Minimill Speed Controler - Help

[Ken I]

My no name Chinese minimill has developed an intermittent fault.

The motor will suddenly slow down (under load) - ie it fails to up the torque (Amps) as you load it.

I compensate by turning the speed up but of course now the speed varies with load and shoots up to whatever you have dialed in when unloaded or when the fault clears.

The fact that it is intermittent is what I find curious.

Before I start trying to fix it I though I would ask the forum if anyone recognises this machine / circuit board / problem and can point me in the right direction or provide a schematic.

This is the board :-

The only marking I can see is LM1H1

This is the machine (if that helps) marked as a CH-10

Any help would be appreciated.

Ken

 

[LongRat]

Not familiar with that particular controller but I would strongly consider the ceramic load-sensing resistor for a close look. In the X2 mill I had, the legs of these resistors were prone to snap from fatigue after years of vibration. The resulting connection/disconnection of the resistor caused the load sensing to intermittently fail, shutting the motor off early or giving behavior similar to what you have described. It might be a simple case of replacing that resistor and everything works again.

 

[Ken I]

That makes sense - the problem first appeared whilst fly cutting and tends to reappear when doing interrupted cutting with a 3 tip shell mill etc.

Thanks.

 

[websterz]

Start simple, re-melt every solder joint on the back of the board. I found no less than a dozen cold solder joints on my x2 controller. One resistor literally fell off due to vibrations. The cooling fan board had a bad joint and I went a year without the fan ever running. If it is cutting out during an interrupted cut then I bet ya' it's a cold solder joint.

 

[Ken I]

Thanks for the input guys.

I've been over everything with a magnifying glass - nothing.

But the problem has gone from imtermittent to permanent - so I hope to be able to dope it out.

I can stop the spindle with my hand and the voltage to the motor just falls away.

 

[dvbydt]

This might help :-

http://www.littlemachineshop.com/Reference/DriveTroubleshooting.pdf

Ian

 

[Troutsqueezer]

This might help :-

http://www.littlemachineshop.com/Reference/DriveTroubleshooting.pdf

Ian

This is exactly why these guys stay in business. They know how to inform and sell at the same time.

 

[Ken I]

Some feedback - I removed the board and dummied it on the bench using a lighbulb for load.

The oscilloscope showed one thyristor not firing - replaced both (as a pair) and the problem seems to be sorted.

Once again thanks for the input.

Ken

 

[dalem9]

That great glad you got it working ,Dale