Another electronics question

[jimbo84]

A friends mini mill is running very slowly with the speed potentiometer in the off position.

I’ve measured the 10k pot and it measures ok.

If you unplug the lead to the pot it runs at full speed.

It also has a switch on the back of the pot that’s switching the neutral. When the switch is off there is continuity, when it’s on there is no continuity, I’m guessing this is acting as the no volt release switch, which does work. If you turn the power on with the pot in the run position it doesn’t start until the pot is turned to the off position.

I’ve tried trimming the small board mounted pot but that just alters the speed in both the off and on positions.

Some pics:
The board with the wires out. The pot goes into the white socket. The brown and blue in there are the output to the motor.






Back of the pot

 

[Guineafowl]

It also has a switch on the back of the pot that’s switching the neutral. When the switch is off there is continuity, when it’s on there is no continuity, I’m guessing this is acting as the no volt release switch, which does work. If you turn the power on with the pot in the run position it doesn’t start until the pot is turned to the off position.

Good visual inspection of both sides of the board sometimes helps.

Has this feature been bypassed by the fault, ie will the machine start running as soon as the power switch is flipped?

Is that relay involved? I’d have a play with the pot, various positions, flipping power switch on and off to see if the relay is working reliably if so. It looks like it’s there to drop out power to the motor unless the pot starts in ‘off’.

 

[MoreWellie]

Have you asked whether it has always been like that?

Has anything else been added or changed recently?

Does the pot control the speed smoothly between minimum and maximum?

 

[voipio]

The potentiometer also has a switch actuated by the shaft. Between which points is the potentiometer resistance being measured, and is the pot disconnected for the measurement?

 

[jimbo84]

Good visual inspection of both sides of the board sometimes helps.

Has this feature been bypassed by the fault, ie will the machine start running as soon as the power switch is flipped?

Is that relay involved? I’d have a play with the pot, various positions, flipping power switch on and off to see if the relay is working reliably if so. It looks like it’s there to drop out power to the motor unless the pot starts in ‘off’.

With the speed control in the off position if you power on at the switch the spindle starts to move round very slowly, you then click on the speed control and it starts to build speed. If you power off the machine with the speed control in the on position then power it back on, the spindle will not turn until you switch the speed control to the off position.

The relay clicks on when you power on the machine at the switch it doesn’t then click off until you switch off the machine at the switch or mains.

 

[jimbo84]

Have you asked whether it has always been like that?

Has anything else been added or changed recently?

Does the pot control the speed smoothly between minimum and maximum?

He’s only just got it recently so it has potentially been doing it out of the box.

The pot controls the speed smoothly.

 

[jimbo84]

The potentiometer also has a switch actuated by the shaft. Between which points is the potentiometer resistance being measured, and is the pot disconnected for the measurement?

Pot disconnected for measurement, measuring resistance at the plug end of the wires. One way 0 to 9.5kohms and the other way 9.5kohms to 0 both smoothly and then 9.5kohms actoss the winding.

The switch on the back of the pot switches the neutral, contacts closed when in the off position and then open when in the on position. This holds it off when switching it on when the speed control is in the on positions.

 

[jimbo84]

I’m beginning to think it may well have been like this from new.