# Motor frequency (how high)???

• posted

How high a frequency can you run a 50hz motor using an inverter, and i assume that you must lose efficiency or something the higher you run it??? Any help would be greatly appreciated.

John.

• posted

Because magnetic field ~ U/f, you enter magnetic field weakening area, so you're loosing on torque, resulting that you can not run some constant torque loads.

• posted

Of course you can counteract this somewhat by increasing the voltage. A constant volts/hertz ratio can maintain the torque curve. But there are limits of course. As voltage increases, insulation breakdown will start to occur. And a given torque at higher RPM means more power out. Thus, more power in. Even if losses stay a fixed percentage, the increase power flow will increase heat generation. Windage losses increase with speed cubed, so that's another issue. And of course you also have bearing limitations.

One could probably get to 60 hz easily (with constant V/Hz). Might even see

90 to 100. But I have doubts about getting much further.

daestrom

• posted

"daestrom" schrieb im Newsbeitrag news:uyEme.30442\$ snipped-for-privacy@twister.nyroc.rr.com...

we are frequently using 3-phase motors of 230/400V 50Hz (German supplier) with the following specs:

Inverter with 400V 50Hz infeed, frequency range = 20 .. 87 Hz, output voltage of 230V@50Hz, 400V@87Hz (i.e. frequency linear voltage curve). Motor connected in delta, current = delta current (see motor spec). This increases the motor power by a theoretical factor of 1,7321, but the motor supplier only allows a practical factor of 1,5 (which is already a nice thing to have). Of course the motor is designed to stand the higher voltage by using increased insulation. Cooling was no problem up to now, because the higher fan speed also gives increased cooling.

Apart from the increased motor usage, another benefit is that you can use the motor (without forced cooling) at a frequency range of more than1:4 at full torque (i.e. at full load). That is what we use them for in the first place (on inclined conveyors).

We ran into problems, however, when we tried to do the same thing at 690V (with motors of 400/690V 50Hz). Now the spikes on the inverter output were so high (above 1kV) that the motor supplier requested to have du/dt or even sinus filters for the inverters, which made the whole thing less attractive.

One common problem with that kind of connection is to convey the idea to the standard electrician (and even engineer) who wires and maintains the whole thing. It happens ever so often that someone - preferably in the night shift - will reconnect the motor because he thinks it was wrong (400V grid connected in delta to a motor of 230/400). Then phone calls come in and we have to explain that over and over again ... it is just not "standard" but clever ;)

Of course the whole thing will also work at 60 Hz, with different voltage/frequency ranges of course.

Mike

• posted

Likely, "some" of the derating has to do with core losses which tend to increase in proportion with frequency. The inverter output might not be perfectly "clean" and this, too, would increase core losses.

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