New VFD's on motors

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steve66

steve66

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Illinois
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Engineer
I've got several motors a client wants to add VFD's for. These existing motors are fed from a MCC right now. They are 25 HP, 50 HP, and 60 HP, 480V motors.

The plan is to just intercept the motor circuits at the motor, and install the VFD right at the motor, intercepting the motor branch circuit right at the motor.

I had planned to replace the motor starter buckets at the MCC with new buckets that have standard circuit breakers, instead of starters. However, that's pretty expensive.

Can I just leave the existing starters in place and run the VFD's from those starters? I

'm a little concerned the overloads won't hold if the VFD draws a little extra current for its inefficiency. I've checked, and the breakers in the MCC don't have a thermal trip, just instantaneous. So I don't think I can bypass the overloads.
 
steve66 said:
I've got several motors a client wants to add VFD's for. These existing motors are fed from a MCC right now. They are 25 HP, 50 HP, and 60 HP, 480V motors.

The plan is to just intercept the motor circuits at the motor, and install the VFD right at the motor, intercepting the motor branch circuit right at the motor.

I had planned to replace the motor starter buckets at the MCC with new buckets that have standard circuit breakers, instead of starters. However, that's pretty expensive.

Can I just leave the existing starters in place and run the VFD's from those starters? I

'm a little concerned the overloads won't hold if the VFD draws a little extra current for its inefficiency. I've checked, and the breakers in the MCC don't have a thermal trip, just instantaneous. So I don't think I can bypass the overloads.
Click to expand...
I would first varify if the motors will be compatable with the drives. My concern would be the integrity of the insulation of the motor windings which may be prone to failure. After you can be assured that this is not an issue then address your concerns. You just don't want otor insulation failure as a result of "I didn't even consider that."
 
steve66 said:
I've got several motors a client wants to add VFD's for. These existing motors are fed from a MCC right now. They are 25 HP, 50 HP, and 60 HP, 480V motors.

The plan is to just intercept the motor circuits at the motor, and install the VFD right at the motor, intercepting the motor branch circuit right at the motor.

I had planned to replace the motor starter buckets at the MCC with new buckets that have standard circuit breakers, instead of starters. However, that's pretty expensive.

Can I just leave the existing starters in place and run the VFD's from those starters? I

'm a little concerned the overloads won't hold if the VFD draws a little extra current for its inefficiency. I've checked, and the breakers in the MCC don't have a thermal trip, just instantaneous. So I don't think I can bypass the overloads.
Click to expand...
No, it is not that simple, your initial plan is the only correct one, replace the starters with feeder breakers. The VFDs will not be listed to have starters as the OCPD. If you read the manuals, they will list either fuses or thermal-mag breakers, maybe both, but not mag-only breakers, even if in a starter with a thermal OL relay. The only valid use of a mag-only breaker is when it is part of a starter that has a thermal OL that is directly feeding a single motor. Even though there is eventually a motor in the circuit, the connection is no longer direct, you are now feeding a piece of power conversion equipment, and IT is feeding the motor. It's like a new source in that regard, so your circuit is no longer a single motor circuit. If its not going to be inspected, you can do what you feel is right I suppose, I'm just stating the technicalities.

But there is another issue: 430.122, the sizing of the conductors feeding the VFD. When the motors were fed from the MCC starters, the conductors had to be sized for the motor FLC from the NEC HP tables, x 1.25. But per 430.122, the conductors feeding a VFD must be sized based on the MAXIMM INPUT AMPS OF THE VFD x 1.25, which may be higher than the motor FLA. Make sure that is investigated, you may need to pull new conductors to be legal.

The alternative would be to get new buckets made with VFDs in them. The conductors FROM the VFD to the motor are still sized the same way. This would however likely result in a major reconfiguration of the MCC.
 
To add to what Tempkll said.
Yes, check that the motor is VFD rated.
If it isn't you may need a filter between the VFD and the motor.

You also need to check cooling. Most standard motors have a NDE shaft mounted fan which is rated to provide adequate cooling at rated load on fixed speed.
Drop the speed and the effective cooling drops considerably.

And then there's the type of cable/conductor arrangement.
Others here will be better able to advise you on that than I can. We do it differently here in Britain.
 
templdl said:
I would first varify if the motors will be compatable with the drives. My concern would be the integrity of the insulation of the motor windings which may be prone to failure. After you can be assured that this is not an issue then address your concerns. You just don't want otor insulation failure as a result of "I didn't even consider that."
Click to expand...

Yes, we've checked into this, and we have gotten conflicting info. on how long the non VFD motors will last. All we can do is tell the owner we aren't sure, you can replace all the motors now, or take your chances, or cancel the whole project (hope that doesn't happen).
 
Jraef said:
No, it is not that simple, your initial plan is the only correct one, replace the starters with feeder breakers. The VFDs will not be listed to have starters as the OCPD. If you read the manuals, they will list either fuses or thermal-mag breakers, maybe both, but not mag-only breakers, even if in a starter with a thermal OL relay. The only valid use of a mag-only breaker is when it is part of a starter that has a thermal OL that is directly feeding a single motor. Even though there is eventually a motor in the circuit, the connection is no longer direct, you are now feeding a piece of power conversion equipment, and IT is feeding the motor. It's like a new source in that regard, so your circuit is no longer a single motor circuit. If its not going to be inspected, you can do what you feel is right I suppose, I'm just stating the technicalities.

But there is another issue: 430.122, the sizing of the conductors feeding the VFD. When the motors were fed from the MCC starters, the conductors had to be sized for the motor FLC from the NEC HP tables, x 1.25. But per 430.122, the conductors feeding a VFD must be sized based on the MAXIMM INPUT AMPS OF THE VFD x 1.25, which may be higher than the motor FLA. Make sure that is investigated, you may need to pull new conductors to be legal.

The alternative would be to get new buckets made with VFDs in them. The conductors FROM the VFD to the motor are still sized the same way. This would however likely result in a major reconfiguration of the MCC.
Click to expand...

Not at all what I wanted to hear, but I understand what your are saying. I wasn't thinking about protecting the wire, but hadn't really thought about the VFD's. But it really sounds like once the load isn't a motor, a motor starter just isn't the right OCP anymore.

I had thought about the VFD input current, and I was looking into that. I got the rated currents, and they are actually less than the motor current. I have to start a new thread about that - its the exact opposite of what I thought.
 
steve66 said:
... I got the rated currents, and they are actually less than the motor current. I have to start a new thread about that - its the exact opposite of what I thought.
Click to expand...

No need for a new thread, it's simple.

MOTOR full load current as shown on the nameplate is based on a measured power factor at full load, usually also stated on the nameplate. But with a VFD, the Power Factor, as it relates to the SUPPLY, is corrected, essentially because the VFD caps are storing the reactive power for the motor, basically like having PFC caps for it. So the ACTUAL current drawn by the VFD is always going to be lower than the current drawn by the motor. So that might make one think that 430.122 then is not going to be a problem.

The problem is actually in the reality of the drives business. Most VFDs, by aggregate volume, come from Asia made by Asian companies as part of a product that is designed to be sold all over the world. But for our North American market, they tend to have a difficult time interpreting the way UL requires testing and listing, plus they do NOT want to pay for multiple tests. So when they design a VFD for "400V Class" and then extend the input voltage range to 480V so they can sell them here, they test and rate them at 400V motor currents, which are higher than 480V currents for the same given power rating. That means when you buy a 250HP Japanese drive, it is going to be rated at 250HP 400V, which more than covers 250HP at 460V current capability. But it means that the MAXIMUM current rating on the VFD nameplate is higher than it could otherwise be, which then gets you in trouble with 430.122. Companies that design their drives in North America for our market, regardless of where they are actually manufactured, will size the current ratings based on the NEC motor HP charts, the same ones used for sizing the conductors. So that's where you tend to see that the VFD maximum input amps are the same or lower than the NEC HP chart amps.
 
I have put VFD's on many process motors starting in ~1995, never lost a motor. Doesn't mean you will not. Just my experience.

We bypassed the contactors, sure don't want them with a VFD. We didn't have instanteneous only breakers, we had fuses. 99% of the time the motors were operating at less than nameplate speed after installing the VFD's - which is why the VFD's!

Our thinking was if we lost a motor, we would simply replace it with one VFD rated. We rolled the dice and won. Sometimgs you don't, but it worked out for us. Saved a boat load of motor $'s and used less power, so we were way ahead.

RC
 
The problem is actually in the reality of the drives business. Most VFDs, by aggregate volume, come from Asia made by Asian companies as part of a product that is designed to be sold all over the world. But for our North American market, they tend to have a difficult time interpreting the way UL requires testing and listing, plus they do NOT want to pay for multiple tests. So when they design a VFD for "400V Class" and then extend the input voltage range to 480V so they can sell them here, they test and rate them at 400V motor currents, which are higher than 480V currents for the same given power rating. That means when you buy a 250HP Japanese drive, it is going to be rated at 250HP 400V, which more than covers 250HP at 460V current capability. But it means that the MAXIMUM current rating on the VFD nameplate is higher than it could otherwise be, which then gets you in trouble with 430.122. Companies that design their drives in North America for our market, regardless of where they are actually manufactured, will size the current ratings based on the NEC motor HP charts, the same ones used for sizing the conductors. So that's where you tend to see that the VFD maximum input amps are the same or lower than the NEC HP chart amps.[/QUOTE]

Excellent piece of information, which is now stored in my memory vault!! :thumbsup:
 
petersonra said:
I seem to recall at elast some VFDs that are listed to be used with MCPs.
Click to expand...
Allen Bradley drive WERE listed with MCPs, up to about 6 months ago. UL decided that they now don't like it and all MCP versions had to change to Thermal Mag breakers from here on out. Doesn't affect equipment already in the field, just new.
 
steve66 said:
Yes, we've checked into this, and we have gotten conflicting info. on how long the non VFD motors will last. All we can do is tell the owner we aren't sure, you can replace all the motors now, or take your chances, or cancel the whole project (hope that doesn't happen).
Click to expand...
Non VFD rated motors operating at 208-240 volts are likely to last longer then same motor/same load operating at 480 volts - the higher peak voltages on a 480 volt application will punch holes in winding insulation much easier then it will on a 208-240 volt application. Motor cooling is the next thing to pay attention to, especially if you are going to regularly run at 50% or less speed on a regular basis.

Ragin Cajun said:
I have put VFD's on many process motors starting in ~1995, never lost a motor. Doesn't mean you will not. Just my experience.

We bypassed the contactors, sure don't want them with a VFD. We didn't have instanteneous only breakers, we had fuses. 99% of the time the motors were operating at less than nameplate speed after installing the VFD's - which is why the VFD's!

Our thinking was if we lost a motor, we would simply replace it with one VFD rated. We rolled the dice and won. Sometimgs you don't, but it worked out for us. Saved a boat load of motor $'s and used less power, so we were way ahead.

RC
Click to expand...
I have done the same. Some work out well, others do not. Like I said before 480 volt drives will be harder on a motor - especially if not wound with spike resistant wire like ones rated for drives are. Definitely want to use a line reactor on the output if a 480 volt drive and not a VFD rated motor.
 
Like the others I have seen many old motors retrofitted with VFDs without any issues.

I can't see replacing the motors unless they do die.
 
iwire said:
Like the others I have seen many old motors retrofitted with VFDs without any issues.

I can't see replacing the motors unless they do die.
Click to expand...
It's called getting everything you can out of it:happyyes:

If you replace it as soon as you put the VFD on it, some can't stand to see that old motor that worked sitting there doing nothing.

Now if downtime because of a failed motor is not worth it, you should definitely put on the correct motor, some instances that down time costs more then equipment repairs do.
 
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