3rd Generation Owner

[dusty]

I found this article that might be applicable (of interest) here. Reading down through the section on Single Phase AC Motors are a couple comments about defective start capacitors.

This sorta confirms my suspicion that it would not be wise to simulate a "stuck closed" start switch.

[JPG]

I found this article that might be applicable (of interest) here. Reading down through the section on Single Phase AC Motors are a couple comments about defective start capacitors.

This sorta confirms my suspicion that it would not be wise to simulate a "stuck closed" start switch.

Who ever accused me of being wise?:D

My curiosity still wants to determine if the start switch stuck on causes an overvoltage to cause said 'cooking' of the capacitor, or prolonged current flow.

I can understand prolonged current flow in the start winding could cook the start winding.



I do believe we have justified our suspicions of the start switch. That 'article' confirmed it!

[dusty]

Who ever accused me of being wise?:D

My curiosity still wants to determine if the start switch stuck on causes an overvoltage to cause said 'cooking' of the capacitor, or prolonged current flow.

I can understand prolonged current flow in the start winding could cook the start winding.



I do believe we have justified our suspicions of the start switch. That 'article' confirmed it!

I am having difficulty understanding where an excessive voltage would come from. The applied voltage is 115vac and the capacitor is rated at that as well.

Unfortunately, I am never had the opportunity to conduct "destructive testing" on a motor. That might have been very informative. I have destroyed just about everything else electrical/electronic but not a motor.

[JPG]

I am having difficulty understanding where an excessive voltage would come from. The applied voltage is 115vac and the capacitor is rated at that as well.

Unfortunately, I am never had the opportunity to conduct "destructive testing" on a motor. That might have been very informative. I have destroyed just about everything else electrical/electronic but not a motor.

My curiosity will not intentionally 'destroy' a motor just to satisfy curiosity. That does not stop me from trying to test non-destructively.

First of all we must understand instantaneous voltage magnitude. Look at a sinusoidal wave form(voltage). The peak(max value) is greater than the rms value that is used to specify the voltage. The peak value is the square root of two(1.414....) times greater than the rms value. Thus the peak value of a 115v source is slightly greater than 162v(162.635).

I need to correct something I stated earlier - A start relay coil is connected across the start winding, not the run winding. That detail helps explain the following.

Now introduce capacitance and inductance to the mix and all that sinusoidal waveform stuff flies the coop!

The start circuit consists of the start capacitor and the start winding and a centrifugal switch that is closed at rest and open as rpm increases. So we have a series circuit that has an inductance(start coil) and a capacitance(start capacitor) across 115v AC. The current becomes the result of two very different behaving loads. The current will tend to lag the input voltage due to the inductance, but the capacitor has a greater effect and will cause the current to lead the voltage.(Being a series circuit the current is the same in the coil and the capacitor). This causes a voltage divider so the capacitor has less than 115v(162+) across it.

As the rpm increases, the start switch opens and all currents in the start circuit go to zero.

Now the start winding however is rotating in a varying magnetic field('slip induced') and becomes a 'generator'. Since the start winding has a large number of 'turns', the voltage will be large. It is this higher voltage that picks a start relay.

I have convinced myself that 'higher' voltage will indeed exceed the rated voltage of the start capacitor(125v) if the start switch sticks on. A start relay picks at around 130v(IIRC) which has a peak value higher(184v?). The question mark since I doubt the waveform is pure sinusoidal.

[dusty]

My curiosity will not intentionally 'destroy' a motor just to satisfy curiosity. That does not stop me from trying to test non-destructively.

First of all we must understand instantaneous voltage magnitude. Look at a sinusoidal wave form(voltage). The peak(max value) is greater than the rms value that is used to specify the voltage. The peak value is the square root of two(1.414....) times greater than the rms value. Thus the peak value of a 115v source is slightly greater than 162v(162.635).

I need to correct something I stated earlier - A start relay coil is connected across the start winding, not the run winding. That detail helps explain the following.

Now introduce capacitance and inductance to the mix and all that sinusoidal waveform stuff flies the coop!

The start circuit consists of the start capacitor and the start winding and a centrifugal switch that is closed at rest and open as rpm increases. So we have a series circuit that has an inductance(start coil) and a capacitance(start capacitor) across 115v AC. The current becomes the result of two very different behaving loads. The current will tend to lag the input voltage due to the inductance, but the capacitor has a greater effect and will cause the current to lead the voltage.(Being a series circuit the current is the same in the coil and the capacitor). This causes a voltage divider so the capacitor has less than 115v(162+) across it.

As the rpm increases, the start switch opens and all currents in the start circuit go to zero.

Now the start winding however is rotating in a varying magnetic field('slip induced') and becomes a 'generator'. Since the start winding has a large number of 'turns', the voltage will be large. It is this higher voltage that picks a start relay.

I have convinced myself that 'higher' voltage will indeed exceed the rated voltage of the start capacitor(125v) if the start switch sticks on. A start relay picks at around 130v(IIRC) which has a peak value higher(184v?). The question mark since I doubt the waveform is pure sinusoidal.

JPG, I don't know which one of us is confused but one of us certainly is. I think you are working in/on a different motor than I am. The Emerson 3/4hp motors in the Power Station and CS do not have start relays. I do not believe the Emerson 1 1/8hp motor has a relay either. The start circuit in all of these are switched in/out by a centrifugal switch.

Your explanation of voltage and current phase relationships is totally acceptable but has very little to do with what is or is not happening in these motors. The capacitor in the start circuit establishes the out of phase relationship needed to get the motor rotating and then disappears from the discussion.

[JPG]

JPG, I don't know which one of us is confused but one of us certainly is. I think you are working in/on a different motor than I am. The Emerson 3/4hp motors in the Power Station and CS do not have start relays. I do not believe the Emerson 1 1/8hp motor has a relay either. The start circuit in all of these are switched in/out by a centrifugal switch.

Your explanation of voltage and current phase relationships is totally acceptable but has very little to do with what is or is not happening in these motors. The capacitor in the start circuit establishes the out of phase relationship needed to get the motor rotating and then disappears from the discussion.

There has been mention of start relays not because their being on either Emerson motor, but because they provide a clue as to the voltage generated by the start winding under running conditions.

The capacitor 'disappears' only if the start switch(or relay contacts) open. This sadly was not the case for the 'star' of this thread.

This is the first time(to my knowledge) this particular fail mode has been discussed.

I tend to ramble I suppose, but do so in the interests of gaining better understanding of funky behaviors of things being contrary and do so with the intent of sharing that understanding.

'Normal' behavior is more commonly understood, it is when things are not behaving that way that mystery enters our horizon.

At this point I do not think I am confused(certainly I am much less so than several posts back).

Having been involved in this thread almost from the git go(as well as its twin)I view the entire discussion(s) when adding later posts. Their relevance can be missed without those earlier posts being included.

I appreciate this thread as it has exposed a different failure than has been raised before. The common 'attitude' before was the capacitor rarely goes bad. Now we know it can be made to fail as the result of failure of the start switch to open.

IF and when I measure capacitor voltage while running with a closed start switch, I will post 'findings' here.

[wa2crk]

Been following this thread but refrained from commenting again because you guys (Dusty and JPG) have covered the topic very well. But my question is "has the start switch/capacitor assembly been wired incorrectly?" Could errors in the wiring cause the capacitor to fail repeatedly?
Time for a comparison between a known good motor and the one in question.
Why is a capacitor for a 115 VAC circuit rated for less than 250 VAC? Capacitors and solid state devices are rated for the peak voltages that are to be expected plus a safety factor. As JPG stated the peak voltages of a 115 VAC circuit will exceed +/- 150 VAC and continuous application of this voltage will cause the insulation of the cap to fail.
Sorry for asking three questions.
BIll V

[dusty]

Been following this thread but refrained from commenting again because you guys (Dusty and JPG) have covered the topic very well. But my question is "has the start switch/capacitor assembly been wired incorrectly?" Could errors in the wiring cause the capacitor to fail repeatedly?
Time for a comparison between a known good motor and the one in question.
Why is a capacitor for a 115 VAC circuit rated for less than 250 VAC? Capacitors and solid state devices are rated for the peak voltages that are to be expected plus a safety factor. As JPG stated the peak voltages of a 115 VAC circuit will exceed +/- 150 VAC and continuous application of this voltage will cause the insulation of the cap to fail.
Sorry for asking three questions.
BIll V

I guess it is possible that I wired it incorrectly but I don't think so. My "defective motor" passes all of the continuity tests that Bill has posted. Given my track record, I am not about to open a good motor to check.

Yes, the capacitor in my motor is rated at 115vac but I trust that this rating is an rms reading. The outlets in your house are all rated for 115vac but the voltage that they deliver peak higher than that. No, I think the capacitor is properly rated for a normally operating circuit. The manufacturer cannot be expected to anticipate or protect against conditions that are not suppose to happen.

BTW, as an update, the defective motor has been "reassembled. It passes all of the recommended continuity tests. But when power is applied, the circuit breaker trips immediately. There was no sign of motor action (the motor shaft did not move at all).

I am not ready to give up on this (as an educational experience) but I do not expect that this motor will ever run again.

[dusty]

Maybe it would be more appropriate to say that my motor has been saved from me.

Bill has made a proposal to accept my motor for scrutiny. I could not refuse.

[JPG]

Recall that 'normal' operation includes being in series with the start coil. The 115v will be divided between the two so the cap will not see a full 115v. I have no idea how much less.

I think Dusty is correct that an ac capacitor intended for this application when rated at 115vac would be capable of withstanding normal peak voltages.

Capacitors rated at 270vac(and higher) are usually 'run' capacitors. An entirely different scenario.


Now I wish I was really closer to Plant City!;)