Motor and Drive Selection

[7JLAman4]

I'm looking for assistance dealing with a motor configuration. I'm looking for ideas regarding continuous low speed operation of an induction motor, or what I think is low speed. The required motor is 700Hp at 100 rpm. I've got a supply voltage of 480VAC 3Ph 3Wire (delta). What I'm thinking of is a step-up phase shift transformer to 600VAC to a 12 or 24 pulse PWM drive. If I select a 14-pole 60hz induction motor, what are the ramifications of operating this motor at the low end continuously? Are there other configurations that I should entertain? The motor may eventually become a custom build. The motor initially will initially brought up to full rpm (100rpm) with near no-load and then full load or torque applied once speed has been reached.

 

[dpc]

I'd be thinking about mechanical speed conversion to get in the ballpark.

 

[7JLAman4]

I would tend to agree, however I'm sorry for not mentioning that the motor is actually part of a larger shaft. This will be part of or segment of a longer drive shaft. Similar to a Power Take Off (PTO) configuration with the exception that the machine will operate as a motor , not generator. The drive shaft is powered from another engine thru a gearbox where this motor will then be connected in line with the drive shaft.

 

[Marke]

Hello jlamann

If you reduce the speed of a motor using VFD, you reduce the effective power capacity of the motor with the speed.

In you your case, you are asking for 700Hp at 100RPM.
If you start with a motor that operates at 1000RPM at line frequency, then you are operating at 10% speed and therefore 10% power. That would mean that you would need to have a 7000hp motor at 1000RPM to achieve those figures.

It is best to use a mechanical speed reduction to get your base speed to your required speed with the motor operating at line frequency and then you can alter from there.
For your example, yous a 700HP 1000RPM motor with a 10 : 1 gearbox will work, and then you can use a VFD to vary the speed from there.

Best regards,

Mark Empson
L M Photonics Ltd

 

[7JLAman4]

Marke- Is it possible to acquire or design a PWM drive that delivers full rated torque or constant torque over the entire operating range of the drive or at low output rpms?

 

[CJCPE]

To put Mark’s analysis another way, if you need 700 Hp at 100 RPM, you need 36,764 lbs-ft of torque. If you select a 14-pole, 60 Hz motor, it would need to be designed to produce 36,764 Lbs-ft at the full-load speed of 500 RPM or so. When you reduce speed with a VFD, you can not get more than full-speed rated motor torque at the reduced speed. At 500 RPM, the motor horsepower rating would need to be 36,764 x 500 / 5252 = 3500 Hp. The physical size of such a motor would be comparable to a 1750 RPM motor rated for 36,746 lbs-ft or 36,764 x 1750 / 5252 = 12,250 Hp.

I don’t think there is any “may eventually become” about such a motor being a custom build. If you really need direct drive, you should go to the people that can build the required VFD and motor and ask them to make a proposal. Those people would be ABB, Siemens and a couple of others.

 

[ScottyUK]

Yes, but motors are typically designed for operation at (approximately) 750, 1000, 1500 or 3000 rpm, equating to 8, 6, 4 and 2 poles respectively. Anything over 8 poles is pretty rare these days. You can slow them down but you will be using (say) a 7000HP / 6 pole motor to deliver 700HP at 100rpm. The drive will be sized based on the current of the 7000HP motor because it will need to deliver rated current at low frequency and low voltage to give you 700HP at 100 rpm, unless you use a matching transformer between the drive and the motor. The economics are awful.

A 14 pole machine will be a 'special' and will be very expensive. It will have a very high magnetising current too, meaning lousy power factor and high penalties or you'll need to install a PF correction system.


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If we learn from our mistakes I'm getting a great education!

 

[davidbeach]

One rule of thumb I've run across is that synchronous motors are probably a better option than induction motors when you get the point that the motor rated HP is greater than the motor rated RPM. 700 HP at 100 RPM is a ratio of 7:1, probably well into the realm of synchronous machines, excitation systems, and the whole kit and kaboodle.

 

[edison123]

At that speed and horse-power, the motor will be a monster and hence, rightly will be killed.

 

[DickDV]

I didn't see the OP directly rule out a gearbox so, if at all possible, figure a 700hp 6 pole motor with a 10/1 reduction ratio. You might want to go to 750hp just to cover some of the friction losses in the gearbox. You are way ahead in physical size, power factor, efficiency, and cost to use this approach.

 

[Marke]

Hello jlamann

Is it possible to acquire or design a PWM drive that delivers full rated torque or constant torque over the entire operating range of the drive or at low output rpms?

The problem is not the PWM drive, it is the motor.
In order to do work, the motor must develop torque and the torque is a function of the flux in the gap. The maximum flux is limited by the iron saturation, so fro a given frame, there is a limit to the flux and therefore an limit to the torque. The design full speed torque of the motor is very close to the limit, so you will not be able to increase the torque from this figure.
As you reduce the speed by reducing the frequency, the torque stays the same and torque times speed equals power, so the power reduces with reducing speed.

Best regards,

Mark Empson
L M Photonics Ltd

 

[roydm]

Sounds like some sort of planitary gearbox to me. 2 input shafts and one output shaft.
Something like a car differential only more robust.
Roy