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DavidJ2 (Electrical) (OP)
1 Jun 12 11:18
We have a small issue w/ a electrical motor installation in the corn fields of Kansas. The situation is as follows:

We recently installed a 480 VAC single phase motor. The setup consists of a solid state 7.5 KVA variable frequency drive (VFD powering a 5 HP motor.
The unit is designed and will operate very well on input power from 440 VAC thru 500 VAC. BUT our situation out in the Kansas is that the utility grid is very unstable. Our input voltage from the utility is periodically running up to 520 VAC for hours on end, causing us issues.

I realize that there are some fairly sophisticated solid-state controlled multi-tap transformer devices available on the market which could provide automatic step changes to regulate the power within a give range. But their cost is $4000 to $8000, way over our budget.

Any thoughts regarding a means of regulate the power from the utility, keeping it within 500 VAC?

Many thanks!
SceneryDriver (Electrical)
1 Jun 12 12:52
First, a question:
How can you have a VFD on a single phase, 5HP motor? Do you mean to say that you have a 480v single phase supply for the VFD, running a 5hp three-phase motor?

Contact the utility and have them lower your supply transformer one tap. Since the VFD can operate on a lower voltage (down to 440v) lower the incoming voltage to keep it rising above 500v.

Install a brake resistor on the drive. This will let the drive burn off excess power on the DC bus and keep the DC bus voltage within limits to prevent overvoltage trips. Keep in mind, you will have to size the resistor's wattage higher than probably specified, as it may be in almost continuous duty use at times. Plan for and mitigate the heat issues that will arise from dumping lots of power into what is essentially a space heater. If voltage levels are wildly[i] exceeded, you risk actual damage to the VFD's electronics, but your issue sounds like it's a borderline issue that happens sometimes.

A line reactor on the supply side [i]may
help, but probably won't be much use in low/no load situations, as voltage drop increases with an increase in current through the reactor.

The utility/transformer solution is preferred, but the brake resistor solution possibly in combo with a line reactor may help. Contact the VFD's manufacturer to seek more guidance.

jraef (Electrical)
1 Jun 12 14:11
Simpler solution:
Buy a VFD rated for 575V input, then program it to put out 460V. The VFD itself will act as the "voltage regulator" because the output voltage will never be higher than what you program it to be, regardless of the input voltage. But by having the higher voltage rated drive, you avoid having it trip off on Over Voltage. The tricky part will be finding a 575V VFD that can accept a single phase input (with appropriate de-rating of course), be programmable to not trip out on Phase Loss, and not trip out on Under Voltage as low as 70% of it's rating. There are some out there that will fit this bill but I work for one of them so I'll remain as neutral as possible here.

By the way, I also am assuming you have a 3 phase motor and are just using a single phase input to the VFD.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)

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DavidJ2 (Electrical) (OP)
1 Jun 12 17:08
Many thanks for the advice to date. I promise to post up the results that we finally obtain, for everyone's benefit. Some clarification from my side:
1) Yes the 5 HP motor is 3 phase, 480 AC rated. The motor is taking the 520 VAC hits OK, it is the VFD which is throwing high voltage error messages and tripping off in the middle of the night.
2) I will check to see if we have a brake relay in the VFD to function w/ an external power resistor. Would seem like a good idea IF the logic is built into the VFD's firmware.
3) Have thought about using a 600 VAC VFD, but an issue popped up. That is we have to communicate to the VFD via MODBUS and we only have drivers written for a few VFD models from Baldor and Allen Bradley. We are kind of limited to which ones we can talk to, writing and testing new drivers is a real pain for us to do.
4) We have had the utility out, they did move the secondary tap to the lowest voltage output. (this corn field has true 3rd world power, the voltage swings from 440 to 520 VAC on an hourly basis)
Still searching for a cure for the root cause (high voltage on the utility side). The Reactor sounds interesting, will most likely buy one. The brake idea is a solid idea, unfortunately our compatible VFD does not have the dynamic brake relay/logic built in.
waross (Electrical)
1 Jun 12 17:27
Use a buck transformer to drop the voltage. Use an adjustable voltage relay to control contactors to switch in the buck when needed. With an adjustable dead band on the relay this should work and may be the cheapest.

"Why not the best?"
Jimmy Carter

Helpful Member!  stevenal (Electrical)
1 Jun 12 17:57
Have you tried taking this higher up in the utility? Most have service standards, sometimes enforced by government regulators. What you are reporting is way outside ANSI standards.
jraef (Electrical)
1 Jun 12 18:19
An A-B PowerFlex 40 is capable of what I described with regards to using the 600V drive. In the 600V drive, the input voltage can drop to I think around 300V, so no problem with it accepting 480V input on a regular basis. After that, your line voltage can surge to 711VAC without causing an Over Votlage trip.

But looking at the spec sheet to get that data, the 480V drive actually may be fine for you. The Over Volt trip occurrs at 810VDC, which equates to a 575VAC input voltage. I forgot that.
If you prefer the PF70, the OV trip threshold for that series is 570VAC.

You will need to double the size of the drive to get enough capacitance to avoid it tripping on excess ripple from having the single phase input, so buy a 10HP VFD.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)

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DavidJ2 (Electrical) (OP)
6 Jun 12 18:31
As promised I am now getting back to close out this thread with our solution.

But first, many thanks for those who took the time to get back with some ideas to assist! The bucking transformer and reactor were certainly some viable options.

WHAT WE DID - Well it turns out the first VFD got toasted due to high voltage. The rural utility did come out and place the secondary on the lowest voltage output tap at the power pole. When I started this thead, we were trying to make the VFD operate, not knowing it had been toasted. On a hunch, we swapped the toasted VFD out and placed a known good VFD in its place. VFD/motor has now been running for 48 hours without any issue, though the utility line voltage still goes up and then down on a regular basis. Will most likely keep it as is without doing anything more to the system. (unless this 2nd VFD gets toasted)
jraef (Electrical)
6 Jun 12 19:50
If you don't have a Line Reactor, get one. When a utility has big voltage swings, they also likely have a lot of grid switching going on and line capacitors being used to boost voltage, which can cause big spikes on the line. Reactors don't do much for sustained over voltages, but they help dampen the impact of spikes from the aforementioned issues. It's cheap insurance.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)

For the best use of Eng-Tips, please click here -> FAQ731-376: Forum Policies

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