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Old 07-30-2014, 08:54 PM   #1
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Old School Motor Overloads Explained


Some of you sparky's will know this.....especially those who do commercial work.

For you residential guys.....you may have never seen this....

So...add this to your 'So What' file.

The more 'modern' motor control centers for motor starters typically use electronic overloads. Smaller and adjustable.

In the old days before the wazu electronic stuff, they used motor heaters. These are devices that open up a starter if one or more legs of a 3ph motor starter pulls too much amperage. This is not to be confused with the fuses or circuit breakers which have a different function....protection for high current/fault conditions.

The thermal overloads are what protects the motor when it's running slightly over it's max current....a condition that fuses would not protect.

It's rare I see one like this....99.9% of the time....the proper over loads are installed and I never have to mess with them.....today I had to install some on a system I'm starting up....so I figured it was a good time to take pics.

This is the actual overload. The strip of metal between the two tabs (holes) is a resistive that gets warmer as the current goes up. The part in the middle that looks like a small gear on a shaft is the protective part. That 'gear' is connected to a shaft that goes inside the tube. The whole mess is put together with a low temp solder...similar to what was used in the old school fire sprinklers. Melts at a much lower temp than regular solder....like around 125-150 deg.

When the current across the unit goes up...it gets hot....when it gets hot enough, the solder melts and the wheel (gear) can turn.



This is the starter with the thermal over loads installed. If you look closely you will see what looks like a metal paddle sitting on top of the wheel. In this pic the overloads are in the 'tripped' position....all the paddles are out and the over load is open (no power to the motor)



In this pic the over load has been reset. See the red handle? You push that in....it pushes the paddles back and they park on the serrated edge of the wheel. Because the solder is not melted...the wheel can't turn and the paddle stays locked in the on position (power to the motor).

If the over load see's too much current, solder melts, the paddle can now turn the wheel...and it pops out....opening the power to the motor.



When it comes to simplicity are reliability....thermal over loads are the way to go....but.....if you need to change the current setting, you have to buy new over loads. Hence, the electronic stuff is taking over.

That includes your lesson for the day.
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Old 07-30-2014, 09:38 PM   #2
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Many Overloads are bimetallic, two dissimilar metals connected on each ends, when the metal heats up the metals expand at different rates. This results in a bending of he heater and this hits a switch result in a opening of the control circuit.

Quote:
a condition that fuses would not protect.
Quote:
This is not to be confused with the fuses or circuit breakers which have a different function....protection for high current/fault conditions.
Actual fuses and circuit breakers can be designed to operate within the same parameters as motor overloads. Just not practical or cost effective.

For feeder and branch circuit protection fuses and circuit breakers offer over load protection, molded case circuit breakers are thermal, magnetic devices and fuses offer the same features with overload and fault protection

Last edited by Know A Little; 07-30-2014 at 09:43 PM.
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Old 07-30-2014, 09:52 PM   #3
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Given the choice, I always use the melting alloy O/Ls. They are time proven, and their failure rate is very nearly zero. Can't say that about electronic ones.

One thing I've noticed al lot over the years is that if the O/Ls trip, a lot of guys will simply turn up the current setting on the electronic ones. This usually results in a burnt up motor. Most guys won't replace the melting alloy ones, they'll usually figure out why it tripped.

Also, given the choice, I will always use NEMA starters (like the one in the pics above) rather than IEC type.

The NEMA stuff costs more, but it's pretty much bullet-proof. I've seen it horribly abused and it just keeps on working.

The IEC stuff is less expensive, but it'll barely hold up under normal conditions, and won't take any abuse at all.

In my experience, Allen Bradley is the top of the line, followed closely by Square D (pictured above), then everything else. A lot of this experience is from mines, rock-crushing plants, concrete plants and other 'high abuse' environments.

If an installation includes Allen Bradley or Square D NEMA, it was built with quality and reliability in mind. If it uses IEC, it was built for low cost.

Rob
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Old 07-30-2014, 10:01 PM   #4
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Micro.....yep....

It's easy to burn up an IEC if you size it wrong.

The newer, younger (no experience) guys want to use the IEC stuff because they can't figure out the FLA on a system and if they get the OL's wrong, don't want to deal with it.

The above is in a Combi Starter/disconnect setup....pretty common and cost effective box to use. I prefer the AB ver...but it's about $100 more and the customer is trying to cut costs.

If I have a system where I know for a fact what my FLA is going to be, I'll pick the thermal over loads.

Like you said, you can't adjust them too high....
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Old 07-30-2014, 10:09 PM   #5
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Another commonly overlooked problem with IEC starters is the AIC ratings which is lower.
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Old 07-31-2014, 07:02 AM   #6
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Quote:
Originally Posted by electures View Post
Another commonly overlooked problem with IEC starters is the AIC ratings which is lower.
You forgot to mention something

Another commonly overlooked problem with IEC starters is the AIC ratings which is lower, they are about 1/3 the size of a NEMA starter, the contacts are cheesy looking and they seem to burn up at a higher rate than NEMA starters
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Old 07-31-2014, 07:10 AM   #7
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Given the choice, I always use the melting alloy O/Ls. They are time proven, and their failure rate is very nearly zero. Can't say that about electronic ones.

Rob
The basic's behind the electronics is a current transformer and a electronic protective relay. Current transformers have been utilized in protective devices for a long, long time, the electronics relays since the early to mid 70's. The biggest issue is how long the electronics will last in a crappy environment were many motor starters are located.

I have tested many Bi-metallic overloads and there are issues. We test OL's in the same manner we test circuit breakers, subject the motor starter with the OL installed to overcurrent, I would have to check the standards but I believe it is 600% of the devices rating.
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Old 07-31-2014, 10:12 AM   #8
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Quote:
Originally Posted by Know A Little View Post
The basic's behind the electronics is a current transformer and a electronic protective relay. Current transformers have been utilized in protective devices for a long, long time, the electronics relays since the early to mid 70's. The biggest issue is how long the electronics will last in a crappy environment were many motor starters are located.

I have tested many Bi-metallic overloads and there are issues. We test OL's in the same manner we test circuit breakers, subject the motor starter with the OL installed to overcurrent, I would have to check the standards but I believe it is 600% of the devices rating.
Yes, CTs have been around for ages, and the larger starters with melting alloy O/Ls use them so the full motor current doesn't go through the heaters. I've seen a few of these fail mainly because they had current run through them without the heaters installed.

Also, the electronic relays usually associated with CTs are not made with low cost in mind. I've seen a few of these fail, but nowhere near as many as the small motor starter relays. It's been a long time since I've seen a melting alloy relay fail.

Motor O/Ls come in different classes of trip. Most are class 20 which means that if the O/L is subjected to 600% of its rated current, it will trip in roughly 20 seconds.

This is satisfactory for the majority of applications. I use class 10 for high strung' motors such as refrigeration compressors, submersible pump motors, IEC motors and anything else that can't take locked-rotor current for long.

Class 30 is for motors that drive high-inertia machines and need more time to accelerate.
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Old 07-31-2014, 10:33 AM   #9
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Quote:
Originally Posted by micromind View Post
Yes, CTs have been around for ages, and the larger starters with melting alloy O/Ls use them so the full motor current doesn't go through the heaters. I've seen a few of these fail mainly because they had current run through them without the heaters installed.

Also, the electronic relays usually associated with CTs are not made with low cost in mind. I've seen a few of these fail, but nowhere near as many as the small motor starter relays. It's been a long time since I've seen a melting alloy relay fail.

Motor O/Ls come in different classes of trip. Most are class 20 which means that if the O/L is subjected to 600% of its rated current, it will trip in roughly 20 seconds.

This is satisfactory for the majority of applications. I use class 10 for high strung' motors such as refrigeration compressors, submersible pump motors, IEC motors and anything else that can't take locked-rotor current for long.

Class 30 is for motors that drive high-inertia machines and need more time to accelerate.
Which also brings into play the ckt protection....fuses or circuit breakers.

You have to pick the right fuse or circuit breaker....not just trip current but also trip curve.

What you use on say electric heaters is totally different than what you would use on a motor.

It's a 'system'.

Breaker/Fuses protect the wiring.
Over Loads protect the motor.
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Old 08-01-2014, 07:19 AM   #10
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and when the thermal issue is resolved, the o/l will re-solidify and can be reset. you did a nice write-up.
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Old 08-01-2014, 09:16 AM   #11
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Ddawg,

For the younger (less experienced) guy.

Can you explain how to size the ol protection to be used in the "system" rather than just slightly over the fla of the motor?

Thanks
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Old 08-01-2014, 10:37 AM   #12
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Quote:
Originally Posted by TimPa View Post
and when the thermal issue is resolved, the o/l will re-solidify and can be reset. you did a nice write-up.
And don't be resetting them too quick. If you do it while still soft they will never reset. You must wait for them the solidify(cool down) before resetting.
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Old 08-03-2014, 05:04 PM   #13
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Quote:
Originally Posted by ritelec
Ddawg, For the younger (less experienced) guy. Can you explain how to size the ol protection to be used in the "system" rather than just slightly over the fla of the motor? Thanks
they can be sized using the heater charts that come with the starter.
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Old 08-03-2014, 06:05 PM   #14
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Ddawg,

For the younger (less experienced) guy.

Can you explain how to size the ol protection to be used in the "system" rather than just slightly over the fla of the motor?

Thanks
Sizing O/Ls is usually easy, but there are a few variables to consider.

If the motor is a basic NEMA frame, the service factor is 1.15 or more and the temperature is the same at the starter and the motor, use the manufacturers recommended heater. (The 'heater' is the melting-alloy or bi-metal part of the O/L relay.)

If the temp is higher at the motor, use one size smaller. If it's lower at the motor, use one size larger. This is close enough if the temp difference is not more than 20F. If it is more, there's a fancy formula that can be used.

For NEMA motors with service factors of 1.0, use one size smaller.

Class 20 are ok for the above motors.

For IEC, spindle, submersible well pump and hermetically sealed refrigeration compressor motors, use the manufacturers recommended heaters (along with the temperature stuff listed above), but use class 10. These motors cannot withstand locked-rotor current for very long, and a class 10 will trip in roughly 10 seconds while a class 20 is 20 seconds.

Class 30 heaters are also available; these will carry the locked-rotor current for 30 seconds and are designed for high-inertia loads (like punch presses, shears and other stuff with huge flywheels). Be careful here, 30 seconds is pretty much the limit for most motors. NEMA design D motors are made specifically for this purpose and can easily sustain locked-rotor current for 30 seconds.

Rob
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