good idea for other reasons, would do nothing toward addressing this problem.
- Given the distance from the VFD to the motor though, I would definitely have AT LEAST a load reactor to help prevent motor winding damage and bearing issues. A Load Filter would be better.
What is the VFD you are using?
If the motor routinely runs at 10Hz a step down gearbox between motor and load might be the answer. Or, if there is already one in place, change it for a higher ratio.
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