Overload protection for air compressor

[Jpflex]

On a previous post, I mentioned wiring up a “3.7” hp 1 phase 230 volt motor/ compressor with 15.7 name plate amperes, S.F 1.0, temp rise 40 degrees, continuous rated

However, on most air compressors with a single phase motor attached in which I came across never seemed to be included or wired with a separate overload protection device but only had a OCPD short circuit ground fault circuit breaker installed upstream to the motor. so I’m not sure if the best way to go about adhering to this NEC requirements.

Motor disconnect and OCPD sizing:

When using a circuit breaker as both the compressors disconnect and OCPD, sizing the breaker to 250% max of the motors FLC (but in this case there is no NEC table stated FLC for a 3.7 hp motor) so 250% of compressor name plate amperes was used and satisfied it to be used as both the motor’s OCPD and the motors disconnect means with a rating meeting and exceeding the required 115% of the motors FLC full load current value. Therefore, a 40 ampere circuit breaker could be used here.

Branch circuit wire size:

A much smaller motor branch circuit wire than 40 amperes at 1.25 x 15.7 ampere = 12 AWG wire SOOW cord used good for 22.75 amperes after ambient temperature correction



Separate Overload protection:

Then a separate overcurrent protective device which I never see installed in compressors over 1 hp and continuous rated should be included at 115% of motor name plate full load current value for a motor with a service factor of 1 (but a temperature rise of 40 degrees? calls for 125%?) which would equal 15.7 i x 1.15 = 18.05 ampere overload protection.

what device would I use to do this and can I go to the next standard size breaker/device for overload protection or must I be exactly set at 18.05 amperes?

Our company only has 3 phase starters with overload protection. I am not familiar with 1 phase starters. In a previous post I used a 3 phase starter to protect a single phase motor. Can I do this here?

Also if the nameplate shows both a temperature rise of 40 degrees Celsius good for overload protection at 125% of name plate full load and a service factor of 1 good for only 115% of name plate full load current, do I choose the smaller 115%?

 

[kwired]

I can't recall ever seeing one of these motors for this application that didn't have internal overload protection.

But if it does not have any you can use what you suggested. Keep in mind old school melting alloy type thermal devices may need to use a different selection table depending on conditions of your application. If you are using just one thermal unit it puts less heat into the enclosure than if using three - most manufacturers do have several selection tables for different conditions, finding the right table is usually a bigger task than making your selection from the table.

Many modern style three pole units may have phase loss protection to some degree, if so you likely need to pass current through all three poles to keep from tripping it on phase loss features.

 

[augie47]

The nameplate on the motor in the previous tread showed it had internal thermal protection

 

[Eddie702]

I agree the motor if single phase should have an internal overload. Usually, small print on the name plate

 

[kwired]

I agree the motor if single phase should have an internal overload. Usually, small print on the name plate

Not really any requirement to have internal protection. Many general purpose single phase motors do have internal overload as standard though. OEM motors varies and is really up to the OEM whether they want it internal or separate when they make the motor specifications.

Definite purpose motors can be either way as well. This air compressor motor would normally be one those DP motors, and as I said earlier most if not all the ones like OP described that I have seen have always had internal overload. Usually a red reset button sticking out the end cap opposite side of the shaft extension on a majority of them is the overload unit for these air compressor motors.

 

[Jpflex]

I can't recall ever seeing one of these motors for this application that didn't have internal overload protection.

But if it does not have any you can use what you suggested. Keep in mind old school melting alloy type thermal devices may need to use a different selection table depending on conditions of your application. If you are using just one thermal unit it puts less heat into the enclosure than if using three - most manufacturers do have several selection tables for different conditions, finding the right table is usually a bigger task than making your selection from the table.

Many modern style three pole units may have phase loss protection to some degree, if so you likely need to pass current through all three poles to keep from tripping it on phase loss features.

How do I know if the motor has internal overload protection if the nameplate doesn’t say so?

And I’m aware that motors with thermal protection allow higher overload current based on a different table

 

[Jpflex]

Perhaps what kwired said, if there is a reset button by compressor switch this would be overload protection

 

[kwired]

Perhaps what kwired said, if there is a reset button by compressor switch this would be overload protection

Upper right portion of your motor tag it says -Thermally protected.

If there is no external reset on the motor itself (which I bet there is) then it probably automatically resets when it cools down again, but that wouldn't be common on this kind of application.

 

[kwired]

Usually a device that looks something like this and the red reset button is all that is exposed on outside of the motor.

 

[Jpflex]

Upper right portion of your motor tag it says -Thermally protected.

If there is no external reset on the motor itself (which I bet there is) then it probably automatically resets when it cools down again, but that wouldn't be common on this kind of application.

As learned in the past, a thermally protected motor is not an overload protected motor. I guess if a motor nameplate doesn’t say overload protected and there’s no reset button then I guess I assume there is no overload protection correct?

 

[texie]

This sounds like a consumer grade compressor given the odd HP rating. As such, the motor is part of the listing for the whole unit. These types of motors are OEM and do not comply with NEMA motor standards. The HP ratings on things like this are meaningless. You don't need a starter and OL protection as with a standard NEMA rated motor. You just supply if with a branch circuit and receptacle to match the cord. Note that the data plate you posted has no HP rating and this is by design. I'm guessing that the HP rating is on the unit itself and is meaningless.

 

[kwired]

As learned in the past, a thermally protected motor is not an overload protected motor. I guess if a motor nameplate doesn’t say overload protected and there’s no reset button then I guess I assume there is no overload protection correct?

Outside of electronic overload relays, all other motor overload protectors are thermally functioning devices.

Don't confuse this with some shaded pole motors that are "impedance protected". That just means the impedance is high enough they won't draw enough current to reach a certain temperature level, they still can get pretty hot if the rotor is stuck, but usually not hot enough to burn out the motor winding so they just continue to give off heat, takes a long time to get a open circuit type of failure out of it Seen them melt down a plastic blower wheel on bath fans before or even in one case a motor mounted in a plastic frame destroyed that frame after being heated for long enough.

 

[kwired]

This sounds like a consumer grade compressor given the odd HP rating. As such, the motor is part of the listing for the whole unit. These types of motors are OEM and do not comply with NEMA motor standards. The HP ratings on things like this are meaningless. You don't need a starter and OL protection as with a standard NEMA rated motor. You just supply if with a branch circuit and receptacle to match the cord. Note that the data plate you posted has no HP rating and this is by design. I'm guessing that the HP rating is on the unit itself and is meaningless.

Yes and it is the motor that usually has integral overload protection. If not, the consumer grade appliance has other provisions already built into it for overload protection.

 

[Jraef]

I agree the motor if single phase should have an internal overload. Usually, small print on the name plate

It must expressly state ON THE NAMEPLATE of the motor or the machine that it is "Thermally Protected", but it can used words to that effect, or in some cases, I have just seen a little logo with "TP" in it, meaning "Thermally Protected".

If it doesn't say it, you must assume it doesn't have it and provide external OL protection.

For a lot of small 1 phase motors, people just use Manual Motor Starters that have a single pole OL heater element in them.
Cheap, effective, small.

 

[Jpflex]

It must expressly state ON THE NAMEPLATE of the motor or the machine that it is "Thermally Protected", but it can used words to that effect, or in some cases, I have just seen a little logo with "TP" in it, meaning "Thermally Protected".

If it doesn't say it, you must assume it doesn't have it and provide external OL protection.

For a lot of small 1 phase motors, people just use Manual Motor Starters that have a single pole OL heater element in them.
Cheap, effective, small.

I’ve seen those switches at work but not familiar with them. Are these rated in horsepower and or is there OL heaters in them? How to size heaters for switches?

 

[augie47]

Check your AllenBradley catalog or website for that series and there will be an expatiation on choosing OL heaters. Basically the same as NEMA starters only a different Table.

OR
Here are links to tha same product by SquareD Catalog on starters & Thermal Unit Selection
https://download.schneider-electric...Type=Catalog&p_File_Name=2510CT9701R01-16.pdf

https://www.productinfo.schneider-electric.com/nadigest/5c51d645347bdf0001f1f280/Master/17716_MAIN%20(bookmap)_0000062583.xml/$/_17716004_67228

 

[kwired]

I’ve seen those switches at work but not familiar with them. Are these rated in horsepower and or is there OL heaters in them? How to size heaters for switches?

There may or may not be different switch sizes.

Most those only good for maybe 2 HP max that I have seen.

Does not come with OL heater, you need to know your motor details and use selection table to pick proper heater.

 

[Jraef]

Does not come with OL heater, you need to know your motor details and use selection table to pick proper heater.

Isn’t that always the case?

 

[LarryFine]

I’ve seen those switches at work but not familiar with them. Are these rated in horsepower and or is there OL heaters in them? How to size heaters for switches?

The post and two screws to the left of the toggle take a standard heater sized for the load.

 

[Jpflex]

Is this the overload button on the compressor? The power on and off switch is located to the right of this