Explain "Supercharging" an AC motor Please !

What we call "Supercharging" is using a 460 VAC VFD with a mtor wired for 230 VAC rather than 460. And the VFD has to have a HP rating of twice the motors HP rating. There is some gain to doing this relative to obtainable speeds.

I always calculate the required RPM/Torque/Gearbox etc then choose a motor size and VFD accordingly. I fail to understand this process of supercharging.

I realize the motor doesnt know or care what size it's suuposed to be. up until it saturates and/or melts, a 2HP motor is only good for 2 HP no matter how you manipulate the voltage / current etc.

The key to understanding what you describe as "supercharging" (a term I've never heard before!) is to realize that a motor develops torque based, not on voltage, current, or frequency, but rather on the volts per hertz ratio.

The coils in your dual voltage motor are wound for developing torque at a volts per hertz ratio of 230volts/60hz or a ratio of 3.83. These coils can be configured with two in parallel which preserves the 3.83 ratio or with two in series which makes the ratio double to 7.66 which corresponds to 460volts/60hz.

The trick that you describe is possible because, even tho the coils can be wired in parallel for 230V operation, the insulation in the motor has to be good enough for 460V.

Taking advantage of that fact, with a motor wired in the 230V configuration (actually the 3.83 ratio configuration) you don't have to stop going up the ratio at 230/60. Keeping the ratio constant, you can continue up all the way to 460V/120hz. At that point, you have to stop because you have reached the voltage limit of the insulation.

Since the ratio has remained constant, the motor will be able to develop constant torque all the way up to 120hz. Nameplate rated torque at double frequency and speed calculates to double nameplate hp.

Of course, you have to have a power train that can deal with the higher speed but the doubling of hp is real and the motor is quite happy doing it. You may get slightly less than double hp due to higher friction and windage losses. Some older motor coils may loose some of their magnetic intensity at the very highest frequencies but high and premium efficiency motors should be ok. And, of course, motor cooling is better and audible noise is higher due to higher fan speeds.

One caution: do not try this with two pole 3600rpm motors. The plastic fans will not tolerate any speed much over 5000rpm generally.

This is a great theory but;

The voltage insulation rating of dual voltage 230/480V motor windings is 240V, not 480. The series connected windings
produce 240V accross each winding when 480V is applied, and so, each winding of a dual voltage 230/480V rated motor
is designed to operate at only 240V.

JRB said:

This is a great theory but;

The voltage insulation rating of dual voltage 230/480V motor windings is 240V, not 480. The series connected windings
produce 240V accross each winding when 480V is applied, and so, each winding of a dual voltage 230/480V rated motor
is designed to operate at only 240V.

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The insulation rating is measured to ground, not line to line. Therefore, any motor rated for 480v has to have insulation rated for at least 480v, no matter how it is wired. Think corner grounded Delta...

jawolthuis said:

Think corner grounded Delta...

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That sounded real bad!
Don't ever wire a corner grounded Delta! My Father-in-law almost got killed on one when he was helping me on a job...
But... it does help us to understand that the potential voltage in a 480 volt rated motor is 480 volts at any point in the winding, no matter how the leads are configured.

jawolthuis said:

The insulation rating is measured to ground, not line to line. Therefore, any motor rated for 480v has to have insulation rated for at least 480v, no matter how it is wired. Think corner grounded Delta...

Click to expand...

You need to check your assumption. 480 VAC three phase is 480 VAC line to line and the insulation is usually rated for more than that. Line to ground on a 480 VAC three phase system is 277 V.

Tom Jenkins said:

You need to check your assumption. 480 VAC three phase is 480 VAC line to line and the insulation is usually rated for more than that. Line to ground on a 480 VAC three phase system is 277 V.

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Tom,
That is what I was trying to say. A corner grounded delta wired to a motor will have 480v nominal to ground on 2 legs. The insulation will have a higher rating than that.

Actually, Tom, jawolthuis is correct. 277v to ground only occurs when the 480V source is configured 480/277 with a grounded center wye connection. Any ungrounded or floating or high resistance grounded system can, as he mentions, go to ground on one leg and the other two will go up to 480v to ground.

Floating power is actually even worse than that. I put a recorder on a floating (at least they thought it was floating!) 480V network at a Ford plant and, in only one week, the network went to 1800VAC to ground at one time and 800VDC to ground at another point.

And, JRB is wrong with his insulation voltage assumptions. A dual voltage motor needs insulation throughout rated for the highest voltage. And, no, it's not "a nice theory", either. It's done all the time and works very well if properly applied. I've done it at least a dozen times with good results.

In IEC motor areas, it is standard practice to arrange the coils in wye or delta to get two different voltage ratings. The ratio between the voltages is 1.73. not 2.0 as with NEMA motors. And you can play the same game with using the lower voltage configuration and run up to the higher voltage while the frequency goes up to 50 x 1.73 or 86.5hz.

Any ungrounded or floating or high resistance grounded system can, as he mentions, go to ground on one leg and the other two will go up to 480v to ground.

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Roger that!!

On our vessels this is a common occurence.
We do not tap to ground so we have no Neutal on our system.
If one leg grounds on our 440, 220, or 110 volt systems then it will pull the other 2 legs of that system high. It doesnt need to be anything that big either for this to happen.

And should you happen to get 2 phases go to ground at the same time, it is not pretty.

With a factory full of 440 volt motors sitting just inches from the deck that has a continuous flow of salt water running across it, and a bunch of, Well, really special people doing high pressure, salt water washdown on everything including the electrical equipment, I end up looking for ground faults all the time.
(And NO they are not supposed to be washing the electrical equipment with HP salt water, but you can talk till you are blue in the face and it does little good)

We have ground fault meters on our main distribution board in the ECR that detect when voltage is going to ground.

BCS

Washing electrical equipment with high pressure salt water

You might as well stand on top of a hill in a lightning storm wearing wet copper armour and shouting "all the gods are bast@rds!" (with apologies to Terry Pratchett).

ASF said:

Washing electrical equipment with high pressure salt water

You might as well stand on top of a hill in a lightning storm wearing wet copper armour and shouting "all the gods are bast@rds!" (with apologies to Terry Pratchett).

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Welcome to my world, it is not very glamorous, but pays very well.
So for now I put up with it.

They dredge people up from all corners of the earth to process in these factories, I think they figure "well its in the factory so it must be water proof why cant I spray it"
My hair is already too thin and gray, so I just roll with it and fix what they break.
It is not my job to manage them, and honestly I would like to keep it that way.
My out box is full of reports about it, so I have M.A.C.

If you ever happen to be strolling around the Bering Sea come by some time when we are in production, its better than center ring side seats at the circus.

Returning to JRB's comments about individual coil voltage not exceeding 240V, he is, of course, correct if you look at the coil in isolation. Worst case would be the first and last turn of a coil laying next to each other.

Unfortunately, when the coils are stuffed into the stator slots, you could easily end up with the first turn of one phase coil laying next to the last turn of another phase coil. Since there is no phase paper in the slots, its up to the individual wire insulation to withstand the voltage.

In actual practice, the insulation is far overrated to handle surge voltages. Plus, if its a NEMA motor intended for sine wave power, it has to pass a 1200VAC test to meet spec. If its an inverter duty motor, that number rises to 1800VDC peak pulse voltage.

I don't know what IEC motors are tested to but the 230, 400, and 690VAC motors are surely tested far above those operating levels.

Hope this clarifies a somewhat complex subject. I've meant no offense to anyone. If I have caused any offense, please accept my opologies.

What is your duty cycle for the application...

milmat1 said:

What we call "Supercharging" is using a 460 VAC VFD with a mtor wired for 230 VAC rather than 460. And the VFD has to have a HP rating of twice the motors HP rating. There is some gain to doing this relative to obtainable speeds.

I always calculate the required RPM/Torque/Gearbox etc then choose a motor size and VFD accordingly. I fail to understand this process of supercharging.

I realize the motor doesnt know or care what size it's suuposed to be. up until it saturates and/or melts, a 2HP motor is only good for 2 HP no matter how you manipulate the voltage / current etc.

Click to expand...

Supercharging – Hands On Experience
I am a long experienced industrial electrician. If you told me to intentionally miss-wire a 3-phase motor to 240vac, while knowing that the system is 480vac, I would tell you that this is guaranteed to FRY.
But, a customer had a unique application. They needed to reduce the time of transport on an assembly line. The motor only needed to run about 15 seconds out of 60 seconds, so approximately 25% duty cycle.
I was EXTREMELY reluctant to Supercharge. But I went to a LAB with the VFD vendor, and we played in the lab.
I went to the plant, which is a critical supplier, so failure was not an option. In five minutes, I changed the connections in the motor, and modified three parameters in the drive and we ran the motor to TWICE the rated RPM all day long. After running the motor this way, I personally touched the motor, it was cool to touch, or almost less than room temperature.
I am a believer in Supercharging.
Here is the theory: A 3-phase motor will develop full torque at 60hz (1725rpm) and give it’s best performance at that level. If you were to push beyond 60hz, which if a simple conveyor, that you might get away with it.
But if the motor has a substantial mechanical load, when you exceed 60hz, there is torque-slip, and the motors actual horsepower will be reduced.
The purpose of Supercharging is to intentionally miss-wire the motor to DOUBLE the windings. The motor is protected because entering the correct parameters into the VFD will CURRENT limit the power to the motor and prevent it from destruction.
When you miss-wire the motor, you achieve full horsepower and torque all the way up to 120hz.
The only thing we were instructed to do, was verify with the supplier of the equipment, that it would be ok to run at twice the intended speed (120hz / 3600rpm) for “x” amount of time.
Our mechanical motor/drive train vendor approved in writing, and the system has run years without issue.

So the next obvious question is can we extend this into the field weakening area (assuming of course that the mechanical system can handle it)? I'm guessing that the motor magnetics are going to start having issues with this as the frequency continues to rise. But this could have some serious physical sizing benefits on low horsepower center driven winders.

Keith

so in "normal" VFD / motor operation, is the volts/ hz ratio maintained by the VFD as we mindlessly crank the frequency up and down to vary the speed of the motor?

this is very interesting... never heard of it before

Am I correct that the whole point of this is to gain RPM?
do the gearboxes have a problem with this?
what I'm wondering is why do this rather than size motors and gearboxes "Normally"?