Quality and Service Since 1936

Compressor Motor Overload Protection

Internal, External and Why it is ALWAYS Recommended

Before we begin on the titled subject understand that this article will focus on why we at General Air Products recommend a motor starter for overload protection with every unit. If you would like to know more about what a motor starter is please view our motor starters. You can also see what happens to a motor that is not protected with a motor starter by clicking here.

Troubleshooting Air Compressor Motors

Overload protection for motors goes by many names, they all mean the same thing. Here are a few of those terms:

  • Magnetic Line Starter
  • Line Starter
  • Overload protection
  • Motor Starter
  • External Overload Protection
  • Manual Overload Protection
  • Internal Overload Protection

You can also count on seeing these terms used interchangeably. This causes a lot of confusion in the field, hopefully the list will help you understand that they are all the same thing.

Now, to the actual article!


Motor Starters

Motor Starters

General Air Products' IEC Line Starter is a rugged, industrial designed, full voltage, non-reversing IEC Rated overload protector. Learn More »

As stated above, we recommend an external motor starters for overload protection to be installed with every one of our units even though some of our units have overload protection built into the motors (internal overload protection). While there is some redundancy we have a very specific reason we make this recommendation.

Riser Mounted Air Compressor with Motor Starter for Overload Protection Riser Mounted Air Compressor with Motor Starter for Overload Protection. Note the blue button which is the "Manual Restart Button"

Let’s step back a minute first to address the larger question of “Is a motor starter required on every dry pipe system air compressor?” The answer is yes. The code that requires overload protection for dry pipe system air compressors can be found in article 430 of NFPA 70 (2008 edition) well as the National Electrical Code.

Now that we have established the requirement for overload protection you should understand the difference between the two types of overload protection offered from General Air Products – internal overload protection at the motor and external overload protection via the motor or line starter.

(For a breakdown of which of our compressors come with internal overload protection and a guide for choosing the right overload protection for the compressor model you are using, see Motor Overload Protection Selection Sheet)

All of our single phase oil less air compressors and our single phase lubricated air compressors below 1.5 HP have internal overload protection at the motor. In these cases an external motor starter is not required as internal is sufficient to satisfy code. However we still recommend using the external overload protection in these instances.

Both internal and external motor overload protection work the same way. When the motor starter senses an electrical problem the overloads (which are coils located within the motor starter) heat up and break the electrical connection to the motor, thereby protecting the air compressor from the type of damage shown in the picture below.

Air Compressor without overload protection

Internal Overload Protection

In the case of internal overload protection when the overloads cool off the motor in the air compressor will restart automatically. If the voltage issue that caused the original shut down is still present the motor will be shut down again. This constant starting and stopping of the motor will continue until the problem is addressed or the air compressor’s motor dies an untimely death. The chances of the problem being addressed are slim as the only alarm that will go off will be the low air alarm and that will not occur until the compressor is dead (in most instances). So even while the internal overload works properly you may never know about the problem until the compressor motor is burnt.

External Overload Protection

External overload protection addresses this issue with a manual restart button (the blue button on the box shown in the picture above). You see when there is an electrical issue with an air compressor where external overload protection is employed the compressor will not restart when the overload contacts cool as it does in the internal overload protection.  In fact the compressor will not restart at all until the manual reset button is pressed - this is the primary and most important difference in the function of internal versus external motor overload protection. With external overload protection, at the time of the electrical issue, the compressor will stop and the issue will have to be addressed by a fire protection professional.

This is why we recommend external overload protection (or an external motor starter) with each of our air compressors in a dry pipe sprinkler system – the problem must be addressed and the life of the air compressor will be preserved!

7 comment on “Compressor Motor Overload Protection”

  • Pressure Pipe

    Thanks for your blog. I just landed up in your blog and I really appreciate your blog. It is full of resourceful information.

    Reply
  • RAFAEL DUNGO

    chiller screw type compressor always trip after few minutes indicate the problem in screen Compressor motor overloaded and high temperature.what is the solution.tnxs

    Reply
    • ray

      Sorry Rafael, our expertise is not with those types of air compressors. I suggest you go back to the chiller manufacturer for assistance.

      Reply
  • Darren Holmes
    Darren Holmes July 29, 2015 at 2:04 am

    Hi, can you please tell me how to install a overload switch into my air compressor motor? My new motor dosnt come with one, but I do have a new overload switch which has two brown wires comming from it, but I don't know where to wire it in to my motor, the motor just has a ground wire and + and - wire, as well as 1 start Compasitor and 1 run compasitor. Any help would be great.

    Reply
    • ray

      Hello Darren - please email Geoff at gpridham@generalairproducts.com with your details, he or one of our Tech's will be able to assist you.

      Reply
  • SchumakIndia

    Thanks for explaining detailed information about air compressor want more details.

    Reply
  • Thompson

    my screw type compressor shutdown on motor overload protection

    Reply
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