[Thomasio]

I'll post more details here in a bit... our 3 axis CNC has a servo motor that drives the spindle (roughly 5 rpm to 5,000 rpm). It is controlled by a simple potentiometer to set the speed. The potentiometer goes to a control box (literally a black box LOL) that then goes to the servo motor. It looks like it has an encoder on it, but not sure whether that is even being used.

Sometimes when the potentiometer is rotated from 0 to any speed, nothing happens. Sometimes after a couple of seconds it will lurch into action. Other times, everything is fine and works as expected. Still other times, if the potentiometer is moved to some speed, if the motor doesn't begin turning, if the spindle is rotated slightly it will lurch into action.

I suppose that it could be the potentiometer, but I wonder if it could be something with the motor. Maybe a capacitor that helps it start to turn when it is first signalled to turn?

I'll post the motor type and model a little later, but anyone have any ideas of where I should begin trouble shooting?

To be clear, the motor is controlled directly / manually with a potentiometer, it is not controlled by the CNC software (Mach3, FYI).

[Al_The_Man]

You say servo motor? if so it is not a traditional induction motor spindle?
The pot can be checked easily by measuring the voltage on the slider WRT the common end, for a smooth variation from 0 to +10vdc.
Make model type of controller and motor is needed.
Al.

[Thomasio]

Okay, I checked the voltage at the potentiometer. It seems very smooth, not jumps or dead spots. Varies from 0v to 3.28 v max. The spindle motor seems to kick on at about 0.4 volts, although every once in a while it doesn't kick on until about 0.8 v. Could it have a dead spot or something?

The motor: Electro-Craft Servo Products (also says "Reliance Electric"), model S644-3A.

What else can I check?

[Thomasio]

Here is a picture of the controller. It says "e2/e2".

The voltage at the red/white/green seem to track the potentiometer voltage exactly.

[Al_The_Man]

That is a simple SCR controller with a DC brushed servo motor.
These drives I believe are a knock off of the KB/Baldor series.
I doubt it is the pot, you can check the motor for smooth operation and same speed in each direction by placing it across and automotive battery.
But it appears you may have a problem in the controller itself.
Also try an ordinary 100w lamp in place of the motor and see if the illumination is smooth.
Al.

[Thomasio]

With the motor still connected, at the controller I watched the voltage as I rotated the potentiometer. It seems to jump around a bit. For instance, it stays at zero for too long before it jumps to a voltage. Sounds like the controller isn't doing well. Where can I find a direct replacement? I just saw an ad here on CNCzone for a digital PID controller (digital screen, that is). Would that work? I need from about 1,000 rpm to about 5,500 rpm control.

Also, when I disconnected one lead of the motor, the "baseline" voltage was 10 volts instead of zero volts with the motor connected. Strange.

Thanks folks for the help!!

[Al_The_Man]

The PID for that motor will not work.
Try KB-125's or the Baldor equivalent on ebay.
Did you do the motor test?
BTW the manual shows 1850 rpm and 60vdc for that motor unless it is an OEM special,
If using a mains driven controller such as these you would exceed the 60vdc, unless the max rpm pot was set.
Al.

[Thomasio]

The PID for that motor will not work.
Try KB-125's or the Baldor equivalent on ebay.
Did you do the motor test?
BTW the manual shows 1850 rpm and 60vdc for that motor unless it is an OEM special,
If using a mains driven controller such as these you would exceed the 60vdc, unless the max rpm pot was set.
Al.

Could you be more specific re: a replacement controller? I don't find anything on eBay when I search for KB-125.

Didn't directly test the motor, should have tried that... but the output of the controller seems sketchy so I'm assuming that is the culprit.

[Al_The_Man]

A couple for starters.
290594849457
300545535232
Al.

[Thomasio]

After testing the motor w/ 12V direct, I believe it is the motor that is the problem.

I removed the brushes, and they are looking pretty short to me... where can I source some brushes in a hurry? Are these standardized, or do I need to order through ElectroCraft (closed today)?

[Al_The_Man]

The fastest would be to try any motor rewinders in your area, that would be about the only way to get them right away.
You may not have much luck with ElectroCraft, BTW Rockwell (Allen-Bradley) owns them now.
They are getting short, but I would still expect the motor to operate with that length?
They seem to be pitted also, which is not a good sign of motor health.
The other source may be Helwig Carbon.
Al.

[Thomasio]

Thanks Al.

Disassembled the motor, blew out all of the cr*p (brush debris I suppose), cleaned the brushes and lightly rubbed them with 2000 grit paper. Motor works now, but I don't have much confidence.

Where could I find a spare? It doesn't even need to be a servo, it can be a plain DC motor. I run my spindle from about 1,000 rpm to about 5,500 rpm. I'd like the same form factor so I can just bolt a replacement in without changing the mount.

Thanks.

The fastest would be to try any motor rewinders in your area, that would be about the only way to get them right away.
You may not have much luck with ElectroCraft, BTW Rockwell (Allen-Bradley) owns them now.
They are getting short, but I would still expect the motor to operate with that length?
They seem to be pitted also, which is not a good sign of motor health.
The other source may be Helwig Carbon.
Al.

[Al_The_Man]

You could search ebay for servo motors, you don't need tach or encoder, but they are generally pricey, the alternative is a high quality Treadmill motor such as Baldor, Leeson, Nordic Track etc.
Al.

[mike_Kilroy]

.....
Where could I find a spare? It doesn't even need to be a servo, it can be a plain DC motor. I run my spindle from about 1,000 rpm to about 5,500 rpm. I'd like the same form factor so I can just bolt a replacement in without changing the mount.

some of us dont know the exact form factor of your motor but may have dc motor that may be same..... w/o data we cannot offer it tho.... want to list form factor that would be good for you so we can check our junk bins? Id suggest diam, length, shaft dia+length (key req'd?), voltage, current, no tach needed?. bolt circle dia, pilot req'd?