Replacing a DC Motor with an AC Motor

You need know the maximun torque at minimun RPM needed. Whit these values find an AC motor useful.

If your application worked OK with DC at or below base speed and your AC has same speed range, it is a match.

However, if the DC had 1200 rpm and the AC is 1800 RPM your Power increases to (1800 / 1200)*4kw to get the same torque.

Just check your torque provided by DC at max RPM and, then calculate AC power with that torque and (higher) rpm.

However, if running DC speed is faster than rated AC, then you have a problem. The AC motor does not run well in the flux weakened range, so you may need a special AC motor with a higher rated frequency, or a new gear box to match the rpms.

Check your speed range and motor cooling to make sure you are in the safe operating area. For small AC machines TENV have a small premium in cost and will run from stall to rated speed at rated torque. Larger motors may need auxiliary cooling.

dear sir,

i have 4 kw dc motor siemens make miniume rpm is 20 and maxiume rpm is 3000 .we want to change it into ac motor and we want to vary speed by vfd. pl suggest the equivalant ac motor rating with base rpm of 1500 max.

Ok, This is a small motor.

If you want a maximum new input rpm of 1500 rpm I assume you are going to re-gear for the lower input RPM. In this case, go ahead and get a 4kw AC machine.

However, if you want to keep the gearing the same then use a 4kw 2 pole 50 hz 3000 rpm machine, or a 2 pole 5kw 60 hz 3600 rpm machine. RPMs given will be the no load synchronous speed, loaded will be lower but the VFD will correct for that. If the DC machine is running less than 83% load then you could stick with the 4kw machine. This could be a good opportunity to examine how much power you actually require by checking the DC amps against the rated NP amps. You may not need the 4kw anyways.

For my money I would get a TENV motor so cooling is a non-issue. Otherwise sustained running at 20 RPM will require a separate blower or some other means of forced ventilation.

Estimated cost would be between $1000 US and $2500 US depending on manufacture and options.