Selection of an AC Motor to Replace the DC Motor

It won't be OK unless the power supply is altered to AC as well.

10KW 2 pole to me looks marginal for this application. How can you tell the starting torque is always low? For such a modification, I'd use more than double the original DC motor capacity. S.M.

A AC motor and inverter as a direct replacement will not work. The rollers on most of these machines do not start up at full speed. Control of the speed on a DC motor is voltage driven on an AC motor it is frequency. More will need to be considered on how to control the ramp up of the speed.

The DC motor was used for a reason in the original design, probably speed control.

I'd say it's an overall cost vs proper application decision.

Why not 7.5KW 4-pole? How often does the motor start/stop?

To your point, why not a 4-pole motor? I'm guessing they looked at the RPM ratings for a 2-pole motor and decided that since it was faster than a 4-pole motor that it would be more suited to their 2700 rpm DC motor speed.

Surely their inverter would go faster than "rated speed" and take a 4-pole motor to 2700 rpm. And I'm thinking (although thinking can be dangerous) that the loading would be distributed better in the motor/bearings of a 4 pole motor verses a 2 pole motor. Then again, maybe that is single phase thinking.....

Typo; I meant 2-pole, and guessed that 7.5kW is a standard size.

Given your present info I do not see why either running a 2 pole motor below its stock speed or running a 4 pole motor above its stock speed via a VFD unit would cause any problems.

As far as starting torque with and decent VFD unit thats not an issue being most will allow a programmable starting torque boost to go 200 - 400% over the continuous running torque and let the user select the overall ramp up and ramp down times to be user programed to whatever works best for the application.

If it was me I would be more inclined to use a 6 KW 4 pole motor running above its normal stock speed over using an over sized 2 pole motor running at below its stock speed.

Everything else is just a matter of personal choice regarding what brands and models of components you use and how you chose to set the system up. No need to over complicate it with 'what ifs'.

I'd agree with a 4 pole motor but 6 kW isn't a standard rating. Assuming it's a 400 volt 50 Hz system as in Europe, I would go for a 5.5 kW motor, star connected at 400 volt. But connect it in delta and set up the inverter to give 230 volt at 50 Hz, and it will deliver 5.5 kW in those conditions. Can then increase the Hz and the volts by √3 times, to 86.6 Hz and 400 volt, giving power √3*5.5 = 9.5 kW, plenty of power, at speed ~ 1450*√3 = 2511 rpm. That's not quite the 2700 rpm required, so Hz has to raised to ~ 93 Hz at constant voltage (and power).

10kw 2 pole is much bigger than your need ,just replace it with 7.5kw -2pole

and a vfd ... initial torque can be set from your vfd parameters .

just a month back i have replaced the 8 dc motors of a cylindrical grinder machine with all ac drives and all are working well on full load................

Ask the conveyor manufacturer to recommend a suitable ac motor.

Apart from saving one conductor why was this a DC motor to start with?

Is this a repurposed locomotive bogie motor or something?

How are you generating the DC and are there batteries involved here?

More information please.

Dear All;

Thanks for input. The DC motor is connected with a DC drive.

If I select a 7.5KW 4pole 50HZ motor and run it above 50HZ, is their any harm for motor? There are almost 40start/stop per day.

Dear Suhail0110

What was the rating and RPM of old DC motor and the rating and RPM of new AC motors?

4 pole motor running above 50Hz is no problem, your 2700 rpm is below speed of a 2 pole motor (at 50 Hz).

40 starts per day unlikely to be an issue, unless it's a very high inertia load. In a typical situation 6 starts/hour is OK.