Snubber

Hi all

I want to start a motor remotely.

The contactor coil is rated 240 volts and I am driving it with the
contacts of 12 volt DC relay driven by a microcontroller.

The relay contacts arc and I thought a snubber will reduce the arcing.
I checked various sites and ended up lots of calculations for which I
have no proper data.

Is there a ball park value I can use for the capacitor and the
resistor?

Thanks in advance for your time and attention

ClueLess

 

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:
http://www.panasonic.com/industrial/components/pdf/abd0000ce30.pdf

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

 

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.


http://webpages.charter.net/jamie_5"

 

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

 

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

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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.

 

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

 

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