# Current imbalance on motors



## micromind (Aug 11, 2007)

The voltage on A-B is high enough to begin to saturate the core of the stator. 

This is where the iron holds all the magnetism it possibly can and the very top and bottom parts of the sine wave become a short circuit. 

This will cause high current.


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## Forge Boyz (Nov 7, 2014)

Thanks micromind. Next question: what can be done to clean up the power?


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## Big John (May 23, 2010)

Silly question, have you taken a voltage reading at the line-side of your phase converter?


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## backstay (Feb 3, 2011)

If the Phase converter is a rotary style it's making the third phase. They are never clean power like the utility. I'd do more checking like John said.


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## micromind (Aug 11, 2007)

Big John said:


> Silly question, have you taken a voltage reading at the line-side of your phase converter?


Since B is the manufactured phase, A-C would be the utility voltage.


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## micromind (Aug 11, 2007)

Forge Boyz said:


> Thanks micromind. Next question: what can be done to clean up the power?


The Currents/Voltages on a phase converter will never be very balanced, it's just the way they are. 

Check to see if any controls or other loads are using the manufactured phase. This is always trouble. 

Most likely, there are multiple capacitors in the converters box. Try disconnecting one of them. Might not help but worth a try. 

A typical motor can be loaded to about 80% of its capacity with a rotary phase converter, anything over that and the O/Ls will trip.


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## telsa (May 22, 2015)

Get rid of the obsolete rotary phase generator -- install a VFD. 

There is no comparison.

Such units -- at that power level -- are fantastically cheap.

Get one rated for 2-3 hp... and be happy.


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## telsa (May 22, 2015)

Get rid of the obsolete rotary phase generator -- install a VFD. 

There is no comparison.

Such units -- at that power level -- are fantastically cheap.

Get one rated for 2-3 hp... and be happy.

Yes, they can generate 230VAC 3-phase out of 120 VAC L-N, let alone 240/120.

( I'll admit the amps will be !!! at 120VAC.)


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## frenchelectrican (Mar 15, 2007)

What brand is the PRC is ? Some case it may need change capaitor setting depens on the load..

The A-B voltage is on pretty high side so it may need remove one cap or swap for amaller cap to knock the voltage down ..


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## Big John (May 23, 2010)

....


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## Forge Boyz (Nov 7, 2014)

Thanks for the replies. I had a feeling it might involve tinkering with the roto phase, something I haven't really done before. Big John, I did check voltage feeding the roto phase and it was good. The motor in question is getting up sized so I think that should take care of the immediate problem but I would like to get the power balanced to prevent future motor failures.


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## telsa (May 22, 2015)

Forge Boyz said:


> Thanks for the replies. I had a feeling it might involve tinkering with the roto phase, something I haven't really done before. Big John, I did check voltage feeding the roto phase and it was good. The motor in question is getting up sized so I think that should take care of the immediate problem but I would like to get the power balanced to prevent future motor failures.


Then install a VFD. Your job is done.

They cost PEANUTS.

FAR less than a rotary phase converter.

A Chinese VFD -- plain vanilla is all you need -- costs well under $200.

That old junk cost thousands, and WASTES energy -- forever into the future.

Solid state power electronics has made that old gizmo obsolete.

Get it ?


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## telsa (May 22, 2015)

Jump to 8:52 -- get a load of the trivial cost -- and the speed of air delivery.






As you can see, he's a machinist, not an electrician... and even he gets it to go.


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## Flyingsod (Jul 11, 2013)

I like the VFD solution but its not as simple as just slapping one in. Some VFD's will not work like that so you need to do you homework on drive choice. You need to go no higher than 25% of the rated drive current. If not spec'd right and you just throw one in both your motor and drive will have longevity issues.


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## telsa (May 22, 2015)

Flyingsod said:


> I like the VFD solution but its not as simple as just slapping one in. Some VFD's will not work like that so you need to do you homework on drive choice.* You need to go no higher than 25% of the rated drive current.* If not spec'd right and you just throw one in both your motor and drive will have longevity issues.


Tell me that's a typo. 

His driven load is a pathetic one-horsepower 3-phase motor.

So a VFD that's rated for 2-3 hp will be well able to handle it.

The device rectifies the 120VAC wave form ( not the 240 -- even though it's being fed 240VAC -- and then voltage stacks it up to create the required 230 VAC.

Re-stated, it takes both legs (2) and generates a third while at the same time shifting the original two so that they are equally 120 degrees apart.

One bumps up the hp rating of the VFD so as to provide plenty of reserve capacity in the electronics. 

But 25% is too severe a de-rating... 33% to 50% is enough.


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## telsa (May 22, 2015)

One might note that the low-ball VFD in the video has the capability of 'standing on its head' in terms of auxiliary tricks.

1) Can dial the speed, reverse direction...

2) Can connect a WIDE selection of low voltage control inputs -- and INTERRUPTS -- ie safety cut-offs.

3) You can turn it on remotely -- ie when the process equipment needs power.

One can place emergency stop buttons all over the joint.

One can place high temperature stop logic... 

Stuff that used to cost Big Bucks is now priced down there with Chicklets.

This progression parallels that of cell phones, PCs, and much else.

DUMP rotary phase converters. They are energy PIGS.


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## Flyingsod (Jul 11, 2013)

telsa said:


> Tell me that's a typo.
> 
> His driven load is a pathetic one-horsepower 3-phase motor.
> 
> ...


 Nope wasn't a typo. I'm going by what the regional Drive rep told me in class. I do admit fully that it could be that they want you to buy a bigger drive than you need OR that they do not want to take any risk for failure and have too severely derated. My only experience in this was trying to get one to run in my house so I could play with it. I do know that some drives WON'T run with a single phase input because i had one in my house  .90% of my admittedly limited drive experience is with ABB. My company doesn't really install anything else unless forced to by the customer. If you've used one of the budget drives with only 50% derate and its been working past its warranty then I am very happy to know that.
I'm sure I can use that knowledge somewhere down the line. thanks


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## Jhellwig (Jun 18, 2014)

There are drives out there specifically meant to be powered by single phase. I think that is what tesla is talking about.


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## JRaef (Mar 23, 2009)

Here's the low down on de-rating of VFDs for single phase supply to run a 3 phase motor. This by the way assumes that you are NOT using a drive that officially has a single phase input rating already.

There are 3 issues in play when selecting the drive for a single phase input; Rectifier size, Capacitance on the DC bus and Temperature

1) *Rectifier size*; When feeding a 3 phase motor, ALL of the power going to that motor on 3 phases will have to come from the 2 lines feeding the drive, instead of being even split among 3 phases. The current drawn by each of those 2 lines feeding the drive then increases by the square root of 3 (1.732x). So if the motor draws 10A from each of the 3 phases on the output of the drive, the input current for the 2 lines of single phase power feeding the drive increases to 17.32A. If the drive was rated for 10A 3 phase output, the input current will likely be a little lower (because the VFD corrects the displacement power factor), so the diodes making up the rectifier section might be rated for only 8A. 

But even if we assume they are rated for the full 10A, the current will be 17.32A and the diodes will eventually fry. So from that standpoint, the de-rate must be _at least_ 58% (1/1.732). You might often hear that as a statement from salesmen about de-rating drives, but that's not all there is to it, see below.

2) *Capacitance on the DC bus*; A VFD rectifies the AC to DC, smooths it out on the DC bus with capacitors and MAYBE chokes, then uses the transistors to fire pulses that make the motor APPEAR to get AC again. In the rectification stage, the AC current is pulled through the diodes only at the peaks of each sine wave, so the resulting DC has a lot of "ripple" in it. If the source is 3 phase, at least ONE of the diodes is conducting all of the time, so the ripple is there, but minimal. Transistors need clean smooth consistent DC, otherwise they can misfire and either damage themselves or fail to provide enough current to the motor. So the smoothing of that DC is critical for the life of the drive. When you feed that drive with single phase, there are now moments in time when NONE of the diodes are feeding the DC bus, so the bus ripple is much much deeper, all the way down to zero volts at every cycle. To get this smooth again, you need MORE capacitance on the bus, in fact you need at least twice the capacitance. So for this reason, the new de-rate must be 50%, meaning for a 10A load, the drive must be rated for 20A. it's overkill on the AC output side, you only need this for the bus capacitance, but you still need it.

3) *Temperature*; when the capacitors have to be increased to supply more current to smooth the ripple, one of their other specs, what's called the ESR (Equivalent Series Resistance) also increases. So just like a resistance of any sore, ESR produces heat and when doing the double-duty of smoothing that single phase ripple, the ripples are there for longer, so the current is higher for longer and thus the heat of the capacitors is higher. When a drive is designed for 3 phase input, the expected heat from ESR is accounted for in the cooling system of the drive. When that same 3 phase drive is fed with single phase, the capacitors will give off a lot more heat. That heat can be dealt with in two ways. You can either use an ADDITIONAL de-rate percentage, usually another 15-20%, OR you can de-rate the temperature by roughly 50% (empirical evidence). But HOW the drive is built make a difference in that too. If the drive has a DC bus choke in ADDITION to the capacitors, that actually assists in dampening the DC bus ripple by slowing it down so that it doesn't dip as low, and that decreases the ESR and heat, meaning you may NOT need to further de-rate. So here's what that bottom line looks like:



 If the VFD _*does NOT*_ have a DC bus choke (most drives from Asia and under 30HP will be this way):
65% de-rate of current at the rated operating temperature of the VFD, or
50% de-rate of operating temperature. So if the VFD is rated for 40C operation, it must be 20C, 68F, which is virtually impossible.

If the VFD *DOES *have a DC bus choke (most US made / designed drives and many European drives over 30HP), 50% de-rate, and no temperature de-rate.
YMMV; meaning if the VFD mfr provides you with a de-rate value, lets say down to 25%, you should believe them, because it likely means they know something about that drive design and the way it cools itself.


Conversely if the VFD mfr does NOT specifically say, IN WRITING, that you can de-rate the drive for single phase input, you must assume tou cannot. If you want to get a REAL answer, call their _*TECH SUPPORT*_, not the salesman, because 90% of the VFD salesmen I encounter have no clue, they will sometimes say whatever they think you want to hear. Not de-rating a drive properly will not show up as a problem right away, so by the time it does it's usually out of warranty.


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