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VFD Help

C

coyotechet

Cast Iron
Joined
Jun 13, 2010
Location
Kansas
What is the slowest safe speed I can run with a 1750 RPM Baldor motor EM31116 3ph and using a VFD With the slow speed pulley of(304 rpm)? Hoping to get close to 50 RPM with this setup. Have not ordered anything yet.
Just Hoping that I can reduce the 304 rpm to some where close to 50 RPM safely before I start ordering things.
To be installed on my South bend Heavy 10.

Thanks Chet
 
I routinely run the motor on my machines at half speed. Less than that, and I find that I can stall the motor, etc. Your machine has backgear. That is how you get to 50 RPM spindle speed.

allan
 
I have been using South Bend Lathes since 1968 (9A,3 of them, 10L, 2 of them and 1, 13) currently only one 9A and one 10L.) I was asking about using a VFD. which I have not used at all. That is why I am asking for help with a VFD.:):)

Chet
 
You can, and I answered your question to the best of my ability. I think you will have trouble if you try to run your motor at less than half speed, unless you are doing something that does not require much power.
 
Coyote, the problems that you will possibly see running at very low speed with a VFD are the motor running hotter due to less air flowing over the motor and a reduction of power in the form of torque. Low speed is better achieved by gearing in most cases,and in conjunction with a drive, almost infinitely variable speed can give you a way of fine tuning surface finish. I put one on my 10L a while back and never regretted it . hope this helps. Jim
 
Not sure what HP motor you're using. But guessing 1 or 1.5hp.

2 speed motors are not as easily found as a single speed, but in that hp range I would think cost not too terrible if you come across one. I want to say an 1800 rpm motor, the low speed would come in about half, say 900rpm. Food for thought I guess.

I have no idea about running a vfd with a 2 speed motor though.
 
In general industrial motors can be run at 25% continually if VFD capable. Does not look like this motor is VFD rated.

How long will you run it at low speed? How hard will you push at at low speed? If lightly loaded absolutely no problem. Heavily loaded it will overheat over time. Does it have a thermal switch? If so, make sure you use it and if it trips you know you pushed it too hard.
 
Based on my own personal experience, you should be able to run a 3 phase motor on a VFD as slowly as you'd like. Running it very, very slow can get to be frustrating because the torque drops off pretty rapidly as you approach "0" or "clocking" speed but if you don't have big torque requirements with what you're doing, I can't see that it's going to hurt anything. Obviously if you do a lot of heavy stuff at very low speed you might want to keep an eye on motor temperature. Remember that an induction motor of "standard" NEMA design will be so hot you can't comfortably put your hand on it and it will still be within its temperature limitations. For example, a 40 degree (Centigrade) rise motor running at full load in a 23 degree (C) environment will be at 63 degrees C or about 145 degrees F which will hurt if you put your hand on it.
 
I have a 10L with the factory Reliance 1hp open 3 phase motor, not VFD rated. It’s running off a FMX Tough Drive TD 200. I have run mine in back gear at 10hz for about an hour parting off a large piece of cast iron without a problem with heating or torque. The motor got warm but not hot to the touch. The belt still slipped before the motor stalled as well.

It was 50 degrees in the shop at the time... I don’t think it would do as well now with 90 degree shop temperatures. It would not be my choice for daily operations but worked in a pinch to control chatter.

Hope this helps.

Ben
 
Dobermann said:
Remember that an induction motor of "standard" NEMA design will be so hot you can't comfortably put your hand on it and it will still be within its temperature limitations. For example, a 40 degree (Centigrade) rise motor running at full load in a 23 degree (C) environment will be at 63 degrees C or about 145 degrees F which will hurt if you put your hand on it.
Click to expand...

I don't believe that this is correct. Most industrial motors are rated 80 to 105 deg C rise (class B or F rise) by resistance and add 10 deg C for hot spots. These motors are probably on the cooler side so at 23C ambient call it 103 C or roughly 215 degF - this is at full load running a long time. Some are rated at 80 deg C rise but actually run cooler, but not 40 C rise cool.........
 
Chet,
have you already purchased a motor?
i ask because with a VFD its the better compromise IMO to overspeed a motor(within its limits) to achieve higher speeds than it is to underspeed one to achieve lower speeds.
 
1) if you want slow spindlw speed, avoid the 1725 motor. Go for four pole not two pole.

2) get a VFD that has sensorless vector capability Hitachi makes a good one.

3) upsize the motor a bit. 1 hp is not out of line.

4) if you will be running the motor for extended time at low speed, add a small muffin fan to help cool the motor. It mounts on the end of the motor.

5) Use the small countershaft pulley and the large spindle cone pulley step.

I think this will work OK.
 
Lot of good information for anybody thinking about installing a VFD Thanks.

I have not bought any thing yet but this the is type of information I was hoping to get. I want to do it right the first time.

Thanks a bunch Chet :):)
 
iwananew10K said:
Chet,
have you already purchased a motor?
i ask because with a VFD its the better compromise IMO to overspeed a motor(within its limits) to achieve higher speeds than it is to underspeed one to achieve lower speeds.
Click to expand...

I'm not an expert, but this is what I did after asking many questions on this forum:

I have no problem running my 4 pole motor at 120Hz with a VFD. I changed the pulley sizes so the motor has to run twice as fast to achieve the same spindle speed. So max spindle speed is the same at 120Hz as it use to be at 60Hz. And now I have twice the torque at lower speeds, which eliminates the need to change pulleys, except for extremely low spindle speeds.

Not the best explanation. I hope that makes sense.
 
ptsmith said:
I'm not an expert, but this is what I did after asking many questions on this forum:

I have no problem running my 4 pole motor at 120Hz with a VFD.
Click to expand...

Is it oversized? You can start running out of breakdown torque at 120 hz - depending on the motor. Which means you can stall the motor. Above the base speed, the breakdown torque decreases as the square of the speed so at some point you run out of torque. Depends on the motor, but can be at 100hz, 120hz, 150hz...........
 
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