We have an application where we are using DC motor with a DC drive. The motor ratings are mentioned below:

7.3 KW
26.8 A
2500 RPM

We want to replace this with an ac motor-drive arrangement. Kindly suggest a suitable arrangement and specify ratings. Is there a thumb rule that we can follow.

Best Regards,

 

As far as I know, there may not be a thumb rule for selection of AC drives. But as the ratings provided by you suggests, you may opt for the nearest AC kw drive available for 7.5 kw, 2pole induction motor with the suitable AC vvvf drive. Of course it should be suitable for the driven load characteristics requirement & the details of the models for Drives can be easily obtailned from the vendors like ABB, Siemens or equivalent.

Regards.

 

You will need to select an AC motor/Drive combination that give you equivalent power at 2500 RPM as you are getting from the DC motor/drive. Since 2500 RPM is not exactly at a standard 4 or 6 pole AC motor rating, you might find that you actually need slightly larger AC motor/drive to get the same amount of power depending upon the application and also the speed range (this is equally important) of operation for the current motor/application. Since I don't know your application, I'd also suggest you ask your drive/motor supplier, they should be able to help you out with this. Once you see how it's figured it's fairly easy to do. Hope that this is helpful.

 

You need to know how difficult it is to start your load. Although the are getting better with their ratings, an AC motor and drive usually do not have the same starting torque as a DC combo. There are differences in starting torque between types af AC drives such as vector and PWM. Usually you will have to up the HP to be safe. I have gone as high as 2 times the DC rating and just squeaked by. Extruders are the worst loads, conveyers are not so bad. The best strategy is to get some MFGS sales reps in and get their input.