# examples of overload devices



## erics37 (May 7, 2009)

Depends. A regular ol' magnetic coil motor starter will have an overload block built-in, or be of a modular design to which you can install various configurations of overloads.

You can also get an overload relay by itself.

If you have a VFD then overload protection is just part of the programmed design, depending on the motor characteristics you input and the parameters you set.

Are you looking for something more specific?


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## erics37 (May 7, 2009)

And typically you install overload elements in series with the ungrounded conductors supplying the motor. One per phase.


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## micromind (Aug 11, 2007)

1) A separate device that is responsive to motor current. This is typically a part of the controller (also known as the starter). 

2) A device that is built in to the motor at the factory. It can be either automatic reset or manual. It can interrupt the current to the motor or it can be a pilot device that doesn't carry the motor current, and must be connected to the control circuit. It can be either a digital device (like a thermal switch) or an analog device (like thermistors).

3) Fuses.

4) Impedance protection. This is where the motor can carry its locked-rotor current indefinitely without overheating. Typically found on very small motors. 

That's all I can think of at the moment.


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## Wirenuting (Sep 12, 2010)

erics37 said:


> If you have a VFD then overload protection is just part of the programmed design, depending on the motor characteristics you input and the parameters you set.


But if the VFD has a bypass function then o/l protection must be in place. 

Also some VFD's that come with a physical o/l block have the o/l function disabled as it may trip due to the VFD output. That style <don't remember the brand> sucks when you need to bypass. A person needs to know to physically re-enable the o/l device.


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## nmackintosh (May 8, 2008)

Thanks for the replies. Ya i came across a small single phase 208v pump think it was like half a horse with an inline fuse holder built into one leg of the cord. I figured that was used as a overload device. I have hooked up a few other small motors but never noticed an overload device on some of them hopefully they had them built in.


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## oldtimer (Jun 10, 2010)

nmackintosh said:


> Thanks for the replies. Ya i came across a small single phase 208v pump think it was like half a horse with an inline fuse holder built into one leg of the cord. I figured that was used as a overload device. I have hooked up a few other small motors but never noticed an overload device on some of them hopefully they had them built in.


 Which half of the horse?? :laughing::laughing:

Sorry, I could not resist. It just struck me funny!

B T W, I hope it was not the back half. :lol:


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## JRaef (Mar 23, 2009)

nmackintosh said:


> Thanks for the replies. Ya i came across a small single phase 208v pump think it was like half a horse with an inline fuse holder built into one leg of the cord. I figured that was used as a overload device. I have hooked up a few other small motors but never noticed an overload device on some of them hopefully they had them built in.


It's really difficult to use fuses as a means of motor overload protection. The rules on where the device opens up are often in conflict with the Short Circuit protection aspects of a fuse. So typically it's a lot easier to let the fuse be the SCPD and get a separate OL device.

In your case it's entirely likely that the motor had internal thermal switchies, the common term for them are "Klixons", which is a brand name from Texas Instruments. All they are is a little metal disc that heats up and snaps from being convex to concave, which breaks the circuit. These are typically limited to about 5A, which for a 208V motor means about 1HP. Larger than that and you go with an external OL relay and a controller.

So in addition to the types already mentioned, there are also what we call Motor Protective Switches. These are like Manual Motor Starters that come from European mfrs, they are a switch, short circuit device and adjustable OL device all in one.


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