Motor Generator Type

[unclesyd]

For many years our site used motor generator sets to supply DC to approximately 60 DC motors, from 1.5 to 30 hp, used to drive PD pumps, mainly gear pumps. At times we had to call for a reduction in throughput on a process line by requesting that the MG room give us 75%,50%,or 25% until requested to resume 100%.

I need to know the type of DC generator that would be used for this type application. The motor was connected to generator by a 4 speed gear box to accomplish this. The MG sets were built in the 40's and 50's.

 

[waross]

The output would be reduced by reducing the voltage. You may reduce the voltage either by reducing the field strength or reducing the speed. Field strength reduction is the method that I am familiar with, however the gear box suggests that speed reduction may have been used in your instalation. If these are self excited generators, A reduction in speed will also give a reduction in field strength. Both of these effects will cause a reduction in output voltage, so reducing speed may give a disproportionate reduction in output voltage on a selfe excited machine.
I suspect that they used the gearboxes to reduce the output voltage and used a fixed exciter generator to excite several generators.
respectfully

 

[unclesyd]

Appreciate the reply.
There reason for the post is that a friend bought the last 5 sets and immediately sold the motors to mine in New Mexico. He has a party interested in the generators who wants to know the generator type. The gear boxes and generators were in storage when someone stripped the large thick cast bronze nameplates to sell for scrap.

The gear box did mechanically reduce the generator speed. As post as 50% speed reduction we would get 50% process throughput of polymer. A 1000 cc/revolution pump would put out 500 cc of polymer at the 50% level. The response was instaneous. At the time this speed change was done by an operator at the generator. As all motors on one line were driven off this generator so they all responded the same.

At 75% we still made a saleable product. Anything below we couldn't maintain pressure control any everything went to scrap.

Not mentioned before there was a small generator driven by the AC motor on the input side of the gearbox or on the outboard side of the AC motor, which may have been the exciter.

I'm also trying to contact one of old DC electricians to get additional information, if nothing but who made them.

 

[edison123]

It seems to be ward-leonard system of speed control, quite famous in rolling mills. The DC generator is separately excited and the field excitation is used to vary the generator voltage and hence the dc motor speed.

Since this system controls normally only one DC motor (for the mill), I am not sure how it will work on a group of DC motors (unless all these motors require the same speed control simultaneously)

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[waross]

My first suggestion would be to check the data on the motors if this is possible. This will give you the operating voltage. Then add the motor currents. The generator current rating should be more than the total motor currents.
If any control gear is still extant try to locate an overload device for the generator. If you are able to determine the setting this will give you an accurate indication of the allowable current
The small generator should accompany the generator(s), or be replaced with a static exciter.
The generator will be a constant voltage, seperately excited machine.
This will give you the voltage and a minimum current rating.
More information is possible but more difficult to obtain.
respectfully

 

[unclesyd]

edison123,
All motors motors with the exception of 2 30Hp motors operate the same. The two 30 hp motors had some type in line speed control as they were control by pressure required at the spinnerets.

waross,
They only protection I was aware of was for the 48 1.5 Hp motors. The circuit breakers were in a very panel box a the end of the machine. As I recall they were marked 15.

I know for sure that there is still some documentation on site if I could get someone out of the office to check around.

Just a though would it be worth it if interested party could send his DC expert to physically look at the generators? Would they be able to tell any thing about the output without the nameplate or again maybe who made them?

 

[edison123]

As for the generator voltage, the nameplates of the motors connected to the generator should give an indication.

As for HP, the frame size could be an indicator. It is an old motor and so the rating in those times was probably very conservative.

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[unclesyd]

He trying to get the motor information from the outfit the sold the motors to. The company has been through several different owners and he again having a time trying to find anyone on the other end that can located the purchased motors.

 

[waross]

Hi edison123
The Ward Leonard system controlled the output voltage of the generator. There is no reason that the variable output voltage could not be sent to more than one motor. You could compensate for speed droop on a group of motors if the motors had approximately equal loading. If one or more of the motors in the group is subject to varying loads, you may have some droop issues. However, in most instances it should work well for multiple motors.
In this instance it sounds as if the exitation was held steady and the generators driven at different speeds by a mechanical transmission to develop the different output voltages. It probably would have been cheaper to change the speed of the exciter (with a much smaller transmission) to get the same effect, and cheaper yet to apply Ward-Leonard circuitry to the existing equipment and save the cost of the transmissions.
respectfully