Motor HP gain by reconnect? Dick DV attn

leitmotif

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Dick

this is all based on a separate post from Kid PLC. I have recopied relevant info to ensure I am not repeating partial truths - I hope.

I put these questions in a separate post to avoid taking Kids post on a tangent.


Sparkz, I find it amusing that you refer to our marvelous dual winding dual voltage NEMA motors as "infamous"! As you point out, they can be handy for getting more hp out of a motor in a way somewhat similar to using an IEC motor to get more than nameplate kw. However, in the case at hand here, we still don't know if the power train reduction ratio can be easily changed or not. If not, your clever solution won't work. If yes, then your solution is easily the best and cheapest under the given circumstances especially if there is no room for a larger motor. Let's see what PLC Kid comes back with for a power train.

DICKDV

Assuming across line starting and no change in the power train the motor drives
I do not see how changing the same NEMA 30 HP nine lead motor from 230 to 460 will gain HP.

Another poster said
QUOTE Surely this is one of those (infamous) 2-voltage, 9-wire motors? If so, you can almost double its power output at twice the speed if you connect the motor windings for 230V and hook it to a 60HP-460V VFD. Base frequency would have to be set to 120Hz; base voltage to 460V. In other words, the motor will be able to deliver (almost) the same torque at 3600rpm (motor power = 60HP) as it does at 1800rpm (motor power = 30HP). UNQUOTE

I sorta follow this
Motor is same ie 30 HP 1800 RPM at 60 Hz
ASSUMING No changes of mechanical setup in the driven load.

120 HZ = 460 volt and 60 Hz = 230 volt
torque is constant because he has set baseline at 120 Hz.
I do not see how he can drive a 230 volt connected motor at 3600 RPM and corresponding 460 volt without burning it up. For sure I do not see how he can deliver continuous double rated motor HP without burning motor up.

I also have done a little with IEC six lead motors and understand what they are doing. Seems like a good option with differing advantages.
I do not understand how HP can change by changing from delta to wye configuration when you change the line voltage of course.

I do not want to waste a lot of your time
- but I do want to gain better understanding of what is being done here and why it can work
- can you refer me to some reading material please?

Dan Bentler
 
Generally speaking motor heating is a result of the current going through the windings. This current (suared) times the winding resisance defines the heating in the motor. I know, there are additional losses but for the purposes of our discussion this is the major loss.

Keep in mind that as the motor voltage goes up the resistive voltage across the windings does not. The motor winding heating is always governed by I^2*r regardless of the motor voltage. So running a 230VAC motor at 230VAC / 60 Hz / 10A will produce basically the same motor heating as running that same motor in the same configuration at 460VAC / 120Hz / 10A.

The thing that is lost in this is that the drive needs to be able to deliver this voltage and current level. So a motor that is listed as 10HP 230VAC will require a 460VAC 20HP drive to run it. In reality you could do the same thing with a 460 VAC motor if you had 920 VAC available (discounting the obvious motor insulation issues of course).

Keith
 
I think I see a dim flickering of comprehension. Let me work up a solution as to how this works in my head and present here for correction.

Thanks
Dan Bentler
 
Here's another way to look at it. 230V/460V motors clearly have to have insulation good enough for 460V. The individual windings in the motor are sized to 230V at 60hz. By that I mean that they will load to the proper amps at the V/hz ratio of 230V/60hz equals 3.83. In fact, the coils will load to the proper amps with that V/hz ratio over a wide voltage range.

A normal VFD application with a 230V drive takes the voltage and frequency down at that ratio but the torque output capability of the motor remains the same. There is no reason why the voltage and frequency cannot be increased in the same ratio expecting the torque output to also remain the same.

That is exactly what happens when a 230V/60hz motor is run on 460V/120hz. The ratio stays the same at 3.83 so the coil loads the same. Since the torque output capability is the same and the speed has doubled (120hz), you have twice the hp.

Pretty crafty, huh!!! And, as kamenges points out, the heat generated is still nearly the same as at 60hz. But with the fan turning twice as fast, cooling should no problem. Keeping the fan from flying apart might be a problem tho. Normally plastic fans are changed out to metal when this is done.
 
Dick

Am working up a spreadsheet and am coming up with results I am having a hard time believing.
In essance I took a 230/460 30 HP 1800 RPM and had it on
1 VFD 460 and 60 HZ at baseline and did a bunch of calcs.
2. Reconnected motor to 230 set VFD to 460 V and 120 Hz - ran those calcs.

Two results surprise me
1. Line current (VFD output) is constant - yet this is not too surprising since constant torque to baseline and current proportional to torque.
2. The reconnect to 230 results in less heat loss in windings -??? I have tried to ensure that I am not entirely treating motor windings as resistors.
3 because I left VFD output at 460 in both cases i get equal line amps but cannot believe the motor amperage does not double. i think I am in error here.

Power is calcd from speed x torque /5252 then current is derived from that. It worked well for the 460 hookup so I left it the same for 230 - therin lies my mistake I believe. I can see right now if I correct for this I will get 60 HP at baseline and above but does the paper math correlate accurately to the real world motor?

I gotta get away from it and let this run around in cranium for a while.

Here is spreadsheet where I did calcs. I think it is correct but have not convinced myself. I gotta try this out - do not have access to large VFD or motor - please send for me to experiment with.

Dan Bentler
 
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I haven't studied your spread sheet, Dan, but remember that the 230V connection puts two coils in parallel. The 460V connection puts the two coils in series. The coils are wound for a V/Hz ratio of 3.83. In series, that would be a ratio of 7.66.

Since you have the motor connected 230V, the coils are in parallel and the FLA is doubled. So, with the motor wired for 230V, at 460V 120Hz the FLA would still be the doubled number. With the motor wired for 460V, at 460V 60hz the FLA would be half that.

And, that makes sense too because the input kw has to double to get double the hp out.
 
Dick

In my second case at baseline freq is 120 volt is 460 and in my spreadsheet the motor rpm at 120 is 1800. Should I have it set to 3600 if so another light bulb came on.


As i think and study this more the slickness of this is getting impressive. Dont waste time studying spreadsheet yet let me find my errors and bad assumptions on my own and present final product.
Dan Bentler
 
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Sparkz
Thanks for reviewing and correcting. While you were doing that I set the baseline speed to 3600 and all the rest of calcs came true.
Increased windage is not considered nor is power factor

CONCLUSIONS
1. I see how this works on paper - most impressed.
2. paper motor winding I2R loss does not increase
3 how you can get double the HP by only changing VFD and motor connections.
4. This should not bother the motor at all since torque never exceeds factory design.
5. The motor winding current and voltages do not change and do not exceed factory design
6. Increased power factor from increased frequency will only be a problem (if any) to the motor itself or the output of VFD. It should have no impact on plant PF.

Uhhh couple devil advocate questions or thoughts
If this is so neat how many people are doing it?
I cannot see any drawback except the driven machinery must be able to handle increased RPM and input HP.
This can be done at the possible expense of more frequent bearing change and possible rotor damage (like Dick said take out the plastic fan and replace with metal).
What are other drawbacks of doing this that I have not even thought of?

My corrected spreadsheet is attached. I think I have it right and it is ready for review.

Thanks for education and patience

Dan Bentler
 
Is there a name for this method?

I guess i have just never ran across it?

Is it common practice?

Is this mostly yielded to a specific application?

Kid

You are mirroring my thoughts exactly.
I just answered one of my questions above - this may not be commonly done because you do NOT get increased torque just increased HP from increased SPEED.

Take a look at my most recent spreadsheet or Sparkz - they should now be identical.

Assuming
you have 460 fed to machine / VFD
gearbox and other drivetrain components can handle increased speed and HP and resulting heating.
this may be a good option for you. Only change would be double the VFD capacity and pulling new conductor. It may not solve torque problems from increased loading of conveyer
- for SURE you will not get production to stop doing this.

Dan Bentler
 
I'll try to answer the questions.

Is this done commonly? Not in North America because double speed has lots of related problems. In Europe, where the voltage ratios are 230/380 which is 1.73, it is much more common. It also is accomplished by changing the single windings from a wye to delta configuration. That requires access to both ends of the coils which is standard on IEC motors. NEMA motors, on the other hand, always have internal connections with the rare exceptions being single voltage six lead motors and dual voltage 12 lead motors. The 1.73 ratio reduces but does not eliminate the overspeed issues.

What problems arise? Basically the same problems as taking any motor to double speed. Some motors especially older standard efficient motors will not maintain constant torque up to 120hz. That makes the hp rise less at higher frequencies. You would almost always need to increase the power train speed reduction ratio since most machines simply can't be driven that much faster. Sometimes this is a real deal-buster. Turning the input shaft of a gearbox to double speed often results in oil foaming which drastically shortens gear life. The larger the motor, the more likely you would run into rotor balance issues. That and bearing issues altho, in my experience, bearings are rarely the problem. Another issue often overlooked is audible noise. Large motors turning that fast tend to scream like a siren.

On the other hand, getting double hp without increasing motor frame size is really compelling. The drive doesn't really care.

But I think that the main reason this isn't explored more in North America is ignorance. I probably worked in this field 10+ years before I had even heard of this technique. It probably was another year before I understood how and why it worked. (Not sure I should admit that!) And, it took a European with patience to make it clear.

So, what's that statement? "Everything I know I learned from someone else"!!
 
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But I think that the main reason this isn't explored more in North America is ignorance. I probably worked in this field 10+ years before I had even heard of this technique. It probably was another year before I understood how and why it worked. (Not sure I should admit that!) And, it took a European with patience to make it clear.

So, what's that statement? "Everything I know I learned from someone else"!!

REPLY Well that sure makes me feel better Dick. I take pride being a good brain tapper. Now if I could just get it into my head as fast as I can get it out of others heads,,,,,

Dan Bentler
 
Ignorance might be a bit harsh, afraid of the unknown is more likely.

But can I make a suggestion? Try not to mention this method too often, just keep it at hand to deal with a (major) engineering flaw or to jump to the rescue when a production manager decides to "crank it up a notch": Always good for a pat on the back.

I would like to think of myself being an engineer, but once the engineering dep has figured this one out, you can bet your (fill in the appropriate word) they'll ruin it and often leave you with nothing else to do but to replace an entire power train, most likely in a spot where it's physically impossible to fit something larger. It has occured way too many times that a colleague seemed to have used a dart to place a decimal point...
 

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