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 that circuit breaker so we can shut things down remotely and manage loads to make
system startup easier. We’ll need a button to turn the motor on
and we’ll want that to light up while the motor 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 protection on our contactors to protect our controllers
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 Productivity 2000 Controller to monitor and control this A/C Motor 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 8
inputs and an output module with 8 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 one. 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 really 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 number one. 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
two 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 number
two. Could we have used this built in aux contact 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 just use the aux contact module to monitor the status of the contactor.
Finally, we definitely want an overload protector and we’ll have the overload reported on
closure of the overloads normally open aux contact into input number four. 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 so 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’s not so bad …
We can monitor and control the Branch Circuit Breaker, We can enable the contactor, monitor
the contactor, protect the contactor 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 number three and we’ll want to add surge
suppression too. We’ll also add a mechanical interlock. We
already made sure 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 this wiring diagram is intended
for guidance only – It’s YOUR responsibility to insure all wiring and installation meets
NEC code, UL codes and any other local agency codes and standards that are required.
And while this has fail safes built in to protect the controller, it does NOT show how
to handle the load end of the system. You need to make sure you have all the requisite
safety relays, ESTOPS, and whatever else you’ll need to ensure your system meets the appropriate
NEC, UL and IEC safety standards. 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.