Over excited motor
Over excited motor
(OP)
I am being asked to run some small motors (0.75 to 4 kW) at up to 100hZ.
My question is what is the long term effect of this on the motor, and on the frequency invertor driving it?
Many thanks
My question is what is the long term effect of this on the motor, and on the frequency invertor driving it?
Many thanks





RE: Over excited motor
if you are using more advanced drives ( vector type, direct torque control type), you shouldn't worry too much then. Those drives can handle the motor at that speed properly.
RE: Over excited motor
RE: Over excited motor
To vary the speed of an AC motor and provide constant torque throughout, you must maintain the Volts-per-Hertz ratio at a constant. So for instance, a 460V 60Hz motor must get 7.6 V/hz, derived from deviding 460 by 60. However, if you drive the motor to 100Hz, and you want the same torque at that speed, you would need to apply 760V. No VFD is capable of INCREASING the maximum output voltage beyond the input voltage, vector or not.
So once you get beyond the 60Hz range, your V/Hz ratio is going to go down, essentially creating a "field weakend" state where the HP remains constant, but the speed increases. Since Tq = 5250 x HP / RPM, as the RPM goes up with no increase in HP, the Tq will go down. If your load remains constant at that speed, you motor will begin to overload. There are tricks you can use to help compensate, but you need to describe your application a little better to get the right recommendation.
"Venditori de oleum-vipera non vigere excordis populi"
RE: Over excited motor
RE: Over excited motor
"Venditori de oleum-vipera non vigere excordis populi"
RE: Over excited motor
If you have a load that needs to be run at 100hz but above rated W/(100*2pi) N-m, jraef is right that no matter what kind of drive you have, the motor is over-loaded.
RE: Over excited motor
A little more info. I am driving a rotary valve through a gearbox, so the load will remain pretty much constant.
The torque should decrease slightly as the speed increases, but I dont want to rely on that. Most of the motor/gearbox drives are 0.75KW so I'm looking at using a 1.5kw motor (ie doubling the motor over the rated kw of the drive).
If I understood what you all said, then by doing this I should be OK, yes?
RE: Over excited motor
"Venditori de oleum-vipera non vigere excordis populi"
RE: Over excited motor
The valve is a vane rotor that delivers a given amount of powder per revolution, at 50Hz this equates to approx 50kg/h. Although the relationship is not quite linear, I need to modulate the speed of the valve to deliver anywhere between 15 and 100 kg/h. As all the valves I am dealing with are already controlled by an invertor (for reasons I wont go into), speed control and overexcitation would seem to be the first step if it can be done.
RE: Over excited motor
Watch out for increased friction issues however. I did something like this on a gravel feed out system, and the rotary valve box overheated. The original design assumed never exceeding the maximum flow with the motor running at 60Hz, so the case could not dissipate additional heat from the gravel friction fast enough. That may not be an issue in yours depending on the makeup of the powder.
Good luck, and let us know how it turned out.
"Venditori de oleum-vipera non vigere excordis populi"
RE: Over excited motor
Many thanks for your help and advice.