Reversing Motor Contactors

[The Great Sarducci]

All,

I am currently in the process of trying to redesign a DC motor reversing circuit due to an essential downsizing of my existing distribution panel. The motors controlled in the panel are 48Vdc/9A each. Our current design includes using side-by-side contactors, which switch the motor poles depending on which contactor in the pair is pulled-in. The problem is that I am limited on space in my new panel, and would like to know if you guys know of any other more compact method of doing this? The most important thing would be performing the same function in a smaller space. With a total of 6 Allen Bradley contactors (3 motors total), its just too much for my new panel.

Thanks,

Ryan

 

[gar]

160413-1309 EDT

A single DPDT relay will switch polariity, but it won't remove power.

If you combine a SPST relay with the DPDT, then you can eliminate the problem of switching the motor while power is applied with the DPDT relay, and a relay like a Potter & Brumfield KUP in a 10 A rating should provide long life. Then you select a SPST relay designed to switch the DC motor load for on-off control of the motor, and to allow changing the DPDT state when no current is present.

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[Jraef]

160413-1309 EDT

A single DPDT relay will switch polariity, but it won't remove power.

If you combine a SPST relay with the DPDT, then you can eliminate the problem of switching the motor while power is applied with the DPDT relay, and a relay like a Potter & Brumfield KUP in a 10 A rating should provide long life. Then you select a SPST relay designed to switch the DC motor load for on-off control of the motor, and to allow changing the DPDT state when no current is present.

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The 10A rating on a KUP relay is 33VDC max, for 48VDC it drops to 0.5A.

TGS, what AB contractor are you using now? If it's the 500 Series NEMA style, you can get an IEC style (100Series) that will be smaller, and you can get one that has 2NO / 2 NC power contacts for DC field reversing. If that's what you already have, it won't get much smaller than that.

 

[gar]

160413-1956 EDT

Jraef:

The reason for the DC specification you provided for the KUP is based on switching under load. I specifically stated that that the KUP relay state would have to be changed when the load was off. The switching off an on of the DC load would be the function of said DC rated SPST relay in series with the supply to the KUP contacts.

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[Jraef]

Gotcha. I mentioned it only because it's really common for people to read PART of the info on a relay, i.e. the 10A rating, and not read on to see that the DC rating is far lower depending on the voltage. I should have known you would cover that detail, which you did. Mea culpa... :ashamed:

An A-B 100-C09 contactor is rated 20A at 48VDC, roughly a 2" x 3" footprint for each one, not really much bigger than an ice cube relay with the base.

 

[gar]

160414-0031 EDT

Jraef:

Your A-B suggestion is probably much better than mine.

Switching DC is brutal on contacts. There is a unidirectional transfer of metal from one contact to the other forming a conical hole in one contact and a conical mound on the other.

This was always a problem on automotive breaker points. Also on voltage regulator contacts. Many different contact materials were tried on the regulator contacts to try to improve life. As I remember a 1000 hours was a typical lifetime. The voltage regulator relay was the primary problem because it cycled many times per second. By comparison the current regulator relay did not operate much. Resistor damping was applied across the contacts, but that still did not prevent metal transfer.

To solve the breaker point problem there is an issued patent, circa 1940, in the names of Henry Ford and Emil Zoerlein that alternated the direction of current flow in the breaker points each contact cycle. Thus, metal was trasferred back and forth rather than in a single direction. The patent having both of their names means it resulted from an interactive discussion between the two men. Zoerlein would have created the actual design detail. Zoerlein's office was immediately adjacent to Ford's in the old engineering labs with a door between the two. They collaborated on many subjects. This idea was never put in production. However, later, 1950s, Ford Motor made breaker points with a center hole in one contact to reduce metal buildup on the contact with the hole.

Zoerlein designed the ignition sysytem for the first Ford mono-block V-8 engine. This engine development was done by just a few men in extreme secrecy in the Edsion Fort Myers lab in Greenfield Village. Zoerlein had to devise a means to test the engine output in this lab. I believe a steam engine was used as an air compressor for the load. Since the Menlo Park machine shop had a dynamometer it would seem that that would have been a better choice, but I only have meager information from Ralph Zoerlein, Emil's son.

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[shull]

The DC motor controller should have a forward and reverse input for the purpose of controlling the direction of your motor. A small control relay with 1 common set of no/nc relay contacts should work. Use the NC contact for forward and the NO contact for reverse. Energize the relay and the motor will reverse. Check out your motor controller wiring diagram. This is best for your motor because you let the DC Controller handle all the heavy work. Energize your relay and the motor should de accelerate, stop and accelerate back to requested speed in reverse. If you are using a "M" contactor, use it for Emergency Stops, and keep it energized the rest of the time. Hope this helps. Ron

Sent from my 831C using Tapatalk

 

[The Great Sarducci]

Although we do have a DC motor controller to handle normal control of the motors, the circuit that I am working on is strictly an override application that bypasses the controller, applying direct line power to the motors to bump them in whichever direction is needed. I should probably also note that these are high torque / low rpm servo motors. Unfortunately I am not at liberty to discuss any specific detail of the application, or any of the actual motor specs. I'll admit though, that I am having one hell of a time finding a SPDT or even DPDT relay which is rated for the inductive 10A DC current that I need. Again, my main design constraint at this point is panel space. If space weren't an issue, the original heavy-duty dual contactor arrangement of before would have worked fine.

To some of the previous posts;
The contactors that are currently employed are AB Bulletin 100 series. I would also like to thank the original post for the suggestion of using a DPDT in conjunction with a SPST. I am definitely working this idea out to see if I can incorporate it into my circuit. I drew it all out and found that it will be a good compact solution to my original dilemma. I just now need to figure out a suitable SPDT or DPDT (which I prefer so that I can have feedback circuits to my PLC).

I appreciate everyone's help so far. Thanks.

 

[gar]

160414-1519 EDT

The Great Sarducci:

Put a shunt reverse biased diode in parallel with the input uni-directional voltage to the DPDT direction switching relay. Thus, on the output side of the SPST relay. Also put a similar diode on the input side of the SPST relay. This will eliminate arcing at the SPST contact. Then you can probably do the on-off switching with another KUP relay identical to the direction switching relay, making them common parts. The on-off relay can have its two poles paralleled.

You need external logic that is different to control the sequencing of the relays which is a somewhat different arrangement than what is required for your present two relays.

The diodes are to prevent arcing from inductive kick. There is still counter EMF from the motor, but that is not a arc source. Use an extra pole on the on-off relay to short the motor armature with a resistor for dynamic braking.

If space allows use Jraef's suggested AB relays.

Whatever you decide create a mockup of the circuit and run some sort of accelerated life test. Possibly cycle direction every few seconds. If you can get as short as 1 second, then that would be 86,400 cycles per day.

Another possibility for the SPSY relay is a liquid mercury relay.
See http://www.wolfautomation.com/produ...rcury-relays?gclid=CKythof0jswCFZSEaQodi44BHQ .

The P&B KUP is available as a 3PDT relay.

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[Jraef]

I've never seen a Servo Motor that can run without a servo drive...

 

[gar]

160415-1244 EDT

Jraef:

What did you mean by this statement?

I've never seen a Servo Motor that can run without a servo drive...

Any DC brush type motor can run by applying a DC voltage to it with sufficient current to overcome losses and load.

Same is true for a DC brushless or AC servo. Here it is only necessary to provide a multiphase AC voltage of the desired frequency, and again adequate current.

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[Jraef]

160415-1244 EDT

Jraef:

What did you mean by this statement?

Any DC brush type motor can run by applying a DC voltage to it with sufficient current to overcome losses and load.

Same is true for a DC brushless or AC servo. Here it is only necessary to provide a multiphase AC voltage of the desired frequency, and again adequate current.

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Right. So if it's a true brush type DC servo motor, applying DC to it directly might make it into an armature controlled DC motor, but with no control of the armature current, it might run away. If it is a PM DC motor with straight DC applied to it, it is no longer a servo; you would have no control of the armature current, torque, speed etc. Starting current on a PM DC motor what was designed to be controlled as a servo might become so high that it would demagnetize the armature (been there accidentally, it's no fun). If you applied straight 48VDC DC to a multi-phase brushless DC servo motor, it is not going to run at all.

Maybe this is just something I've never seen or even contemplated, I don't know.

 

[just the cowboy]

Crydom

Crydom

Crydom makes a solid state reversing relay for thatpurpose about 3"x4". Part # DP4R60D20 it is 48VDC13A motor load works great only problem is 4.5-15vdc control so you need 12vnot 24v. Order plug separately.

 

[gar]

160415-1721 EDT

just the cowboy:

The Crydom reversing contactor looks like a good solution for the application. A datasheet is at http://www.crydom.com/en/products/catalog/d_p4_r.pdf .

How much above the 48 V maximum voltage limit can the contactor operate needs to be determined. I assume the logic control is isolated from the load, but that needs to be determined. Acceleration and deceleration is fixed by the model number. A simple voltage regulator can provide the logic voltage from the motor power supply, or that may already be available from the control source.

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[gar]

160416-0900 EDT

Jraef:

A wound field armature type DC motor will never experience permanent demag. The same motor with a PM field is probably good to about 10x overload without demag. A DC armature type motor with low field intensity will runaway. You don't need a controller to prevent demag, just some current limiting.

DC brushless motors are really an AC synchronous motor. Thus, appropriate AC excitation will make them run.

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