RE: [CAD_CAM_EDM_DRO] VFD and inverter duty 3phase motors

Inverter rated refers to certain attributes of the windings. Inverters
create low frequency AC waveforms (below 400 hz) using high frequency (8 to
20 khz) PWM waveforms. The dv/dt on the PWM is tremendous. This creates
spikes that exceed the normal voltage. A 460 volt motor may see 1800 volt
spikes on the edges of the PWM wave.

Cheap motors will have degradation of the insulation due to the high
frequency spikes, such motors will meg out at less than 1000 volts.
Inverter grade motors should meg at more than 2000 volts and will withstand
the crap put out by inverters.

While many factors complicate this, the fact is that short wires between the
inverter and the motor (15 feet or less) and use of 230 volt (not 460 volt)
power makes the problem go away. Spikes stay below 1000 volts, which cheap
motors can withstand. Inverter grade motors are not needed if you keep to
those two rules.

G. Jackson

> -----Original Message-----
> From: Lance Hopper [mailto:snaggletto@...]
> Sent: Thursday, July 29, 2004 8:38 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] VFD and inverter duty 3phase motors
>
>
> Hello,
> I've got a 5HP Automation Direct GS2 AC (0-400Hz) drive. I'm
> looking to buy a 3-5HP 220VAC 3phase ~3500RPM motor for a small lathe
> application. I've been doing some looking and it seems that there
> are motors that are "inverter duty" rated. The inverter duty motors
> seem to have a rated speed like 1725RPM @60Hz, and then have a 'max
> safe speed' usually rated much higher like 3450+RPM. I'm guessing
> this max speed is at 120Hz. Also it seems that inverter duty motors
> have a much better spread of torque than standard motors made to
> operate at or close to the rated RPM only.
>
> Should I just get a regular ~3450RPM @60Hz motor, and limit my VFD
> to 60Hz, or should I go for an inverter duty motor where I can run
> much higher frequencies/speeds. Any comments would be appreciated.
>
>
>
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