DC generator works OK as a Motor, but does not generate?

[Archibald Tarquin Blenkinsopp]

a

If it has brushes, you need to maintain a magnetic field for the thing to operate. A voltmeter as a "load" will not draw enough current to maintain the field, assuming of course you dont have a magnet in the thing.

I have not come across a device such as yours, but to maintain current through its field coils, you would have to apply DC to the generator and load together, then remove the DC. Disconnecting the load or replacing it with a meter would not draw enough current to magnetise the field.

AB

[Archibald Tarquin Blenkinsopp]

have

As you obviously have not killfiled me, why do you post exactly what I have pointed out around an hour before you added your gem?

Is it that you didn't understand the explanation?

I strongly suspect that that was the case and your lack of understanding stretches from the technical to the political also.

Knuckles...................Elastoplast :-)

AB

[George]

Afternoon chaps,

An old friend of mine who collects scrap and often brings bits round to me for identity and possible sale sold me what said on the label was a DC generator of some 110 volts. As a quick check I tried feeding it with about 50 volts of DC and away it went. I dismantled it as far as necessary, checked it all for brushes, bearings and internal wiring, it was all fine, but the first time I tried to use it, it was spinning around quite fast on the belt drive from a more powerful mains motor. but no sign of any DC output. thinking that it might not be spinning fast enough, I cranked up a big battery charger I had made capable of over 100 VDC and away it went. At top speed, I put a voltmeter across the input and measured the voltage, however as soon as the DC was switched off the voltage died straight away, although the generator continued to rotate for a bit. I did this to prove that it was not an excitation issue as sometimes I have found that generators of various kinds need to have a jolt to get them going.

In fact, I have an ac alternator of some 5 KW 240 VAC and this does a very similar fault I have tried all sorts of ways to get this going as it is a US Wisconsin (?) petrol engine set and I would love to get that to work OK

Come to think of it I have also a set with a 2 cylinder Armstrong Siderly diesel engine of about 10 KW 3 phase and if I put a load on of more than a Kw or 2 it just switches off? There is no overload device or anything.

Is this something to do with me and my suspected magnetic personality or is there a more logical explanation. Or is it as I have always suspected all mechanical devices have secret minds of their own and the best thing is to find ways to live with them. Perhaps they are just getting TOO excited, as getting them excited has been uppermost in my mind.

If anyone has experienced this peculiar behaviour and knows the answer(s) I should be very glad to hear of it.

Thanks and regards George.

[The Natural Philosopher]

You need to draw current from generators for them to generate.

[Dave Plowman (News)]

Pretty well all generators need DC applied to the field windings before they will generate. Some older dynamos could start up on residual magnetism then use their own output for the field - but not all. Alternators often can't.

[AJH]

When we tested dynamos in old cars we would wire the output to the field and test the voltage between output and ground at low revs, till we saw no more than 18V. As you say this depended on residual magnetism in the field to get it booted up (a term which I would not have used at the time or before 1970).

This was the period when older (often 6V) vehicles would be positive earth. To change the system to negative earth it was necessary to flash the field with 12V before it would generate.

AJH

[harry]

DC generators depend on "residual magnetism" in the field coil to "boot them up" The field coils are an electromagnet driven from the armature.

But on initial start up they depend on the bit of magnetism left in the field to get going.

This means that if you run it in the wrong direction or have connected the field in the revers direction, you will kill the residual magnetism on start up. So you need to renew this residual magnetism from a car battery by momentarily "flashing" the field ONLY..... Disconnect the armature. (Lift a brush) If it doesn't work, try again with a reverse connection flashing. The field's gotta be the correct direction.

[newshound]

+1

Brought up with the feeble 6 volt dynamos of British Bikes in the 1950's and 60's. My first car had a dynamo too.

[Brian Gaff]

Well not strictly true. You should if its dc be able to measure a voltage. Are we sure its not an ac generator which uses rectification to produce dc and using it as a motor has trashed the rectifier?

After all when I used to have a scalextric, if you put a meter across the tracks when a car was speeding and removed the dc, you could in fact read a dying dc voltage as the car and motor slowed down and acted as a generator, indeed, some slot car controllers used this as a brake by shorting the track out when the plunger was released. Which brings me to the second comment. if you put a load on this generator then it should be harder to turn if any current is being drawn, even if its just a light bulb or resistor. Brian