DC Spindle Motor Help Please

andrewmawson · Nov 11, 2018

I have a CNC lathe with a Mawdsley 27 kW DC motor and I'm having problems running it slow enough for M19 Spindle Orientation to work. It should be run (from parameters I got with the machine) at 5 RPM for M19, but I can't get it going slower than 10 RPM.

Field is supplied by a separate adjustable thyristor supply rated up to 5 amps. Motor plate says excitation should be 170 volts. I measure (at DC) a field resistance of 23 ohms (23 ohms at 170 volts = 7.4 amps). If I tweak the field current up to maximum I can only achieve a field voltage of 165 not the 170 on the plate, and the MENTOR-1 armature controller errors when the motor starts up - ok when tweaked back.

Now I know that DC motor speed can be controlled by field current but am getting confused by what's happening here.

I'd like to be able to confirm that the 23 ohms I'm measuring on the field is reasonable, but the manufacturer went out of business in the mid 90's

Anyone any experience of these things - I'm wallowing at the moment !

thermite · Nov 11, 2018

andrewmawson said:
I have a CNC lathe with a Mawdsley 27 kW DC motor and I'm having problems running it slow enough for M19 Spindle Orientation to work. It should be run (from parameters I got with the machine) at 5 RPM for M19, but I can't get it going slower than 10 RPM.

Field is supplied by a separate adjustable thyristor supply rated up to 5 amps. Motor plate says excitation should be 170 volts. I measure (at DC) a field resistance of 23 ohms (23 ohms at 170 volts = 7.4 amps). If I tweak the field current up to maximum I can only achieve a field voltage of 165 not the 170 on the plate, and the MENTOR-1 armature controller errors when the motor starts up - ok when tweaked back.

Now I know that DC motor speed can be controlled by field current but am getting confused by what's happening here.

I'd like to be able to confirm that the 23 ohms I'm measuring on the field is reasonable, but the manufacturer went out of business in the mid 90's

Anyone any experience of these things - I'm wallowing at the moment !

I'd need to know a LOT more about that Field supply to be of any help. Ex: Whether it is in "Current mode" vs "Voltage mode" (Eurotherm-now-Parker-SSD AKA Shackelton Systems Drives can be configured to EITHER), or if it is also serving as Field Regulator, wherein it controls the Armature supply as well as itself? Shackleton used a whole separate "box" to do that in about the same era as your drive - or even earlier. Your drive is not really "old" as far as designs go. Just from a company that left the building and orphaned it.

Most of the currently-shipping DC Drives are about 20 years old as to initial design release, some are closer to 30 years. The needs just have not changed much.

Further on that point, at typical UK supply voltages, that 165 VDC should be about 190 VDC "actual" when under no load and 180 VDC even at full load.

SSD drives have min and max potentiometers SEPARATE FROM the main variable control simply to set the upper and lower "bounds" for that. Does your one have those also?

JST · Nov 11, 2018

No clue on the machine.....

BUT, normally, higher field voltage slows a motor. Field weakening is used to achieve high speeds.

The difference of 165 vs 170 does not seem like enough to change the slowest speed by 2:1, but then I do not know that machine.

How is the armature voltage? Might it not be getting low enough?

Is there any feedback loop on the spindle motor to set speed? Tach feedback can be "iffy" at slow speeds for some types of feedback, particularly an analog feedback vs a "rate" type digital feedback that sends x number of piulses per rev. Might not be working right.

If there is a tach feedback, AND it should be able to do 5 RPM, it might be necessary to check the "error signal".... if the control fails to reach the set point, then the error signal gets very large. Easy to view if analog, not easy if digital, but if digital, there may be a monitoring function that senses the large error and sets an error code.

Do you have a manual?

andrewmawson · Nov 11, 2018

The original field controller / supply was an FXM-3 from KTK . That went bang shortly after a 0.1 uF suppression capacitor on it spread it self about with lots of smoke and noise.

It was replaced by a re-engineered one from a small support company ( again out of business) but apparently these replacements are more reliable than the originals (!)

As I understand it the field supply board monitors the armature and weakens the field for higher speeds, hence the armature volts pot on it

thermite · Nov 11, 2018

andrewmawson said:
The original field controller / supply was an FXM-3 from KTK . That went bang shortly after a 0.1 uF suppression capacitor on it spread it self about with lots of smoke and noise.

It was replaced by a re-engineered one from a small support company ( again out of business) but apparently these replacements are more reliable than the originals (!)

As I understand it the field supply board monitors the armature and weakens the field for higher speeds, hence the armature volts pot on it

Yes, that would be "at least" a basic "Field Regulator" function AND make it a "non trivial" exercise to replace that unit. Most present-day ones (20 years and counting?) live right on the mainboard, ELSE a plug-in daughter card of the 3-Phase-ONLY DC Drives.

SSD has the best explanation of it I have ever seen ANYWHERE in a mere paragraph or two of their manual on their one, so I shall try to find that for you (I have a copy on-disk, but among rather a LOT of HDD) rather than fail to convey it as well as they have already done.

Big file. Scant interest in general in DC at all. We should probably move it by email.

Cheers

Bill

[email protected]

andrewmawson · Nov 11, 2018

Thanks Bill, I've dropped you an email

JST · Nov 11, 2018

For slow speeds, the field should be at full voltage. It's possible but not super likely, that it might go higher for very slow speeds. I doubt it in your case, but one can bear it in mind.

The ARMATURE voltage would be varied to change the RPM. You probably need to check it to see if it is getting down to where it is supposed to be (assuming you have a spec on it)

thermite · Nov 11, 2018

andrewmawson said:
Thanks Bill, I've dropped you an email

Responded, thanks. I must go dig for the file now.

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