replacing DC motor with AC motor
replacing DC motor with AC motor
(OP)
I have heard that it is recommended to upsize the hp when replacing a DC drive with a 3 phase AC drive. Is this still a recommended practice?
I have a 240VDC 10hp 1750rpm drive to be replaced with a 575VAC 3ph drive w/inverter. Should I select a 12.5 or 15hp inverter and drive?
Thanks.
CAF
I have a 240VDC 10hp 1750rpm drive to be replaced with a 575VAC 3ph drive w/inverter. Should I select a 12.5 or 15hp inverter and drive?
Thanks.
CAF





RE: replacing DC motor with AC motor
What is the application?
Barry1961
RE: replacing DC motor with AC motor
If it were a 2500RPM DC motor however, the torque would be 10(HP) x 5250 / 2500 (RPM), or 21 ft. lbs. The nearest AC motor would be 3550RPM, so 10(HP) x 5250 / 3550 = only 14.8 ft. lbs., requiring you to oversize it to get the same torque.
If your application involves continuous and/or accurate operation at the low end of the speed range, I suggest using a Vector AC drive to match the performance of DC down to 1Hz. If the DC motor/drive was on a hoist or winch that needs full torque at zero speed, make it a closed loop vector drive. Also consider external cooling for the motor because at slow speeds the motor fans provide less cooling.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: replacing DC motor with AC motor
Thanks again.
CAF
RE: replacing DC motor with AC motor
One of the major differences between the AC and the DC is that with the AC, you get rated HP at rated speed only. As you reduce the speed, the HP drops with the speed, where a DC machine does not have the same limitation.
If you require high torque at low speeds, you may need to oversize the AC to achieve this.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: replacing DC motor with AC motor
Does the load require high starting torque. What is continous torque. What is peak torque. What is the speed accuracy required. What is the speed range. How does torque vary over that speed range--increasing, constant, decreasing, or impact/cyclical.
Cover those requirements and you will have a happy system. I can't tell you how many old drive/motor systems I've encountered over the years that were grossly over or undersized. Why risk duplicating the original errors.
RE: replacing DC motor with AC motor
RE: replacing DC motor with AC motor
In the case of a shunt wound DC motor, the load current and the shaft torque are in direct proportion. That's quite simple, actually.
And, as I mentioned above, sometimes you get quite a surprise---motor running in continuous overload, or motor under half loaded under all operating conditions. In either case, sizing based on the motor will lead to less than optimum results, clearly.