[rutexus]

Dan,

I wanted to verify my judgment on your motor with Vladimir before I responded. I apologize for the long delay in responding.

The motor you specified has a very low armature resistance, and would blow
even the R90H (200 volt 40 amp) without some resistors in the motor wires.

Let me explain:

At the instant before the drive stops the motor completely, you could have
100VDC from the drive, plus 100Volts DC generated by the motor (BEMF) in the
motor circuit. Take this 200 volts and divide it by .5 ohms resistance and
you have the potential of 400 amps momentarily peak. We try to keep the
current down under 50 Amps. Therefore, 200 volts /50 amps yeilds a
desirable motor resistance of 4 amps.

If the motor only provides .5 ohms, you still need to provide 3.5 ohms of
resistance in the motor circuit. One 2 ohm resistor can be put in series
with one motor wire and a 1 ohm resistor in the other leg.

The resistors will need to be high wattage. Here we look at the average
current. Say it is 13 amps. Watts = I squared *R. So, the 1 ohm resistor
needs to be at least 169 watts. The 2 ohm twice that rating.

As hard as it may seem to believe, you would see little reduction in motor performance by putting these resistors in the circuit. I have put resistors in motor circuits where the motor resistance is too low before, and we could not tell the difference on a lathe we fitted them with. VFD manufacturers may have some you can use. I only needed 100 watts for the ones I used on a R990H drive configuration. Digi-key has them that large. (You can parallel them to get the wattage) 2 each 2 ohm 100 watt resistors paralleled yields 1 ohm, and 200 watts.

These are large resistors, but...these are large motors. This is why VFD
manufacturers use huge "braking" resistors. The BEMF energy has to be
dumped somewhere. You will have to do a web search to find some.

I am beginning to work on a FEQ section for the new Rutex web site that will
soon be on line. It will help users through this motor drive selection
process.

Tom Eldredge,
Rutex LLC