# Sizing motor overloads



## jmsmith (Sep 10, 2011)

subelect said:


> Customer keeps buying 240 volt, 3 phase motors for use in his 208 volt factory.
> After about 2 years of operation, one air compressor motor is being to cause issues with the original bi-metallic overloads.
> 1. If I do replace the bi-metallics, I have no nameplate data for 208 volts. Do I just use the chart in 430 (208 volt column) for sizing them?
> 2. Or, do I use the max amps that are indicated when the compressor operates?
> ...


Don't know if this is much help, but if memory serves me correctly, a 240V motor will draw a little more current on 208V than one designed for 208V or even one designed for 208/240V. So the chart you are referring to may be borderline. At the plants that I worked at, we used your second option. Your starters should have overload guides in them. I would check the loaded current, then select the overload from the chart that is supplied by the starter manufacturer. Hope this answers your question...

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## 636Sparky (Jun 24, 2011)

FWIW: Every dual voltage motor I have ever seen, the current and voltage are proportional. I would suggest applying the ratio to your 208 voltage. 
Also, I hope you are referring to a 230 volt motor.


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## jmsmith (Sep 10, 2011)

636Sparky said:


> FWIW: Every dual voltage motor I have ever seen, the current and voltage are proportional. I would suggest applying the ratio to your 208 voltage.
> Also, I hope you are referring to a 230 volt motor.


My apologies... Meant 230V or 208/230V rated. But full-load current ratings, although in the norm, are not always so cut and dry. Rebuilds, and manufacturers can vary. This likely won't be much, but could be borderline as I said before. It also depends on your voltage. If your voltage isn't EXACTLY 208V (which you know is seldom the case), the full-load current can go either way. All these factors considered, your current can vary as much as 5-10% either way. Not much, but may be enough to create nuisance tripping in thermal overloads. That is why we used to base overload sizing off of motor load conditions rather than off the table. It is not a perfect world.... Have a good day!
:thumbsup:

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## JRaef (Mar 23, 2009)

Sizing OLs off of the measured running current of the motor sets you up for nuisance tripping, which typically ends up frustrating the user who solves the problem by over compensating and you lose the motor. You should always use the motor nameplate FLC and in your case, adjust by the voltage difference as suggested. In other words since the voltage ratio is 230/208 = 1.10, multiply your NP FLC x 1.10 as your OL setting. And buy a solid state OLR if you are going to change anything.


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## jmsmith (Sep 10, 2011)

jmsmith said:


> Don't know if this is much help, but if memory serves me correctly, a 240V motor will draw a little more current on 208V than one designed for 208V or even one designed for 208/240V. So the chart you are referring to may be borderline. At the plants that I worked at, we used your second option. Your starters should have overload guides in them. I would check the loaded current, then select the overload from the chart that is supplied by the starter manufacturer. Hope this answers your question...
> 
> Sent from my iPhone using ET Forum


This is the advice that we had gotten from motor rewinders. As I said, your option no. 2 is what they suggested. The other gentleman is right in what he says... HOWEVER, if you are going by a multiplier, CHECK YOUR LINE VOLTAGE, THEN BASE YOUR MULTIPLIER ON 230V divided by that voltage. A difference of tenths in this figure can make a difference of a couple of amps... This could mean that you really need an overload that is at least one size higher than what would be called for if you were running 208V. Any way you look at it, you need to know either your voltage, or what the motor is pulling at full load. That is why the starters that use thermal overloads are provided with an overload chart! As I said before, this is not a perfect world, and things don't always go by the book!

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## 636Sparky (Jun 24, 2011)

I have to disagree with above post. 
Basing your thermal overloads off max amp while operating seems foolish. There are too many variables with the load on the motor. As the air filter or lube oil gets dirty, the motor is likely to draw more current. Does this mean the motor is bad? Perhaps, but it may not be. 
When you say base it on line voltage, are you referring to actual voltage measured with a meter? When has a motor load been based on measured voltage, and not the nominal voltage? Your system voltage; especially in an industrial atmosphere, can vary greatly at different points of the day, sometimes even instantly.
I still believe using the ratio is the best method. If you look at table 430.250, you will see the numbers correspond to this method. It will take a proportional amount of current to generate the same amout of horsepower at a lower voltage.
That is my $.02.


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## BBQ (Nov 16, 2010)

jmsmith said:


> As I said before, this is not a perfect world, and things don't always go by the book!


Not following the book in this case may well lead to a toasted motor.


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## BBQ (Nov 16, 2010)

JRaef is the man when it comes to things like this.

Also FWIW



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


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## Wirenuting (Sep 12, 2010)

We have 120/208 here and I like to keep with the 200v motors especially with compressors. The other motors do work but I have seen them not last as long. 
Sizing the O/L's is based on the book and since the load can begin to vary over time, you can get a different class of O/L's. It will give you a longer duration before she trips. 
It doesn't matter if they are bi-metallic or electronic, you can get different classes of them. I'm not talking about ambient compensated units ether.
I believe the classes are 10,20 & 30. I'll look in my book after I get in my truck. 

Also if your tripping on start up, check your unloading valve.


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## jmsmith (Sep 10, 2011)

636Sparky said:


> I have to disagree with above post.
> Basing your thermal overloads off max amp while operating seems foolish. There are too many variables with the load on the motor. As the air filter or lube oil gets dirty, the motor is likely to draw more current. Does this mean the motor is bad? Perhaps, but it may not be.
> When you say base it on line voltage, are you referring to actual voltage measured with a meter? When has a motor load been based on measured voltage, and not the nominal voltage? Your system voltage; especially in an industrial atmosphere, can vary greatly at different points of the day, sometimes even instantly.
> I still believe using the ratio is the best method. If you look at table 430.250, you will see the numbers correspond to this method. It will take a proportional amount of current to generate the same amout of horsepower at a lower voltage.
> That is my $.02.


If you will retread my post, you will see that I AGREED with JR about the proportional business, current is based on voltage impressed. Chart is based on NOMINAL conditions. If all conditions were by the book, there would not even be a need for power correction! And in NO way was I even remotely talking sizing an overload grossly over full load current.

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## jmsmith (Sep 10, 2011)

BTW, op never mentioned age of overloads..... Could the right size units be in there already? Corrosion can take place over time, allowing tripping at a lower current. In this case, new units THE SAME SIZE as what is currently installed might cure your problem.

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## joethemechanic (Sep 21, 2011)

JRaef said:


> Sizing OLs off of the measured running current of the motor sets you up for nuisance tripping, which typically ends up frustrating the user who solves the problem by over compensating and you lose the motor. You should always use the motor nameplate FLC and in your case, adjust by the voltage difference as suggested. In other words since the voltage ratio is 230/208 = 1.10, multiply your NP FLC x 1.10 as your OL setting. And buy a solid state OLR if you are going to change anything.



What does running a 240 V motor on 208 do to the slip? Is there a formula to tell me how much more?


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## jmsmith (Sep 10, 2011)

Wirenuting said:


> We have 120/208 here and I like to keep with the 200v motors especially with compressors. The other motors do work but I have seen them not last as long.
> Sizing the O/L's is based on the book and since the load can begin to vary over time, you can get a different class of O/L's. It will give you a longer duration before she trips.
> It doesn't matter if they are bi-metallic or electronic, you can get different classes of them. I'm not talking about ambient compensated units ether.
> I believe the classes are 10,20 & 30. I'll look in my book after I get in my truck.
> ...


You have made a valid point here, because every starter manufacturer has their sizing charts. Not only does motor full load current matter, but the APPLICATION. Is it continuous duty, what the motor is driving, RPMS, etc. A compressor is one piece of equipment that is usually protected by other devices in order to prevent it from running fully loaded on a continuous basis. It could be a faulty unload valve, pressure switch, anything else that is not allowing the motor to unload.

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## JRaef (Mar 23, 2009)

joethemechanic said:


> What does running a 240 V motor on 208 do to the slip? Is there a formula to tell me how much more?


 TECHNICALLY, you will likely be hard pressed to find a motor that is actually rated 2*4*0V on the nameplate. 240V is the _*distribution*_ voltage, the motor design will be based on a _*utilization*_ voltage. The utilization voltage in North America is 230V (or 460V, or 200V, or 115V) in order to allow for voltage drop at the motor connection. I know what you mean though, I'm just being pedantic in case someone else who reads this over interprets.

So what happens to slip on a 230V designed motor that is given 208V? It increases at roughly twice the value of the voltage delta, although I'm not finding the actual formula right now. But here is a chart that shows it:








From there you can see that at -10% voltage, slip increases roughly 20% (that's a 20% increase, not 20% slip!). So if you had 3% slip at rated voltage, you have 3.6% slip at -10%. That means it will draw more current for the same load and if the motor is not designed to accommodate this, it can be damaged. Fortunately NEMA MG-1 design specifications call for +-10% voltage variation in motor designs, so an increase in current based on a 10% delta is not supposed to affect nominal performance.

In addition MANY motors have a 1.15 Service Factor precisely for this reason, it gives you that "fudge factor". The problems people experience with this issue are often the result of an OEM who gets cheap about their motor selection and CONSUMES the service factor based on performance at nameplate voltage, so when connected to a 208V system, they end up overloaded. Happens a LOT with compressors for some reason, probably because of market conditions for them.

Related Side Issue: When you see a motor nameplate that says 200-230/460V, that is usually a motor design compromise. What they do is design the motor windings for 220/440V. At 208V supply they are less than 10% under, at 240V they are less than 10% over. At 480V they are less than 10% over, so all is right with the world. You just have to be careful on those that your ACTUAL voltage is not already on the fringes. For example if your line is 504V, that is less that 10% high for 480V or even 460V, but it is almost 15% high for a 440V design. Or if your 208V nominal line measures out at 188V that is within the 10% for 208V but is -15% for 220. So you just have to be a little more careful in those extreme cases.


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## Wirenuting (Sep 12, 2010)

In 430.32 (c) allows motors with a 1.15 sf to be set no more then 140% of full load. Motors with a 1.0 sf may be set at no more then 130% of full load. 
Use of this option is discouraged and a better solution to change the o/l class. 
Class 20 carries 6 times it's rated current for 20 seconds. Class 10 is 10 seconds and class 30 is for 30 seconds. 

ie, trips on o/l during the first 20 seconds of running with properly sized elements you can switch to a class 30 and address a legitimate problem with failure to start. 
<from the 08 handbook>


But remember that the unloading valve plays a big part in a compressor starting issue. Some compressors wait several seconds for the unit to come to speed before closing. Others close at start up. But most open when pressure is satisfied and unit cycles down.


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## jmsmith (Sep 10, 2011)

JRaef said:


> TECHNICALLY, you will likely be hard pressed to find a motor that is actually rated 240V on the nameplate. 240V is the distribution voltage, the motor design will be based on a utilization voltage. The utilization voltage in North America is 230V (or 460V, or 200V, or 115V) in order to allow for voltage drop at the motor connection. I know what you mean though, I'm just being pedantic in case someone else who reads this over interprets.
> 
> So what happens to slip on a 230V designed motor that is given 208V? It increases at roughly twice the value of the voltage delta, although I'm not finding the actual formula right now. But here is a chart that shows it:
> 
> ...


The above is why I was stating what I did, in the OEM world, they have concentrated so much on making the packages smaller that they have had to resort to the "fudge factors". That is why I offered the suggestion of using a "KNOWN" voltage as opposed to nominal if he was going to use the ratio method to derive the full-load current, that is all. I did not make any statements here to take away from what you are saying, and that is not intended in the least. I hope it is not taken that way, I am just going with my experience, and what I was taught to do through the years. After years of use and abuse, not all the nameplate info is there to go off of anyway.

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## JRaef (Mar 23, 2009)

jmsmith said:


> The above is why I was stating what I did, in the OEM world, they have concentrated so much on making the packages smaller that they have had to resort to the "fudge factors". That is why I offered the suggestion of using a "KNOWN" voltage as opposed to nominal if he was going to use the ratio method to derive the full-load current, that is all. I did not make any statements here to take away from what you are saying, and that is not intended in the least. I hope it is not taken that way, I am just going with my experience, and what I was taught to do through the years. After years of use and abuse, not all the nameplate info is there to go off of anyway.
> 
> Sent from my iPhone using ET Forum


I don't have any problem with anything you said, I was addressing the question asked by joethemechanic. I actually think your suggestion to modify what I had said about using the ratio by measuring the actual line voltage is fine, but it should also be noted that voltage changes. It's not supposed to, but it does (as you already alluded to). So even a measured voltage should be taken with a grain of salt. I prefer to use what the nominal voltage is SUPPOSED to be only because if it is in the weeds, you can point a finger at the utility when a problem arises. If you adjust everything to what you actually read in one snapshot one day, then the utility "fixes" their issue and it results in some other problem because of your adjustments, you have no recourse. But again, it's a judgement call and something that needs to be decided on based on actual circumstances in the field, which I interpret as being your overall point. :thumbsup:


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## RIVETER (Sep 26, 2009)

subelect said:


> Customer keeps buying 240 volt, 3 phase motors for use in his 208 volt factory.
> After about 2 years of operation, one air compressor motor is being to cause issues with the original bi-metallic overloads.
> 1. If I do replace the bi-metallics, I have no nameplate data for 208 volts. Do I just use the chart in 430 (208 volt column) for sizing them?
> 2. Or, do I use the max amps that are indicated when the compressor operates?
> ...


If it is taking two years to cause a problem then that should be long enough to to get a good reading on the max load of the motor. Why not just go 25% over that? If the motor doesn't appear to be harmed then possibly the OLs were inferior. IMO.


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## jmsmith (Sep 10, 2011)

JRaef said:


> I don't have any problem with anything you said, I was addressing the question asked by joethemechanic. I actually think your suggestion to modify what I had said about using the ratio by measuring the actual line voltage is fine, but it should also be noted that voltage changes. It's not supposed to, but it does (as you already alluded to). So even a measured voltage should be taken with a grain of salt. I prefer to use what the nominal voltage is SUPPOSED to be only because if it is in the weeds, you can point a finger at the utility when a problem arises. If you adjust everything to what you actually read in one snapshot one day, then the utility "fixes" their issue and it results in some other problem because of your adjustments, you have no recourse. But again, it's a judgement call and something that needs to be decided on based on actual circumstances in the field, which I interpret as being your overall point. :thumbsup:


Well, at least neither one of us told him to get a water hose and keep water flowing on the motor, and this, too, shall pass!!
:laughing:


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