I have noticed for some time now, that occasionally the mill does not start when I push the button, but just hums. I immediately switch off. If I turn the spindle a quarter turn then try again, off it goes. I have been told that if you push the spindle with a stick in these circumstances, it will start, but decided against that for the obvious reason.
On a recent occasion however, the acceleration was quite slow and it took several seconds to reach normal speed. I decided it was time to investigate further.
The mill has a single phase, 4 pole motor, which on our 50 Hz supply has a synchronous speed of 1500 rpm, but as it is an induction machine it runs with some slip, and nominally 1400 rpm. I took the cover off the terminal box, which revealed two capacitors, so it is a two capacitor motor. One of the capacitors is the start capacitor and the other one normally stays in circuit during normal running. The start capacitor is in circuit when the motor is stationary, but is typically switched out at about 75% running speed. This arrangement gives quite a high starting torque which makes it suitable for high inertia loads.
I tried to measure the capacitance of each with my voltmeter, but while it works on general capacitors used in electronics, I had trouble making sense of any sort of reading. It seemed like the capacitors were both faulty, the start capacitor worst, but even the run capacitor was well down on the labelled value if my readings were meaningful.
I rang the warehouse where I bought the machine and was pleasantly surprised that their service department had the replacements for my machine (which looks like it comes from the same factory as the Grizzly). They also confirmed that the symptoms I described sounded like a capacitor problem. They put a set aside for me, and I drove out to pick them up next morning. Always good retail therapy to browse the showroom, and this seemed like as good an excuse as any.
Of course, the new capacitors were a different brand and different physical size from the originals. I also noticed that the start capacitor they supplied was only 125 V whereas the original was 250V.
Now to me, neither voltage rating really makes sense, as our rated voltage of 220 V, meaning the rms voltage, has a peak to peak voltage of about 380 V if my memory serves.
The new run capacitor was rated at 450 V like the original, which seems appropriate.
Now my work experience always involved three phase motors, and I never did get to grips with how a single phase motor works. I did quite a bit of reading but didn’t find anything really clear. I do hope that today’s students have better text books available than I was able to find.
As far as I can make out, the start capacitor, is in series with the start winding, and provides a phase lag in the current to that winding which provides the rotating torque necessary to get the rotor moving. It actually provides a quite high starting torque, potentially a peak much more than twice full load torque, but this torque falls off as the motor comes up to speed. At about 75% rated speed, the start capacitor is switched out by a centrifugal switch. The run capacitor is a much smaller value, and wired in parallel with the start capacitor. Parallel capacitors add, so when the motor is initially running up, the run capacitor effectively increases the value of the start capacitor, so helps with breakaway torque and early run up torque. When the start capacitor is switched out it leaves only the run capacitor in series with the start winding. The motor characteristic torque swaps to the curve appropriate to the lower capacitor value with about 150% pullout torque somewhere around 85% synchronous speed, reducing to zero at synchronous speed, in the normal way, with the name plate full load at about 1400 rpm, which means about 7% slip. Roughly like the sketch in the photo.
As the capacitors are in series with the starting coil, the voltage is presumably divided between the capacitors and the coil, so at least during the initial acceleration, there is some justification for the lower voltage rating of the start capacitor.
I hope that some of the forum members with more expertise in these small motors can confirm that I have understood it correctly.
The two capacitors were easily installed and ready for testing.
What a difference. It’s along time since the machine was new, but I have the distinct feeling that the machine has never started so well. It starts with a real bang and almost instantly up to speed. And starts beautifully on the highest speed range, on which the run up has always seemed a bit slow. Probably faulty capacitors from the start if I had been able to recognise it, but such is the life of a beginner.
Now I am wondering if the lathe has the same problem. It definitely does not like starting at the maximum speed which is not ideal for small components. Certainly a high starting torque would be helpful for running up the 8 inch four jaw chuck. Unfortunately the capacitors on the lathe motor are not so accessible. They are tucked into a space behind the headstock, under the electrical junction box. I will have to completely remove the motor from its mounting to get at them. The vertical motor at the top of the mill was so much easier. But a project for the near future.
The capacitors are supposed to be rated for some thousands of hour of running, but the point not so often mentioned, is a maximum of 20 starts per hour. Now as a hobby machine, it will probably never exceed the expected number of hours of operation, but with machining tiny components, potentially less than three minutes between starts is not so uncommon. I expect that this could significantly shorten the life of the capacitor, and may be a factor in capacitor life.
So quite a lot learned about single phase motors, and the satisfaction of having diagnosed a problem and successfully fixed it. I even found that the capacitor manufacturer, while probably manufacturing in China, had an office here in Melbourne, and were able to provide some technical advice.
I hope this little adventure is of interest and helpful to others.
MJM460
