Zero crossing power switch - Page 1
Forum section: electronics / projects

author: Rachie5272
author_rank: Regular Contributor
timestamp: April 11, 2014, 06:01:05 pm
content: My table saw uses a single phase capacitor run AC motor, but has a woefully inadequate power switch. Despite being double pole and rated for 15 A, the contacts have welded themselves shut on me twice. Needless to say, it's a very bad thing when deadly spinning metal things can't be turned off.I ameliorated the danger by building a triac circuit. Now the switch contacts only see milliamps running through them, and the actual switching is done by a beefy heatsinked triac. This works great, but there's one minor problem: the power can be switched on when the AC line is at maximum voltage. This means the motor suddenly gets hit with a burst of current, instead of slowly ramping up over a quarter period.How can I build a switch which only activates at a zero crossing? Lots of optoisolators have zero crossing detectors built in, but for some reason triacs never do. I thought about using said optoisolator, but I don't need isolation, and it means I would need to rig up a separate power supply. Solid state relays seem to have the same problem.Any idea what to do?

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 11, 2014, 06:20:32 pm
content: can you connect an zero crossing opto isolator to trigger a TRAIC?What do you mean, you need more voltage rails doing this? I think its the common method of doing it.http://www.nteinc.com/specs/3000to3099/pdf/nte3097.pdf

author: Marco
author_rank: Super Contributor
timestamp: April 11, 2014, 06:27:32 pm
content: You can just buy zero voltage crossing SSRs (generally made with two SCRs rather than a Triac).

author: Fank1
author_rank: Regular Contributor
timestamp: April 11, 2014, 10:39:16 pm
content: You are getting way too complicated.Get a 40 A rated relay and switch it.Follow the KISS principle.

author: dannyf
author_rank: Super Contributor
timestamp: April 11, 2014, 10:42:05 pm
content: QuoteGet a mechanical switch from HomeDepot and flip it on whenever you feel like it, including at zero crossings.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 11, 2014, 11:03:26 pm
content: With the Zero crossing SSRs, they seem to measure zero crossing on the voltage.What if you have a lagging load, ideally you would switch on the zero crossing on the current.Are there any that do this?

author: bronson
author_rank: Regular Contributor
timestamp: April 11, 2014, 11:26:48 pm
content: Are you talking about switching on or switching off? Sounds like the OP is worried about switching on, so there's no current.To the OP: the original switch didn't have a zero crossing detector did it? Typical AC motors couldn't care less where in the waveform you turn them on. They have enough inductance that they'll force a ramp onto whatever you give them.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 11, 2014, 11:45:07 pm
content: QuoteQuoteI was talking about switching off, and I thought the OP was too.Still my question remains, how much does switching off on a voltage zero crossing help on a severely leading lagging load? Can it get the the stage where it actually hinders?

author: Marco
author_rank: Super Contributor
timestamp: April 11, 2014, 11:59:30 pm
content: Quote from: HackedFridgeMagnet on April 11, 2014, 11:03:26 pmThe zero voltage detection is irrelevant for switch off, the SCR's will switch off at zero current.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: April 12, 2014, 12:52:16 am
content: QuoteOk that's what I was confused about.I only looked at one data sheet it said.QuoteI took that to mean they switched on a zero crossing of the voltage. Obviously Zero crossing of the current makes much more sense.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 12, 2014, 01:34:22 am
content: I love the whole zero crossing thing... how to avoid playing Russian roulette with your designs.

author: calexanian
author_rank: Super Contributor
timestamp: April 12, 2014, 01:46:20 am
content: How about a small motor contactor? The are not very expensive and can be had with either 24VAC, 12VDC, or 120VAC coils. 30 amp is a pretty easy find. Also you can just use a couple poles of a 3 phase Fwd/Stop/Rev lever switch. Those are beefy.something like thishttp://www.ebay.com/itm/60-Amp-Drum-Switch-Forward-Off-Reverse-Motor-Control-Rain-Proof-Reversing-60A-D-/170710854649

author: Rachie5272
author_rank: Regular Contributor
timestamp: April 12, 2014, 07:30:01 am
content: To clarify, I was talking about two different problems. First was with the crappy old mechanical switch which could be shut off at peak voltage/current, resulting in welded contacts. I fixed this with a triac, which has no contacts to arc, and inherently shuts off at a zero crossing.The triac can still turn on at peak voltage though, which dims the lights and makes the motor jump. The sawblade drive belt drive uses the weight of the motor as a tensioner, so the motor can actually jump quite a bit. This puts extra stress on the belt, and makes the whole table jerk.Sometimes I get lucky, and happen to switch it at a zero crossing. The difference is significant. No dimming lights, no jerking, and the motor spins up to full speed very quickly and smoothly. Definitely something I want to be repeatable.Another reason for not wanting another mechanical switch is the original one is designed to switch off just by bumping it.How exactly would I wire up a optoisolator or solid state relay? They require a voltage input to drive the internal LEDs, not just closing a switch. I would have to use a separate low voltage power supply, which seems crazy.

author: miceuz
author_rank: Frequent Contributor
timestamp: April 12, 2014, 08:24:45 am
content: You always can make a simple nonisolated low volgate supply by dropping voltage via series capacitor, rectifying and regulating it (you might be able to skip this step).You just have a mains rated switch to provide line voltage to this contraption that will ligth up the led in the optocoupler.

author: mjkuwp
author_rank: Unknown
timestamp: April 12, 2014, 12:50:46 pm
content: Is there any chance the start-up of the motor is related to the angle it lands at?otherwise- seems like zero-crossing SSR could work. If I were doing it I would have a couple concerns.1. how reliable is the power-off of an SSR. If it were me I would build in a relay to the circuit somehow for turn it off - or other safety rated mechanical switch.2. How much heat is dropped through the SSR - but of course this is related to the size that you purchase.I have seen others go to ebay/china for inexpensive packaged SSRsPersonally, I used bare component Sharp SSR, zero crossing and connected them to a heatsink. However! I was driving a resistive load and this is nothing at all like a motor as you know.

author: Marco
author_rank: Super Contributor
timestamp: April 12, 2014, 01:01:45 pm
content: There are SSRs with AC control.http://www.digikey.nl/product-detail/en/A2425/CC1014-ND/139499

author: turbo!
author_rank: Regular Contributor
timestamp: April 12, 2014, 03:11:26 pm
content: You do not want zero crossing for inductive load. That's the worst possible inrush. You want a peak turn on. Better option is to just use a "motor starter" as commonly used on larger machines.

author: TerminalJack505
author_rank: Super Contributor
timestamp: April 12, 2014, 03:52:14 pm
content: They make such a thing as a triac driver with zero-crossing (for example.) The output is designed to drive a triac. The input looks like an optoisolator.The datasheets mentionAC motor driveas one of the applications. Whether or not you want to switch an inductive load at zero-crossing, I can't say. I don't know why it would be a problem, personally.

author: Rachie5272
author_rank: Regular Contributor
timestamp: April 12, 2014, 05:04:30 pm
content: Another project of mine uses a big SSR to drive a bunch of solenoids, and so far it's been completely reliable with next to no heat dissipation.Is a low voltage DC circuit the only good way to drive these things? I know they can be small and simple unisolated things, but it seems rather strange to do it this way.Why exactly would a motor need to be switched at peak voltage instead of zero? I know the current in inductive loads is out of phase, but the current has to be zero when it's first starting.

author: TerminalJack505
author_rank: Super Contributor
timestamp: April 12, 2014, 05:25:33 pm
content: A capacitive power supply might be the way to go if you use a zero-crossing TRIAC driver or an optocoupler. They make TRIAC drivers and optocouplers with anti-parallel LEDs too so using one of them might help reduce component count.You're right, though, there should be (and probably is) a component that does it all in one package.

author: richard.cs
author_rank: Super Contributor
timestamp: April 12, 2014, 05:56:10 pm
content: Quote from: turbo! on April 12, 2014, 03:11:26 pmThis is correct, the zero-crossing switching is the worst possible thing to do for a motor. The current in the stator starts at zero and integrates voltage over a whole half-cycle, the flux follows, the iron saturates sometime after the voltage peak and there will be an enormous current spike as the motor inductance almost disappears. Switching on at the peak means that the current only increases over a quarter-cycle, the saturation flux is never reached and no current spike occurs.I'm not aware of any SSRs that will do voltage-peak switching but maybe they exist or a capacitively derived supply for the control part as TerminalJack505 suggests could trick one into working that way. Or just use a chunkier mechanical relay or SSR.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 12, 2014, 07:12:25 pm
content: Use a contactor with a 120V coil. Connect a hair dryer or something in series with the saw motor and use the contactor to bypass it. Connect the coil across the saw motor so that it pulls in once the motor is up to a good fraction of its rated speed.

author: calexanian
author_rank: Super Contributor
timestamp: April 13, 2014, 05:13:35 am
content: Sounds like what you really need is a soft starter.. They make motors, switches, and breakers happy. Particularly for woodworking tools that have no extra load at startup.

author: turbo!
author_rank: Regular Contributor
timestamp: April 13, 2014, 02:08:18 pm
content: Quote from: NiHaoMike on April 12, 2014, 07:12:25 pmWhy that arbitrary voltage instead of other arbitrary voltages like 100, 200, 400, 415, 575 etc?

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 13, 2014, 03:17:36 pm
content: It should have the same rated voltage as the mains. In the US, it's usually 120V. On 240V mains, use a 240V contactor.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2014, 06:18:17 pm
content: The MOC30xx series of optically isolated thyristor drivers which detect zero crossings are what I normally use for this type of thing whether galvanic isolation is needed or not.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 13, 2014, 06:32:27 pm
content: It is extraordinarily difficult to find an AC SSR (Solid-State Relay) which does NOT have zero-crossing switching.I am working on some remote-control lamp dimmers (which need random-triggering to implement phase-angle control). If you look at Ebay, 99.99% of all the SSRs for sale there have built-in zero-cross switching (which makes the unsuitable for lamp dimming, but perfect for power switching).I would NEVER use a SSR for something as hazardous as a table-saw (or anything with sharp moving blades). This is why many places still require old-school (and ultra-reliable) "motor contactors". A "motor contactor" is effectively an over-rated mechanical relay with contacts beefy enough to take the turn-off transient abuse.

author: Rachie5272
author_rank: Regular Contributor
timestamp: April 14, 2014, 08:26:11 pm
content: Well, what I have now is equivalent to a random fire SSR. I'm guessing the motor jerking actually happens when it turns on at a zero crossing, instead of a peak like I thought.How would I make a peak firing circuit? I'm thinking it would involve a diac to delay triggering the triac until the voltage gets high enough, similar to a dimmer switch circuit. I don't know how to make it trigger continuously thereafter though.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 14, 2014, 11:10:23 pm
content: It seems far more likely that "jerking" is coming from switching on somewhere WITHIN the cycle (especially at the PEAK!) vs. turning on at the ZERO-crossing. If you are controlling a motor you WANT it switch at the zero crossing, it is easier on every part of the circuit including the motor.What you want is a zero-crossing switch. By far the easiest (ant probably cheapest) is to simply use a power-mains-rated SSR (solid-state relay). These will switch at the zero-crossing by design.If you want to roll your own, you will need to add a circuit to monitor the mains power and indicate when the zero-crossing happens. Then delay your gate signal until the zero-crossing.To make it trigger continuously after that, you simply leave the DC gate signal applied for as long as you want the power to be applied.But, repeating, using an SSR for a large motor on a saw is NOT RECOMMENDED!And using a home-made SCR circuit without zero-crossing detection, and snubbing components, etc. etc. IS REALLY REALLY NOT RECOMMENDED!! It's YOUR fingers or hand at risk.Note to others: Don't do this!!!1) SSRs are readily (and inexpensively) available. Rolling your own doesn't make any sense.2) Using an SCR (or SSR) for a large motor on a saw is just asking for trouble.

author: Anks
author_rank: Frequent Contributor
timestamp: April 15, 2014, 01:46:59 am
content: A current limited or soft start is really what is called for as far as I can see. Say a cap a NTC or a bulb and a relay.

author: richard.cs
author_rank: Super Contributor
timestamp: April 16, 2014, 02:38:33 pm
content: Quote from: Richard Crowley on April 14, 2014, 11:10:23 pmCounter-intuitive though it may be, this is not the case as I explained in a previous post. An ideal inductive load (i.e. purely L and R with L independent of I) cannot have a switch-on surge because I = integral( V ) dt, the current cannot instantaneously increase. A real motor is wound on an iron core that greatly increases it's inductance, but which can saturate giving a current-dependant inductance. It's this saturation, causing the inductance to fall to around 1/8000th of it's initial value (for sufficiently hard saturation the iron "disappears"), which causes the switch-on surge as the only thing then limiting the current is the d.c. resistance of the windings and the supply impedance. The saturation occurs when the current exceeds the normal peak-running-current by some amount (perhaps 20% for a well optimised motor design).The cause of this excessive current is switching on before the peak, the zero crossing being the *worst* time because that allows the current to increase for the whole half-cycle. Take a look at this explanation, it's for a transformer but inrush on any iron-cored inductive load works like this:http://www.electrical4u.com/magnetizing-inrush-current-in-power-transformer/

author: Rachie5272
author_rank: Regular Contributor
timestamp: April 18, 2014, 04:14:45 am
content: On further inspection, I'm pretty sure the motor already has a soft start, or at least a starter winding. It has two capacitors, and the telltale click of a centrifugal switch when spinning up or down.

author: SeanB
author_rank: Super Contributor
timestamp: April 18, 2014, 08:49:45 am
content: Best is to go get a soft starter for it.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 18, 2014, 09:35:00 am
content: Just use something like this;http://www.ebay.com.au/itm/32A-Two-Pole-Double-Throw-Knife-Disconnect-Switch-/290614814007?pt=AU_B_I_Electrical_Test_Equipment&hash=item43a9fe2537&_uhb=1even if the contact "weld" you can just give it a nudge and it will break the weld anyway...

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: April 18, 2014, 05:35:43 pm
content: You don't need any sort of zero crossing circuit - just use a magnetic motor starter switch. They are purpose built for what you need. If you live in the US and have a commercial shop, you'd have to have one of these anyway, if you have fire / labor insurance.This way the equipment can't be powered on if the line cord is pulled or the power goes out. The only way to start your saw is with the green button - order one appropriate for you motor size and # phases. You said your motor has a centrif. switch, so it is single phase:http://www.grizzly.com/products/Magnetic-Switch-Single-Phase-220V-Only-2-HP/G4572You can get these from other electrical supply houses, and from Delta / Square D, but they are all the same contactor (fancy word for heavy relay with much more contact closing force) inside. Grizzly is much cheaper. Square D costs much more for the same components inside (at least that's what I got) Generally you also get a circuit breaker function - handy if your motor is stalled out for some reason.I have these on all my shop equip - table saw, milling machine, CNC gear, Lab benches, etc. and have never, ever seen one "weld shut" - if that happens, you've got some sort of cruddy contacts in the switch, or very low contact pressure. Some of these are used daily for 20 years with zero issues.These contactors will clank shut with a lot of force, and there is nearly zero current at the moment the contacts close due motor inductance - no matter what's going on in the AC cycle, but it will rise quickly. Opening the contacts will create an arc but the good thing about AC - and if you have a good contactor - the arc is quenched at the next AC cycle zero crossing. Generally AC is much easier to turn on and off at high currents than DC, and the contacts last much longer.

author: SeanB
author_rank: Super Contributor
timestamp: April 19, 2014, 11:08:31 am
content: I have regularly had contactors fail, generally as open circuit. If they do weld they are fail safe with dual contacts per phase, and the release springs are either going to open the weld or open the other contact and bend it so it will break the welded contact free. They only fail from old age or from massive overload, as in the wiring melted off type, or the coil was given 5 times rated for a few hours.

author: David Hess
author_rank: Super Contributor
timestamp: April 19, 2014, 07:44:20 pm
content: Quote from: richard.cs on April 16, 2014, 02:38:33 pmThanks for mentioning this. Sometimes I forget what I have forgot.I have never seen a switch optimized for this kind of load making me think that the inductive worst case is not as bad as the zero crossing worst case.