BRX Series PLCs (Stackable Micro Brick) Videos

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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)