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Join Date: Aug 2010
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Burned Relays

08/09/2010 3:12 PM

We are having some unexplained issues with our motor controller and 3 phase motors.

We produce a motor controller with two 30 amp three pole relay's. There is no mechanical interlock, but only a 1.5 second delay from our logic board so only 1 relay may be engaged at anytime.

The relay's are confiqured as a reversing contactor with the jumpers hard-wired. We utilize .50-1.0 HP single phase motors and .50-2.0HP three phase motors. The highest current draw for 3 phase motors is approximately 6.0 A. and 12 A. on start. Most of the time our motors draw 1-3 amps.

We have been seeing relay boards returned with 1 or sometimes 2 relay's burned out. When I say burned out, I mean totally destroyed and char-broiled. Other times the relay's survive, but the 25 amp tracking on the board is burned off. This problem has never appeared with single phase motors where there is much higher current draw. Three percent of the units have been returned and 97% are without issues. When the controller is replaced we never see the problem again on that site.

Obviously, the first conclusion is that somehow both relay's had both engaged. With the logic board returned and a new motor controller installed we have not been able to simulate the failure even with 500,000 test cycles. Even with 2 motor lines shorted or a dropped line we do not see the failure. We recommend 10 Amp fused disconnects, but that has been hard to verify and police. Even-so why such a catastrophic when the applied current is lower than the ratings of the relay or board tracking.

Any ideas would be appreciated.

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#1

Re: Burned Relays

08/09/2010 5:03 PM

Given the 1.5 second delay, is the motor still rotating, for instance forward when the reverse relay pulls in?

Were the (failed) relays/boards operated in a high humidity or high conductive dust environment?

What is the coil voltage of the relays?

Let us know what you find.

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#2
In reply to #1

Re: Burned Relays

08/09/2010 5:23 PM

The coils are 24VDC.

As far as the enviroment, there is sometimes humidity.

You really never get a good answer for fear that blame would be placed.

Thanks for the input. We will explore the drift or momentum of the motor.

Roy

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#3
In reply to #2

Re: Burned Relays

08/09/2010 5:46 PM

Hello Roy2312.

Out of curiosity as much as anything, what sort of real quantity are we discussing? Are there 6 failed units a month?

The bad thing about an attempt at designing something that proves to be imbecile-proof or at least idiot-resistant is, we underestimate the determination and cleverness of our adversaries!

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#4
In reply to #3

Re: Burned Relays

08/09/2010 6:03 PM

GA Doorman.....for knowing what it is like in the real (my)world and making my day. thank you doorman

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#13
In reply to #3

Re: Burned Relays

08/10/2010 8:54 AM

Over the last 3 years we have had 30 returns out of 2500 units, so the average is less than 1 per month.

Eventually we would like to see 0 returns.

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#5

Re: Burned Relays

08/09/2010 6:21 PM

Could there be welded contacts on the output relay(s) from the logic board? I'm grasping at straws here, but I wonder if the inrush current to the coils of the power relays (motor contactors) is enough to overheat your logic board contacts.

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#6

Re: Burned Relays

08/09/2010 8:42 PM

Just to be clear, are these problems exclusive of one particular location?

Do these controllers share the same power supply with unfiltered (input) VFDs?

The curious detail is the problem goes away with the replacement, something is missing.

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#7

Re: Burned Relays

08/09/2010 10:43 PM

From the small amount of information you've given it is clear that these motors can convert some considerable electrical energy into mechanical energy. So from what I see here this leads to a few possibilities for these relay contacts to be toasted.

  • The wiring information you provide do not indicate clearly how these motors should be wired and when the back EMF from the motor might overload an opening contact. Many assembly documents can have an inadvertent ambiguity in them that maybe causing a few of your customers are miswire your assembly.
  • You and your customers have found the hard way a few counterfeit relays in your assembly.
  • The relay manufacturer has made a subtle change in their product that has made it unsuitable for all scenarios that your motors can provide.

Now which of these scenarios will take some careful forensic or reverse engineering of the failure mode.

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#8

Re: Burned Relays

08/09/2010 11:50 PM

We are a service contractor and have seen alot of this through the years. Had the same problem with relays in our own configuration. Relays are not breaking clean(fast enough) and the contacts start to weld causing the partially open ones to arc for extended time. This causes the track to heat as well. We replaced with better relays and the problem stopped. Our preference is electronic relays when the cost is justified ( 2.5 times the cost). There are uncounted manufacturers and supliers but my first choice is idec, best quality for the money. We used a well know brand for 400 relays and 2 year failure rate was 97% really great for our reputation but we ate the relacement and have at least 4 years with no more failures on our applications.

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#9
In reply to #8

Re: Burned Relays

08/10/2010 1:24 AM

Your perseverance in tackling this problem seems to have paid off in the long run. Well done.

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#10
In reply to #8

Re: Burned Relays

08/10/2010 3:40 AM

On the coil side of the 24vdc relays do you have flywheel diodes to quench the electromatic field?

Flywheel diodes assist in dropping the control relays as fast as possible, but that is probally not the problem. The problem is probally to do with the load connected to the relays, do you know if the control circuit connected is AC or DC some DC loads can cause arching hence a slow break to the circuit, you need to ask the people you have supplied a few more questions about thier circuit.

Cheers

Joe

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#11

Re: Burned Relays

08/10/2010 4:09 AM

I repair control boards used in Fuel dispensers, (Petrol pumps) and they have 30A relays to switch 1hp motors at about 6A. The relays are always burning out the contacts and I have a box with several hundred replaced over the years. When the relay casing is opened the contacts are obviously overheating - some relays are completely destroyed with damage to the surrounding tracks and components.

This is a known problem with electromechanical relays where the contacts open and close at all points of the ac waveform causing arcing. The arcs eventually weld the contacts 'on' causing the motor to run continuously. This I suspect is what is happening to your control boards with both relays becoming 'on' at the same time!

The answer is to use a solid state relay with zero switching, costs a bit more, but saves a fortune in the long term.

If you decide to use SSRs watch out for the conducted noise caused by the thyristor/triac switching which will exceed the EMC limits and require filtering on the mains connection. If you want to use a SSR which will operate well within the EMC limits, (30db below the standard SSR) I can recommend the ones made by Root2 Ltd.

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#16
In reply to #11

Re: Burned Relays

08/11/2010 10:21 AM

I like the post!!

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#12

Re: Burned Relays

08/10/2010 8:50 AM

Common problem with mechanical relays is bounce. Basically when the relays open there is a arc which over time will cause the contacts to weld together. One solution is gold plated contacts which will increase the relay cost but, it will add some life to the circuit. Gold plated relays might get you approximately 1 million cycles. The best solution is solid state relays. Solid state relays will most likely solve your issues. Solid state relays may require a circuit redesign which will increase your upfront cost but, it will certainly increase your product cycle life and quality.

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#14

Re: Burned Relays

08/10/2010 11:18 AM

Conwaymech has a GA from me. Which brings me to the 1.5 sec delay: What is the nature of this control circuit? If the switched 24VDC coil source is capacitive, then that would explain the problem. In other words, it would cause a slow release of the relay(s) and therefore burned contacts.

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#15

Re: Burned Relays

08/10/2010 12:11 PM

As an addition to all the good answers you got allready:

Try to introduce an electrical interlock between the two relays in addition to the 1.5 sec delay.{a N.C. contact on each relay in series with the coil of the opposite relay}. This will prevent the closure of both relays in case there was a welding of the contacts or any sluggishness in the movement of the relay contact block.

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#17
In reply to #15

Re: Burned Relays

08/11/2010 10:22 AM

Good post.

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#18

Re: Burned Relays

08/11/2010 10:29 AM

I agree with many here, the contacts are welding together and you have no interlocks.

Also, when reversing a 3 phase motor, is 1.5 seconds enough to allow the motor to stop? I doubt it.

3 Phase motors starting can draw up to 8 x the running current, that is far more than the contacts are rated for. Generally 6 x is the average......but even that is more than they can take....

I have never personally tested what current flows when a running (turning still !) motor has the direction reversed but :-

a) the current is not going to be less that starting current!!

and:-

b) its going to flow longer......

I personally like interlocked SSRs.....but get them very heavily rated.....

I would guess that you need relays with a far higher current rating than 30 amps......for proper reliability anyway.....start with 60 amp rating!!

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Andy Germany (3); Bill ML (1); CONWAYMECH (1); Doorman (1); jamesw (1); Joe Sparky (1); KJK/USA (1); klearzen (1); LAA_Lucke (1); redfred (1); Roy2312 (2); tom (1); Tornado (2); Unredundant (1)

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