PWM vs SCR for Elecrtocraft 703 servo

[bilinghm]

I recently bought a bargain servo motor on EBAY with the intention of using it to power the spindle on an X2 mini mill. The servo is an Electrocraft 703 brushed permanent magnet, 90 volt, 7.7 continuous stall amps, 35 maximum stall amps.

I called a tech representative at KB and asked for advice on the most appropriate drive to run this motor. The tech rep walked through all of the motor specifications and also asked for some kind of "factor" that should be listed in the motor specs. This factor number was not listed, therefore the rep said that he would avoid SCR type controllers and stick with a PWM type. He specifically recommended a KBWT-110 with 10 amp capacity.

While this is O.K., there a many inexpensive 90 v 10a controllers out there on Ebay that match up well with a KBWT -110, but they are almost always SCR not PWM. Do you think that I must use a PWM for good results with this motor?

Thanks in advance,

Bill

[Al_The_Man]

I have used a KB 1 phase SCR drive with a SEM brushed DC motor, the one I have is four quadrant as I preferred the dynamic braking and I wanted bi-directional rotation with either a pot or analogue input. Rather than reverse the armature leads, most of them also have a tach input for really precise speed control.
Al.

[bilinghm]

Well Al, I rigged everything up with the 700 watt Electrocraft motor and a Leeson SCR drive. Everything seems to be fine until I get up near top speed where the motor starts to surge and hunt. I have played with the trimpots but I still am getting bad results. I have pulleys in place to speed up the 4,000 motor to 10,000 rpm at the spindle. Is it posslble that the motor is being overtaxed?

[Al_The_Man]

If this is a used motor, how are the brushes and commutator?
Does the Leeson accept tach input and are you using it? If the motor has a tach, that is.
So far I have been running mine with no problems at all.
Al.

[bilinghm]

Both the brushes and the commutator look good. The motor doesn't have a tach, nor does the Leeson. As a test, I'm going to remove the belt and see if the results change. I would welcome any other suggestions. Thanks, Bill

[Al_The_Man]

Do you have a variac? If so, for test purposes, you could put a bridge on the output and bring it up to 120v into the bridge and see if the motor runs ok, this will simulate the SCR drive full on.
Al.

[bilinghm]

I do have an old WWII vintage two wire Variac. When you say "bridge" do you mean I should set up a bridge rectifier?

[Al_The_Man]

Originally Posted by bilinghm I do have an old WWII vintage two wire Variac. When you say "bridge" do you mean I should set up a bridge rectifier?

Yes, a bridge rectifier on the Variac AC output, no capacitor.
Al.

[bilinghm]

Al, I am a total novice at this. Please excuse my ignorance! For the bridge rectifier, will a Radio shack unit do the job? What specs should it have? Thanks, Bill

[Al_The_Man]

A 150-250v at 10 amps should be plenty.
Al.

[bilinghm]

Al,

The testing was successful, so the motor is certainly O.K.

I spoke to a leeson engineer and he suggested several adjustments to the trim pots on the drive. The max speed, torque and IR comp pots could all be part of the solution (or problem). First, the max speed pot needs to be set to match the voltage of the motor. The drive may be adjusted to output more than 90VDC . Leeson suggested using a multimeter to calibrate the max speed pot for desired 90vdc. If there are still problems, cut the torque and IR comp pots back to minimum and work back upward in small increments. I'm going to try this tonight.

Bill