sizing thermal elements

[nizak]

When sizing thermal overload elements for a starter should you use the nameplate of the motor or what the actual amp reading is from your meter? I have a motor that is 230V and it's running on a 240V system(actually 248V). Enough difference to cause a 1 size change in the SqD thermal selection.

 

[jumper]

I think it is nameplate. 430.32.

 

[LeeB]

Use the nameplate

Use the nameplate

You should definitely use the nameplate. If the motor is running outside the mfg's nameplate rating you will want the thermal overloads to trip.

 

[Sierrasparky]

Name plate. However I think you would adjust for voltage ariances.

 

[iwire]

Name plate. However I think you would adjust for voltage ariances.

I may be lacking in theory but I do not think you would want to do that. Often (always?) service factor comes into it.

 

[renosteinke]

Nameplate? Only if you're the original installer and have no way of telling what the actual use will be.

Thermal overloads ("heaters") are there to protect the motor- unlike the fuses / breakers that are pretty much useless in protecting the motor. Heaters, with their slow response, completely ignore the usual starting currents, etc.

The usual procedure is to operate the equipment in it's "normal" manner and size the heaters as small as you can, until nuisance tripping becomes an issue. This is just about the only way to protect the motor from single-phasing, without designing an entire control cabinet. It also lets you know when a bad bearing, etc., is placing additional load on the machine.

Why else would the heaters be made in such a variety of settings?

 

[iwire]

Nameplate? Only if you're the original installer and have no way of telling what the actual use will be.

Thermal overloads ("heaters") are there to protect the motor- unlike the fuses / breakers that are pretty much useless in protecting the motor. Heaters, with their slow response, completely ignore the usual starting currents, etc.

The usual procedure is to operate the equipment in it's "normal" manner and size the heaters as small as you can, until nuisance tripping becomes an issue. This is just about the only way to protect the motor from single-phasing, without designing an entire control cabinet. It also lets you know when a bad bearing, etc., is placing additional load on the machine.

Why else would the heaters be made in such a variety of settings?

Do you just make this up as you go??????hmy:

You do not adjust the thermals to match the running load of the motor.

The NEC requires we use the name plate to size the thermals.

 

[kwired]

Nameplate amps along with service factor and other heat compensating instructions provided with the manufacturer sizing tables will give you maximum size overload element allowed. Nothing says you can't use a smaller element if the motor is not fully loaded if you want closer to actual running current protection. If you have a motor that is normally only 75% loaded and a bearing goes bad, how hot does the interior of the motor get before the actual current reaches 125% full load trip point? You likely have winding insulation damage in this situation

 

[iwire]

Nothing says you can't use a smaller element if the motor is not fully loaded if you want closer to actual running current protection.

I agree you could code wise, seems like a poor idea in real life.

How do you know the motor was at it's max load the day you took the readings?

If you have a motor that is normally only 75% loaded and a bearing goes bad, how hot does the interior of the motor get before the actual current reaches 125% full load trip point? You likely have winding insulation damage in this situation

The motor is designed to handle the nameplate amps regardless of the drag being from the driven load or a bad bearing.

As far as the heat generated by the bad bearing itself the OLs will not do anything about that no matter how you set them. You would need a motor with internal thermocouples to protect against that.

 

[renosteinke]

Remind me ... where does the NEC require the use of the nameplate to size the thermals? (As opposed to OCPD's).

Likewise ... how do you set the electronic overloads? All you have there are the tiny set screws with microscopic 'percentage' markings around the dial. Makes it a wee bit hard to say just what the exact setting is.

 

[Cow]

Why would anyone size to the running amps of a motor, unless say it's a submersible, and there's no way of knowing?

Size to the nameplate. Just because a motor is pulling 5 amps instead of the 8 amps listed on the plate doesn't matter IMO. Unless I hear someone post otherwise with some good reasons, I think you're doing your customer a disservice sizing the OL's so tight there is a potential for nuisance trips.

 

[kwired]

I agree you could code wise, seems like a poor idea in real life.

How do you know the motor was at it's max load the day you took the readings?




The motor is designed to handle the nameplate amps regardless of the drag being from the driven load or a bad bearing.

As far as the heat generated by the bad bearing itself the OLs will not do anything about that no matter how you set them. You would need a motor with internal thermocouples to protect against that.

The more expensive the motor the more likely you may want some early indication of something being wrong.

Constant torque loads a bad bearing is usually an obvious problem. I have seen centrifigual fans and pumps that will run for a long time with a bad bearing, they often never know the bearing was bad until there was winding failure. Just replaced a 10 HP fan motor a couple weeks ago where this was the case. A thermal protector in the motor is the best protection but if it does not have protection in the motor and the fan does not normally draw full load, nothing wrong with lowering the protection a bit to possibly catch something like this. My replacement motor on the 10 HP did have an internal thremostat.

 

[renosteinke]

Cow, I understand your point.

It's easy to forget that there are two type of electricians: the guys who put stuff in, and the guys who run it.

It's the 'maintenance' or 'industrial' electricians who will 'fine-tune' the overloads to their specific needs. They're the guys who handle the trouble calls. I agree that there is no way for the construction guys to know what the actual conditions will be.

 

[iwire]

Remind me ... where does the NEC require the use of the nameplate to size the thermals? (As opposed to OCPD's).

This happens to be from the 2011 but has been in place for a long time.

430.6 Ampacity and Motor Rating Determination.
(A) General Motor Applications. For general motor applications,
current ratings shall be determined based on
(A)(1) and (A)(2).

(2) Nameplate Values. Separate motor overload protection
shall be based on the motor nameplate current rating.

 

[iwire]

Cow, I understand your point.

It's easy to forget that there are two type of electricians: the guys who put stuff in, and the guys who run it.

It's the 'maintenance' or 'industrial' electricians who will 'fine-tune' the overloads to their specific needs. They're the guys who handle the trouble calls. I agree that there is no way for the construction guys to know what the actual conditions will be.

Another word for that 'fine tuning' is hacking, or DIY work. It is common in maintenance men, they start to think they know better than the manufacturers or codes.

For what it is worth I was a maintenance electrician /mechanic for about 5 years at a place with a low budget, I know how it works.

 

[iwire]

The more expensive the motor the more likely you may want some early indication of something being wrong.

Yes, and they way to watch for bearing failures is to buy a motor with thermal couples for the bearings.

I have seen centrifigual fans and pumps that will run for a long time with a bad bearing, they often never know the bearing was bad until there was winding failure.

Yes, without a doubt.

I also doubt that the bearing added enough current draw that you could count on it tripping a closely set OL.

Just replaced a 10 HP fan motor a couple weeks ago where this was the case. A thermal protector in the motor is the best protection but if it does not have protection in the motor and the fan does not normally draw full load, nothing wrong with lowering the protection a bit to possibly catch something like this.

Code wise I think it is OK, design wise I think it is wishful thinking. JMO>

 

[kwired]

Yes, and they way to watch for bearing failures is to buy a motor with thermal couples for the bearings.



Yes, without a doubt.

I also doubt that the bearing added enough current draw that you could count on it tripping a closely set OL.



Code wise I think it is OK, design wise I think it is wishful thinking. JMO>

I don't know, maybe it is wishful thinking. I think the user is the problem most of the time. If an overload frequently trips, they find a way to bypass it, turn up the setting, leave the cover off, put a fan on it, you name it. Then they wonder why the motor burned out.

I had a customer that had a product auger that every so often I would get a call that it would keep tripping. By the time I would get there that motor would be so hot you could cook your lunch on it because they kept resetting it as quickly as the overloads would cool enough to reset. Every time it was when they were running a product with higher fat content than most of their other products and it would gum up in the auger. It took 3 or 4 of these calls (usually sevaral months apart) before I caught on and told them when receiving the call that they need to make sure the auger is cleaned out before I come. You know how production employees are, if the motor will not run it is an electrical problem.

I work on a lot of grain storage equipment, drying fans seem to be one of the more common motors that just burn out. Internal thermostat is probably the best way to protect them as I don't know how else you would really know what the winding temperature is - especially on a motor that is located in the air stream. Sad thing is most OEM motors do not have thermal protector, but most of the replacement motors do, and I run into a lot of burned out replacements where the thermal protector was not connected to the control circuit. Conventional overloads for these motors (on a starter) are really only useful for single phasing conditions, or on single phase units for starting failures like a bad capacitor or something.

 

[jim dungar]

According to Square D's sizing instructions (I know how dare I read the instructions).

1.) Determine motor data
a. Full load current rating
b. Service factor
NOTE: if motor FLC is not known refer to page 16-129 [for tentative selection]

2.) Motor and controller in same ambient temperature
a. 1. For 1.15 to 1.25 service factor use 100% of FLC for thermal unit selection
a. 2. For 1.0 service factor motors use 90% of FLC for thermal unit selection

3.) Motor and controller in different ambient temperatures
a. Multiply FLC by multiplier in table A [see page 16-125]

So, the answer to the OP is: Use the nameplate.

 

[kwired]

According to Square D's sizing instructions (I know how dare I read the instructions).

1.) Determine motor data
a. Full load current rating
b. Service factor
NOTE: if motor FLC is not known refer to page 16-129 [for tentative selection]

2.) Motor and controller in same ambient temperature
a. 1. For 1.15 to 1.25 service factor use 100% of FLC for thermal unit selection
a. 2. For 1.0 service factor motors use 90% of FLC for thermal unit selection

3.) Motor and controller in different ambient temperatures
a. Multiply FLC by multiplier in table A [see page 16-125]

So, the answer to the OP is: Use the nameplate.

That is the easy part. If you are trying to size them out of the catalog it is another process finding out which table to use. If you have an enclosed starter they have the appropriate table inside the cover. If you purchased an open starter they usually give you at least two tables and you need to determine which one is the correct one for the application.

 

[renosteinke]

Iwire, that comment regarding industrial electricians is unworthy of you.

The industrial maintenance field is most definitely a field of trained professionals .... who else did you think got "ladder logic" going with PLC's? Simple answer: the union (UAW) electricians at GM, when Allen-Bradley started making PLC's.(The A-B intent was to use Boolean symbols). Are you seriously suggesting that these guys don't know how to power motors?

"Based upon" does not mean that you are limited to using only the number on the nameplate. Please note that the same language is not used regarding OCPD's. In this regard, "based upon" is no more than another way of saying 'should.' That FLA figure is nothing but a starting point. Start at FLA and work your way down.

Now, if you meant to say that putting LARGER heaters in was a bad idea, I'll agree with you.

As an aside, the FLA figure is an important figure. Motor makers are extremely willing to fib (their term is 'relable') about the horsepower of their motors. I just replaced a 7-1/2 hp motor that turned out to have been a 10 (larger) motor that Baldor had 'relabled' for some reason. Something to keep in mind the next time you wonder about those wire size charts in the motor tables ...

You're hardly 'hacking' things, or making them less safe, by protecting the motor more closely. True, one can find exceptions where other design considerations take precedence (lifting magnets, fire pumps, etc.) .... but that's why they're called 'exceptions.'