DC Motor arcing across relay contacts 1

[Cbrunner]

I'm working on a winching/hoisting application. The load in question is being raised/lowered by a spool, driven by a DC motor. I expect the motor is a simple series wound DC drive, but I don't have one here at the desk to be certain. I do know that the motor is not a permenant magnet, or three pole (only has two brushes).

My problem is in the control circuitry for this drive. The winch operates on 110v DC, and uses a reversing switch for direction control. When the control pendent is in neutral/off, the winch poles are shorted, and the power poles are open. Pressing extend or retract performs as expected.

I can't see any filtering/suppresion devices on the circuit; by meter or eye.

Using two DPDT relays, we attempted to build a magnetic reversing switch (so we could operate with 12v signal). The winch circuit breaker is rated a 7 amp. My first contactor choice was for a 20amp relay (as they were common here). We found after about seven activations the contacts would fuse. My expectation was simply a case of surge current from a "bumping" of the controls.

We stepped up the size of the relays, and exchanged them for solid state type going on the advice from a vendor. The new 50amp solid state relays are able to handle the switching; but are a bit expensive.

It has come time to design another control circuit for a similar application. Instead of forging on with the SSR's and high cost, I wanted to see if I could find the source of the arcing contacts.

Could the contact spacing on the relays just have been too small, causing the arc? Could the arc have been caused by the EMF of the motor start/stop rapidly combined with the motor being driven by the load (these units do not have any dynamic braking). Is there a combination of cap/resistor I should be using to help prevent the arcing? How do I then calculate the cap/resistor value?

Thanks for any help;
Chris

 

[BrianE22]

Do the contacts spark while running? Or only when the motor is either being shutoff or started?

 

[ScottyUK]

I would give up on 'relays' for this type of application and start looking at contactors.

Inductive DC is tough on switching devices. There are contactors designed for DC switching but they are expensive, and the ratings start at fairly large values. Often they incorporate a magnetic blow-out coil which helps force the arc up onto the arc chutes where it extinguishes. Smaller contactors sometimes use multiple poles in series to split the arc into smaller and weaker parts. That might be a path to explore.


----------------------------------

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

 

[potteryshard]

According to the original post, the hoist poles are shorted when off... That sounds like dynamic braking to me; that could be causing some of the contact erosion. Also; since this type of control is usually done with two or more relays, one has to be absolutely certain that both open at the same time. If reversing, for instance, and the forward relay is a few milliseconds behind the reverse relay, it is dumping into a still-shorted contact.

Do you have a schematic available?

 

[Cbrunner]

The relays had an interlock on them to prevent dual actuation. See poor quality schematic for wiring info.

I had the relays wired in the same configuration as the switches in this diagram.

 

[rasevskii]

If you are controlling a DC motor with contactors, then you have to use contactors with blowout coils and arc chutes. Ordinary relays of whatever size will not work as the arc will not be blown out. This is nothing new. DC contactors with arc chutes have been around since the earliest days of electricity, and were the de facto standard item in all DC motor controls until more modern thyristor drives (for example) became avialable in the 1960s.

Therefore the manufacture of such contactors has been largly discontinued, at least by the major companies.

For a supplier try Cableform, Inc. in the USA. They have to do with DC mining locomotives and tramway controls (I do not work for them)(google "Cableform) I think they have an online catalog.

rasevskii

 

[potteryshard]

If the relays are indeed interlocked to preclude simultaneous operation, that should be pretty secure, and the full-wave rectified DC shouldn't present servere switching problems.

Releasing the control will however, short across the terminals of a running motor, which will 'plug' brake it. That might be the source of your contact wear.

If positioning accuracy on stopping is important, the braking is best left as is. If accuracy is not important, the dynamic braking could be eliminated by placing a diode facing toward the negative side of the rectifier in each switch return leg.

This also may be an operator problem... Often, users of hoists like to repeatedly pulse the controls to finalize position. This is of course very hard on the contacts. I've threatened to put time delays in the control circuits to block repeated operation...

 

[rasevskii]

for Cbrunner & Potteryshard.

IMO you would still be best advised to use proper DC contactors as I suggested above, irrespective of the DC source. In fact full wave rectifier supplied DC would be the worst case (other than pure DC ).

Also a low ohmic resistor should be put into the braking circuit to limit the current in the short-circuit to a level that will not flash over the commutator, especially if there is a lot of mechanical inertia in the drive.

Another resistor may be necessary in the main circuit for the same reason.

Otherwise your motor may be smoked rather quickly...

rasevskii

 

[ScottyUK]

raevskii,

Just for future reference:

ABB and Telemechanique still make the old-fashioned bar and shaft contactors for severe duty DC switching. Siemens also make them but appear to only make NO variants, where the others make both NO and NC types. I posted some references a while ago in a thread where someone was looking for replacements for contactors in his AVR.


----------------------------------

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

 

[Cbrunner]

Potteryshard:
I'm having a little difficulty picturing the diode placement in the circuit.
Are you suggesting the connection of a diode on the negative side of the switch, to prevent the motor from "pushing" positive current when it is driven by the load? Sorry for being a little slow on this one, I've been working with this for so long I fear I have developed tunnel vision.

By the way, the solid state relays that we have been using naturally incorporate a delay between activations.

 

[Lakey]

Hello,

I’ve always understood that SSR’s are not considered fail safe for this type of application. Perhaps someone can enlighten me if I have been misinformed.

Is this a series wound motor(kW)?

What is the nominal running current, and expected ‘in-rush/start-up current? How inductive is it (or time constant)? How many switching operations should the switch be expected to survive?

As mentioned, time delays between Fwd & Rev will reduce the amount of energy seen by the contacts when they change state. Sometimes the time delay can be set to suit the rating of the contactor, or its ability to extinguish the arc.

Also mentioned, is the braking resistor - the resistor’s value can be selected to reduce the energy across the contacts. Obviously any change will effect the performance of your device…

You may find that a lower voltage rated , but higher current rated contactor would switch nicely.

Google DC Contactors….. Albright makes MB60, PC60, or SW66/DC66 contactors which may fit the bill. You could them ask them directly about your circuit etc…

Regards,

 

[potteryshard]

Yes... A diode in each switch B contact return to the rectifier; cathode towards rectifier. Normal run currents (with only one B contact closed) will not be affected by diode, but with both contacts shorted, the diodes will be facing each other and will block self-generated current.

No dispute that proper DC contactors would be better; I also like the idea of installing a braking resistor, however, that will require a somewhat different scheme. If flashing at the commutator is evident, a large MOV across the terminals may help if a resistor is not used.

None of the suggestions were intended to be 'best' practices, but instead to point out that dynamic braking was indeed in use, and that it might be contributing to contact problems.

 

[Skogsgurra]

DOLing a DC motor of any size above a few hundred watts is a very bad idea. It destroys the commutator and ruins your contacts.

With 110 V DC and controls in the 10 A range, it seems that your motor is at least 1 kW.

We do not know much about your application. You say that "I expect the motor is a simple series wound DC drive, but I don't have one here at the desk to be certain. I do know that the motor is not a permenant magnet, or three pole (only has two brushes)" and my guess is that your motor is a shunt motor.

If that is so, you have several things to consider:
1. When you connect the motor to DC, there is no excitation (takes some time to build in the highly inductive field winding) and during that time the armature current is only limited by resistance in the circuit. And that alone is usually enough to create problems.
2. Even if you apply excitation voltage before you apply armature voltage, your motor is not running and is not creating any EMF. Again, the current is limited only by resistance in the circuit.
3. There is some mentioning of breaking inductive current and that that would be a problem. Not so much of a problem compared to the first two items above. Actually, if there is a shunt winding connected parallel to armature, the latter acts as a parallel path and will reduce kick back voltage quite well.

My advice is that you get someone that knows about DC motors and controls to do the job. Fear and guesswork do not produce a good result.

At least, make sure what kind of motor you have (size, type etcetera). One tip: there is no such animal as a DC motor with three poles (or three brushes).

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

As the above venerable members have said, you cannot just willy-nilly grab a "relay" and use it on a DC circuit, especially a DC motor circuit. Just to give you an example to help make their points, Siemens makes both AC and DC contactors. An AC motor contactor that is rated for 22A at 600VAC, is rated to make and break only 0.1A at 110VDC! You MUST use a device that was designed from the outset to switch DC and at the voltage you want to use. Switching DC circuits is much harder on components than AC is, with AC the current is naturally crossing zero V 120 times per second, which helps extinguish arcs. DC does not, the arcs linger and destroy contacts unless they are designed differently (magnetic blow-outs, etc.).

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376