Overload protectors keep motors from burning out when under prolonged heavy loads. That
could be due to a jammed conveyor, a bad bearing or even too many rapid starts or forward and
reverse cycles. Pretty much anything that could increase the winding and motor temperature.
While it is typically sold as an add-on accessory, you should consider it indispensable and inexpensive
insurance. And of course -- it's required by code.
An overload coupled with a contactor gives you the beginnings of a true motor starter.
And they are easy to use. Here is an overload for a Fuji contactor. You just insert it on
the load side of the contactor and tighten the screw clamps and then set the current
where you want it to trip. The load wiring now connects here at the output of the overload.
To test it, just press this button, the overload will trip its contacts, and it will set a
little indicator in this window. To reset it, just press the blue button here.
It's important to understand that the overload doesn't cutoff power to the motor -- it just
opens and closes its own contacts. For example, this Fuji unit has a Normally Open and Normally
Closed contact. Wire the normally closed contacts in series with the contactors coil to cutoff
power to the motor. Wire the normally open contacts to the PLC so it can monitor the
status of the overload. On most overloads, you will see an option
like this where you can set the overload to Hand operated, or Automatic. Right now this
is pointing to the hand operated setting and to rotate this dial to automatic you actually
have to put a screw driver in this little slot and break off this little tab. That should
be a hint that normally you will want to keep this in hand operated mode. That way the overload
can't automatically reset itself while you are working on the machine. If you do want
to do an automatic reset, then break this little tab, depress the blue dial and twist
it to lock it into automatic reset mode. Here is a WEG Overload. It's the same thing.
You clamp it in and dial in the trip current you want.
Because the WEG overload blocks access to the Contactors A2 terminal and the aux contact,
those are brought out here so you can still access them.
Also, resetting the WEG mini-contactor is a little different too. There are TWO hand
modes and TWO automatic modes. H Mode and HAND Mode both reset the overload after it
has tripped when you press this button. The difference is if the overload hasn't tripped,
HAND mode will toggle the overloads contacts so you can test your systems response to an
overload fault by pressing this button. That's really handy. But it could also be dangerous,
so you have H Mode which doesn't do anything when the overload isn't tripped. You would
use H mode if you want to be sure the overloads contacts can't be toggled during normal operation.
In AUTO reset mode, the contactor automatically resets so pressing this button just toggles
the contacts so you can test the systems response to an overload fault. In the "A" mode, pressing
the button doesn't do anything because the overload automatically resets and the button
doesn't have any effect on the contacts. Again, you will normally want to use one of the manual
reset modes and avoid using the automatic reset mode so your machine doesn't start itself
back up unexpectedly. Here's an overload for a GH series contactor.
Same thing. Clamp it into the GH series contactor and dial in the current you want it to trip
at. The Overload contacts on this one are over here and here's the reset button.
The Eaton overload is a little bit different. Instead of dialing in the current you want
it to trip at, you plug in little heater packs here. You just select the module with the
trip point you need. Otherwise it is still the same animal.
Here's an overload for a larger Fuji contactor. Again, it's the same thing, just bigger.
Overloads also typically detect phase loss, so if one leg of your power circuit goes down,
the overload will trip on that too. Does that mean you can't use overloads on single phase
motors? Not at all -- just be sure to run one of the lines in series through two legs
of the overload like this. As you can see, these are all basically the
same - they just have different shapes and styles and the contacts are located in different
places. Keep in mind that while overloads and circuit
breakers and fuses are all system protection devices, they are not the same thing. Overloads
protect the motor from prolonged heavy loads and take time to trip. Circuit breakers and
fuses detect things like short circuits and react very quickly.
You really need both to properly protect your system.
If you need any help with overloads, please call AutomationDirect's Free, Award Winning
tech support during regular business hours. They will be happy to help you out.
And don't forget to checkout the forums. They are not monitored by AutomationDirect's Tech
Support so don't post any support questions there, but there are a lot of experienced
users out there that love helping others.