Motor input frequency 4

[mikeengurs]

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!

 

[waross]

They may have used Hz when they should have used RPM and then multiplied when they should have divided.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mikeengurs]

Can we run the motor at 75Hz as they indicated? No were in the rating indicates max rmp.

 

[waross]

Usually not but sometimes yes.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Above base speed - 60Hz - the motor operating mode will move from constant torque to contant power, so the available torque will fall away as speed (frequency) increases above base speed. Maybe your load behaves like this, but generally they don't. You could size the motor based on meeting the load torque at 75Hz, or you could get involved in tricks such as the one Bill described.

Mechanically the motor should be fine at 75Hz.


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If we learn from our mistakes I'm getting a great education!

 

[jraef]

Or it could be that the motor is slightly oversized so that the slight drop in torque at 75Hz is irrelevant to the task at hand. In other words it depends on their definition of "rated torque". Rated from the machine or rated from the motor?

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.

"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

 

[mikeengurs]

Thanks a lot guys. jraef, I agree with your numbers. After talking to the vendor, it looks like there is an other 50 to 1 gear box in series, so that makes the numbers correct. They also claim that the amps would drop when adding the gear box as torque would increase. Is that accurate?

 

[TheVedge]

waross...yes you can indeed maintain constant torque past the rated voltage by increasing the supplied voltage thus increasing the amount of power available to the motor and yes i have seen this used in compressor motor applications. Beware that more power will increase the heat generated by the motor and you risk damaging the insulation in the windings, so the motor will no longer be safe for continuous duty!!!! (which is why it is not rated for this).

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.

 

[jraef]

After talking to the vendor, it looks like there is an other 50 to 1 gear box in series, so that makes the numbers correct.

Minor detail they left out there eh?

They also claim that the amps would drop when adding the gear box as torque would increase. Is that accurate?

They seem to be confused. Amps follows load, as in kW or HP. What you do between the motor shaft and the final load shaft can change the speed or torque in a manner inversely proportional to each other, but the load is the load. If your speed goes down because of a gear box the shaft torque goes up, but the HP at the load shaft remains the same, and that is ultimately reflected all the way back to the gear boxes to the motor. If anything, the addition of two gear boxes and their associated mechanical losses (heat) will actually INCREASE the amp draw on the motor.


"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

 

[waross]

TheVedge;
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