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75HP 380v 60Hz motor

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Kiljoy

Electrical
Apr 15, 2003
132
OK, we have an order for two machines going to Brazil. The voltage there is 380V/3PH/60Hz. We use Weg motors that just happen to be built in Brazil. For some strange reason, they are telling us this is a long lead custom item that will take months to get. IT'S BRAZIL'S STANDARD VOLTAGE AND THE MOTORS ARE MADE THERE!!
Anyway, my question: Is it possible to use 460V/3/60Hz motors and re-nameplate them to 380V/3/60Hz? Would you use the high or low voltage taps (208-230v or 460v). There has been a mixup with our purchasing department and we may need to get these motors ASAP.
I'm thinking that the widings should be large enough on the 230V taps because 380V will draw less amps. I'm sure there are V/Hz ratio considerations and such, but the only information I could find was going between 50Hz and 60Hz, not 60Hz to 60Hz.
 
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There are no problems if you use them on the 460v tap and derate them by 380/460.

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There is also an effect on the torque curve which will decrease by approx (380460)^2. Affects starting and momentary overload capability. We don't know much about your application, and what kind of requirements have been specified for the 460 motor, but I'm guessing that's not a problem if you're driving a centrifugal pump.

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There is one potential trap to fall into. If you apply rated torque to the motor shaft, you will have almost fifty percent more slip - that is 50 percent higher rotor losses. And heat.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
Unfortunately, because our hydraulic tank design, we need to keep the same NEMA frame size as the 75HP motors. These motors are driving hydraulic pumps. It's and industrial baler and load is variable, but starting torque is fairly low. We are using soft starters as well.
We are waiting to hear from our motor supplier as to what they plan on doing. We knew that they could be long lead custom motors, but the PO's got "lost in the mail".
 
Gunnar - I don't see where you got the 50% more slip.

Assuming original full load nameplate torque is Tnp and the associated slip is Snp. Original slope is Snp/Tnp

Reduce the voltage and the torque at original Snp is Tnp * (380/460)^2 = Tnp*0.83^2.

The new slope is [Snp/Tnp * (1/0.83)^2]


Apply the derating -> We will limit the applied torque to the new torque [T*0.83]

The slip at the rated torque is new torque times new slope = [T*0.83] * [Snp/Tnp * (1/0.83)^2] = (Snp/Tnp)/0.83

About 17% more than the original slip.

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You have 480 volt rated motors and you have 380 volts available. give each motor two small transformers connected in open delta auto-transformer boost configuration. 380 volts in, 480 volts out.
This is probably your fastest and cheapest solution. I'm short of time just now and can't do the calcs. but if you're interested I will probably have time on the weekend to give you transformer sizes and voltages.
respectfully
 
Pete,

I said "If you apply rated torque to the motor shaft"

That will give you 46.537.... percent more slip.

I took the liberty to express that as 50 percent. Sorry.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
Gunnar - ok I understand what you were saying. But I think it's a little misleading in that you said there is one more trap directly after my comments. But my comments already suggested to derate the motor.

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The voltage change on a 240/480V motor puts coils in parallel or series so, each coil sees the same voltage and current on both supply voltages. If you put a 240V motor on 380V it's very likely you will saturate the magnetics and cause high currents.

Use a VFD instead and then you can use a 240V motor with the proper setup of the VFD.

 
Lionel - I agree it's a problem to hook up on the 230v connection. But you're not objecting to using the 460v connection with 380v system are you?

Gunnar - going back to your comment. I believe the iron losses in a rotor are insignificant. If we assumed they go as frequency squared, then at 1% slip the watts per pound loss in rotor would be 0.01% of that in the stator.

So the bottom line, I think the motor can be derated by a factor of 380/460 without any further derating associated with concerns about slip. Agree/disagree?

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I will mention also that high efficiency motors typically have low slip, but the reason has nothing to do with rotor iron losses. It has everything to do with rotor resistive losses. The lower efficiency associated with the old motors is not created by the high slip... it's created by the high rotor resistance (the high slip just comes along for the ride)

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And the bototm line remains as before:

I think the motor can be derated by a factor of 380/460 without any further derating associated with concerns about slip. Agree/disagree?

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Well, if the application requires 75hp and there is no way to install derated motor to still get 75hp then I just didn't see it as a solution.

I believe derating as you suggested is correct. I've never done this practically though to really know the effect. The starting torque reduction could be a problem on a hydraulic pump.
 
Thanks guys for the info. Unfortunately, the machines are sold and there is no margin to add transformers or drives. The two balers each have two 75HP motors driving two pumps. Often our hydraulic expert sizes the pumps to use into the service factor. I'm not at work today, so I can't check the exact HP requirements. This power unit may be sized under 75HP. Due to nature of the application, the full load is not required constantly and a slight de-rating probably won't hurt.
In all reality, it's the purchasing department's problem since the motors were not ordered in time. I'm thinking the solution from the motor manufacturer is going to be re-nameplate a 460/3/60 motor.
I still can't figure out why a Brazilian motor company has trouble supplying motors for their own voltage.
 
Oh, by the way, the starting torque will be low and we are using soft starts. That probably won't be a problem.
 
Re-nameplateing motors for the wrong voltage will certainly be the fastest and cheapest solution in the short term. Let us know how it works in the long term. This may be one of those times when we regulars on the forum learn something new, to our chagrin.
Or maybe not.
respectfully
 
I suspect the reason Weg is balking at building 380/60 motors is that the real service is 380/50. That would be a standard motor and a common South American service.

Better doublecheck to see if a mistake wasn't made with that 60Hz. Good chance they meant 50hz.
 
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