Motor input frequency
Motor input frequency
(OP)
We have a 4 pole inverter duty motor with a 5:1 speed range and 700 Powerflex VFD. The vendor claims a 29.27 to 1 ratio gear box, will allow us to run the Drum at .24 RPM @ 12 Hz, and 1.2 RPM @ 60 Hz, and running @ 75 Hz would allow us to run at 1.5 RPM with no drop in torque. I'm not sure how they came up with these numbers. Also, can we run a 1800 RMP inverter duty motor at 75 Hz? Thanks in advance!





RE: Motor input frequency
Bill
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"Why not the best?"
Jimmy Carter
RE: Motor input frequency
RE: Motor input frequency
Above rated voltage and frequency you cannot maintain the Volts per Hertz ratio and the torque drops off above rated speed.
BUT
If a VFD is supplied with a higher voltage, it is able to maintain the V/Hz ratio above rated frequency.
For instance if a 230 volt rated motor is driven by a 480 Volt VFD supplied from a 480 Volt source it is possible to maintain rated torque to about 120 Hz (on a 60 Hz base.)
I have heard rumors that some compressor manufacturers have used this trick to double the Hp of a motor.
I would be interested to hear if anyone has first hand experience with this technique.
Bill
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"Why not the best?"
Jimmy Carter
RE: Motor input frequency
Mechanically the motor should be fine at 75Hz.
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If we learn from our mistakes I'm getting a great education!
RE: Motor input frequency
The rpm math is wrong though. 12Hz is 1/5 speed on a 60Hz motor, 4 pole is 1800rpm synch. 1800/5 = 360rpm motor speed, divided by 29.27 gear ratio is 12.3 rpm at the shaft.
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RE: Motor input frequency
RE: Motor input frequency
Current demand by the motor will drop if your load is the same, but the output speed drops. output power is a function of torque and speed while input power is a function of current and voltage. Power in = power out (ideally) and in this case voltage is constant and torque is constant, therefore a drop in output speed will mean a drop in current demand. This is a very basic explanation.
RE: Motor input frequency
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Motor input frequency
When a VFD is used to drive a motor at higher voltages and frequencies, the current should not exceed rated current. The same current times twice the voltage gives roughly twice the power.
If the current does not exceed the rated current the heating will be normal but the cooling fan will be spinning twice as fast.
Over-voltage may be an issue with a 230 Volt motor but most 230 Volt motors are dual rated 230:460 Volt motors. These have all windings insulated for 480 Volt service and the 230 Volt windings are safe up to 480 Volts as long as the Volts per Hertz ratio is correct.
Bill
--------------------
"Why not the best?"
Jimmy Carter