residual magnetism?

D

donQ

Senior Member
I have a generator that runs well as a motor on the bench, but when installed in a car and polarized, it will not charge. I measure the voltage at the generator output terminal so the cut out is not a factor. I was told that the problem is that the generator has no residual magnetism and therefore will not build up voltage when running as a generator.
If this is true, what part of the generator needs to be reworked or replaced to make the unit put out voltage?
donQ
 
The pole shoes are soft iron and should hold magnetism no problems once polarized. There is nothing you can do to change that.
Jump the cutout with a jumper while running to simulate a functioning cutout, and to make a complete circuit, and see what you get on the ammeter. Jumper can stay in play until you shut engine off.

There is a similar thread running right now with good info from Tom W, the electrical master
 
donQ said:
I have a generator that runs well as a motor on the bench, but when installed in a car and polarized, it will not charge. I measure the voltage at the generator output terminal so the cut out is not a factor. I was told that the problem is that the generator has no residual magnetism and therefore will not build up voltage when running as a generator.
If this is true, what part of the generator needs to be reworked or replaced to make the unit put out voltage?
donQ
Click to expand...

This ONLY verifies that you have battery voltage at the output, as it should. You need to measure the voltage at the INPUT terminal, which is the generator output.

If the field isn't working, the residual magnetisn will only let the generator output reach about 1/2 volt.
 
I had a similar problem to this a while back that had me scratching my head. I measured what I thought was a good voltage on the output terminal, but it was actually battery voltage. I depressed the cut out manually and it immediately blew my safety fuse. Some more scratching my head and I decided to take the generator apart and I found the problem was that 1 of the long screws was shorting to the output wire due to someone twisting the terminal post on re-assembly and then not bench testing correctly after re-assembly. Not sure if this helps or is just a good story?
 
Residual magnetism is not needed in order to have the generator operate as a motor. This is because the current going through the field winding will create the needed magnetic field that the armature's magnetic field interacts with in order to turn the shaft.

As a motor, the generator isn't a good design, but it's a good way to test the device.

Marc
 
I measured the voltage at the output of the generator ( input to the cut out) not the output of the cutout. Tom seems to indicate that the field is not working. Could this be due to poor contact between the field brush and the armature? With the field brush on the armature I measure 1.4.1.7 ohms. The field brush is good and when I lift the field brush the field impedance is 1.9-2.0 ohms. I polarized the generator by jumping across the cutout to the generator output. Should I leave the jumper on while the engine runs as suggested by tbirdtbird or do I need to replace the pole shoes or the field coils?
donQ
 
I need two voltage readings with the engine running at fast idle (about 900 RPM).

What is the voltage on each side of the cutout? Measure each terminal using the negative meter lead and have the positive meter lead clipped to a good ground.
 
Tom,
Thanks for the reply. The engine is run at fast idle. The positive lead of the digital voltmeter is grounded The negative lead is connected to the output of the cut out. This is the battery voltage and is 6.12 volts. The negative lead is moved to the input of the cutout ( generator terminal) and is ~0.23 volts.
The brushes are good. The readings are fairly constant even if the engine is revved up higher.
donQ
 
One other thought I had was that perhaps the cutout was shorting the generator to ground. I measured the impedance from the input terminal of the cut out to ground and it was 49.3 ohms. I tried a new repro cut out with a no charge result as well. The impedance of the repro cut out input terminal to ground was 35.1 ohms.
I checked the impedance from the tip of the field brush to the field winding lead on the brush holder and found it to be 0.8 ohms.
donQ
 
The cutout ohm readings sound correct. I always use an original cutout. They are the best quality.

Make sure you don't have cutout mounting screws that are too long and shorting the field to ground. In fact you can remove the cutout, then check the generator output post voltage at fast idle. If it's still low, then take the generator apart and see what's wrong.

You can also lift the adjustable brush off the commutator, or slip a piece of paper under the brush, then connect a battery charger to the case (+ground) and the brush (-). The meter on the battery charger should show about 4 to 5 amps.

If your multimeter has a 5 amp or higher range, then you could use that instead of the battery charger. Just connect the meter leads to the wire going to the firewall and to the field brush with the piece of paper under it. Again, it should show about 4 to 5 amps. This will confirm a good field winding that isn't open nor shorted to ground.

Also make sure your cutout has the insulators on both sides and make sure the terminal screws aren't too long and shorting the generator output.
 
"make sure the terminal screws aren't too long and shorting the generator output."

I have actually run into this.

Not sure why this baby would motor on the bench for him and then not work, problem would seem external to the gen. The fields HAVE to be good for it to motor
 
I just went to the shop to check. The generator WILL motor with the field wire disconnected from the adjustable brush. In fact it motors a little faster but much weaker than with the field wire connected.

I worked on a wiper motor once that had one of the field windings shorted to ground. It ran the same as if it wasn't shorted to ground, but it drew more amps.

The first thing I would do is remove the cutout, then check the output at fast idle. Remember that your generator is unregulated and the voltage can rise quite high with the cutout out of the circuit. If you get 6 plus volts at more than 500 RPM, then I suspect the cutout was the problem. Remember to check the screws and insulators on it. The thick insulator goes on the output side.
 
"I just went to the shop to check. The generator WILL motor with the field wire disconnected from the adjustable brush. In fact it motors a little faster but much weaker than with the field wire connected."

Tom, this is really wild. Thanks for checking this; your role as the chief electrical guru will never be challenged! dave
 
I raised the third brush and energized the field and the current was 5 amps as Tom indicated. I ran the generator as a motor and the draw was 9 amps. I raised the field brush while it was running and the speed increased and the current draw increased to 17 amps.
These tests confirm that the field is probably OK and that the field brush is making good contact with the armature.
The insulators on the cutout are good and the screws apparently are not shorting out the field.
It seems like it should charge but it doesn't.
donQ
 
I've worked on a couple where everything checked out and yet the output was very poor or almost zero. I suspected the air gap was too large from someone using a belt sander to remove the rust. Anyway a different armature fixed the generator.
 
Here is a possibility that hasn't been mentioned. When the generator motors, does it motor in the direction that the engine turns when it runs ? If the wires that come off the field coils are hooked up backwards, the generator will motor backwards and won't charge. It may be worth taking a look at the direction that the generator motors.
 
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