dc excitation

[jcole]

Hello guys and gals

I have an old dc motor and drive. I replace drive recently with Powerflex DC by allen bradley. The old drive had and analog out to a percent dc excitation display. This display had contacts that closed at certain set points which would shut machine down. The machine would shut down on torque overload when this contact closed.

I am not much on dc machines. Could you explain to me what exactly percent of dc excitation is?

I dont know how to program the analog out on new drive. There is no option for dc excitation.

Any comments or suggestions would be appreciated.

 

[Jraef]

In a DC motor, speed is dependent upon armature VOLTAGE, and torque is dependent upon armature CURRENT. To compensate for droop under varying load conditions, you would monitor both and feed that information back into the drive so that it can regulate the output to maintain either (or both). It sounds as though your old system had an armature current feedback for torque, but was probably "open loop" meaning the operator was relied upon to monitor it and make adjustments, but there were set points at which it would just shut down. Modern digital DC drives will have a closed loop system; all of that is done internally to the drive.

If you need to use the output analog signal for a meter, the equivalent output from your digital drive would likely be % motor torque. The set points for tripping however are most likely programmable within the drive now (I don't know much about the AB DC drives however).

 

[hillbilly]

Hello guys and gals

I have an old dc motor and drive. I replace drive recently with Powerflex DC by allen bradley. The old drive had and analog out to a percent dc excitation display. This display had contacts that closed at certain set points which would shut machine down. The machine would shut down on torque overload when this contact closed.

I am not much on dc machines. Could you explain to me what exactly percent of dc excitation is?

I dont know how to program the analog out on new drive. There is no option for dc excitation.

Any comments or suggestions would be appreciated.

What are the motor specs.?
If that motor is old, it most likely has a wound field, and requires field excitation before it will operate.
Does the motor plate show armature and field voltage?

The AB drive should have a built in field supply.
This should be wired to the motor field connections.
Make sure that the field output on the new drive matches the nameplate voltage on the motor.

For a wound field motor....if the field voltage is less than the motor spec, the motor will run too fast (above it's rated base speed).
If it's too high, the motor may not run fast enough.

There should be enough parameters on the new drive to allow you program any special operating modes that your application may require.

I'm not familiar with the Allen Bradley drives, and it's been quite a while since I worked on these type drives, so my info may not be correct.

Just a idea
steve

Edit for spelling

 

[jcole]

Thanks for the replies.

The motor FLA is 65a. Thats all I can remember on the spec. It is about 25 years old and it does have field windings.