Motor Selection Help 1

[Planets]

I need to size an electric motor for a traction winch appliction. the motor will be controlled by some kind of VFD and will be operated at either 900 RPM or 1800 RPM. At 900 RPM the required output torque is 14000 lbin and at 1800 RPM the desired motor torque is 7000 lbin. For VFD speed control do I have to size the motor at the MAX torque and Max speed conditions. This would result in a 400HP/1800RPM motor. The actual horsepower requirement is for the two load points is only 200 HP so does this mean the VFD would be sized at 200hp? Any comments are welcome........

 

[CJCPE]

You can use a VFD to operate a motor above nameplate frequency at a constant voltage and provide constant horsepower. That type of operation is usually only recommended for a speed range up to 150% of nameplate. You could operate a 1200 RPM motor up to 90 Hz at constant voltage for 1800 RPM and down to 45 Hz at constant V/Hz for 900 RPM. The motor would need to be sized for 14000 lb-in at 1200 RPM or 266 Hp. That would make it a 300 Hp system operating at 88% of rated load. There are other options, but they would require a special motor.

Constant horsepower seems to be a rather unusual characteristic for this type of application. How does that come about?

 

[Planets]

I don't really know for sure why these particular values were set this way since it comes to me as a specification that is several groups/companies removed from the actual users of the equipment. In fact, the data was presented as winch line speeds with corresponding line pull specifications that represent a low speed and high speed operation. It may be the result of translating a hydraulic drive spec for a similar winch application. It is relatively common to set up a hydraulic power system with a fixed speed/HP electric driven pump coupled to a variable displacement motor. This allows the control system to use all available horsepower as either predominantly speed or torque.

Can you tell me if I would then need a VFD sized for 200 or 300 HP? I assume 200HP but this is not really my area of expertise (although it is interesting)

Don

 

[CJCPE]

For the system that I described as 266 Hp at 1200 RPM, you would need a 300 Hp motor and VFD.

I think it possible to use a 200 Hp 900 RPM motor and operate it between 60 Hz and 120 Hz, but constant voltage between 60 Hz and 120 Hz would likely leave you short on torque at 120 Hz. The motor would need to be wound for some voltage less than the input line voltage to the VFD. It is the sort of thing that the drive supplier should design and supply as a motor-VFD package.