3rd Generation Owner

[dusty]

The few that I have seen have identical start switch/mechanisms IIRC.

I think you might have missed my point.

Did the contacts fail to open when run speed was achieved? If they remained closed at run speed, could that be what caused the capacitor to blow?

[dusty]

Does the centrifugal switch mechanism from a 1 1/8hp Emerson look any thing like this?

[ATTACH]22556[/ATTACH]

[ATTACH]22557[/ATTACH]
Meter Connected

[ATTACH]22558[/ATTACH]
Switch Closed (no rotation)

[ATTACH]22559[/ATTACH]
Switch Open (motor rotating - simulated switch action)

[tvidnoviciii]

Dusty,

Yes it does and it operates the same way and I see similar performance when I operate it manually. The problem is (I think) the spinning armature is getting stuck (intermittently) and leaving the switch closed, causing bad things to happen w/the capacitor as it would be continuously charging and discharging and most likely overheating.

[dusty]

Dusty,

Yes it does and it operates the same way and I see similar performance when I operate it manually. The problem is (I think) the spinning armature is getting stuck (intermittently) and leaving the switch closed, causing bad things to happen w/the capacitor as it would be continuously charging and discharging and most likely overheating.

If it leaves the switch closed, the capacitor would never discharge (be continuously charged). I don't know, for sure, what that would do. I'm thinking that this would be the same as simply connecting the capacitor across 115vac and leaving it there.

Additional Thought: I was sitting at the bench this morning playing with this; trying to figure a way to actuate the centrifugal switch due to rotation. With the motor as it is seen in these photos, if the shaft could be rotated (an electric drill isn't fast enough), the switch action could be seen.

Hmmm...."Mark V Rotation Test Bed"

[tvidnoviciii]

Dusty,

The capacitor should shut off the current in the circuit once it gets to a full charge, so there must be something that is discharging (the start winding) the capacitor in order for the circuit to continue to operate.

[dusty]

Dusty,

The capacitor should shut off the current in the circuit once it gets to a full charge, so there must be something that is discharging (the start winding) the capacitor in order for the circuit to continue to operate.

I don't follow the logic.

It is not the capacitor being fully charged that terminates the flow of current through the start windings. It is the action of the start switch. The start switch begins closed. It is through that "closed" circuit that the start current flows and the motor begins rotation. When the motor reaches some speed (run speed), the centrifugal switch opens. Opening that switch is what "stops" the start current. The motor is now at run speed, the switch is open and the capacitor simply holds its charge. There is no completed circuit attached to the capacitor.

Now if the start switch "never" really disconnects, there would be a constant charge current even if the motor is at run speed. If that should happen, I don't know what would happen to the capacitor.

You reported that when you heard the pop and saw the smoke, you removed power. If you had not removed power, would the motor have continued to rotate. The capacitor is, at that point, no longer and there certainly would be no run current.

[tvidnoviciii]

You are correct about the operation of the circuit. The motor would have continued to run if I hadn't removed power. The start circuit is really only there to get the rotor up to operating speed. The capacitor is there to enhance the torque on the rotor at start up so it can reach operating speed under load. At this point, I think I'll remove the components of the start circuit and see if the motor will start and run without it as a test. I realize that this will not solve the problem, but it should be a definitive test to verify that the start circuit is the problem.

The logic I was trying to explain is flawed as I was thinking of a circuit that had a diode in it. Nevermind.
Clear as mud?

Just a note, a relay is a much better component to use instead of the centrifugal switch as it eliminates moving parts. In the relay there are two circuits set up (start circuit and run circuit). At start up, the start circuit switch is closed. Once the rotor reaches operating speed, there is enough current in the run circuit to trigger the relay to open the "switch" on the start circuit.

[dusty]

You are correct about the operation of the circuit. The motor would have continued to run if I hadn't removed power. The start circuit is really only there to get the rotor up to operating speed. The capacitor is there to enhance the torque on the rotor at start up so it can reach operating speed under load. At this point, I think I'll remove the components of the start circuit and see if the motor will start and run without it as a test. I realize that this will not solve the problem, but it should be a definitive test to verify that the start circuit is the problem.

The logic I was trying to explain is flawed as I was thinking of a circuit that had a diode in it. Nevermind.
Clear as mud?

Just a note, a relay is a much better component to use instead of the centrifugal switch as it eliminates moving parts. In the relay there are two circuits set up (start circuit and run circuit). At start up, the start circuit switch is closed. Once the rotor reaches operating speed, there is enough current in the run circuit to trigger the relay to open the "switch" on the start circuit.

I too might have done it different but they didn't ask me.

If you do try to operate without the start circuit, you may have to spin the shaft manually to overcome that initial load. If I was going to do that, I'd attach something like a pulley wheel or the sanding disk (realizing the either would tend to increase that dead load).

[billmayo]

Just a note, a relay is a much better component to use instead of the centrifugal switch as it eliminates moving parts. In the relay there are two circuits set up (start circuit and run circuit). At start up, the start circuit switch is closed. Once the rotor reaches operating speed, there is enough current in the run circuit to trigger the relay to open the "switch" on the start circuit.

I will have to disagree with you. The early Shopsmith 3/4 HP AO Smith motor had a start current relay that did this function. These current start relays fail more often then the centrifugal switch fails. The start current contact points are closed when no power is applied bur must have the voltage from the run windings to stay energized and open the points once the motor is operating. This constant current tends to cause the energizing coil windings to fail and allow the points to close putting power back to the start windings which does cause problems. The centrifugal switch points can corrode or burn resulting in no power to the start windings and the motor to buzz when power is applied. If you quickly quit appling power to a buzzing motor, you will normally not damage it. The start current relay failure will result in the coils overheating and many times a small fire will result leaving a destroyed motor.

[JPG]

Capacitor lesson # 1.

A start capacitor is an AC device and passes alternating current so it is charging and discharging and at a 120 HZ(over here anyway) rate and reversing polarity at a 60 HZ rate..

Induction Motor lesson #1.

When the rotor is not turning, no torque is produced(thus it just sits there and 'hums'(loudly)). It also acts like a room heater.

Start circuit lesson #1.

The start winding and capacitor produce a magnetic field that leads the magnetic field produced by the run windings. Since the start coils are spaced between the run coils the magnetic field rotates at 3600 rpm. That rotating field induces current in the armature which create an opposing magnetic field. This causes the armature to attempt to 'follow' the rotating field. The rotating armature will produce torque and the race begins. As speed increases, the run windings induce currents in the armature and the start coils are no longer needed. The armature will reach a balance between rpm and torque output, but will never catch up with the rotating field, thus their rpm rating is less than 3600 rpm. This is a simplified description, but is reasonably accurate(there is other stuff going on). If the start winding polarity is reversed, the rotating field will be reversed.

I am not sure, but believe if the start switch is held closed, the voltage developed across the capacitor may exceed that which it experiences when starting. If that be so, a stuck start switch may indeed be the cause of the capacitor blowing due to exceeding rated voltage.

I will try to duplicate a stuck switch and measure the capacitor voltage when I can physically get to a non-installed motor. May need to take it apart to access terminals of interest. Do not hold yer breath while waiting!;)