Motor flyback current re-engaging starter relay.  How to stop it? - Page 1
Forum section: electronics / projects

author: calzap
author_rank: Frequent Contributor
timestamp: January 20, 2016, 08:25:12 pm
content: I have a table saw in my shop with a 1.5 HP, 120 VAC motor that receives almost daily use. I was bothered by the fact that it used a simple DPST switch for starting/stopping. If it was unplugged, there was a power failure, circuit breaker trip, etc, it would start running again as soon as power was re-applied, a dangerous situation. So, I installed a starting relay in a typical self-latching configuration and retained the original switch as an enable/disable switch (see pics). Everything worked well until a couple of weeks ago. If the stop switch was pressed and immediately released, the saw restarted in 1-2 seconds. If the switch was pressed for 4-5 seconds and released, the saw did not restart.I'm assuming the spring in the relay has weakened with use, so the flyback current from the motor re-engages the relay. I'd like to get a scope picture of the event, but don't have a differential probe or battery-powered scope. I'm not about to float my Rigol, and can't float a large table sawI thought of several alternative solutions:1. Replace the relay with a new, identical relay.2. Replace the spring in the relay with a stronger spring.3. Replace the relay with a heftier relay with a stronger spring.4. Place a RC snubber across the motor leads.5. Place a resistor in parallel with the coil of the relay.6. Use a small DPST relay in a latching configuration to operate the present relay. One concern was that flyback current from the coil of the larger relay might re-engage the small relay. This could be avoided by using a relay with a 5 VDC coil powered from the switch lamp PSU.7. Replace the present relay with a 3-pole relay. The third pole would be used for latching the relay. Currently, latching shares a pole with the motor, root cause of the problem.Some of the alternatives would require a larger switch box or replacing the present relay, something I wanted to avoid. Because it would be cheap, quick, and fit in the present box, I decided to try a resistor in parallel with the present relay coil. The relay coil consumes 11 W sealed, which corresponds to 92 mA and a resistance of 1300 ohms. So, I tried a 3 kohm, 10 W resistor, which should draw 40 mA and consume 4.8 W at 120 VAC. It worked amazingly well. In fact, the merest touch of the stop switch resulted in the motor stopping more quickly and quietly than ever (subjective impression) and no self-restarts.For a permanent solution, I'll use a 2 kohm, 25 W resistor in a metal case that can be bolted to the metal box for heat dissipation. It will draw 60 mA and consume 7.2 W. I'm not concerned with the extra power consumption. Although the saw is used almost every day, it normally runs for a total of less than 20 minutes/day, and that is in multiple short bursts. The power consumption of the relay coil and resistor together will be minuscule compared to the motor.Comments?Mike in California

author: tautech
author_rank: Super Contributor
timestamp: January 20, 2016, 08:56:25 pm
content: You need to convert it to a DOL (Direct on Line) configuration.You'll need a NO contactor (relay) with 1 more pole than the conductors that supply the motor and a NO push button for Start and a NC button for stop.The 2 buttons need to be wired in series and supply current for the contactor coil when Start (NO) is pressed and the spare pole closes and maintains current to the coil that can be ceased when the Stop (NC) is pressed.Like in this Google image:Power cuts will then have no effect, it just reverts to an OFF (Stop) state.

author: 128er
author_rank: Regular Contributor
timestamp: January 20, 2016, 09:28:50 pm
content: Quote from: tautech on January 20, 2016, 08:56:25 pmThat's the classic way of doing it. In german it's called "selbsthaltung" witch translates cruedly to "self hold on" "self latching", HOWEVER . . . .There is another option which is widely used in table saws and machines like that. A On/Off push button switch with electromagnet, like the KJD17 types. These switches don't have the problem with automatic restart after power outage. And they have a small form factor and are available with a cap in an emergency switch style, which is great especially for table saws. And no need for contactors and extra buttons.You can look here for different types:https://www.e-switch.com/product-catalog/pushbutton/product-lines/kjd17-series-heavy-duty-industrial-power-pushbutton-switchesHere is the emergency switch type:https://www.carbatec.com.au/machinery-and-accessories/electrical-switches-and-leads/nvr-switch-40x74mmLike I've said, it's one more option that is available. Maybe in your case it's easier to rewire your circuit.

author: calzap
author_rank: Frequent Contributor
timestamp: January 20, 2016, 09:41:37 pm
content: Quote from: tautech on January 20, 2016, 08:56:25 pmThis would be option 7 in my original post, and in hindsight, the way I should have done it in the first place. If the parallel resistor works (it seems to), is there some reason (safety, likely to fail eventually, ??) not to continue with it?Mike in California

author: calzap
author_rank: Frequent Contributor
timestamp: January 20, 2016, 09:54:29 pm
content: Quote from: 128er on January 20, 2016, 09:28:50 pmThanks for the links. I wasn't aware of this type of switch and can see uses for it in the future.Mike in California

author: C
author_rank: Super Contributor
timestamp: January 20, 2016, 10:03:25 pm
content: Calzapcompare your circuit to tautech's for the stop case for safety.For tautech's two things would have to happen to prevent turn-off.1. The contractor or relay contacts welded closed. If this happens only removal of power stops motor.2. The Stop button broke or will not open connections. Here the stop button is only carrying contractor current, chance is small.Your circuit is not so simple.For safety circuit should be as simple as possible so less likely to fail.If you look at emergency stop buttons. You will find that when pressed latch holding button in keeping circuit open. many can be in series and when open prevent power being applied.A part of the problems is that your relay is wired wrong in your circuit. You have one connection for your motor relay connected to the motor side, of relay contacts. This connection should be connected to line side of relay contacts.You are missing one set of relay contacts with your existing relay. Just use a DPST relay to control the existing relay. One pole of new relay is power to existing relay. Second pole is new relay holding path.You have tautech's circuit but using two relays.

author: rich
author_rank: Frequent Contributor
timestamp: January 20, 2016, 10:30:36 pm
content: ^ yup, what they said.It's curious what changed to cause the new failure though - perhaps contacts arcing more than they once did? Or maybe the neutral contact failed closed circuit. I'd be interested to see the type and ratings of the relay.

author: tautech
author_rank: Super Contributor
timestamp: January 20, 2016, 11:27:32 pm
content: Quote from: calzap on January 20, 2016, 09:41:37 pmIt's been done DOL for decades, for 2 good reasons; safety and simplicity.QuoteQuite so, and any ANY chance of this happening MUST be prevented. Correct selection of the Contactor; voltage, current, motor rated and AC should be observed and when correctly done can give decades of multiple daily start usage.Why would one want to f**k with simple proven design. Apply the KISS principle and be safe.One another note; selection of the contactor coil voltage will determine where the other leg of the coil must be terminated, be it: Neutral or another phase (3 phase) but NEVER mains GND. To use mains GND would break most mains wiring standards/codes AFAIK and possibly elevate the plant to mains voltage should the Earth (GND) fail.

author: tautech
author_rank: Super Contributor
timestamp: January 21, 2016, 12:05:04 am
content: Further notes FYI:InsomeDOL starters the Start button forces the contactor contacts closed thereby not needing an electrical connection to start and the Stop function is achieved in the normal way by breaking the supply to the coil.