Fault Isolation, Mark V Power

[dusty]

[ATTACH]10423[/ATTACH]
Switch OFF

[ATTACH]10424[/ATTACH]
Switch ON

From time to time we read here on the forum that a member has a Mark V that will not turn on or one that just hums when turned on. With that I started looking into fault isolation techniques for such problems.

I am very interested in hearing from some of you what you see when you perform resistance and voltage tests on your power cords.

Examble: The above two images show what I get when I connect an ohm meter to the Mark V power plug. One is with the switch off and the other with the switch on.

If I change the meter to measure AC Voltage, I read something I cannot fully explain and I would appreciate feedback from you all. What do you get? I measure a very low AC voltage that changes when I rotate the sanding disk.

[damagi]

A motor and a generator are basically the same thing...just a perspective thing. the generator gets power from another source (ie: water, steam, bicycle, you) and the motor gets power from the utility.

I would expect that rotating the sanding disk would generate a small charge.

[dusty]

I suspect that you are correct but there is a low level voltage present without turning the sanding disk. That voltage could well be the charge from the start capacitor.

If it is determined that the motor should act like a generator when unplugged and the sanding disk is turned, does the existence of the measured voltage then indicate that the start winding, start capacitor and centrifugal switch are working?

[JPG]

I suspect that you are correct but there is a low level voltage present without turning the sanding disk. That voltage could well be the charge from the start capacitor.

If it is determined that the motor should act like a generator when unplugged and the sanding disk is turned, does the existence of the measured voltage then indicate that the start winding, start capacitor and centrifugal switch are working?

1) The motor will not act like a generator. There are essentially no magnetic fields present when off, and no way to get any induced voltage to the power plug.

2) You are constantly in a myriad of electrical and magnetic fields caused by all sorts of electrical devices and wiring. If the meter is sensitive enough, it will indicate their presence. Remember the stereo/hi-fi/PA days when occasionally an input plug would come loose in an amplifier input and the shield became 'unconnected'? Remember the 60/120 hZ noise that resulted at the speakers??? Caused by the same fields.

Depending upon the meter used, the indication across the linecord(pix 1) may not indicate 'ol', but instead a very high reading(Megohms).

Also of interest both with the switch off and on is the resistance from the 'hot' conductor(narrow blade, nickel plated...) to the chassis. An equally high resistance should also be indicated both on the actual chassis and the grounding connector in the linecord.

This pretty much exhausts all one can do by making dc resistance checks with the power cord other than verifying a low resistance connection from the plug grounding pin to the chassis.


I am curious why you jumpered the neutral(white) wires since you are so adamant re switching both sides. Trying to simulate others condition???

[dusty]

You are correct in saying that the "OL" indicated in one of the pictures may be an extreme high resistance reading and not an absolute open (infinity). But when I switch the power switch on, the low resistance reading is the DC resistance of the power cord (round trip), the switch and the dc resistance of the motor windings. These two contrasting readings clearily indicates that the switch works.

As far as generator action is concerned - I need to take a few more readings but I believe that the voltage will rise significantly if I could spin that sanding disk faster. The residual charge on the start capacitor (which you have warned about) and the residual magnetic field set up around the armature would be enough for some voltage generation (would it not)?

JPG, you have mentioned a couple times when talking about the start capacitor that it could be tested. The start capacitor is in the circuit along with the centrifugal switch and the start windings. How would expect that capacitor to act with an ohm meter connected to it and then the polarity of those leads suddenly reversed.

[JPG]

You are correct in saying that the "OL" indicated in one of the pictures may be an extreme high resistance reading and not an absolute open (infinity). But when I switch the power switch on, the low resistance reading is the DC resistance of the power cord (round trip), the switch and the dc resistance of the motor windings. These two contrasting readings clearily indicates that the switch works.

As far as generator action is concerned - I need to take a few more readings but I believe that the voltage will rise significantly if I could spin that sanding disk faster. The residual charge on the start capacitor (which you have warned about) and the residual magnetic field set up around the armature would be enough for some voltage generation (would it not)?

JPG, you have mentioned a couple times when talking about the start capacitor that it could be tested. The start capacitor is in the circuit along with the centrifugal switch and the start windings. How would expect that capacitor to act with an ohm meter connected to it and then the polarity of those leads suddenly reversed.

I doubt it could be 'detected' from the linecord. The motor windings will cause those extremely low resistance readings.

However if one does have access to the capacitor(isolated from other motor parts) then depending on what range the ohmmeter is set it will tend to indicate a momentary lower resistance followed by open circuit indication. Immediate reversal of the meter leads will cause the momentary lower resistance to repeat and possibly for a longer duration. This will occur since when initially connected to the ohmmeter, the capacitor is charged by the meter(very low voltage). Reversing the leads causes the meter to first discharge the capacitor and then charge it to the opposite polarity.

Depending upon the meter internal circuitry, this observation may or may not be possible. Back in the D'Arsonval meter days this was more predictable.

Checking the start switch involves first identifying where its 'parts' are and where to connect to it. One side is fairly easy. It will be connected to one of the capacitor leads(usually only that lead).

[dusty]

Does my Simpson 260 date back to the times of D'Arsonval meter movements.

[JPG]

Does my Simpson 260 date back to the times of D'Arsonval meter movements.

Affirmative!