replacing DC motor with AC motor 2

[cfordyce]

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

 

[Barry1961]

The main problem when changing from DC to AC is the speed or speed range. Sometime there is also a starting issue but not often in my experience. If you have a 5hp DC motor that always runs at 1750 rpm you can replace it with a 5hp AC motor that runs a 1750 rpm. If you have a 5hp DC motor running at 50 rpm you may have a problem.

What is the application?

Barry1961

 

[jraef]

HP is HP at a given speed. If the DC motor were faster you might have to oversize an AC motor because the torque would be different, but in your case the DC motor and AC motor are going to be the same speed. Torque = HP x 5250/RPM, AC or DC is not part of the equation. So in this case, the torque at base speed will remain the same.

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

 

[cfordyce]

Thanks. Yes that makes sense. HP is HP. I'll keep the same hp for the AC motor and Inverter. by the way the application is a polymer extruder pump.

Thanks again.

CAF

 

[Marke]

Hello cfordyce

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

 

[DickDV]

In my view, the whole approach here focusing on the old motor is wrong. Much better to look at the load and size the AC system based on load requirements.

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.

 

[UKpete]

DickDV, that makes perfect sense. But I guess the reason people focus on the old motor is because they don't usually know what the detailed load torque requirements are, unless they go to the trouble of measuring it.

 

[DickDV]

UKPete, its not that hard to discover. Use the old motor and monitor the current levels. If you know what kind of motor you have, finding actual loads under varying conditions is actually quite simple.

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.