DC Shunt Motor Question

[podobing]

Hello All,
We have an elevator application that uses a low speed (55 rpm), 100 HP DC shunt wound motor. We have a lot of problems with contactors in the shunt field circuit that fail. These contactors are being used for required testing purposes. The contact tips burn severely when the contactor opens. The shunt field current is 30 amperes, and the voltage is 120 volts. We are looking for a replacement contactor. Most DC contactors specify an L/R ratio that they can operate within. I am curious what would be the approximate inductance of the motor shunt field? Would it be close to 500 mh, 1H, 5H or ??

We are currently using a freewheeling diode in the circuit, but the contactor must still open at some point in time.

Thanks,
Dave

 

[waross]

Has your diode failed?
Is there any resistance in the diode circuit?
Is the diode connected in the proper part of the circuit? (Across the field terminals, not ahead of or across the contactor contacts.)
Is the contactor failing completely or just looking very bad.
As I remember some of the early contactors, the contacts would be burned and have "grape" clusters on them. They were designed to function in that condition. Trying to "dress" or clean the contacts so that they looked good was a waste of time and money.

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

 

[podobing]

Bill,
The diode is fine, and was installed as a means to protect the contact tips. The system is working properly now (with the diode), but we would like to find a properly rated contactor to do the job without the diode (for political reasons). The original contactor was never rated for this job, and now we would like identify a properly rated contactor for this job. Unless you get into the mill duty contactors (which will physically not fit in the enclosure), most dc contactor ratings specify an L/R of less than 15 msec. (for example). From the data that I know, R is approximately 4 ohms (120 volts/30 amperes). However, I have a suspicion that L is much larger than the 60 millihenries required for proper operation of the example contactor specifying an L/R < 15 msec. I am just trying to get an approximate handle on the level of inductance that we are dealing with.
Dave

 

[waross]

When an inductive circuit is opened, the induction tries to maintain the current, in your case 30 Amps. As the resistance across the opening contacts increases, the voltage rises to try to maintain the current. With fast opening contacts the voltage can reach very high levels. The current tends to burn contacts and the high voltage may damage the field coil insulation.
With a freewheeling diode the voltage doesn't rise and the current safely decays to zero in about 5 time constants.
Good engineering would suggest some form of arc suppression on a contactor interrupting 30 Amps to a highly inductive load.
Can you avoid the politics by using a self protecting solid state switching circuit?
When politics and engineering collide, good engineering generally loses.

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

 

[melspuds]

podobing,
Check the arc suppression of the arc chute for the contactor. If in doesn't an arc chute then that is your problem. The arc vaporizes the tips (contacts) and after "n" amount of breaking the load operations the tips will be worn to not able to work correctly. Also misalignment of the movable arm is also problem caused by arcing. Check the alignment, replace the tips, the arc chute, and try again. Do not look at the arc, its like a welder arc and keep your hands away also for burns. If unrepairable I suggest changing the to a different contactor. I believe the best DC Contactor for your problem is a EC & M Class 7004 Line-Arc. Google www.ecandm.net were formally Square D until square d was brought by Schneider.
I have used them for about 42 years with 35 in steel mills. IF they hold up in steel mills and other large dc equipment they should work well for you.
Good Luck,
Dave

 

[ScottyUK]

The way this problem is handled on generator field windings is to use a two pairs of contacts operating in antiphase such that as the main poles open the auxiliary poles connect a field discharge resistor into the circuit providing a safe method to dissipate the stored energy. By careful selection of the resistor the discharge time can be minimised.

 

[podobing]

Guys,
Thank you all for the information, as it is all good information to have and use as necessary.
But does anyone have a feel for how much inductance is in the field winding?
In my mind, this information is what is needed to select the proper dc contactor, or the proper discharge resistor.
I know that I am asking for something that probably no one can answer with the limited information that I provided. I was hoping for a ball park number. Is it closer to 500mh?, 1H, 5H, or ??
Dave

 

[waross]

Apply an AC voltage and measure the current. (120 Volts @ 50 Hz or 60 Hz should be good.)
Calculate the impedance.
Calculate the resistance. ( The shunt field current is 30 amperes, and the voltage is 120 volts. )
Use Pythagoras to calculate the Inductive reactance in Ohms.
Transpose the formula: X[sub]i[/sub]=2pifL to calculate the inductance in Henrys.
There should be zero or very little voltage at the brushes.
After a motor or generator rebuild, the initial brush position may be determined by applying 120 Volts, 60 Hz and moving the brushes to the null voltage position. This test may cause hysteresis heating of the pole pieces if left energized for extended periods of time.
I have never heard of excess heat developed during the time required to set the brushes but I wouldn't leave the AC connected while I took a long lunch break.

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

 

[podobing]

Bill,
I never thought of using AC to do the measurement. I was focused on DC, and once I get focused, it's like putting blinders on a horse.
What you suggest makes a lot of sense. I'll have to give that a try.
Thanks,
Dave

 

[waross]

You're welcome. Glad to help.

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

 

[melspuds]

Bill,
I forgot about that way to calculate the impedance. Thank you!
Dave