Re: Testing DC lathe motor

On 17 Aug 2006 12:25:15 -0700, "Steve" <oldnoccer@xxxxxxxxxxxxxx>
wrote:



Thr DC output of the 220V full wave rectified supply to the
field is 0.9 RMS plus rectifier losses - which were pretty high
with the old westinghouse copper oxide and selenium rectifiers.
The field inductance provides all the smoothing that's necessary
- adding a capacitor would result in excessive field voltage. In
fact, if you replace the Westinghouse rectifier with a modern
silicon component you'll need to add some dropping R to get near
the 170V nominal.

Speed control by varying the field supply of the generator
system is an excellent system. It's just possible that your
generator is a Ward Leonard type - these use a complicated
internal feedback system which makes it possible to control the
generator output by a relatively low power potentiometer
controlling the field excitation.

If yours is a straightforward generator you may find that
supplying the field from a Variac driven full wave rectifier
gives a better wide range control.

Jim

Jim.

Nice to hear from someone who clearly knows about these things. I am
learning as I go.

After replacing the run capacitor on the AC motor and adding extra
insulation over the fabric/rubber covered wires on the generator I have
successfully driven the generator and with 12Volt field coils at
2800rpm it gave 21.8Volts output. So it works at low voltage !

The rectifier system looks home made so may not be original. Will have
to check that out - thanks for the info.

However I have hit a problem, and wonder if you can help. I have opened
up a small vented enclosure about 6inches by 1inch by 2 inches and
found inside a broken electrical component, presumably a resistor. It
is 4 inches long and seems to be a ceramic tube, now in two parts. I
guess it gets very hot and that is why it is enclosed. The electrical
drawing shows two resistors, one for the DC motor armature and another
across the DC generator field windings (marked Field Discharge
Resistance). Both of these seem to handle the field that develops if
you were to suddenly throw the reversing switch to open circuit. Though
they may have more subtle effects if a sudden change in speed is called
for.

I already have a large resistor composed of resistance wire wound on
ceramic formers - thats about 20 ohms, which can't be permanently
across the generator field windings as they are 350ohms. So the broken
resistor must the generator field discharge resistor. Have you any idea
what value this should be (its green if that counts for anything), and
do you know where I might get one ?

Thanks, Steve


Can't give a definitive answer here but 5 to 10 times the
field resistance is a reasonable guess (and consequently 1/5 to
1/10 of the field watts). Can't positively identify your resistor
type but a common high power resistor type is wirewound on a
ceramic tube and covered with a green vitreous glaze. If yours is
this type you can usually find the exposed end of the wire at the
break and get a good enough connection for an ohmmeter
measurement of the value.

Maplin or RS Components stock a pretty wide range
of resistor types - both have good on line catalogues.

Jim


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