The specifications of a 4-pole, 3-ph induction motor: Rated- 3.7 kW, 2.52 kg-m, 1430 rpm, 415 V, 7.27 A. Torque, Starting- 225%, Pullout- 275% of FLT.
It is required to control the speed of this motor in the range 3 to 1200 rpm. Whether it is possible to get the lowest speed of 3 rpm through VFD? or a gear box combination is required?
Experientially I have used equipment where the low-speed was below 1rpm - maybe 0.1 rpm and the high-speed was either 1800 or 3600rpm
1HP motor on a chemical injection pump
Almost certainly the motor would need to be inverter-grade and oversized and possibly blower-cooled depending on the torque requirements across the rpm range.
Dear Chandrashekhar Dharankar,
For a commercial Variable Frequency Drive, it is recommended more than 50% of the rated speed to avoid over heat at full torque. The lowest recommendable speed is 10%, providing external forced cooling. For lower than 10% rated speed, it is necessary to implement advanced torque ripple compensation strategies to reduce speed ripple. Please find attached some references for PMSM.
http://www.mdpi.com/1424-8220/13/3/3831/pdf
http://www.mdpi.com/2076-3417/6/9/254/pdf
Regards,
Alfonso Gómez
Dear Chandrashekhar Dharankar,
A gear box can help to get the lowest speed of 3 rpm, although you probably will have to set the VFD for more than 100% speed to achieve 1200 rpm.
Regards,
Alfonso Gómez
VFD's can be used in open or closed loop. (Closed loop need an encoder connected on the motor shaft). In open loop you will have a good speed management only when the speed will be higher than the slip speed (70 rpm in your case). If you wnat to run your motor near 0 RPM the only solution is to use a VFD in closed loop.
Following on Long Wang answer, according to Chandrashekhar Dharankar information the motor is rated for 1430 rpm at 60 Hz, so 3 rpm means 0.126 Hz electrical frequency, and some speed ripple could arise.
practically, below 10 Hz (50 Hz motor) motor cannot start with full load when using V/F method. one solution is to add some boost voltage proportional to speed but it may result in saturating the motor (flux = v/f). but if you want to run the motor down to zero speed, use either Vector Control (most drives are having this control mode) or use the Direct Torque Control mode (only ABB drives as they are PATENT it).
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