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Testing Field Coils


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Guest Silverghost

There is a very special meter called a Meg-Ohm Meter or "Megger" for short.

This meter tests electrical Insulation for "shorts" and bad electrical insulation.

The older style "Meggers" have a hand crank generator internal to the meter.

It is used to put a higher than normal voltage on the coil windings or insulation to be tested to check for poor leaking electrical insulation.

The insulation varnish of a field coil can be tested successfully in this manner.

Extreme Care must be taken when using a meg-ohm-meter as improper use can in fact blow a hole in the otherwise good electrical insuation.

I have several Navy Surplus Meg-Ohm Meters.

They often appear for sale on eBay !

New units often cost $1000. or More !

A good multi-meter with several ohm

scales can do a pretty good basic test of starter & generator field coils~~~

All seperate field coils should have about the same ohm-meter resistance readings if they are not internally shorted-out !

A Growler is a special AC electromagnet type tester used to test the armatures of DC starting motors & generators as well as Universal brush-syle motors for shorts & open windings , bad commutator connections etc.

This AC electro-magnet assembly actually induces a voltage in the armature coil windings themselves for testing at the commuttor !

They are often used along with a steel hacksaw blade, And,or, a light bulb & meter for some tests !

These are NOT used to test the field coils.

Only rotating Armatures !

I have several older growlers also !

It may be a good idea to test field coils & armatues that are slightly heated to simulate actual auto operating temps !

What are you trying to test ?

Possibly I may be able to help ?

Or at least talk you through some simple electrical tests !

Edited by Silverghost (see edit history)
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Growlers induce ac into an armature, normally the coil shouldhave just the induce voltage in them, if on the other hand if a coil is shorted within the winding it creates a high reverse current in that coil and the hacksaw blade will viberate different,,,, field coils , transformes we had another way of testing, which we used about a 1 amp 6V transformer to apply a 60hz signal to a coil through a capactor, then watched the signal across the capactor, and the coil should "ring" on the scope, like a bell being struck, start off with a large sign wave and drop off gradually like a bell,, if it has a shorted turn thought, it will act like a cracked bell,, and the wave form will not drop off evenly due to the reverse emf off the shorted coil..

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When using a megger you put one clip on a lead, the other to the core. It's best to get inside a bolt hole where it's clean. When you crank the megger you should have a minimum of 1000 ohms(1K ohm). usually it will read infinity. That checks it for a ground fault. It can also be open. for that you put both clips on the leads. It should be zero resistance. In some cases it is shorted between loops which is harder to check. for that you need to apply a voltage to it and check the magnetism created compared to a known good one.

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Thank you all very much for the response. I have a few Auburn three brush AutoLite generators on my bench that I am restoring. I have a growler that I use for checking the armatures. It really works quite well, as I found a bad one. I have a good Fluke multimeter which is more meter than I am electriction. It was suggested to check each coil individually and note the resistance. Of the eight field coils I have checked they all are .8 - .9 This would indicate to me they are probably all OK.

The megger sounds like a fun tool to own!

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the Growler's normally have a 110V light on them as well for testing shorts to ground or poor insulation,, the netural (white) of the 110v is connected to the frame of the field, and with both ends of the field not grounded to the frame, you feed the hot side (black) through a 110V 25W light and probe the end of the field coil to check for shorts to the case,, as the same way you would check the armature for shorts between the shaft, and the communitator,, I think it was the Dykes auto repair went into a lot of detail on testing and repair of generators and starters,,

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Guest De Soto Frank

From what I have always read on MoToR's and Chilton's, the Growler had two functions: one was the 120-volt continuity light with two test leads/prods, and the transformer coil & hacksaw blade for testing armatures for shorts.

The continuity light feature that Albert described will allow you to check for open field coils or a short to ground, but would not tell you whether you had turns shorted to each other within the coil itself... that would show-up only with a resistance test or the "Megger", as described elsewhere.

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