Testing DC lathe motor

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
the former.
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 ?
Steve
Reply to
Steve
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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 reverse diode.
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.
Jim
duplicate post - the original seems to have gone astray!
Reply to
pentagrid
Its physically a big heavy motor.
Thanks Jim,
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 Dewhurst switch.
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
Reply to
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.
Jim
Reply to
pentagrid
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
Reply to
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
Reply to
pentagrid
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.
Steve
Reply to
Steve

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