Having a brain fart, help

[DACMACHINE]

I'm cnc'ing a ~16" x 60" lathe

have the breakout board, relay, etc done wired & ready (will be using Mach3)

have motors
http://www.baldor.com/products/servo...=MT-4090-BLYCE

not sure about drives and power supply?

looking at Rutex 100 20A, will this be OK or should I look at 200v 40A??

Power supply 100v 20a,?? KVA???

Read lots, but not sure what direction to take

Any help would be much appreciated

Thanks

Dan

[ynneb]

I would write to Baldor and ask them for the advisable drives, and power supply to use.
Ask them to be specific. Tell them your senario. Ask them what the output voltage/Amperage of the power supply should be. If they give you an answer you dont understand then tell them that, and ask until it is made clear.
I have asked the same questions many times until I have totally understood. You might look like a dork, but at least you finally get the right answer.

There are many parts in the CNC world that are intimidating. You are not expected to know every thing about it.

I looked at the specs you posted and dont understand them either.

[Al_The_Man]

Looking at the specs it would seem that the 20a board would be sufficient, can you set the Rutex drives to limit the 13 amp stall current of those particular motors? If you have the motors then I would think Rutex would be the one's to ask, also you should ask them what the minimum motor inductance can be used with their drives, the motors show 2.5mHy. If the motor inductance is lower than the amp recommended then series choke should be added.
Al.

[DACMACHINE]

Thanks guys

I appreciate your help!
I'll make some calls and get more info

if still not clear I'll pass on the info
and go from there

Dan

[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

[DACMACHINE]

Tom thanks for the reply

you mentiond "desirable motor resistance of 4 amps"

assuming I used the 100v 20a drive


50v x 2 = 100v / 5ohms = 20amp
I'd have the run it at 50v using a 5 ohm resistor

Is my math correct??