info on selecting starting resistors for series field DC motors

[eeinpa]

Curious if anyone has applications info (probably historical in nature!) on choosing starting resistance values for series field DC motors. I am sure GE and Westinghouse once had notes on how to do this.

Eaton, SquareD (EC&M), Hubbell are all willing to quote controls for these as well as resistors, but they aren't very forthcoming about the resistors they will supply. I would like to (1) understand how they were selected, and (2) be able to go to other vendors for competitive bids.

If you can point me to a recent Electrical Machinery book, I might be able to find it. Unfortunately, the large state university near me has pretty much forgotten what rotating machinery is! An application note from a manufacturer might work better.

Thanks!

 

[JERRYROBINSON]

I would suggest calling Huntingdon Electric, If they are still in business. They used to specialize in the split case steel mill motors which were usually series wound.

Huntingdon Pa. 814-643-3921.

 

[eeinpa]

I know Huntingdon Electric Motor Service (Huntingdon, PA) quite well! It's their number that you have listed. But they are not Huntingdon Electric in Indiana that makes grid resistors. That Huntingdon Electric may be found at

http://www.heiresistors.com/.

Their Milwaukee Resistor division still does grid resistors, but no applications info on their website. I have calls in to various manufacturers, and hopefully one will yield results.

Other companies have some info on these resistors, but so far I have not found an application describing how to determine values yourself. Basically they all say to call them for design assistance.

Thanks for the suggestion.

 

[waross]

It's been a long time since I did this.
The first step is fairly easy.
At rest, the armature circuit has virtually zero resistance and zero back EMF.
Decide what you want the starting current to be and select the resistor accordingly.
Use Ohm's law.
Example:
Supply 250 volts.
Limit current to 750 amps.
Resistance = 1/3 Ohm.

As soon as the armature starts to turn, back EMF will start to reduce the current.
Second step, Pick at speed to connect the second step resistor:
Lets use 30%.
At 30% speed, the back EMF will be close to 30%, so the effective voltage will be close to 70%. If we use the same example and still limit the current to 750 amps, the example becomes:
Effective supply 250 x .7 = 175 volts.
Limit current to 750 amps.
Total resistance = 0.233Ohms.
A tap at the .233 Ohm position of the 1/3 Ohm resistor will work.
Third tap, let's use 75% speed.
Back EMF will be 75% so effective voltage will be 62.5 volts.
Limiting the current to 750 amps at 62.5 volts will take 0.083 ohms.
A 1/3 Ohm resistor grid with taps at 0.083 Ohms and at 0.233 Ohms should work.
This is not exact but it should get you in the ball park.
Sources of error:
The resistance of the armature and the comutating and series fields. If we are limiting the starting current to about 300% this resistance should be small in comparison to 0.333 Ohms. The current will be slightly less than calculated.
At full speed the back EMF is less than the applied voltage by a few percent. This will tend to make the actual effective voltage slightly more than the calculated effective voltage. This will result in a slightly higher current than calculated.
Remember though that the motor is accelerating and the back EMF and the effective voltage are both changing as the motor accelerates.
The loading of the motor will affect the calculations more than these sources of errors.
The current calculations for an assumed speed of 75% should be correct at somewhere between 70% and 80% actual speed.
This should give you some ballpark figures to double check the suppliers submissions.
Or, take the values of resistors available and work backwards with Ohm's law to determine the current at any chosen speed.
respectfully

 

[eeinpa]

Thank you! This is super! Brings back a simpler, quieter past, when things could be done without a computer, doesn't it?

Talking to one of the apps guys at PowerOhm yesterday, he said it is typical to pick a starting torque of say 50%, calculate resistance accordingly, then cut out resistance by halving it at each step. Naturally he said the values may not work out precisely for the resistors you have.

PowerOhm has a Motor Control Solutions document (

http://www.powerohm.com/appnotes.php)

with tables of resistor "classes" for various applications which list duty cycle and starting torque.

Thanks again for the clear example.