More motor stuff

[Roger]

You know you dont have much to do on the Holiday when your fooling around doing motor circuit calculations. compute_1

You want to use dual element fuses for your overload protection. What size fuse do you need for a 50 hp 460V 3-phase motor that has a temperature rise of 39 C? The nameplate FLA is 60A.

1) 40 amp
2) 50 amp
3) 80 amp
4) 70 amp

[Mr T]

WHO CARES!! Theres no truck prize involved.. Let the motor burn up. I dont give a crap!!!!!







Sorry just had to say it. :D Carry on!

[Roger]

Now.. now... Mr.T....have you taken your pills today? Maybe you could use a little relaxation time in Iraq. I understand there is some good money to be made by motivated electricians disarming IED'S. I think all you have to do is cut the red wire. :)

[Homer]

You want to use dual element fuses for your overload protection.Do you really want to do that?

What size fuse do you need for a 50 hp 460V 3-phase motor that has a temperature rise of 39°C? The nameplate FLA is 60A.Based on §430.32(A)(1) the overload size should be 125% of the nameplate FLA maximum. This results in 75A which is rounded up to 80A by §240.4(B).

Since you are using a fuse for both overload and overcurrent protection, the resulting fuse size may not permit the motor to start.

A more practical solution is to have a separate overload relay. It could be set to 75A and a time delay fuse for overcurrent protection with a maximum value of 125A could be used. This would allow the motor to start reliably. Furthermore, if it were determined that it couldn't start reliably then the overload setting could be bumped up to 84A by §430.32(C).

Homer

[Roger]

Yes, you have a valid point. My understanding is that this is permissable if you have overcurrent protection ahead of these fuses, the fuses being on the load end close to the starter serving as overload protection instead of heaters. Now that you mention it I havent seen this arrangement, however this is a valid "test question". So I'm going to rephrase the question, however I have a motor calculation example that poses a question very similar to this from an article in ECM magazine that is suppose to be preparing you for the journeyman test. It seems that these "test questions" expect you to assume a great deal!

[Roger]

Ok I'll change the question somewhat Homer. However the first question is phrased the same as a supposedly valid test question.

You want to use dual element fuses for your overload protection. Overcurrent protection exists upstream in the form of a three phase breaker. What size fuse do you need for a 50 hp 460V 3 phase motor with a temperature rise of 39C? The nameplate FLA is 60A.

1) 40 AMP
2) 50 AMP
3) 80 AMP
4) 70 AMP

[Homer]

Ok I'll change the question somewhat Homer. However the first question is phrased the same as a supposedly valid test question.I don't dispute the validity of the question.

You want to use dual element fuses for your overload protection. Overcurrent protection exists upstream in the form of a three phase breaker. What size fuse do you need for a 50 hp 460V 3 phase motor with a temperature rise of 39C? The nameplate FLA is 60A.

1) 40 AMP
2) 50 AMP
3) 80 AMP
4) 70 AMPThe answer is exactly the same. You can use time delay fuses of 80A in this case for overload protection.

The problem is also the same. The motor will probably blow the overload fuses on startup, especially if it's started under any real load.

Once again this is a classic test question. The answer that you are guided to provide is technically code compliant but it is not a realistic application.

Homer

[mdshunk]

There are some super lag (TT) fuses that have a time/current curve very similar to a eutectic overload. Still, nothing beats a genuine overload. At least you can reset an overload if need be. You're not gonna catch me using fuses for overloads... permitted or not. I can understand the special influences that might cause a person to do this however. Compare the cost of a fused disconnect and contactor to the cost of a combination motor starter. I'd say the combination motor starter (the right solution!) is about 3 or 4 times the price.

[Roger]

Homer I agree that 75 amps is your calculation but my understanding is that if fuses are used [240.6(A)]and 430.32(A)(1)] the next size up rule doesnt apply, overload cant exceed 75 amps, since this isnt a standard fuse you must choose the next size down 70 amps. Is my thinking flawed? The wording in 430.32(A)(1) is "shall be selected to trip or shall be rated to trip at no more than the following percent of the motor nameplate FLA."

[Homer]

The type of fuse used doesn't seem to impact anything in §240.4(B).

However, you are correct that I should not have upped the fuse size based on §240.4(B). Technically article §240 is titled OverCurrent Protection so anything in §240 about upping fuse sizes wouldn't apply if used for overload protection.

Homer

[Roger]

Yes, I agree Homer. I was just using 240.6 as far as the 75 amps being a non-standard size.

[Wgoodrich]

You guys are hitting it correctly at the end of your discussion.

While overcurrent would allow the fuse size to be upped to the next normal overcurrent device listed in 240.6 the overload has a maximum overload size limiting to the 125% of name plate flc rating not allowing that upping in overcurrent device size due to the dual use overcurrent device / overload use of this fuse.

Now example in real life where this would or would not work is starting time of real life condition.

If you had a grain bin direct drive squirrel cage motor with the fan blade being cast iron being real heavy the start time with high amps will most likely blow the fuse before the motor starts to speed.

However the same fan blade if it were of magnesium being a light atomic weight the motor would easily start without blowing the dual use fuse.

Just an example and confirmation of your discussion.

Wg