MoToR mOtOr

Hi all,
I want to replace old DC motor in application with equivalent 3-ph AC one.
How to choose the AC specifications?
Any suggestions will be appreciated

I would start by answering at least the following... I'm sure my collegues will have more.

1) Horsepower required
2) Supply voltage available
3) Mounting type (frame size)
4) RPM's during normal operation
5) Any other special requirements (for example, speed needs to be varied widely or motor must run at very low RPM's at times)

That is a good starting point. Many DC applications use tach feedback for extended rpm. Most AC drives today can run the motors in vector control with out tach/encoder feedback and provide better speed regulation than the old DC had.

DO NOT forget torque. IF AC motor does not have adequate torque (both starting and running) you are dead in the water.
DO NOT automatically assume you can replace a 5 HP DC with a 5 HP AC - becuase of torgue.

ALSO if you are going to run teh motor very slow be cautios toensure adequate cooling. The motor runs slow and so does the integral fan therefore there is little cooling - motor burn out.

I would recommend that you proceed as follows. Determine the load torque requirements over its operating range. (Notice, I didn't say motor torque but load torque) Next, determine what the load operating range is and what that represents in motor speed range. Finally, determine what the load speed stability or error tolerance is when torque varies over its operating range.

Come back to us with those three things and we will help you pick the right motor and drive.

Just a little note here. The reason that the data is to be taken from the load is to avoid any errors that might have always been part of the DC system. It also gives you and your operators a chance to IMPROVE machine operation or production possibly with the new drive system. Then you would be a hero and maybe even earn attaboy points!!

Thank you very much for all.
I have few information about the DC motor:
Nominal Voltage: 200 VDC
Nominal Current: 180 A
Nominal Speed: 1500 rpm
Could that data help me to determine the equivalent 3-ph AC motor?How?
With thanks

That's not enough information to do the job right. Of course, with that limited data you could radically over-specify the new system and probably be safe but it would cost way more than doing it with the information I asked for.

Is it your money?

One of the main formulae you will need to use is
HP = torque x RPM divided by 5252.

Need more data
write down all -- ALL -- nameplate data on motor
Tell us what motor does and if reduced the speed reduction

Better yet have a motor and VFD guy come out and take a look at the motor

Dan Bentler

DickDV could state this better than I could. Technically some of the older DC systems were pretty good all things considered but they can be a nightmare to maintain.

It would be easier if you could provide as much detail as possible. The 180 amp suggests to me a fairly large motor, possibly 40HP. If that is the case can you state what brand/model DC drive and motor is being used it could help.

As DickDV stated the issue can be with the torque and speed relationship, its best to determine this curve and match motor and drive as needed.

Also mentioned was the fact you could just oversize the system but note that a jump from 40HP to 50HP could mean an additional cost of a $1000 or more.

If you need some in depth assistance with this project then Dick is a drive specialist that works with ABB products. This means he (or another from his company) can actually determine the best fit and provide a quote for the equipment.

I like the ABB drives (http://www.abb-drives.com/) but if you were going to upgrade the DC drive I would suggest Reliance (http://www.reliance.com).