VFD - Minimum Speed

[mnike000]

Hello everyone. I have a similar application and am wanting to know what I can expect from my motor as far as running time at low frequency. I will be mounting this on a Powermatic 45 lathe with a Reeves drive, and a countershaft between the two. @Jraef I saw that you said not to expect better than a 1:4 drop from a motor that is not specifically inverter rated. Mine is not, but it is fairly old. So, I'm wondering if older motors can still be expected to drop that low. It is also a very physically large motor at 130 pounds, probably 14" in diameter, with a cast iron housing. So, I would expect this to work in my favor as far as cooling. I've attached image links of the nameplate and motor.

https://imgur.com/e1i7oFW

https://imgur.com/ZltvkST

https://ibb.co/c6LBw3H

https://ibb.co/gwnJJ2d

 

[ptonsparky]

Jeez guys, change the sheave on the motor. Let the motor run fast enough to cool itself.
No, I didn't read the other replies.

 

[mnike000]

Jeez guys, change the sheave on the motor. Let the motor run fast enough to cool itself.
No, I didn't read the other replies.

I know. I'm just trying to determine the lowest rpm I'll be able to eek out with a single countershaft. The sheave on the reaves drive spindle is not changeable. The smallest sheave I can find with a 1" bore to fit the motor is 2.5". So, these are my constraints, and I'm looking to see what I can get with a single countershaft where only the motor will can be moved to adjust for the step pulley. The countershaft will not be able to be moved, and therefore I cannot have another step pulley driving the Reeves spindle.

What are your thoughts on the slowest I can go with motor and retain adequate cooling. I can also run an external fan if need be. I should not need to run at these low speeds for long.

 

[GeorgeB]

The operators of this machine have requested that the minimum speed setting be lowered even more. I have limits set on the touchscreen for 10% - 100%. They are wanting to go slower than the 10%. The motor current increases if the speed decreases. So, how slow can I actually go without harming the VFD or Motor?

Any thoughts would be greatly appreciated!

You have lots of good answers; most likely 10% is already "unhealthy" for the motor. The drive will have no problem. This was well covered by others.

I think a very important question is if the operator wants a lower speed to OPERATE, or to setup. If operate, the high (100% limited) speed needs to be known. If the range operations wants is 3% to 30%, say, a change to the mechanical drive system would help. If they need the existing full speed, that option disappears.

As @Jraef said, the inverter drive won't have problems being set lower, the motor probably will. At the least, I'd monitor motor temperature and let them try slower. An external fan will help. Make sure that the people who spend the money understand the tradeoffs.

So, the first question is the needed (well, desired) operating range.

 

[mnike000]

You have lots of good answers; most likely 10% is already "unhealthy" for the motor. The drive will have no problem. This was well covered by others.

I think a very important question is if the operator wants a lower speed to OPERATE, or to setup. If operate, the high (100% limited) speed needs to be known. If the range operations wants is 3% to 30%, say, a change to the mechanical drive system would help. If they need the existing full speed, that option disappears.

As @Jraef said, the inverter drive won't have problems being set lower, the motor probably will. At the least, I'd monitor motor temperature and let them try slower. An external fan will help. Make sure that the people who spend the money understand the tradeoffs.

So, the first question is the needed (well, desired) operating range.

I'm planning the setup with the assumption that I can take it to 25% hz. I'll be able to monitor temps and see what can be done below that, but it would be nice to know ahead of time as it may affect my choice towards favoring the low end or high end. I do need the full range for operation up to 3k rpm at the lathe headstock. 3.5k would be nice. The low end I'd like to get into the 100 range, but the lower the better (for other applications I don't necessarily need.) I'm wondering mostly if 15hz for periods of 10-30 min is a reasonable expectation for this motor.

 

[Jraef]

Check what the motor mfr says about the “turn down ratio” of the motor. As a gross general rule, non “inverter duty” motors are considered to have a 4:1 turn down ratio, meaning they can generally handle being run at 1/4 their design speed. Good inverter duty motors have a 100:1 ratio, the best can have 1000:1, but those usually have a separately powered blower. Size makes a big difference though, you will probably not find a 500HP motor with a 1000:1 turn down ratio, blower or not.

 

[GoldDigger]

... If you want to run it slower than that, it needs a higher ratio. So a 10:1 turn down ratio means you can run it at 6Hz, a 100:1 ratio means you can run it at 0.6Hz etc. If your motor was NOT selected as inverter duty and does not state a turn down ratio, I typically tell people to assume no better than 4:1, so 15Hz as the slowest speed. You say you are already running at 10Hz, so hopefully this is not the case and someone knew what they were doing.

Post back motor nameplate data data if you want more help with that, including make and model.

Actually, the OP is saying that he is already running at 10%, not 10Hz.