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Author Topic: Switchs x Contactors x Relays Luciano Brigite Master Film Handler Posts: 277 From: Sao Paulo, SP, Brazil Registered: Jan 2002 posted 03-19-2003 05:12 PM                     Hello all, I'm on my way to finish the wiring of one projector I'm rebuilting and I'm in doubt about what to use to control it's motor. The simples way would be a switch but since it's a 3 phase motor the idea of having a set of 6 wires very close to eachother doesn't really pleases me. As per request,the kind of switch I'm going to use doesn't seem to be available in the 3 pole version then I'll have to put there a relay or a contactor,since it's only a motor about 1/3cv ( or something.. someone yanked off the specs plate of both then I'm judging by the size)running on 220V 3phase what should I use without adding some extra expense to it? I'd stick with the switch on this but it's not my projector and the guy already sent me the switchs he wants on it. nice 16amp, 2 pole .. square! switches ...it takes only a few minutes to install a round one.. it's just one hole.. but takes at least an hour to fit a square one in it's place . Also.. is it worth spend some extra $$ with contactors or relays to power up motors,rectifiers or anything else and use some fancy back-lighted color switchs with them or keep things simple and use some "buff" switchs? ( at least for motors and fans.. most of the rectifiers I know comes with a contactor somewhere inside them) Thanks  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 03-19-2003 06:07 PM                     I'm thinking "Contactor". Essentialy, we're simply talking about a 3-Pole, Single Throw (SPST) relay that's specially built to withstand the kind of voltage, current and duty cycle that a load like that would produce. A relay is a lighter duty device and often can't withstand the constant duty cycle that a movie projector would need. Also, contactors are designed to minimize (or shield) arcing between the contacts when they engage or disengage. Simple relays may or may not be, depending on the type. You can also get solid state devices which work just like a mechanical contactor. Most people prefer mechanical devices, though. Anyway, the contactor you would use would have 3 poles (or sets of terminals) to hook the projector's power lines through. There would be a fourth set of terminals through which you pass a voltage to energize the magnetic coil and pull the contacts shut, thus energizing the motor. Simple, really. What I might suggest is putting a kill switch or an emergency stop into the system. Hooking up (or leaving a provision for) a failsafe would be a good idea too. A simple way would be to hook the coil in series with the failsafe circuit and an on/off switch. Be careful that the failsafe can take the voltage you're putting through it and KEEP taking it for at least two hours. Better, would be to use a 2P2T relay and set it up to "latch" itself into the closed position when the coil is energized. You would hook a N/O pushbutton switch up to the first set of contacts on the relay and through the coil. Pressing it would energize the coil and close the relay. Once it is closed, the other "throw" of that "pole" is used to keep it energized. In series with this is your failsafe circuit. The other set of contacts is simply used to energize the coil of the three phase contactor. Pressing the pushbutton switch would attempt to close the relay but ONLY if the failsafe is closed/engaged. If it is, the relay sends voltage to the contactor which does its job and closes the circuit to the motor. If, at any time, the failsafe opens/disengages the relay "unlatches" and stops the motor. Essentially, before we got our automation system this is what we did to run our projector. Ours uses a single-phase motor, though. One thing we did was use a 12 volt DC power supply to operate the Kill/Run/Start switches and the latching relay. That way we didn't have to worry about the 120 volt mains voltage overheating a contact, a switch or a wire and causing a meltdown mid-show. You can get contactors that "latch" themselves shut if you like that idea. It's a bit less complicated but more expensive. Your average 3-phase contactor will cost $20-$50 (U.S.) A latching contactor will cost more than that. ($50-$100??) Solid state relay/contactor will cost between $50 and $100. Parts, wires, switches and relays will cost you $20 or so, depending on how resourceful you are.  |  IP: Logged John Hawkinson Film God Posts: 2273 From: Cambridge, MA, USA Registered: Feb 2002 posted 03-19-2003 08:58 PM                     Umm, Randy, generally it's a bad idea to use latching relays for failsafe purposes -- the problem is that latching relays can get stuck in the latched position. That's why the opposite of a latching relay is a "failsafe." It's much better to wire your NC failsafe in series with the coil of the failsafe relay. --jhawk  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 03-19-2003 09:18 PM                     You're right. I get this stuff bass ackwards sometimes. I have the right picture in my mind but when I go to explain it, things don't come out right.  |  IP: Logged Luciano Brigite Master Film Handler Posts: 277 From: Sao Paulo, SP, Brazil Registered: Jan 2002 posted 03-19-2003 09:58 PM                     Thanks for your input Randy, The wiring system is pretty simple, what is messing things up is the owner of the thing..he didn't even asked me for a parts list or anything and sent me a box full of switches and other stuff he wants to be used in his projectors mostly because he thinks they looks nice.. the switchs aren't crappy stuff but none of them are 3 pole and I'm thinking that using a contactor just to power a motor is somewhat a waste of part and $$ because it won't draw more than 3, maybe 4 Amp. then the idea of using a relay to power the motor came to mind instead of a contactor. Initial duty cycle for them will be about an hour on, one hour off ( initial setup is changeover but will change to single projector with tower later) then will change to a full show on,lets say 2 hours on, 15~20 minutes off .failsafes will come later with the tower as it's part of the tower system. Now I have a bunch of parts i'll have to find a way to use them just because the owner wants it there and is complaining that has spent a lot more than he thought on this .. And.. don't think that a latching contactor is a good idea on a failsafe system too. don't want one sticking closed on me. already had a not so good experience with a common one sticking closed and not opening after the coil were powered off and it had to be the rectifier one, doing that on the last show, late at night  |  IP: Logged Randy Stankey Film God Posts: 6539 From: Erie, Pennsylvania Registered: Jun 99 posted 03-19-2003 11:15 PM                     One thing you need to do is ensure that all three phases of the motor get turned on/off at the same time. I suppose you MIGHT be able to use 3 single pole switches to just turn the motor on or off but all three of them have to work as a team. If, somehow, only one or two of them got turned on but not the other(s) then you would have trouble. If you must have a "switch on" and "switch off" arrangement, you should probably just hook a switch up to the coil of a contactor/relay.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 03-20-2003 01:54 PM                     Since it is a 3 phase motor you will require a timming relay and 3 slow start resistors or otherwise the odds are you will rip the gear train apart on start up  |  IP: Logged Luciano Brigite Master Film Handler Posts: 277 From: Sao Paulo, SP, Brazil Registered: Jan 2002 posted 03-20-2003 05:36 PM                     Well, using the switchs to power up the coils was what I did. just hooked the switchs in series with hte contactor's coil. the hard part was to convince (sp?) the owner to buy two extra contactors and show him his idea wasn't going to work.. Now Gordon pointed something I forgot to think about.. the slow start.. I remember seeing the resistor setup on Cinemeccanicas for that purpose and then later a modification on a vci 8 using the resistors ,relay,timer and contactor to power it up at distance. The motor I have on this porjector doesn't seem to start at full speed when powered up but I'll need to check it better. I'm without 3 phase power since I had to move to another place to work on my stuff and i'm testing it with a "3rd phanton phase" created using some capacitors. are these slow start resistors a requierement for all projectors with 3 phase motors? what values should I use for them? If I'm right, I'll need to hook them in series with the motor then use a timer and relay/contactor ( ugh.. another) to bypass (short)these resistors, that's it? I'm starting to dislike the 3 phase thing again.. and this time it's not because any wire will make my hair rise in case I forget to open the breakers and touch any wire .. it makes things more complicated than they should be. I'll go on vacation after all this is done for sure  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 03-20-2003 10:22 PM                     Those capactiors are robbing the motor of starting torque somehwere around 30-40 ohms  |  IP: Logged Philippe Laude Film Handler Posts: 79 From: Longueville, Belgium Registered: Jul 99 posted 03-21-2003 03:02 AM                     Gordon, capacitors , 30-40 ohms ?!  |  IP: Logged Luciano Brigite Master Film Handler Posts: 277 From: Sao Paulo, SP, Brazil Registered: Jan 2002 posted 03-21-2003 08:26 AM                     Philippe, The capacitors Gordon was refering to are the ones I used to create a phanton 3rd phase in order to perform some tests since I had to move my stuff to another place and no longer have 3 phase power available . The 30~40ohms are values for slow start resistors to be used in order to prevent the motor from starting up at full speed and damaging anything inside the projector (breaking gears and all that) I believe that 50W wirewound resistors are ok for this since they'll be in use only for a second or two.  |  IP: Logged

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