Explain "Supercharging" an AC motor Please !

[kamenges]

Originally posted by realolman:

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?

Yes...more or less. This is more true in volts/hertz mode than in the other modes but it is largely correct. FVC and DTC will modify this some to maintain the correct motor flux and torque levels but on the whole this is correct. Motor speed is based on applied frequency. In order to maintain the correct motor flux the applied voltage needs to increase at the same ratio as the applied frequency. Those darn drives are doing all sorts of stuff under the hood.

Originally posted by realolman:

what I'm wondering is why do this rather than size motors and gearboxes "Normally"?

Size, cost and performance. I am basically getting twice the horsepower capacity out of a given size motor. Granted, you need the larger drive to take advantage of this. But in some applications size matters. And as others have said, assuming the rest of the powertrain can handle it, this method can get you out of a sizing snafu reasonably painlessly with very little downside.

Keith

 

[realolman]

would this be NEC compliant?

 

[kamenges]

I don't know why it wouldn't be. You aren't exceeding the motors current rating or voltage rating. You may need to be careful with some motors that you don't exceed their rated maximum speed but that would be it.

Keith

 

[Gene Bond]

Agreed. It's been done for years w/o problems. As mentioned, attention must be paid to mechanical speed ratings such as sheave and gearbox ratings, as well as motor maximum speed ratings.

Overall, it is a much better practice than 'normal' overspeeding by simply turning maximum frequency up to 120hz, where the voltage can't go higher, and thus the windings are starving. In this scenario, the motor is in a 'field weakening' mode, thus reduced torque capability results in constant horsepower above base speed (or nearly constant, discounting the losses).

There are many advantages, not the least of which is motor size. Just as a 2pole motor is in a much smaller frame from a 4pole motor for a given HP, this method utilizes a 'standard' off-the-shelf 4pole motor to replace a potentially long leadtime 2 pole motor.

Yes, 'normal' overspeed can still be used above the 120hz point. This constant-HP operation is often used on spindle drives, centrifuges and other high speed, low torque application to go to incredible speeds. Granted, some of the higher speeds require special motors or at least modifications to run higher speeds, but it's more common than most might think.

Dick has explained it very well. It's just one of the many ways VFD's make our lives so much easier!

 

[DickDV]

I think it is helpful to understand that a motor produces torque and speed, not hp. HP is a calculated value based on torque and speed.

Available motor torque is a function of volts divided by the frequency. On a NEMA motor, that torque can be calculated from the nameplate data. For example, a 10hp 230V 1760rpm motor has rated torque of 10hp x 5250/1760rpm = 30ft-lb. That 30ft-lbs is based upon two coils in parallel and a v/hz ratio of 3.83.

We can vary the speed of the motor all over both up and down from the nameplate as long as we don't exceed the maximum voltage rating of the insulation. As long as we vary the applied voltage in the same proportion as we vary the frequency (to keep the ratio constant) we will have 30ft-lbs torque available at all those speeds.

It follows then that the above motor makes a calculated 10hp at 1760rpm and 60hz. It also follows that the same motor makes 5hp at the same torque at 860rpm and 30hz. And, it follows that the same motor makes 20hp at 3560rpm and 120hz.

The main benefit to doing this is when space is very tight the "supercharged" motor will be somewhat smaller. It can also be handy when you want to take a motor a bit overspeed but you can't have the available torque falling off as in normal field-weakened operation.

I know that this is a rather strange concept. The first time I heard about it, I was sure my tail was being pulled. But, after studying it a little, and then trying it, I'm persuaded that it is an entirely legitimate way to use a dual voltage motor.

 

[boneless]

very interesting!

so would you be able to control the speed with a FC? If so, what would be the appropriate range when you compare it to regular wired motors for example 30-60 Hz.

 

[Tom Jenkins]

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.

Thanks for the correction, Dick!

 

[NetNathan]

You need to check with motor manufacture to verify max speed of the motor.
120HZ is possible but that does not mean the motor is rated to run 200% overspeed.
Motor frame construction and bearings play a roll here.
I have used VFD for overspeed a few times and the motor mfg has never recommended 200%. Think about it....a 3600 rpm rated motor running 7200 rpm....

 

[DickDV]

Netnathan is correct, certainly for 3600rpm motors and many cases NEMA motors in 400 or 440 frames. Aside from these, any NEMA motor with a cast aluminum rotor can be taken to 90hz for sure and probably 120hz with no mechanical issues. As I cautioned earlier, you may begin to get some additional losses above 90hz so you may see slightly less than double hp at the top speeds. I would not expect to see any serious mechanical issues with any of these motors except as noted above

 

[NetNathan]

I use VFD for different types of pumps and fans. There iis advantages to running these motors faster to achieve higher pumping and cooling speeds.
I always call the motor mfg with the motor nameplate info to find out the maximum speed they recommend. They are aware of this use of a VFD to over speed, because they are almost always aware my question.

 

[sparkie]

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.

I work for a packing plant, so I completely understand this. I spend several days a week fixing lights and replacing the guts in local disconnects. Your job sounds like it would be pretty fun though . I wanna do something like that.

 

[milmat1]

Well this has been an interesting reply and I thank all of you. You are correct as I understand this process. The whole purpose of "SuperCharging" IS so that the VFD can keep ramping the frequency without hitting a voltage ceiling. As I have explained to our techs, you can manipulate the voltage and torwue. But maintaining the V/HZ ratio is critical.
So the motor is wired for 230V but everything is setup for a 460V motor.Then the VFD ramps the speed up to as high as 120hz, without running out of voltage.

 

[milmat1]

This is the best explaination I have seen, This comes from WINNIE at Mike Holt's

The mechanical output power of a motor is given by the product of torque and speed. The _torque_ is limited by the physical size of the motor, but you can adjust the speed with a VFD. So for a given motor the peak output power is proportional to speed.

The voltage needed to drive the motor varies linearly with speed. To double the speed you need to double the voltage. The output voltage of the inverter is limited, so you can only increase the speed so far.

If you connect the motor for half voltage, then you essentially double the speed you can reach before you reach the inverter output voltage limit. But by connecting the motor for half voltage you are also double the motors current requirements. This double current requirement is what forces you to have an inverter with double the output current capacity.

But since you can now drive the motor to double speed (presuming the motor is rated for that), you can get double the output _power_ from the motor. You are not increasing the motor torque capability, but doubling speed, doubling power, and doubling the required inverter capacity.
Winnie...

 

[Mill_Control]

very interesting!

so would you be able to control the speed with a FC? If so, what would be the appropriate range when you compare it to regular wired motors for example 30-60 Hz.

The lower limit stays the same so you gain a much wider useable speed range. Using your example 30 to 120 Hz.

The following is a pretty good read if anyone is interested.

Operating_motors_at_higher_frequencies_V2_en.pdf

 

[Gene Bond]

MC:

Typical German approach to confusing the obvious with a bunch of facts

Just Kidding. They do a good job of explaining the math and physics, but intimidation via a bunch of data and charts, etc. is typical.

The point remains, if a motor has a certain voltage and frequency rating for continuous operation, you can derive other operation points which are well within the design parameters of the motor. The perfect example is the 50hz, 380-400v motor running on 460-480v... It's fat and happy, since the torque (current) is the same, thus, we American get 20% power for free! (a joke, but it can almost be construed this way, since the currentis the same and we get the bonus HP.

It's math and physics.