Snubber

I would pick a resistor that passed a peak current that is about the same as the highest peak current the contactor draws. Unfortunately, that number may be hard to come by. But you might estimate it as several (say, 3) times larger than the rated coil current. So if the contactor coil current is rated to be about 0.1 amp, I would pick a resistor that is the peak line voltage divided by 0.3 amps or 240*1.414/0.3=1.1k, so lets say, 1k ohms. That way, if the contact happens to close at peak voltage, the contact won't have to deal with a resistor inrush current much greater than what the relay will draw within a half cycle.

Most RC arc suppressors you can buy have lower resistances in them, most from 220 to 47 ohms, but most are intended for loads heavier than a relay coil.

Ideally, the capacitor value is based on the inductance of the coil and its energy storage (proportional to the peak current squared), but too large a capacitance is safe (will absorb the inductive energy at a safe voltage peak), as long as it doesn't overheat the resistor. So lets say you use a

1 watt resistor (more the peak voltage rating or 300 volts than anything else) and pick a capacitor that will dump a half watt into it. by W=I*I*R, 0.5 watt=I*I*1k, so I=22 milliamps. If that current is limited by the capacitor across 240 volts, that implies a capacitive reactance, by Xc=1/(2*pi*f*C) and Xc=240/0.022=10700 so C=0.25 uF. I would probably go with a 0.22 uF, 600 or 1000 VDC capacitor in a pinch, but the right kind is one rated X1, for across a 240 volt AC line. This sort of thing:

I hope that helps you (and I didn't hit a wrong calculator key).

--
Regards,

John Popelish

100F in series with a .1 uf 600v cap maybe? You could use just the cap across the terminals how ever, if the cap becomes fully charged and then you induct even more, it could short the cap. Using the R in series helps with a slower charge.

"

Yep, there is switch bounce and the motor has stray inductance. Your easiest option is a surge suppressor type device, like MOV or ZNR varistors. In the old days, a resistor-capacitor pair with some size adjustment to the inductance was recommended, but that's too much like engineering...

Would still be .1 µF.

-- Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85.

Michael A. Terrell Central Florida

"Jamie" wrote in message news:5qTAj.14$ snipped-for-privacy@newsfe06.lga...

I use self-contained snubbers that are 100 ohms in series with 0.1 uF for contactor coils, and that seems to be a good combination for most sizes.

At 240 VAC 60 Hz, the resistor will dissipate about 8.8 mW with 9.4 mA.

I did a simulation with LTspice for a 240 VAC coil with 5H inductance (which draws 130 mA or 31 VA sealed-in). This is probably about right for a good-sized contactor rated at 40 amps or so.

Under best conditions, if you apply the voltage at a waveform peak, the inrush current is 180 mA peak, which is the same as continuous waveform. In actual proctice, however, the peak inrush will be much higher (perhaps

5-10x) because the inductance is much lower when the clapper is separated from the electromagnet. If the circuit is opened near the zero current point, there is just some ringing but no voltage overshoot.

Now look at worst case conditions. If you apply voltage at the waveform zero crossing, you get a peak current of 355 mA, and a "DC offset" of coil current that persists for several cycles. This illustrates why zero-crossing triacs are not the best for inductive loads. But the worst condition is turn-off, when the relay opens at peak current. There is a

1.25 kV peak, and ringing at about 220 Hz for about 35 mSec until the peaks are reduced to 600 volts.

You can play with this simulation and find a better value for the snubber. I was surprised that the model for a voltage-controlled switch does not allow conduction in both directions, so I added a second switch in anti-parallel. But the simulation appears very realistic.

In my circuit breaker test sets, I am switching primary currents of several hundred amps at 480 VAC into highly inductive loads, so you can imagine the transients that can occur. But we use a phase-delay fired SCR switch which limits the initial surge, and it continues to conduct until the current is nearly zero, so turn-off transients are not bad. But there can be problems when the circuit breaker trips and the load instantly changes from nearly a short circuit to an open. It is very common to see arcing on breaker contacts.

Paul

================== LT Spice ASCII File Follows ============================

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1.001 5 1.50 5 1.501 0 2 0) SYMATTR InstName V2 SYMBOL sw -16 -96 M90 WINDOW 3 -27 190 VLeft 0 WINDOW 0 26 16 VLeft 0 SYMATTR Value MySwitch SYMATTR InstName S2 TEXT -88 496 Left 0 !.tran 2 startup TEXT -392 344 Left 0 !.model MySwitch SW(Ron=.1 Roff=100Meg Vt=2 Vh=-.5 Lser=10n Vser=.6)

I tried again with a 0.47 uF capacitor and a 270 ohm resistor, and the transients are now limited to 600 volts peak. The snubber power is only about 1/2 watt (44 mA), and peak current will be limited to 360/270 = 1.3 amps. The worst case current for the contacts will be when they first close into the initial inductance of the coil, which could result in 3-4 amps of inrush current. And the most damage may happen as the contacts bounce on closure. But the snubber will greatly reduce the arcing upon opening, which is really metallic ions transferring from one contact to the other, or onto the surrounding insulation, causing the familiar metallic deposits that are seen in relays with pitted contacts.

There are some trade-offs with puttong the snubber across the coil (as I normally do), or across the relay contacts. In this case, when energized, the snubber draws an additional 44 mA, but the total load on the supply is only 85 mA, while the relay coil itself draws 129 mA. So the snubber actually helps the power factor and reduces load on the supply. Placed across the switch, it will apply a constant leakage current to the contactor coil, which could be enough to cause humming, and an unwanted potential when it is supposed to be deenergized. But in other cases, such as for SCRs, it is customary to put the snubber across them.

A better solution might be to use a solid state relay (triac or SCR) which can be driven directly from the microcontroller. You can get them for inductive loads, but even the zero voltage turn-on types will not cause too much problem for a motor load. Of course, a soft-start full featured PWM controller is best, but probably overkill.

Paul

(snip)

I can only guess that all these currents are coil currents. What is the worst case snubber current at turn on? It should exceed 3 amperes, right when the contacts are bouncing closed. For such a light load, I think a 100 ohm snubber resistor is well below the optimum compromise for contact life.

--
Regards,

John Popelish

The problem with MOVs and ZNR varistors is that they have a limited lifetime, and deteriorate with every surge they absorb. As they begin to fail, they get more leaky, and conduct at lower voltages, until they overheat and explode. You must put a fuse in series with them, and when the fuse blows, you no longer have protection. There are some zener type TVS protectors that do not deteriorate, but RC snubbers are still the best for controlling transients. The ideal solution, however, is to eliminate the transients by using solid state controls such as triacs and SCRs.

Paul

This is true, but the surges one usually associates with varistors are lightning-strike kinds of events, much higher currents than a little spark on a motor-start relay.

In this duty, I'd expect a long useful component life.

There is some confusion, though, on the snubber function: this calls for a snubber or varistor across the MOTOR leads, not across the relay contacts (which would result in running the motor if it fails short) nor across the winding (winding current wasn't arcing, as I read the situation). The varistor, if it fails short circuit, pops the circuit breaker just like a shorted motor would.

This is true, but the surges one usually associates with varistors are lightning-strike kinds of events, much higher currents than a little spark on a motor-start relay.

In this duty, I'd expect a long useful component life.

There is some confusion, though, on the snubber function: this calls for a snubber or varistor across the MOTOR leads, not across the relay contacts (which would result in running the motor if it fails short) nor across the winding (winding current wasn't arcing, as I read the situation). The varistor, if it fails short circuit, pops the circuit breaker just like a shorted motor would.

It is not totally clear whether the arcing was on the contacts of the small relay being driven by the microcontroller, or the larger contactor that controlled the motor. But I think it was the small relay, and for that I think the snubber across the contactor coil is best. A motor is a reactive load as well as a regenerative load, so it will produce a high voltage spike if the AC current is interrupted at a peak, but it may also tend to act as a generator and try to inject voltage back into the supply. A snubber would function somewhat as a dynamic brake.

MOVs should be protected by a fuse much smaller than the fuse or circuit breaker of the mains supply, as they usually have fairly small leads, and if allowed to absorb the full short circuit instantaneous trip current of a mains breaker (say 150 amps), at full voltage (240 VAC), that is 36 kW, which can do a lot of damage even in a few milliseconds. The MOV protector should be something like a 1 amp fast blow fuse.

If the motor will be turned on and off only occasionally, there may never be a problem, but for repetitive use, deterioration of the MOV is inevitable, and it is not that difficult or expensive to use a proper snubber (or better yet, solid state phase control).

Paul

I am really overwhelmed at the many responses my question evoked. Thanks to all of you for giving me some leads and I am looking forward to the weekend on my workbench toying with what I have learnt.

Thanks again to all of you

ClueLess

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Yes, arcing is on the small 12 volt relay. This relay is the totally enclosed variety and for testing I cut away the outer casing so the contacts are visible. There is arcing when driving the contactor coil and I wanted suppress that.

Thanks for your interest and help

ClueLess

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