Motor & VFD Help Needed

[Val3ntin0]

Hello, this is my first post after reading these forums for several months but I'm stumped and need some help (just searching didn't help much).

I have a motor that initially during the project was going to be started across-the-line. However, the client convinced the then electrical engineer on the job to add a VFD to it. However, now much later, I'm trying to figure out 1) is the motor compatible with a VFD and 2) how slow can I run the motor without burning it up?

During my investigation I found a 20:1 VT and 10:1 CT rating on the motor nameplate so I'm assuming it's VFD compatible. The motor is for a cooling fan (variable torque) so I'm guessing I should use the 20:1 turndown ratio. This said, does this mean the slowest I can safely run the motor is 180 RPM (23V, 3HZ) without fear of burning it out?

Do I need to check anything in the VFD to make sure it'll handle the load as well?

This is my first go around with motors on VFDs and I have no one to use as a reference at my work place and even the motor vendor wasn't very helpful...HELP!

 

[petersonra]

I suspect the load on the motor will approach nil at 180 rpm, so I would not be real worried about it.

If what is on the nameplate is indeed a turndown ratio, and I do not know that it is but it seems plausible, it seems like a more than adequate range for a fan application.

 

[Besoeker]

Hello, this is my first post after reading these forums for several months but I'm stumped and need some help (just searching didn't help much).

I have a motor that initially during the project was going to be started across-the-line. However, the client convinced the then electrical engineer on the job to add a VFD to it. However, now much later, I'm trying to figure out 1) is the motor compatible with a VFD and 2) how slow can I run the motor without burning it up?

During my investigation I found a 20:1 VT and 10:1 CT rating on the motor nameplate so I'm assuming it's VFD compatible. The motor is for a cooling fan (variable torque) so I'm guessing I should use the 20:1 turndown ratio. This said, does this mean the slowest I can safely run the motor is 180 RPM (23V, 3HZ) without fear of burning it out?

Do I need to check anything in the VFD to make sure it'll handle the load as well?

If the motor nameplate gives a VT and CT ratio, that would apply only to running it at variable frequency so that is a pretty fair indication that it is VFD compatible. That should answer your first point.
For the second point, if it is a variable torque load that infers that it is a centrifugal fan. The power for such a fan is proportional to the cube of the speed so it is almost no load other than losses at 5% speed. But, by the same token, the fan will produce almost no airflow compared to its rated output. I've come across very few such applications where the minimum speed was a turn down ratio of greater than 2:1, 1800rpm in your case. Could you run it at 180 rpm? From the information you've given, you probably could. Would you ever need to? Almost certainly not.

 

[Jraef]

If the motor nameplate gives a VT and CT ratio, that would apply only to running it at variable frequency so that is a pretty fair indication that it is VFD compatible. That should answer your first point.
For the second point, if it is a variable torque load that infers that it is a centrifugal fan. The power for such a fan is proportional to the cube of the speed so it is almost no load other than losses at 5% speed. But, by the same token, the fan will produce almost no airflow compared to its rated output. I've come across very few such applications where the minimum speed was a turn down ratio of greater than 2:1, 1800rpm in your case. Could you run it at 180 rpm? From the information you've given, you probably could. Would you ever need to? Almost certainly not.

+1 on Besoeker's response. Take a look at the fan curves, you will probably see that below some value, usually 40-50% speed, the fan stops moving air at any appreciable volume. So like he said, yes you can, but it's likely a moot point.

What that information on the motor is telling you is that, with a VT load, the motor cooling design is such that you need not worry about it over heating down to 1/20th (5%) speed. That does not mean it is intended to be used that way.

 

[Val3ntin0]

Thank you very much for your replies! It cleared up a lot of my confusion.

 

[Besoeker]

Thank you very much for your replies! It cleared up a lot of my confusion.

Thank you for your feedback - that's one of the things that makes it a pleasure to post here.
:thumbsup:

 

[Besoeker]

+1 on Besoeker's response. Take a look at the fan curves, you will probably see that below some value, usually 40-50% speed, the fan stops moving air at any appreciable volume. So like he said, yes you can, but it's likely a moot point.

A little off topic...or maybe not.
We do a fair number of drive systems for a company involved in the petrochem industry. Mostly they are for centrifugal applications. On some they have a "slow roll" feature which is typically one third of rated speed. I don't know what it does. It's just part of the technical spec. We provide it. Along with all the other control requirements.
They are generally pretty involved projects both technically and commercially. I have to commit to all manner of technical performance and delivery dates before the contract is placed. Failure to meet either means financial pain at an agreed rate. Don't provide GA and foundation drawings within two weeks from placement of order...negative ching ching. etc.
Pressure. We cope. And perform.