VFD overfrequency II

[electrolitic]

The idea it was:To "create" a new value of standardized AC motor rated power in order to optimize motor choice.So, I started a thread in this forum. After helpful tips I think that my initial idea is consolidated.Please check if I am right. Just as an example, here in Brazil standardized values are:22, 30, 37, 45 kW. There is a power gap of 36,23, 21% respect.
If an hypothetical load requires a value of 25kW, the motor choice would be 30 kW ( 20% more than necessary).With an adequate reducer gear ratio a 22kW motor running at overspeed and nominal torque would reach the load requirements without significative problems.So, 22kW, 380V AC motor, 6 poles, 60Hz(1200rpm) driven by a VFD 480 V, 75 Hz(1500rpm) can generate up to 28kW.
Separated fan motor guarantees correct flow at any main motor speed and releases more net power if main motor fan blades are supressed. A dv/dt filter at VFD output would be convenient. Of course, VFD rated current shall be defined at 380V motor base. It is a point to be analised case by case in order to find economical advantages.
If feasible, this idea surely is not new. Let me know.
Thanks

 

[DickDV]

Your plan only works if the 380V motor insulation is rated for 380V. Is it?

 

[electrolitic]

Hi Dick
Yes. Here, 220/380V (frames 90S up to 200L) and 220/380/440V (225 S/M up to 355 M/L) inverter duty motors are standard (WEG motors as an example).
460VAC is also an usual system voltage for industrial distribution.
Best Regards

 

[electrolitic]

Hi Dick
Just now I noted that you typed 380 instead of 480.Sorry.
You are right. Motor maker must be consulted.
Anyway, I suggested dv/dt filter that what I do when replacing direct starting control by VFD keeping the original motors.
Regards

 

[waross]

Don't forget that many motors are more efficient at 75% loading than at 100% loading.
I suggest that you investigate motor costs and VFD costs.
Then you may want to consider energy costs. Check your motor data for the motor efficiency at various load levels and don't forget to factor in the losses of the VFD and gear box.
For the range of speeds that you are proposing I suggest that you forget about supplementary cooling. I think that you will see significant cost disadvantages for both first cost and operating costs compared to just using the next larger size motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DickDV]

Yep! That's an oops! Typed a 3 instead of a 4 on the last voltage.

 

[OperaHouse]

With the world turning to VFDs, why hasn't industry come up with a new frequency standard for motors used on this duty. Seems there out to be some advantages. Reading the first 100 years of IEEE history found that first generator standard was 166.67Hz.

 

[LionelHutz]

Your numbers sound OK. I would not bother with forced cooling for a 25% over speed unless you are also planning on running at low speed with full torque output. Available power output may be just a little lower because of higher losses.

You also have to factor in the supply voltage. If the area you are in is mostly 380VAC, 60hz then where will the 480VAC come from? The better solution is to delta connect that motor for 220VAC and run the VFD on 380VAC allowing up to approximately 104hz operation. Well, you may only allow 75hz but the V/hz would remain linear up to 104hz.

Then, as Bill has pointed out, often a motor has pretty much the same efficiency from 75% to 100% load. So, doing this to save energy is a bad idea if you are planning on just using a VFD to run a smaller motor at a fixed 75hz. The VFD installation would likely use more energy compared to just picking the next bigger motor and running it full-voltage on 60hz. Only if you are planning to speed control the motor would it be a good solution.

I agree with you OperaHouse. But it would only make sense if the motor is being speed controlled.

 

[electrolitic]

Thank you all, specially to Warross and Lionel-lutz.
My reply:
1.Data taken from WEG motor catalog
5,5 kW, 6 poles, 60Hz
eff at 75%:85,5% power factor: 0,7
eff at 100%:86,0% p.f. 0,77
22 kW, 6 poles, 60Hz
eff at 75% 90,7% p.f. 0,81
eff at 100% 91,0% p.f. 0,85
2.Reducer gear efficiency if compared both ratios:
Highter input speed/ smaller input torque, highter ratio/same number of stages, same output torque- No significative different effic. values.
3.Motors/Voltage systems available:
Here, 220/380V is usual for urban/small industries distribution. 460V is usual for industrial LV distribution.
4.VFD rated current
Higher current, losses and cost. I agree with you.
Solid state components tend to be cheaper.Commodities like copper tend to be more expensive, I guess.
5.Separated ventilation is, in this case, a consequence of the process(roller tables for shearing line, hundreds of equal motors subject to run at low/high speed, severe acc/decc cycles).
Conclusion: Probably I will never apply this "plan"(as Dick said). It was an exercise of thinking and you made it more clear to me. Thank you very much indeed.