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

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"The Natural Philosopher" wrote in message
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On 15/09/2019 14:24, George wrote:
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.

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




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