Tech Tip: 3-Phase Motor Reversing with Contactors at AutomationDirect

To reverse a three phase motor you just swap any two power leads and the motor will run in the opposite direction. To reverse a 3-phase motor using contactors, you just run one set of wires straight through one contactor, and a parallel set of wires through another contactor where you swap one set of wires. When this contactor is engaged, the motor turns one way, and when this contactor is engaged it rotates the other direction. You can swap either the line OR the load wires -- not both, of course -- but most folks seem to like to swap the wires on the load side of the contactor to keep things clean and consistent. That looks simple enough, but there's a problem: what happens if you accidentally engage BOTH contactors? Well, now you're shorting out two of the three phases of your power system. That usually makes bad things happen. The way you prevent that is to add interlocks. The quickest and easiest way is to add a mechanical interlock like this Fuji module. When one contactor pulls down one side of this interlock device, the other side gets locked up and can't be puled down. When the other side is pulled down, the original side is locked up. If the coils energize at EXACTLY the same time, the two sides will bind and neither will activate, but the odds of that happening are slim because usually one side slips ahead of the other. To install this mechanical interlock, you just insert it between the two contactors. This Fuji Interlock module even comes with a spacer that locks the two contactors together so the interlock stays coupled to the contactors. Now when one contactor engages, the mechanical interlock prevents the other contactor from moving. And when the other one is engaged, the original contactor can't move. Perfect. The other way to interlock contactors is with an electronic Interlock. You add a normally closed aux contact to each contactor and wire them like this. So if this contactor is active and this aux contact opens, there is no way coil power can get to the other contactor. Likewise if this guy is active, coil power can't get to this contactor. Do you need both types of interlocks? Depends on your application. For example, the National Fire Protection Association code -- which the National Electrical Code is a subset of - does requires both kinds of interlocks when contactors are used for opposing motion. UL508 on the other hand requires one or more interlocks. And since they are so cheap and easy to do, why wouldn't you do both? It's an inexpensive way to protect your valuable equipment. Can you wire all of this yourself? Sure, but why would you when you can get prewired reversing adapters like this that will do it for you for just a few bucks? There's a parallel wiring one for the line side and a line swapping one for the load side. They give you a nice clean solution and you don't have to worry about getting it wired correctly and messing something up. You just insert them, tighten the screws and you are done. For the Fuji family of contactors, select motor controls, Fuji, and Fuji accessories, reversing kits. There's one for the line side and one for the load side. There's three sizes for the line side and three sizes for the load side and another three sizes for the load side if you're using an overload. You'll see why in just a minute. This type is for setups where you are NOT using an overload. They're nice because they do the whole job with one molded part you just slide in, but they get in the way of adding an overload. For example, I can't add an overload here because the reversing module gets in the way. So if you ARE planning to use an overload, you'll need a kit like this for the load side -- the parts are flatter so they don't get in the way of the overload. Notice how the middle one sets above the other two and that these mount in the lower holes so the over load can mount in the upper slots. Of course you'll have to add the aux contacts and extra wiring yourself to implement the electronic interlock with this type of contactor. One suggestion -- make sure you wire your coils first -- the screw driver can't reach the terminals after everything is installed. The WEG Mini Contactors on the other had, have a really slick way of doing all of this in one fell swoop. Look under Motor Control, WEG Mini Contactors, WEG Accessories, Component type, Wiring Kit. Look, there is only one part number. The reason for this is the reversing kit works for ALL three pole WEG mini contactor sizes with or without an overload. Why are there so many posts on this module -- shouldn't there be just three for one contactor and three for the other, like we had with the Fuji part? This one has 8 leads and this one has 10! What's that all about? Well, since the 3 pole WEG contactors have the aux contacts built in, all of the wiring for the electronic interlock is built into these modules. If you look at the wiring diagram on the reversing kit documentation, it looks just like the diagram we had earlier -- doesn't it? Except now, we don't have to add any aux contactors or do any of the wiring ourselves. You do need to make sure you get contactors with a normally CLOSED aux contact. The mechanical interlock is provided by this module which just snaps in here. If we put all of this together -- don't forget the little tabs that lock the contactors together, they come with the interlock module - and add in an overload, then look at this -- we have a complete reversing motor starting solution with both mechanical AND electrical interlocks all in one clean easy to setup, inexpensive configuration for the 7, 9, 12 and 16 AMP, 3-pole WEG Mini contactors. Awesome. Well, that ought to be enough to get you up and running with contactor reversing kits. Reversing kits for other contactor families will just be a variation of the two versions we looked at here. If you have any questions, please don't hesitate to call Automation Direct's FREE award winning Tech Support during regular business hours. They'll be happy to help you out. And don't for get the forums. There are a lot of folks that hang out there and love to share their years of experience. Keep in mind that AutomationDirect's support staff doesn't monitor the forums every day, so don't post questions to AutomationDirect support on the forums -- they may not see them.