Fuse rating and type?

 

Cody:
Get a new motor. Sometimes it's best not to wonder why. Use Ohms law instead. Many techs don't understand this but it can help you troubleshoot.
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I not going to explain ohms law. This is what is taught on the first day of Tech school. You have two of your numbers. Do the math. For whatever reason when the motor has a load you have no restistance. That is the only way you could have 60 amps at start up. Was it 60 amps exactly? It just seems like a weird number. You probably didn't read the true amperage. 60 just flashed for second. Probaby was higher. Anyway just get a new motor.

 

How would ohms law play into an inductive load? I know it's part of it but that isn't going to give any accurate info in this scenario


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more money.

 

Yes just "Mo Money".


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Timebuilder could you explain the calculations. No sarcasm just know your really good at that stuff. His numbers seem weird. Of course the fuses are important. Delayed fuses for sure.

 

Is it pulling 60 amps on every leg? Have you checked your voltage on each leg at start up? Sounds like single phasing to me.

Or It's just plain overloaded. It's not always easy to spot a failed bearing.

Other thought that just popped into my head... A centrifugal fan without any static will pull extra power to move all that air.

 

Sorry I can't explain that it would take forever and I don't know how to make you understand. Its hard to understand but there people who see numbers in a very different ways. Like three great men who we can thank for the Hvac industry.
Edison: for DC electricity no controls systems without that.
Einstein: for simply saying one thing. You cannot get more energy out of something, than you put into it. What does that have to do with Hvac? Everything!
Tesla: my favorite for AC electricity and his wireless experiments.
These men where different than most people. They could see numbers different.
When I look at ohms law. His numbers will not work. This is the only way I can explain.
1. You need Volts, amps and resistance to turn on light bulb.
2. If you take away one does not matter which one it will not work.
To me this has everything to do with ohms law.
https://en.m.wikipedia.org/wiki/Ohm's_law

 

Was the fuse discolored?
And what is the amperage trip out point on the breaker.

Ohms law tells you nothing in this situation.

The nameplate has a listed amperage rating at full load, and a rated voltage - we know when the motor is not under load, it runs well below rating, and under load it does not.

My first instinct would be a problem with the fuse block or a bearing failure or some type.

How difficult is the load to spin manually, and does is coast to a stop when you stop pushing? how does it sound?


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Heatingman:
Sorry that you don't understand this:
480vac
9.4 FLA = 28.2 LRA
Forcing the motor to stop will not produce 60amps.
The only way that is possible is to change your resistance or voltage.
And I just used ohms law to tell you that.
If you could do this then Albert Einstein's is wrong and you would be producing more energy then you put in.
It's against the laws of physics.
I think I will just take Alberts word for it.
Physics are way to hard.

 

Maybe I'm mistaken, you can throw out ohms law on the start of an induction motor. A motor will draw way more current on start up. The fuses are 20 amp @ 600 vac and have been there for over fifty years. That's what makes me wonder. When I started the fan there was nothing else running. The fan was at rest.

 

I'll wait until the OP answers the questions.

 

What questions do you want answered?

 

Is op me. Not sure?

 

Cody: yes you are the OP.
Timebuilder wants to know your fuse type and size. Before he weighs in on this. And he is a Hvac electrical specialist. So you might wanna let him know.

 

Cody no you can not throw out ohms law on the start of motor, heat elements, compressor or whatever. The split second it takes your motor to start. Your resistance is increased. There is actually a way to calculate that. Volts, amps and resistance all work together. They all effect each other. If you install a bigger motor. 10hp let's say. That's increases the resistance of your motor windings. This increases your amperage. Cody when I read your post I new immediately if your 60 amp reading is true.( how and where did you measure 60amps.) Only two things can cause that. Changing voltage or resistance.
Voltage- improper power supply, single phasing.
Resistance- shorted or grounded motor windings
If you learn and understand what I'm talking about.
You can use this a troubleshooting tool.

 

...If you install a bigger motor. 10hp let's say. That's increases the resistance of your motor windings. This increases your amperage...

a bigger horsepower motor with the same voltage rating will have a lower resistance.

 

Ace you are a warehouse of misinformation here. Resistance of a motors windings CANNOT be used in ohms law. All inductors create a calculated resistance called inductive reactance. Once the reactance is known then that value can be used in ohms law. The formula for inductive reactance is XL= 2πfL. XL is inductive reactance (which can be used in ohms law equation), f is frequency, L is the Henry's of the inductor. We in the field don't need to know or calculate this. Motor manufacturer does this for us because there is probably much more to motor design than just this formula. You have made clear that we are not Einsteins. All I am trying to undo, as others have, is to remove he notion that you can plug in the resistance of a winding and use it in ohms law equation.

 

First thing.

There is a lot more info in the Pro section. I have two articles in the Educational Forums that you can benefit from. The Educational Forums are unlocked to Pro members, and our threads go into much more detail than in the open forums.

Look at the very bottom of my posts. See the blue letters that say, "How to become a professional member"?

That's a link. Click on it and apply now. You already have the necessary 15 posts.

You mentioned the L2 fuse. I have to assume it is in a disconnect. Is this a single phase motor (two fuses) or a three phase motor (three fuses)?

What is the amperage value of the fuse(s)?

What is the code designation on the fuse? Is is a "NON-15"? Is it a FRS-R-15?

Is there a voltage drop from the load side of the disconnect and the line side of each phase?

If any of these questions sound like Greek to you, I would start like this:

Have your lead technician or a highly experienced friend look at this issue with you, and show you and ask you about the relevant ideas in play here.

Do an online search for "Fasco Facts." Download the pdf and read all of it.

Current in an AC motor is a function of two main factors:

1) load, the force the motor is being asked to overcome, and

2) rotor RPM.

These are related ideas. As rotor RPM slows, there is less opposition to current flow by counter electromotive force. When a motor first starts, rotor RPM is zero, and the only initial opposition to current flow is conductor resistance. Then, there is an immediate counter EMF which begins to oppose that initial flow. Then, as the rotor begins to turn, even more counter EMF is developed until current is limited by full rotor RPM and the amount of load.

LRA is usually about 6 times the FLA on the nameplate. It is the instantaneous initial current when the coils are first energized.

So, if you can give me a little more info about this unit (the questions I have already asked, for example) we can create a series of steps that you and your lead tech can follow.

 

AC Motor info

https://drive.google.com/open?id=0B6YkyHRLP2ToUnpyRmQ3dHprYmM

Baldor Cowern Papers

https://drive.google.com/open?id=0B6YkyHRLP2ToYTdxRU9SemxCZ2M

Fasco Facts

https://drive.google.com/open?id=0B6YkyHRLP2ToaDVqV2JwbFQ4YU0


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Thank you for the lesson timebuilder. Sorry I was wrong. No problem admitting that. Going off your lesson sounds like he is drawing LRA.
9.4Fla X 6 = 56.4 amps
Close to his 60 amp reading.
Would you mind explaining sometime. How adding a vfd and changing the frequency to soft start the motor And how that effects the motor amps. Does the voltage and resistance stay the same?

 

I do know that the voltage is also varied based on the hz output of the vfd. I can't remember the exact figure but is it was something like:

460v: 7.2v for every hz
208-230: 3.6v for every hz

Definitely take that with a grain of salt until somebody who knows for sure chimes in, but it's right along those lines.

I don't know why it's necessary to vary the voltage like that? I remember reading something along the lines of it being necessary to provide a constant torque.

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Yes timebuilder. It is a three phase motor with the fuses located in the disconnect. The fuses that are in it are FRS-R-20. I am not measuring any voltage drop across the starter contacts. This check was performed while running the motor unloaded. As far as how I measure the 60 amps at startup with a load on the motor I used the max hold setting on the meter. I don't have enough time to make any checks with a load on the motor as the fuse is blowing.

 

There is always drop. Should be well under 50 mv for adequate contacts running at full load.

 

If you find something else on further examination,. post back here and let us know....

 

You could also have a loose connection somewhere.

I've had this happen once. Motor megged ok, ohm'd ok, resistance all the same, no connections to ground, blew fuses on start up.

 

Dont forget

Meg the motor

Meg the feeds

This is why I want him to have an experienced man with him to check all the things that were not enumerated in the original post.

 

Dont forget

Meg the motor

Meg the feeds

 

Megohms check oke

What exactly were the readins as anything under 3megohms is failing

 

...anything under 3megohms is failing

this is an incorrect statement by itself. i have started many brand new compressors around 1 megohm and they were perfectly fine.

 

A note to those following this thread, and all others too.

In your first post of an issue, never tell us that something was tested and was "okay." Tell us about the test AND the specific results. Let us decide if the result is "okay." The more detail and train of thought you can share is all the better.

Measure all phase legs for voltage and current. Do NOT measure voltage to ground. Give us the results. If the unit can run, check for voltage drop from line to load.

If a unit cannot run. put good fuses in the disconnect, and with the feeders de-energized, measure resistance from the line to load terminals with the disconnect closed in.

If you don't understand what the words "feeders de-energized" means, ask someone with more experience to help you right away, so you don't get hurt. This is not residential equipment, and many guys are seriously unprepared for this part of the business.

Most of the time, your "entre" into this type of work needs to be done with an experienced person next to you, giving instruction AND asking you questions.

This is NOT a part of the trade where you should be tossed in and asked to swim.

 

I'd like to add a couple things to the list of requests:

if you're having an issue with some type of overcurrent device.... whether it be a circuit breaker or fuses in a disconnect..... please also supply things like the wire gauge and a fuse or circuit-breaker setting, the unit's minimum circuit ampacity and the maximum overcurrent protection device values, which are given on the unit label.

The more information you provide in a post, the more cogent and insightful will be the reply that you receive.

 

Also make sure the motor is rated for inverted duty...and not just continuous. With continous you will have problems too. Inverted is rated for the varying frequency being sent to motor.

Seen my share of how continous motors on frequency drives ends up putting flat spots in the bearings or shaft itself..grounds..sheesh

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