VFD overfrequency 4

[electrolitic]

Sometimes, caused by motor + gear ratio specified , the AC motor needs to run above its base speed without torque reduction. This means more output power. Which is the criteria to specify the VFD?

 

[electrolitic]

Thank you all
So, if: 1) I have an medium size AC motor, 6 poles, 60 Hz,
220/380V (easily found in my country), P [kW}, Tn[Nm] nominal torque;
2)The available reducer gear ratio requires motor to run up to 1420 rpm keeping the nominal torque to match the process load requirements.
So, if I solve the motor overheating applying motor frame forced ventilation (separated fan) and feed motor(380V connection) through a 460V, 72 Hz inverter drive, I can get around 1,2 times P[kW}. basically without problems.
Am I correct?

 

[waross]

DickDV;
Pick a frame size and check the HP at various speeds. It has been a few years since I went over the tables but although the same frames are used for TEFC motors at 1800 RPM and 3600 RPM you will see an increase in frame sizes for a given HP at the lower speeds. eg; 1800 RPM vs 1200 RPM vs 900 RPM.


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

 

[gepman]

Sorry for the incorrect answer. I am going to give this a try just for fun although I don't know how useful this will be to me.

Voltage will essentially be linear from 0 to 460V from 0 to 120 Hz since it is a constant V/Hz. Can someone say what the current to the motor look like as a function of speed from 0 to 120 Hz? I have my hunch but I don't want to show my ignorance.

As I have posted before, the publication at this link gives the maximum overspeed for NEMA motors on page 15.

http://www.nema.org/stds/acadjustable.cfm#download

It looks like you can overspeed motors up to around 50hp, bigger than that at least NEMA doesn't like it.

 

[LionelHutz]

You are correct. You also should be able to do that without worrying about the cooling.

The cooling can be an issue when running at a low speed and at rated torque. Some fans can "stall" and quit moving air at speeds well above rated. Also, it is possible the fan will put more load on the motor when running above rated speed. Remember, the power to turn the fan has to come from the motor.

 

[LionelHutz]

gepman - the load will likely change as the speed changes and the current will depend on the load. However, if you want to call the load a constant torque then the current will be fairly constant. It will change some due to motor efficiency changing as the speed changes.

 

[PaulBr]

Does anybody know if this technique of overfrequency has been used in the center winding application, where speed of the driving shafts varies significant with the change of size of the winding roll?
Will it work for drive in sensor vector mode?
What is the current of the motor running at 120Hz connected for 230V and drive at 460V if the load torque at or less than rated motor torque?

PaulBr.

 

[LionelHutz]

The application really does not matter as long as the motor is capable of supplying the required load torque at all speeds.

It should work even better with encoder feedback.

The current will be approximately the rated motor current at all speeds if the load on the shaft is the rated motor torque. So at 120hz you still get rated motor current when the load is the rated motor torque.