Starter Capacitor on a 1950s bench grinder

Apologies if this has already been mentioned in another thread.

I've picked up an early 1950s 8" bench grinder which is doing the classic 'hum when switched on' works if I give it a helping hand but obviously not good on the motor long term.

Now I replaced the original starter capacitor like for like with a new one of same value. It still hums but starts slightly better, I then bought one with twice the farad rating and low and behold it almost starts fine. my question is can I just keeping going up in value until I find one which starts it with no help? I've also noticed the higher the value capacitor, the quicker the motor slows down when turned off... is this normal?

Any help would be much appreciated

Thanks

Simon

Hi Simon,

Can you provide the make & model of your grinder?

Bear........

On an old item of unknown condition, it is possible that the capacitor was replaced with the wrong value at some point. If it runs without becoming unduly hot, it should be OK with a value of capacitor that works. However, it may not have a centrifugal switch and may be designed with a permanent run winding and capacitor. If so, be more wary of overheating, and the capacitor must be continuously rated.

Unfortunately, the other possibility is shorted turns in one or other winding, and this would likely cause overheating. When testing, be sure to have a senible low value breaker or fuse in the supply.

If the option is there read the running current and see what it shows but heating will be the give away. An 8" grinder will hum as it gets up speed, might take 2 seconds. It reaches proper running speed - 2800 or so ? IT IS single phase and not a 3 phase with a steinmetz connection, hence the capacitor ? Just a few thoughts. Noel.

First, it would be good to know whether the capacitor is a ‘start’ or ‘run’ item. Like, is there a ‘centrifugal’ switch on the motor armature?

It is entierly possible that it's actually a three phase motor that's had a capacitor added to allow it to work from single phase.

I say this because the 8" Wolf pedestal grinder in my shed has had exactly that done to it. I will be re-connecting it from delta to star and fitting a new cable and three phase plug on the end at some point in the forseable future.

Thankyou for all the feedback and apologies for the delay! I can identify it's a Wolf TG8 C but the serial number I'm struggling to read so it may be a slight varient on that number. The plate states its 240v with 1ph supply 2950 rpm. I don't think it has a centrifugal switch as it doesn't seem to click in and out when it fires up? I could be wrong in assuming this however?

Best not to make any assumptions! There are many motor variations, and it will be lucky if anyone knows for sure what should be on a 70 year old grinder.

Some photos of the beast would help. Especially the wiring and terminal marks inside the connector box.

Nothing to be lost by trying more capacitance, fingers crossed. Otherwise it will be necessary to confirm the motor hasn't got a centrifugal switch by looking inside, and making sure the contacts and mechanism are both OK.

If the thing has been in storage, the windings may be damp, and keeping it warm for a few days might help. Worst case, the insulation has failed and some turns are shorted out inside: if so it will get hot quickly - the cure is a rewind or replace the motor,

Above all make sure it's properly earthed! Ancient electrical insulation is often iffy.

Dave

A further thing to check. Some older motors had the second (start, or phase shifted) winding as two halves with the capacitor in the middle. If the capacitor has been incorrectly replaced, the motor will be unhappy!

Simon, when you installed the lower value capacitor it wasn't passing sufficient current in the start winding to speed up the motor, when you doubled the value it permitted more current to flow. Its all about Xc = 1/ 2 x pi x f x c.

I've noticed some of these el cheapo small grinder have swing connections, with both windings having the same resistance as Malcolm above mentioned. Very handy if you wire a change over switch to give forward and reverse as for a small tool grinder. John

Sorry, but I can't let that one past.

If you reverse a bench grinder there is a good chance the fastening nuts each end will come unscrewed, and let go of one or both wheels.

Exciting, but bad for the nerves.

By the way an old motor such as this needs a proper insulation test at at least 500 volts DC before it can be considered safe to apply mains voltage to it.

RGds Simon

‘I don't think it has a centrifugal switch as it doesn't seem to click in and out when it fires up? I could be wrong in assuming this however?”

‘Nothing to be lost by trying more capacitance, fingers crossed. Otherwise it will be necessary to confirm the motor hasn't got a centrifugal switch by looking inside, and making sure the contacts and mechanism are both OK.”

Check for the switch making on slow-down, not breaking on start-up! Without power, it should be slowing quite gently with no added sound from a heavily loaded motor.

But as per Dave, visual is the definitive check (to avoid having the start winding overheating the motor)

Edited By not done it yet on 27/08/2023 14:54:00

Just to be clear, I suggest checking the winding resistances to be quite clear about the circuit of the windings.

For example, this circuit is a possibility. Connecting the capacitor from C2 to N would produce symptoms similar to those reported.

Edited By Macolm on 27/08/2023 15:25:29

Correction - C1 to N