Centrifugal switch replacement

[Myles Moran]

Short version is i need to buy a new centrifugal switch for my bandsaw. What I'm not sure of is what I need? Saw is an older Hitachi (cb13f) and unfortunately the parts diagram ends at the motor, and I can't find a part diagram for the motor. So my question is what do I need to look for in a certificate switch? eBay has them listed by bore size and I'm assuming that's the shaft diameter. Otherwise, if it looks the same as the on I've got, is it a simple match the bore size and swap it, or is there more i need to know?

As far as I can tell it had moved a little on the shaft and that caused the switch to arc. I'm guessing a replacement and setting it properly will be the key to getting the motor to run smoothly.

On a side note, I checked the other machines to see how their centrifugal switches were working. Discovered my $100 bench top delta drill press doesn't have a centrifugal switch but does have a capacitor. Is it common to see a PSC motor in a low cost tool like that?

[Bill Dufour]

Some high end European saws do not have a centrifugal switch. You have to hold in the start switch until the motor gets up to speed. Or you can buy a solid state starting switch. I think it is just a timer that powers the start windings for a few seconds at every start up. A delay off relay would fit the need.
Bill D

[Mike Henderson]

There's a bit of a problem with implementing Bill's suggestion. The starting winding has the starting capacitor and the centrifugal switch in series with the winding. At startup and at low RPM, that circuit is completed and current flows through the starting coil. At high enough RPMs, the centrifugal switch opens and stops current from flowing through the starting coil.

To implement Bill's suggestion, you'd need to access the two wires going to the centrifugal switch. Add wires to lengthen those wires and connect the two wires to a normally open, momentary closed switch.

Then when you go to start the motor, you hold that switch down and turn on power to your motor. Once the motor is up to speed, let go of the momentary switch. What you're doing is replacing the automatic operation of the centrifugal switch with the manual switch.

Mike

[Rod Sheridan]

Hi Mike, a double pole start push button is used, one pole operates a latching run contactor, the other runs a non latching start contactor.

It’s a great system, no centrifugal switch or capacitors in the motor, everything is in the electrical enclosure for easy servicing……Regards, Rod

[Carl Beckett]

Short version is i need to buy a new centrifugal switch for my bandsaw. What I'm not sure of is what I need? Saw is an older Hitachi (cb13f) and unfortunately the parts diagram ends at the motor, and I can't find a part diagram for the motor. So my question is what do I need to look for in a certificate switch? eBay has them listed by bore size and I'm assuming that's the shaft diameter. Otherwise, if it looks the same as the on I've got, is it a simple match the bore size and swap it, or is there more i need to know?

As far as I can tell it had moved a little on the shaft and that caused the switch to arc. I'm guessing a replacement and setting it properly will be the key to getting the motor to run smoothly.

On a side note, I checked the other machines to see how their centrifugal switches were working. Discovered my $100 bench top delta drill press doesn't have a centrifugal switch but does have a capacitor. Is it common to see a PSC motor in a low cost tool like that?

I can understand why you might want to simply replace the switch instead of completely rewire the motor operation (good info to know though!).

You would want to be sure the switch will fit physically: bore size, and have the space/room for the arms to swing out and actuate the contacts. You would also want the contacts to be rated for the size of your motor current wise. Otherwise it is as mentioned in the other posts, simply a way to switch a winding on/off.

If you spin the motor by hand or a string then hit it with power, does it run? (just wondering if you know for certain it is only a switch issue)

[Myles Moran]

Yes it runs even with the current centrifugal switch. My concern is i want to replace it since it has clearly been arcing and is damaged. I figure I about replace it before the damaged part breaks and shorts out or causes damage to other parts.

The momentary switch for startup seems like an easy solution, i could probably find one off the shelf with the proper rating and wiring it up shouldn't be difficult at all. The only concern I have is this is an extra step in powering on the machine, but I don't think it'd be a big issue at all.

A push button switch with a momentary contact for the start coils sounds cool, but it also sounds expensive.

[Bill Dufour]

I am sure someone makes a twist maintained "on" switch that has a third position connecting a momentary switch contact while keeping the run connected. Just like a car ignition switch. Could even be a toggle type switch.
Bill D

[Charles Lent]

All centrifugal switches arc as the contacts open when the motor reaches about 1/2 of it's rated speed. That is not cause to replace it. Just keep the pitting to a minimum by shaping the contacts with a jeweler's file whenever the motor needs servicing for other reasons. When the contacts are totally gone, it's time to replace the centrifugal switch.

Charley

[Myles Moran]

I'm not an expert at electric, but you you say this is worn enough to replace? I think there was a piece in there that was causing arcing in more than the on-off switch, i think it caused it to arc against the terminal when it should have been open (it looked like a small melted glob of metal fell out during disassembly)

That burnt portion of the board goes about halfway through it's thickness.

[Mike Henderson]

Hi Mike, a double pole start push button is used, one pole operates a latching run contactor, the other runs a non latching start contactor.

It’s a great system, no centrifugal switch or capacitors in the motor, everything is in the electrical enclosure for easy servicing……Regards, Rod

Your comment about the start cap being with the start switch is good. Using that approach, someone could just wire the two wires that go to the centrifugal switch together. Then cut one of the wires going to the start capacitor and put the momentary switch there. They could also move the start capacitor to the same location as the switch, if they wanted to.

A good way to get rid of the centrifugal switch. If you don't have a switch as you describe, they'd have to remember to hold the momentary switch until the motor is up to speed.

Mike

[Charles Lent]

You will have a good chance of getting a replacement centrifugal switch by taking the motor label information and your old centrifugal switch to a local electric motor repair shop. They stock parts for many motor brands, and can match up yours for a new one. Yes, your switch in the photo shows it is about due for replacement and I would look for one while you have the motor apart.

If you go with a manual switch for the capacitor circuit, a high current switch is going to be required. Motors can draw 4-6X their running current while the start circuit is connected. I doubt that you will find a toggle or push button switch with a large enough contact rating for this circuit. Look at the diameter of the start contact of that switch that you removed. The replacement switch is going to need contacts at least that large to work properly for any length of time.

Most European machinery is 3 phase. Many use a three position switch to start them, if there is a high inertia load connected. These switches have Off-Wye-Delta positions, and you step through the Wye position to get the motor up to speed before switching to Delta. A single phase USA power tool does not have this kind of circuit and almost always requires a centrifugal switch and capacitor type starting circuit, which makes a temporary second phase to get the motor turning, but then turns off once the motor is spinning, then resets as the motor slows to a stop, to wait for the next start cycle of the motor..


Charley

[Mike Henderson]

If you go with a manual switch for the capacitor circuit, a high current switch is going to be required. Motors can draw 4-6X their running current while the start circuit is connected. I doubt that you will find a toggle or push button switch with a large enough contact rating for this circuit. Look at the diameter of the start contact of that switch that you removed. The replacement switch is going to need contacts at least that large to work properly for any length of time.


Charley

The current through the "starting switch" is much less than the total current through the motor during startup. The current through the starting coil is limited by the impedance of the start capacitor which is in series with the starting coil. The me, the big problem of the manual switch (and the centrifugal switch) is the arcing caused by the collapsing field in the starting coil (when the switch opens), not the value of the current through the switch during actual start-up.

Most of the high current seen at start-up is current through the field coils and does not pass through the manual or centrifugal switch.

The starting coil of most induction motors is wound with smaller wire than would be required if the current was constant. Some motors have a run capacitor which is much smaller than the start capacitor and is left in series with the starting coil so the the starting coil remains activated and provides some additional torque for the motor. Also, a run capacitor bridging the switch (manual or centrifugal) could help reduce the arcing when the switch is opened. (Note: a smaller capacitor will have higher impedance and not as much current will flow through it.)

It should not be too difficult to obtain a switch which could be used in series in the starting coil, as suggested for manual starting.

I've added a diagram of how an induction motor is wired. You can see that the current through the main field coils does not flow through the start-up switch.

Mike

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