DC motor drive ideas desired.

[itsmoked]

Today's railcar puzzle:

We have a 1hp 120V 8A two-ended DC motor on a car that was ENTIRELY 120VDC. The car has to be converted to AC and - with the exception of the aforementioned motor. Replacing this motor with an AC equivalent would be a nightmare of epic proportions. It's the size of a 25hp 3ph motor and has non-standard mounting bosses.

When 120VAC is fed to this motor it turns about 10% its rated 1750RPM in the correct direction while making lots of noise.

So. What do you guys suggest to get this motor running correctly with 120VAC?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Are you going through a simplistic rectifier to get it to DC?

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[wayne440]

How about a 125VDC rectifier/charger as used in utility substations? 10 and 20 A are typical ratings and the surplus market might yield one at a reasonable price.

 

[Skogsgurra]

Pulling legs? What does the commutation look like?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

Jeff; ? We are doing nothing now. The car was run from batteries charged by an axle generator.

Thanks Wayne. Essentially you're describing a transformer/rectifier filter scheme, as a charger would probably freak at not seeing any batteries as a load.

Just rectifying 120Vac would get us ~160V which would be too much. I can't think of a transformer that's readily available that would give us ~80V that we could rectify.

Gunnar: Pulling legs? That swished over my head. No clue what you're referring to.

Commutation? It has no less than four brushes and not more than eight. We can only see it thru a small access hole.


Do repulsion-induction motors run on pure DC?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[wayne440]

Kludgy, but how about a Variac in front of a common 120v/120V isolation transformer, and then rectify the output? Hook it up, turn the Variac until you have 120VDC, done.

 

[jraef]

My comment was based on this:

When 120VAC is fed to this motor it turns about 10% its rated 1750RPM in the correct direction while making lots of noise.

Surely you were not connecting AC directly to a DC motor were you?

And I know, DON'T call you Shirley...

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

Wayne: That's where it looks like we're heading.. These motors probably heat up a lot with pulsating DC so I'd need more than transformers and diodes.

Jeff; Yes, certainly, someone, did try that. Universal motors have brushes too! (Can't say I've ever heard of a 1kW universal motor.) Hopefully the ancient insulation didn't stroke-out. I'd probably have heard about funny smells and blown fuses when they did it, if it didn't like it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

I can't even find a 120Vdc power supply. The 'SSNR' (Search Signal to Noise Ratio) causes all loss of signal. Enter "120" and you will be swamped with every 120Vac appliance ever conceived of.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[wayne440]

http://www.mdspower.com/utility/majorps+.html

 

[ScottyUK]

Keith, HP have some monster dc thyristor supplies which periodically appear on ebay. I think you're probably better off looking for a dc controller though. They turn up from time to time too - an old spindle drive from a worn out cnc machine might be a starting point. Have you considered using the outer case of the motor to 'hide' a modern machine? The guys who run showman's engines quite often use this trick to use a modern alternator on an antique steam engine.

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If we learn from our mistakes I'm getting a great education!

 

[OperaHouse]

120 RMS AC raw full wave bridge rectification should be the same as 120V DC. I would put a small oil filled cap on the output like 10-20uF just foe spike absorbsion. Don't think the pulsing would bother the motor. I would consider adding a small 1KVA 24V buck transformer to lower the input voltage and upping the capacitance to 100uF or so after the rectifier. Do any switching on the AC side.

 

[waross]

The field inductance will do a lot of filtering for you. Add a free wheeling diode and it will do more. Hysteresis losses at a given ripple will depend on the quality of the iron. You may have to add capacitors to the field circuit. Don't forget "kick" protection for the field. A free wheeling diode AND surge suppression may be considered. This probably has solid iron field cores. The armature will be laminated and much less susceptible to hysteresis heating than the field.
Starting will be a greater problem than ripple. This will be almost a dead short if started DOL. With some design ingenuity you may be able to use the transformer impedance to limit starting current. A series reactor on the AC side may help.
A 120:48 Volt transformer as an auto transformer will give about 85.7V.
(120V + 48V = 168V... 120V / 168V = .7143.... 120V x .7143 = 85.7V)
If you use a series reactor you may be able to size it to drop the voltage closer to the 80 volts that you want.
How heavily loaded is this motor? Can you accept a little extra heat? External fan?
You may consider two rectifiers and a slightly higher voltage to the field. You may then use field strengthening to slow the motor down a little to shed some load if possible. A separate field supply will also allow you to add capacitors to the field circuit alone. The value may be an order of magnitude less than that required to filter both field and armature current.
I would be investigating the possibility of having a good motor shop clean, dip and bake the rotor and field.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Thanks all!

Looking like a lotta hassle here.

How's this for odd: The two end-shafts are both over two feet long!

Now I find out the car has a Gen-a-motor on it. An axle driven over-riding clutch started spinning the generator at speeds over 28MPH. It charged a bank of batteries that could carry the car for quite a while. When pulled into a station the car was plugged into shore power which would run the same generator with a 240Vac motor.

A recent "upgrade" was to remove the batteries and install Head End Power which is a 460Vac ~5kA buss that runs the length of the train. They installed transformers to run the Gen-a-motor. Now they want to add new refrigeration (240V 3ph) and they've be running a 120Vdc Amplidyne to AC power the lights and outlets. The transformers are not cutting it as they're TOO SMALL and they're certainly not going to support new refrigeration. Their proposed solution was to ACize the aforementioned fan motor and switch everything to AC and dump the Amplidye and the Gen-a-motor.

I'm suggesting they just upgrade the transformers to correctly sized ones - which will avoid the need for creating DC for the fan motor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Yes, Smoked. I thought you were kidding. "We have a 1hp 120V 8A two-ended DC motor" and then trying to run it on AC. That's definitely a (bad) joke.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

I have had a few customers like that. Start by going cheap on a good idea and then call you when the slip hits the fan. Probably cheaper to have called you first, but what is, is.

Bill
--------------------
"Why not the best?"
Jimmy Carter