Stanley 257 bench grinder - capacitor

[imc188222]

I'm hoping someone can tell me what capacitor to buy for this bench grinder. When I bought the grinder it wouldn't start on its own, you had to give the wheel a spin first. So I removed the capacitor to order a new one. That was quite a while ago and I'm not sure what happened to the cap but I can't find it or my notes. I remember it was a huge square cap but I don't remember if it had any numbers on it. I've finally gotten back around to it and would greatly appreciate help getting it back up and running.

It's a run cap as there is no centrifugal switch. Any one know what cap I should order? Here are a couple pictures of the grinder.
Thanks,
-Isaac

 

[Stuart in MN]

I don't know the size for that specific grinder but there are selection guides out there that may get you in the right ballpark, here's one: https://inspectapedia.com/electric/Motor_Capacitor_Selection.php

This doesn't help either but it's cool to look at - here's a 1954 Stanley catalog with the model 257 grinder in it. https://archive.org/details/StanleyElectricToolsCatalog12/page/n7

 

[neilreeveszz]

Stuart,
I think that your catalog contribution is a great deal of help. The description of the No. 257 specifically states: "Capacitor start, induction run motor". This dovetails nicely with Issac's statement: "When I bought the grinder it wouldn't start on its own, you had to give the wheel a spin first", a classic short term solution to a failed start cap.

Issac,
I think that you are looking for a start cap as your induction run motor does not have a run cap.

Merry Christmas to all, Steve

 

[imc188222]

Thanks for the help guys.

Correct me if I'm wrong but my understanding of start capacitors is that they are only briefly part of the circuit and are then disconnected by means of a centrifugal switch. This grinder does not have a centrifugal switch, the cap is part of the circuit all the time.
-Isaac

 

[TRWham]

Permanent Split Capacitor (PSC) motors do not switch the capacitor out of the circuit. There will be 2 windings with the cap on 1 to create a difference in phase between the two.They are common on fans and other low starting torque loads so perhaps it’s a PSC motor.

 

[exmaxima1]

I built a belt grinder based on one of those same grinders (mine was branded Atlas) and iit absolutely has a centrifugal switch. Maybe that is your primary problem.

 

[6PTsocket]

There are other means of disconnecting the start cap besides a centrifugal switch.. Some use a start relay. Does one of the wires from your old capacitor go to a small rectangular can with terminals on it? The contacts are electrically in the same circuit location as those of a centrifugal switch but there is one big difference, they are normally open with the motor stopped. The relay coil is in series with the motor. When the motor is started, the high start surge inmediately pulls the relay closed and connects the capacitor. As the motor gains speed, the current drops and the relay drops out. The idea is to get the relay or centrifugal contacts to open at around 80% of full speed. My Craftsman block grinder and plenty of other grinders use a start relay instead of a CS. As the catalog listing says "capacitor start" and your need to hand start and then run normally suggest that it is a cap start problem. When a run cap is used, there would very little power while running, if it was bad. With your cap missing you should still be able to push start it and get normal running power, if it is a start cap. It is more than likely the cap but, just like CS's, the contacts on start relays go bad, too. This would give the same symptom. That leaves you with the same problem of not knowing the value of the start cap. It is not super critical. Too small and starting torque is low. Bigger and you actually get a better kick. You can go too big and cause problems but I would not hesitate to try a start cap around 8-12mfd if I could not get some better info.

Stuart,
I think that your catalog contribution is a great deal of help. The description of the No. 257 specifically states: "Capacitor start, induction run motor". This dovetails nicely with Issac's statement: "When I bought the grinder it wouldn't start on its own, you had to give the wheel a spin first", a classic short term solution to a failed start cap.

Issac,
I think that you are looking for a start cap as your induction run motor does not have a run cap.

Merry Christmas to all, Steve

Sent from my SM-G900V using Tapatalk

 

[imc188222]

Thanks for the help guys. I probably should have put in the first post that I have worked on quite a few grinders/motors and have the basics down.

I put new bearings in this grinder a few years ago but never finished it by getting a new cap.
You guys have me questioning my memory on whether or not this is a capacitor start motor. This afternoon I started tearing it back apart and we will soon know for sure. I should get to it tomorrow and I'll post up what I find. Usually you can feel/hear the centrifugal switch dragging when you spin it by hand but I'm not hearing anything on this one.
-Isaac

 

[exmaxima1]

Thanks for the help guys. I probably should have put in the first post that I have worked on quite a few grinders/motors and have the basics down.

I put new bearings in this grinder a few years ago but never finished it by getting a new cap.
You guys have me questioning my memory on whether or not this is a capacitor start motor. This afternoon I started tearing it back apart and we will soon know for sure. I should get to it tomorrow and I'll post up what I find. Usually you can feel/hear the centrifugal switch dragging when you spin it by hand but I'm not hearing anything on this one.
-Isaac

My Atlas doesn't say Capacitor Start on it, but it definitely is. And it draws the same amperage as yours. The centrifugal switch is extremely high quality, and I don't believe you can feel it when you turn the shaft by hand. Good luck getting it resolved.

 

[imc188222]

Well what do ya know, there is a centrifugal switch in there! It is very high quality compared to some of the ones I've seen and has the friction dealt with so the drag is pretty much imperceptible.

Thanks for making me question my memory!

Exmaxima1, any chance you can easily check the value on the start cap in your Atlas grinder?
-Isaac

 

[imc188222]

Stuart,
Thanks for those links. I have a new 320-400mfd start cap in stock which lines up perfectly with the chart in the first link. I will just use that unless exmaxima1 comes back with something significantly different.

I think I might even give the old girl a coat of gloss dark machinery grey while I'm at it!
-Isaac

 

[B17E1943]

My 2 cents.

My 1962 C-man grinder needed a little wheel spin to get it going - I ASSumed it needed a start cap.

I did some looking and a couple wires had green corrosion on them; my electrical wizard bud replaced them and now it starts and runs like a champ.

Food for thought.

 

[exmaxima1]

Well what do ya know, there is a centrifugal switch in there! It is very high quality compared to some of the ones I've seen and has the friction dealt with so the drag is pretty much imperceptible.

Thanks for making me question my memory!

Exmaxima1, any chance you can easily check the value on the start cap in your Atlas grinder?
-Isaac

Not so easy to check now that my Atlas grinder is mounted to a stand, and it's damn heavy. I'll take a look tonight and see if I want to spend the time....

BTW, my capacitor is not original. When I got that grinder it had a round cap (not flat) installed and it might not even be the factory value. Since the load to start a grinder is not all that much, and no point in having a huge in-rush current at startup, I would try a smallish value like 80-125 uF as a first guess unless someone knows the original value.

 

[imc188222]

Not so easy to check now that my Atlas grinder is mounted to a stand, and it's damn heavy. I'll take a look tonight and see if I want to spend the time....

Don't go to all that trouble, I was thinking it was sitting on a bench and as easy as tipping it on its back and looking at the cap. I think I have enough info now to make a more educated guess. Good point about a grinder not having much starting load. Now that I think about it more that 330-400mfd cap I have was bought for a 1hp bandsaw motor.

Just as an added note, Mcomb Supply has been my go-to source of capacitors in the past. I have no affiliation with them other than a satisfied customer, fast and cheap shipping and good prices has been my experience with them. Hope that helps someone.
-Isaac

 

[imc188222]

I ended up going with a 145-174mfd 250v capacitor. It starts just right in my opinion, quickly but not so fast that it's violent.
Here it is on its way back together in primer.

 

[Will McRay]

I just got done putting mine back together. It’s a three phase and has no issues starting.


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