Voltage Imbalance Solutions 1

[X49]

Hey everybody. I'm new to the forum.

I am having a problem with voltage imbalance at a small facility. We have 15hp 208V motor for a water pump connected to a soft start. The facility is in a rural area where the power quality from the utility is very poor. Our soft start has been tripping on voltage imbalance, and the imbalance has been measured to be about 6%.

I've considered increasing the size of the motor so that it is overrated by at least 50%. Alternatively, I was thinking about replacing the soft-start with a VFD under the premise that the VFD output voltage should be balanced even if the input voltage is not. However, I'm concerned that the VFD may get damaged if it is not designed for voltage imbalance at the input. Does anybody know if this is the case?

Any comments or suggestions would be appreciated.

Thanks

 

[PeterKsiazek]

A small imbalance in the input voltage to the VFD should not be a problem, especially if it is oversized for the application. I have used VFDs to drive 3 phase pumps off of a single phase service. You just have to size the VFD appropriately (Hitachi, I believe suggests sizing the VFD using 2x the FLA of the motor). You wouldn't need to oversize quite this much.

 

[waross]

I had a similar issue with a badly unbalanced supply. The soft start was tripping out on phase angle error. (Phase angle error is another issue with unbalanced rural lines.)
I found some breakers with ratings very close to the FLA of the motors. We connected them to bypass the soft starts. We could usually start the motors, but several times during the day the soft starts would rip.
As I remember it was a 200HP, a 250 HP and a 400 HP.
This was in a remote area of Central America and I didn't have a lot to work with. I had to use what I could find in the warehouse.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

BTW, I agree with PeterKsiazek on the VFD sizing. for a 6% unbalance you don't have to go as much over size as you have to for single phase.
If you go with the soft starts, consider oversizing the motor. Motors don't like unbalanced voltages. If you use a VFD, the motor will be getting a good supply and will not need to be oversized.
I like the VFD solution myself.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[X49]

Thanks for the responses. It looks like we'll be going with oversized VFD drives. I've been trying to determine how much the VFD's have to be oversized, but without much luck. I do know that small voltage imbalances will cause large current imbalances. I'm planning on using a 25hp VFD for the 15hp motor, and a 40hp VFD for the 20hp motor to be on the safe side.

I'm planning on using 5% line reactors to help correct some of the voltage imbalance and to help with harmonics. The electrical service is only 225A and the two VFD's are by far the largest loads, so I'm hoping I don't have enough current imbalance that I will trip the main breaker or have problems with harmonics. Any thoughts?

Thanks

 

[stevenal]

Why not oversize enough to ride through single phasing events? Common enough on the rural lines.

 

[waross]

Unbalanced voltages are a problem for motors. The motor acts as an induction generator and tries to balance the voltages. Hence the high currents when motor voltages are unbalanced.
On rural lines the voltage unbalance may be accompanied by a phase shift. The soft start may not like that either. Again the motors will draw excess current trying to correct the phase shifts. Voltage unbalance is usually not a problem for breakers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Why not oversize enough to ride through single phasing events? Common enough on the rural lines.

The output of a 3-phase rectifier has a relatively low amplitude ripple voltage when unsmoothed, whereas ripple from a single phase rectifier goes to zero twice per cycle. DC bus capacitors become huge in high power single phase applications because they have to hold the bus voltage up between successive charging pulses. It isn't too hard to oversize the input rectifier but physically it's inconvenient to add big capacitors to the DC link. I guess you could use a PFC input to draw a sinusoidal current from the supply and boost it to a stable DC voltage, but I'm not aware of any which can arbitrarily adapt from a three-phase to single phase input.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[stevenal]

Scotty,

I'm unaware of the design details, but there are VFDs that can be single phased. I attended a demo where a fully loaded three phase motor was run from a three phase VFD. A primary fuse was pulled, and the motor never wavered.

 

[jraef]

Most mfrs of 230V VFDs 3HP (2.2kW) an under use a front-end and capacitor system design that is capable of 1 phase input without derating. This is mainly because at that size, the difference in cost for the components is so minimal that it makes good economic sense. In fact, some mfrs have gone so far now as to have 1 phase input as standard for those small sizes; i.e. there is no longer an option for 3 phase input (ABB ACS-50, Teco E-2). A select few mfrs extend that size up to 4HP / 3kW (Siemens) and 5HP / 3.7kW (Lenze/AC Tech) without derating.

Above that, MOST VFDs CAN accept 1 phase input, but need to be drated by 50% (doubled in output current size compared to the motor FLC rating). Some people think that the size increase only nees to be 1.732x so that the diodes can take the extra current, but that doesn't account for the added capacitance necessary to smooth the additional ripple, so 2x is the best way to make sure you get that. Your mileage may vary however, so check with the mfr.

But accepting 1 phase by derating is not universal. Some VFD mfrs tap the control power to run the electronics from a separate SMPS connected to the line (AC) side of the rectifier, which means if single phased, you may not get control power to run the drive regardless of oversizing. Others, because the design engineeer had no forethought of using it with a 1 phase input, install Phase Loss detection to prevent damage to the rectifier. Although this is especially common on 400V+ drives, most mfrs allow a way to defeat that if you intend on using 1 phase input, but several do not. Allen Bradley is one that does not allow defeating of the Phase Loss detection.

So bottom line, you can use 1 phase input on almost all drives, small ones without derating, but always check before purcahsing; nothing is universal.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[LionelHutz]

Is the voltage imbalance causing a problem that can't be fixed by just increasing the voltage imbalance trip point?

Installing VFD's just because there is a voltage imbalance seems to be an expensive fix for an apparent problem which may not be a real problem.

 

[X49]

LionelHutz,

The voltage imbalance has not yet caused any problems, but I would like to do something as a preventative measure. With a measured imbalance of 6.5%, the motor would have to be derated to 67% of nameplate. It it running up to around 90% of nameplate at the moment (not including 1.15 service factor), and I'm concerned with premature failure of the motor.

Please let me know if you think there are better options that would be more prudent.

Thanks

Motor Derating:

http://www.engineeringtoolbox.com/electrical-motor-voltage-imbalance-d_648.html

 

[LionelHutz]

OK, I was just wondering.

How long have the motors been in operation already?

Do you have any means to measure the temperature of the motors and determine if they are showing ill effects from the voltage imbalance?

If you do install VFDs, your 25hp for the 15hp motor looks ok but I would just use a 30hp for the 20hp motor.