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- Part I
The Hardware: Learn how to implement a full AC Motor On/Off control system with a DoMore PLC. Includes circuit breaker, shunt trip, forward and reversing contactors, safety tips, mechanical interlocks and more. Everything you need to implement a safe motor control system.
Turning a 3-phase motor on and off with a PLC is easy enough. The PLC enables the contactor and the contactor allows power to flow to the motor. While that does work, it’s not very realistic. In most practical applications there’s a whole lot more to it. We’ll want to be able to monitor the status of the branch circuit breaker and be able to trip the circuit breaker so we can shut things down remotely and manage loads to make system startup easier. We need a button to turn the motor on and we’ll want that to light up while it is running. We’ll need a way to stop the motor and let’s have that button blink red for 5 seconds while the motor spins down and then turn solid red when it is ok to re-start the motor. Of course we want to be able to monitor the status of the overload and be able to test that it kills the power to the motor when it trips. And finally, we want to be able to reverse the motor and have all the necessary interlocks to ensure we don’t change the motor direction while it is powered. And of course, we’ll want surge suppression on the contactors to protect our controller from the voltage spikes. That’s a little more involved than just turning a motor on and off, but it really isn’t that bad. In this video we’ll review how all of this gets wired together and then in the next video we’ll program an Automation Direct Do-More Controller to monitor and control this reversing A/C Motor on off system. At first glance the wiring looks intimidating, but if we take it one step at a time … it really isn’t that bad. Our controller has an input module with 16 inputs and an output module with 4 relay outputs. And we want to use those to control and monitor a 3-phase motor on a branch circuit. We’ll start with the circuit breaker that provides us the 3-phase power. We want to be able to monitor the status of the breaker so we add an aux contact to the circuit breaker and bring that into input X0. While we could use either the normally open or normally closed contact here, we prefer to use a normally open contact so loose or broken wires will be detected while the circuit breaker is powered up. This serves as a failsafe for our circuit and is a good habit to get into. We want to be able to trip this breaker remotely, so let’s add a shunt trip which we will drive from output Y0. We can now monitor and control the branch circuit breaker – check that off the list. Next we add a contactor – we’re using a WEG mini contactor in this demo because they are so easy and convenient to use. And they’re very capable - they can handle up to 25 amps. We’ll control it from output Y1 and we’ll call that the Forward Direction. We definitely want surge protection, which for this contactor is just a plug in module, and we want to monitor the status of the contactor so we’ll add an aux contact module with a normally open contact going into input X1. Could we have used this built in aux contact that the WEG contactor has for this? Sure, but we chose this particular contactor with the normally closed built-in contact because it will make adding reversing easier when we get to that step, so we’ll leave that one alone for now and use the aux contact module to monitor the status of the contactor. Finally, we’ll need an overload protector and we’ll have the overload reported on closure of the overloads normally open aux contact into input X3 on the controller. Remember – the overload doesn’t kill the power – it just reports that there is an issue. So we’ll use the overloads normally closed contact to kill the power to the contactors coil. We want to use the normally closed contact for this because it is connected when the overload isn’t tripped, but also because if there is a loose connection or a broken wire it will also keep the motor from running. If we used the normally open contact to somehow interrupt the contactors coil, we wouldn’t be able to detect any wiring faults. So use of the normally closed contact helps make the system “fail safe.” Well, that’s all we need to control a motor in one direction. That wasn’t so bad … We can monitor and control the Branch Circuit Breaker, We can enable the contactor, monitor the contactor, protect the PLC from surges, protect our motor against overload and monitor the status of the overload. Perfect. To add reversing, we just add a second contactor and do two more things to the wiring. First, if the motor is powered through this contactor and rotating in the forward direction, we don’t want the reverse contactor to engage, that would short out the wiring and bad things would happen. And likewise, if the motor is going in reverse, we don’t want the forward contactor to engage. To fix that we modify the control wiring a little bit. Instead of controlling the forward contactor directly, we run the control wire through the aux contact on the revering contactor. So if the reversing contactor is engaged, there is no way the forward contactor be turned on. We do the same thing for the reverse contactor – run his control wire through the forward contactor’s aux contact. So if the FORWARD contactor is engaged, there is no way the REVERSE contactor can be turned on. And finally we need to run a copy of the power lines through the second contactor taking care to cross one set of wires to get the reversing action. The good news is this wiring is already done for you in these reversing kits – just plug them in. Some reversing kits require YOU to do the fail safe wiring yourself. With these WEG contactors we are using, all you need to do is run the one wire to each normally closed contact and the single neutral – the fail safe wiring is all built into the bus bars. We also want to monitor the status of the reverse contactor so we’ll add another aux contact to that guy and run that into input X2 and we’ll want to add surge suppression to the reversing contactor too. We’ll also add a mechanical interlock. We already made sure that one contactor can’t engage when the other is active by wiring the coils through the other contactors aux contact. This mechanical interlock does the same thing. When one contactor is engaged, it physically prevents the other contactors armature from moving – same function as the wiring, but this time it is purely mechanical. It adds an extra layer of protection. And having both electrical AND mechanical interlocks is required by some codes. And there you have it. A full reversing AC motor start stop control circuit with the ability to remotely trip and monitor the breaker, remotely monitor the overload, electrical and mechanical fail-safes built in to protect us from wiring shorts if the controller accidentally tries to engage both contactors at the same time and over load protection. Keep in mind that while we didn’t show it in this simple wiring diagram, proper grounding and wiring in accordance with the national and local electrical codes is assumed. Here’s a photo of the test rack and all of the parts used in this video. There’s a lot of extra wires and connectors on this guy so we can swap out controller and subject matter panels for other videos, but functionally, it’s identical to the wiring diagrams we created in this how to video. Join us in Part II of this video where we will write the PLC code to control and manage all of this. If you need more details on any of the items we are using here, check out the companion Tech Tip videos – they go into great detail on every one of these items. If you have any questions about any of this, please don’t hesitate to call Automation Directs free award winning tech support during regular business hours – they will be happy to help you. And check out the forums – there are lots of folks there that love to share their years of experience – just don’t ask and support questions there – they don’t monitor the forms on a regular basis Draft comments (Joe Kimbrell)