DC Relay & inductive loads

[DavidHP]

Hello,

I am seeking help in specifying the necessary components to protect three DC relays while switching off a DC inductive source. The battery is 12 VDC and the motor draws up to 50 amps (until a 25amp push to reset circuit breaker pops) but normally draws from 16 to 30 amps. Also, the motor is reversible which is why I am using three DC relays.
I have found information about using an R-C network across the relay contacts and also information about using a diode across the inductive source. Being an ME I don't understand the differences between the two types of suppression and which is best for my application.

Thanks you for your help.

 

[BobM3]

Are you trying to protect the relay contacts or the device that turns the relay's coil on/off?

 

[gepman]

The RC surpressor works with AC loads. The diode surpressor will work with DC loads. Are you using the supressor on the relay coil or across the contacts?

DC contact ratings are a different animal than AC and you must base your selection of the relays on DC, not AC ratings. DC duty is more difficult since there is no zero current point to stop the arc while the contact is opening. Your contacts will last longer if you put two or three contacts in a series (they have to open at the same time), the rating of two contacts in series is essentially double one contact. The breaking (opening) rating of the contacts will be much less than the making (closing) rating of the contacts.

I think that I would use a starter solenoid for a small engine starter instead of using a relay. That is what a starter solenoid is designed for.

You are lucky that you are at 12V, I had an almost impossible time finding relays and other devices rated at 120VDC for hydropower plant control systems.

 

[DavidHP]

I am trying to protect the relay contacts. This is for a new product so I'm not trying to solve a one time problem. There are currently 60+ units in the field and from what I have read I should be protecting the relay contacts so they will last a reasonable amount of cycles.

 

[ScottyUK]

Gepman,

As an FYI, Telemecanique and ABB both have a decent range of 110V DC controlgear from small mini-contactor style control relays through to huge contactors for heavy current switching.


David,

Bear in mind that if one of the contactors sees the polarity reversal when you reverse the motor direction the diode is of no use.


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

 

[BobM3]

gepman -

You stated something I had not heard. Maybe I misunderstood. Are you saying that if you put (2) 20 amp relays in series you basically have a 40 amp relay?

 

[itsmoked]

No Bob.
As you put the contacts of a single multi-pole relay in series it gets better at interrupting a DC current because you have multiple interrupting gaps in the circuit, distributing the heating and arc suppression. Multiple relays may not work this way as they would likely open at slightly different times.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[gepman]

ScottyUK
Thanks for the lead on Tele and ABB. I eventually found some 120VDC power contactors. The hardest part is finding devices such as pressure switches, etc. that have even a 1A 120VDC inductive break rating. It really opened my eyes to the differences between DC and AC.

I thought of something that I did for the field coils of a group of three 18MW generators. This was 250VDC but it would work in DavidHP's case also. They (Cutler-Hammer) made something called a field breaker which essentially was a three pole DC rated molded case circuit breaker that had the center pole reversed (NC instead of NO) from the outer poles. The center pole was connected to the load side of the outer pole on one side and to a large (about the size of subzero refrigerator) power resistor on the other side. If the field breaker ever tripped the current from the collapsing magnetic field of the field coil would be shunted to earth through the power resistor. You could do the same by adding some additional NC contacts onto multipole relays that you are using to control the motor. It is more complicated since yours is a reversing application (you would need to wire this through a NC contact on the reversing relay) but it should work.

I think however that having a relay which is rated for the switching duty would be the easiest.

 

[dpc]

http://www.selinc.com/sel-9502.htm

 

[armandh]

like any dc inductive device the kick back on circuit opening is the problem. spike over voltage.... just what movs are designed for and they wont be polarity sensitive either. cheap. give em a try

 

[ScottyUK]

MOVs have a finite life. A diode should last 'forever' if used within its ratings.

Look for contactors which have a magnetic blow-out as part of the contact assembly. This uses a magnet to force the arc up a widening gap onto arcing horns where it weakens and eventually breaks. This technique is used on large contactors.

The discharge resistor proposed by gepman is also fairly common on high energy DC systems such as the field of a generator. Often a pair of antiphase contacts on a common shaft are adjusted so that a resistor is inserted in circuit fractionally ahead of the main contacts opening. This dissipates the stored energy quickly into a high power resistor bank.


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

 

[DavidHP]

Thanks all for your suggestions. I am about to test some bidirectional diodes.


Other than testing the relay with the diode repeatedly to see how long the contacts last, how do I know that the diode is working?

 

[jvjtech]

I am looking for similar information and found a thread that maybe OT but I think offers some intersting insights and links to external sources.
thread240-13068
Regards.