Variable speed DC Motors 1

[zpiderman]

I have a Woods VFD for a 2 HP DC motor:
Model J200
Ultracon SCR control for 2HP motors
Power supply: 230V, 1 ph, 50/60 Hz
FLA: Input AC RMS 19, Output DC Avg. 11.2
Field: 200V, Armature: 185V

This drive has an RPM indicator but no Tachometer probe.

Question #1: How does it read the RPM?

Question #2: I would like to use this VFD with a 1 HP DC motor (compatible with the VFD). Can this be done? How?

Question #3: If I want to vary the RPM with reasonable accuracy, can I use a 1 HP 180V permanent magnet DC motor or should I use a 1 HP 200V field, 180V Arm. DC motor?

Since I'm not very experienced in DC motors, any info would be much appreciated.

Thanks,

Spiderman

 

[jOmega]

Typically, on inexpensive DC drives, they would use the armature voltage as an approximate indication of motor RPM. The accuracy could be between 2-10% of actual speed, depending upon several factors. If you need accuracy, (you neglected to state what % accuracy you require... and if this accuracy relates to maximum speed or set [operating] speed) you will need either a dc or ac tachometer or an encoder that is driven by the motor..

You should be able to run a 1 hp PM dc motor with the drive...assuming that it does not have any protective features such as field loss. Just connect the armature output of the drive to the armature terminals of the motor.

I would suggest that you recalibrate the current limit of the drive for 150% of the 1 hp dc motor nameplate armature amps...

I would further suggest that you provide a proper overload relay for the 1 hp motor.

 

[electrageek]

Ojmega is right about everything. Another way to improve the speed accuracy of an armature voltage regulated drive is to adjust the IR droop compensation potentiometer, if the drive has one. The idea is that the speed is directly proportional to voltage except for the losses created by the armature resistance and the brush drop. The brush drop is usually small and fairly constant. The more current the motor draws the more losses you have and the slower the motor will run for the same armature voltage. If you adjust the IR droop pot it will raise the armature voltage as the load is increased and tend to keep the speed of the motor more constant as load is increased. Usually this will allow for at best a 2 to 5% speed accuracy.