C2/Warco lathe Motor Problems

Hi,

Going back to my youth and acquired a a broken Chinese Mini Lathe, unfortunately it had blown the fuse and on replacement it remained dead.

Using this site I have completed some diagnostics and repair, but would really appreciate the help of those more experienced.

What I have done so far:-

1. Removed 250W Brushed Motor ZYT-250 and connected to a 12v supply. Runs not that smoothly but draws 850mA, Stall current 2.2A at 12v. At 36v it draws 2A. Seems excessive.

2. Opened motor tested commutator at 180 degree and adjacent windings for resistance. Seemed OK. Brushes seemed OK. Nevertheless suspect motor.

3. Removed controller board SMT FC250J. Tested FETs as described on this site, one was shot. Replaced both FETs and tested into a 60W bulb load. Now works but hums with a 'inverterish' noise, if you know what I mean! Have ordered the electrolytic caps as described by others on this site.

4. Have not connected the motor to the controller as I suspect the motor could cause damage again. New motor is £90!

Any thoughts would be appreciated.

many thanks

John

A bit more data needed about the supply voltage and motor before anyone could make a suggestion.

If the motor is a 12V dc type then to draw 250W as rated that equates to about 20A. I guess the motor supply must a lot higher than 12V from your measurements. It probably uses a PWM speed controller as you mention FETs in the controller board.

So give us a bit more to work with and someone might be able to point you in the right direction.

The motor is the standard 180v dc offering that comes from the Chinese factories....

The current draw figures seem a little high to me compared to a motor I was bench testing the other day.....at 30v it was drawing around 0.4 amps, maybe a little high but the motor had just been rebuilt with new bearings....

The noise from the controller board could be the bulb filament being excited by the frequency of the pwm signal?

To ensure long term reliabilty a new motor and board........££££££.....s...

Am not an expert, (Far from it) but as I understand it, the motor for the C2 is a brushed 300 watt unit, with Pulse Width Modulated speed control. From what I have read, the FETs are likely to have failed.

A friend has just acquired a lathe in a similar condition. Our estimate is that spares to return it to working order are likely to cost just under £200, based on Arc Euro prices.

If you want more info, contact Ketan, he is most helpful, and has provided a lot of info about these machines on other threads.

Howard

Be careful about reading things into running current of a permanent magnet dc motor without a mechanical load when run well below the normal supply voltage. The motor when spinning acts like a dc generator and produces an opposing dc voltage. So the running current will be higher than normal when operated on a lower voltage. The Back EMF generated tries to stop the normal supply current from flowing.

Permanent magnet dc motors draw a lot more current when first connected to the supply as the inherent dc generator voltage only builds up when it has attained close to full speed. So the current and torque is much higher when starting, which is exactly what you need for a machine to get things moving.

Of course, things like shorted turns in the armature are a possibility if the motor has been overheated and making accurate resistance measurements on a static motor is fraught with problems. The dc resistance of the armature winding will be very low, perhaps as little as 1-ohm and you need good measuring equipment to test them. A standard piece of dc motor or generator testing is a thing called a "Growler", which allows you to see if all the commutator segments and windings are working correctly.

The other thing to be aware of with PWM controllers is the transient protection diodes fitted, if these go open circuit then the FETs will be severely stressed and new ones will probably fail. It is a waste of effort just to replace the FETs, all the other components need to be checked to ensure they are still OK.