I want to bench test a 1HP DC motor before installing it. Its shunt
wound, 220V motor with 170V field excitation. The four wires show
130ohms on one circuit, 2 ohms on the other, so I presume the field is
I have three car batteries. Can I use these to run it up ? Will there
be a big inductive voltage when I break the circuit which could damage
me or the motor (I see it has discharge resistors in circuit in its
wired-in configuration) ?
Maybe I shouldn't bother and the continuity test and turning by hand is
enough - what do you think ?
The figures look a bit strange - 170V and 130 ohm field
implys 220W field dissipation which is a bit higher than I would
expect for a 1 HP motor.
That sort of field dissipation is likely to also store
enough energy to erode switch contacts. By far the best way of
controlling the inductive overshoot is to shunt the field with a
With a shunt wound motor of this size motor it's ESSENTIAL
to allow time for the field to build up before applying power to
the armature. With 3 car batteries the field will be well below
nominal but should be enough for the motor to run light.
Shunt motors of this size often also have a small series
winding in the armature circuit to reduce the peak starting
current. If this is the case direction of rotation is
important.Correct connection and direction is the direction in
which the series field ADDS to the shunt field - lowest peak
starting current and slightly lower operating speed.
If it is necessary to reverse a motor of this type,
reverse only the connections to the brushes leaving the shunt and
series field connections unaltered.
duplicate post - the original seems to have gone astray!
Its physically a big heavy motor.
I decided I was being a bit of a fairy and I should just get on with
it, so pulled the original note. Your reply must have slotted
inbetween, my apologies, and thanks.
I ran it up with 24volts OK. Made the field circuit first, and let that
settle, then the main circuit. So unless there is some sort of
breakdown at high voltage I seem to have a working motor. The motor
should run equally well in either direction as the armature voltage is
provided by a DC generator and the lathe reversing switch changes the
polarity of the variable voltage supplied to the generator field coils,
which presumably will change the polarity of the generator's output
(the generator is driven at constant speed by an AC motor).
In service the wiring diagram shows the field coils for the motor run
direct off rectified 220V AC - it simple shows a 'metal rectifier' so I
assume no smoothing capacitors (maybe that would be an improvement on
the original design?). The field coils are supplied from the first
moment the machine is turned on. The direction and voltage supplied to
the field coils of the DC generator is supplied from the same source,
but dropped by a potentiometer and switched in polarity by a venerable
All of this is in kit form after a previous owner cut all the wires and
heaved it out, so I am slowly re-assembling it. I am very careful with
electrickery, everything will be step by step. Next step is to bench
test the generator. If I keep the field voltage low then the output
voltage will be low, so should be able to drive it at 1425rpm using its
companion AC motor.
Can post a picture of the wiring diagram on the web if my ramblings
make no sense. Its quite a neat system - or it would be if I could get
it back together (and the aim is to restore this DC drive system rather
than plug in an electronic frequency converter).
Thanks for the advice, Steve
Pretty big and heavy but should be a nice system when
you've got it sorted.
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.
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
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 ?
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, you're a star. The wire was so fine I couldn't see it, but I
managed to scrape away some of the outer coating and get a contact. Its
a 1k resistor, presumably designed for constant service at the max
voltage of 200V DC, so thats 40 watts. No wonder its in a little cage.
I went to Maplin the other day and described this 4 inch long resistor
and they claimed the only ones they had were like 'grains of rice'.
They had no idea what I was on about.
At least I know what I am after. Thanks.