# Undersized VFD



## sparkiez (Aug 1, 2015)

Looking at the input rating from the PowerFlex 700 manual and a 40HP rated drive, and assuming 60 Hz I get the following:

Single phase 480V x 1.73 = 59A

You need an input rating (for single phase) of at least 59A

PF700 40HP model input ampacity is 52A. This is Not enough to handle the 59A needed.

PF700 50A model is JUST above the the threshold (at 65A) which is pretty close for my comfort, but this is on a recirc pump. I would need to know more about the motor and if it is a variable speed pump or not, but you mentioned it is rarely running at full load.

Given this, you MAY be okay for a while with a 40HP model but if possible I would wait for a 50 at minimum, perhaps even upsize to a 60HP model (depending on how long this past one lasted. If less than 5 years, I would just upsize it).

Link to manual: Page 13 for table
http://literature.rockwellautomation.com/idc/groups/literature/documents/um/20b-um002_-en-p.pdf


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## Cow (Jan 16, 2008)

I'd use the current limit parameter to protect the drive.

Set it to a limit you're comfortable with and let it rip Glen! I wouldn't even think twice about it.

I just did it the other day with a 700 when a customer had a 60HP pump die on them, but they had a temporary 75HP pump available. I set a current limit just to make sure they didn't kill the undersized drive while the 60HP was getting repaired.


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## glen1971 (Oct 10, 2012)

sparkiez said:


> Looking at the input rating from the PowerFlex 700 manual and a 40HP rated drive, and assuming 60 Hz I get the following:
> 
> Single phase 480V x 1.73 = 59A
> 
> ...


This drive was fine for the last 9 years, so I don't think it is a size issue.
From ALB:
Listed by UL to operate on single-phase input power with the requirement that the output current is derated by 50% of the three-phase ratings

The one that failed was a 20BD065 (25 hp, 51.6 amps input and 32.5 amps output) which are 50 hp at 3 phase. I have a couple of spare 20BD052 (20 hp, 41.3 amps input and 26 amps output) which are 40hp at 3 phase.


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## JRaef (Mar 23, 2009)

Use the Current Limit setting in the drive to limit the output current to 50% of the VFD rating. But understand that in order to limit the current, the drive will also limit the speed and/or torque by overriding whatever you are asking of it. Depending on the machine involved, that may or may not be a problem, only you can answer that.

What would happen to the VFD if you don't limit the current is that under too much load, the DC bus ripple increases and the drive will trip off line, usually on Phase Loss. In the A-B drives you can't disable the Phase Loss, but the only way it detects it is via the DC bus ripple. So over sizing the drive adds enough extra capacitance to take care of that ripple. Under sizing the over size results in SOME added capacity, but not enough to run at the full rated HP of the motor.


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## glen1971 (Oct 10, 2012)

Thanks for the replies!

Update:
So the drive I grabbed was set-up as a 20 hp drive, so I opted to not mess around with any of the motor parameters. I did the swap today and monitored the current. At 60 hz it was about 26.5 amps, which is over what the drive is rated for. I changed the max speed (P82) to 53 hz and the output current was <20 amps. The process was ok with the reduced speed and all seemed ok after an hour of monitoring.
This is only a temporary measure until the proper drive can be installed. But on the Thanksgiving weekend, it'll work..


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## paulengr (Oct 8, 2017)

JRaef said:


> Use the Current Limit setting in the drive to limit the output current to 50% of the VFD rating. But understand that in order to limit the current, the drive will also limit the speed and/or torque by overriding whatever you are asking of it. Depending on the machine involved, that may or may not be a problem, only you can answer that.
> 
> What would happen to the VFD if you don't limit the current is that under too much load, the DC bus ripple increases and the drive will trip off line, usually on Phase Loss. In the A-B drives you can't disable the Phase Loss, but the only way it detects it is via the DC bus ripple. So over sizing the drive adds enough extra capacitance to take care of that ripple. Under sizing the over size results in SOME added capacity, but not enough to run at the full rated HP of the motor.


50%? Uhh no. That would price AB even more out of the market.

Drives and really thyristors are rated in RMS values which is approximately 50% of their DC rating.

When running into the drive current limit, you use just that. Part of the current is magnetizing flux (and is nearly 100% inductive) and the rest is torque (100% real power). There are two places running at 100% current limit can get you in trouble. The first is with an overhauling load. When in 2nd kr 4th quadrant (generating aka braking) the drive runs into a problem with most drives because even though in the real world flux and torque currents are clearly linked, within the drive these are separate paths. So sometimes you need to reserve enough current to achieve flux...you're setting the torque limit only. The second problem area is when the drive is running wide open at full voltage and there is so much inertia that the flux can't be estimated accurately, even with encoder feedback. Without limiting things again when you try to slow down the load, it can run into problems. The solution is to voltage limit a little bit.

This all usually only shows up in servo operations with high inertia loads.

Sent from my Galaxy Tab 2 using Tapatalk


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## Ghostnotes1221 (Oct 20, 2017)

Had a client wondering why they would trip their drive at 70% input all the time. Come to find out they were running a 35HP motor on a 25 HP drive. Worse still, it was an induction fan motor with squirrel cage, heavy sucker with a butterfly damper instead of a vane type. Every time they tried to make a change in the setpoint, the inertia was too great, even with a slow rate change. It would try to correct and.....

Also when they would open the damper and increase load same thing. Had me stumped for a few. Did not expect a drive that undersized. Figured one or the other was a spare replacement but no, it was initially installed that way.


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