By Steve Bush 30th March 2022
A popular upgrade for the lowest cost ‘3018’ desktop CNC machines and their close relatives is a ‘500W spindle’ or ‘300W spindle’ for about £80.
These consist of a brushed dc motor and a mains power supply.
What not many people realise is that some (not all) of the power supplies that are supplied with such spindle motors are not isolated, meaning that all of the wires including those running to the motor and the control potentiometer (which is on flying leads) can be at live mains potential.
This is a similar state of affairs to the generic ‘DC-51’ motor controller I took apart – the dangers are not pointed out and the surrounding insulation is not up to mains-connected standards.
To be clear to the uninitiated – non-isolated power supplies with accessible metal parts are not properly safe – they lack the layers of safety features expected of consumer products sold in the UK and Europe. Check before you buy as some 500W spindle power supplies might be built to safer standards (and brushless motor spindles and their power supplies are another matter entirely).
Returning to the non-isolated power supplies in question, also like the DC-51, there does not appear to be any emc filtering whatever on the mains input nor the output to the motor – a situation which is electrically antisocial, and something that might upset your cnc stepper motor controller too – if the noise from the un-suppressed brushes does not get there first.
The circuits appear to be fairly simple mosfet-based PWM choppers. Although something extra might be going on as there seem to be more passives, transistors and ICs than absolutely necessary for simple open-loop PWM control.
Someone called Tyler Gerritsen has helpfully posted partial circuits on-line – the are linked from this blog. I think there might be some errors in the circuits (the ground symbol appears to be connected to chopped live mains, for example), and I would not advocate doing what Tyler did as his analysis was not complete before he made his modification, but hats off to him for making the effort to prepare even first-draft schematics.
By the way, Gerritsen seems to have been one of the people who get motors that vibrate like crazy due to the fan on the back being out of balance – worth a check if you have one and it vibrates.
Tagged with: ac-dc EinW Engineer in Wonderland motor control power supply PSU
These sound to be exactly the type of product “Big Clive” on Youtube would enjoy reviewing.
Morning Duncan Agreed, from another member of the Big Clive fan club 🙂
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