Functionally, a magnetic contactor is just a giant three pole single throw relay. They
both have and electromagnet that pulls two pieces of metal together so current can flow.
The difference is contactors are specifically designed for heavy loads like motors and have
lots of accessories that help make motor control easier and safer. Let's take this one apart
so we can see what is going on. If we remove this cover, we can see the power
comes in on these terminals, and when these contacts are pulled down by a big electromagnet
in the base, the power crosses over to these terminals. The electromagnet in the base is
powered over here on these terminals. Looks like this one needs 200 to 250 Volts AC or
DC to engage the electromagnet. Some contactors use as little as 12 to 24 volts dc.
This contactor also has some contacts on the side. When the main contacts are pulled down
by the electromagnet, these contacts are also engaged. You could then connect the aux contacts
to a PLC so the PLC can monitor the status of the contactor.
You can get a better view of the electromagnet on this smaller contactor. Here's the coil
of wire and it's terminals. Looks like this one uses 24volts dc to operate. When the coil
is energized, it pulls the contacts down so they can make contact and let power flow across
the terminals. When you release power to the coil, the contacts are no longer held down
by the electromagnet and the spring pushes them back up which cuts the power flow.
It's important to distinguish between a Contactor and a Motor Starter. A contactor is just the
mechanical switch and the focus of this video. You can use a contactor by itself for small
loads, but in industrial applications you will probably be using the contactor in a
motor starter which is just one of these contactors with other stuff added -- typically overload
protection as a minimum. We cover overloads in a separate video, but
in general it's just a device you add in series with the contactor to protect the motor from
loads that are too heavy for the motor to handle for long periods of time. You just
wire the overloads contacts in series with the contactors coil. When it senses overload
condition it kills power to the contactors coil, which opens the contacts so the motor
won't burn itself up. Here is an Overload for WEG mini contactor and here is one for
a Fuji contactor. Again, those are covered in the contactors accessories video.
Automation Direct has hundreds of contactors -- how do you know which is best for you?
The main differentiators are usually how much current they can handle and cost. If we plot
that we see the WEG contactor family runs from about ten to twenty bucks and cover 7
to 22 amps. The Fuji family of contactors starts at about
15 bucks and covers 9 to over 300 amps. The GH series Contactors Start at around $45
and run out to 315 amps. The Eaton Contactor family looks a little
pricy at first glance when compared to these other guys, but they come with higher end
features like a Trip Class of 20 -- which means they will handle a 600% overcurrent
for up to 20 seconds as opposed to the 10 seconds the other guys typically do- and you
can purchase the Eaton Contactors as full motor starters with the overload included.
With the other guys you need to purchase it separately and build it yourself. See the
Contactor accessories Video for more on that. So don't shy away from the Eaton just because
of the price -- if you need the Eaton brand or some of the high end features they have
- then definitely check these out. Hopefully this gives you some perspective on where the
different families sit Once you pick a family, then on the AtomationDirect.com
Website under Motor Controls, select that family. Select the Fuji Contactors -- and
just work your way down this check list to narrow your search. Let's see, before any
filtering it looks like we have 158 possible contactors in the Fuji family alone. Coil
Voltage -- we'll control it from a PLC at 24 volts. Then we can pick current capacity
or frame size. Lets pick the current we need of 32 amps, and just like that we are down
to a single contactor in this family. If you want to, you can do the same thing in the
other families to compare. Easy. Other things you will want to take a look
at are agency approvals, and durability -- that is how many mechanical operations can you
expect out of this device. It looks like this contactor is rated for 2 million electrical
contacts with as many as 1800 closures per hour that's a contact closure every 2 seconds.
The mechanicals are rated for 10million operations. So if you are only doing a couple motor starts
an hour, these things will last a very long time ...
Which is an important thing to understand about contactors. They will wear out eventually.
Each time the contacts open the current wants to try and keep going so you get an arc. That
arc creates pitting and burning on the metal contacts which eventually stop conducting
electricity. So these Durability numbers and operating cycles numbers are really important
to take into consideration. There are a number of accessories you can
get for each of these families like overload protection, surge protection, latching relay
assemblies, reversing kits, aux contacts, etc. Check out the individual videos on those
to learn more. Well, that should be enough to get you started
with contactors. If you have any questions, please contact AutomationDirect's FREE technical
support during regular business hours. They will be happy to help you out.
And check out the Forum, it's not monitored by AutomationDirect's tech support, but there
are a lot of folks out there that love helping people, so don't be shy about posting questions
there too.