frequency vs voltage vs current draw

[Fred Raud]

absolutely it helps,,thats the answers i was looking for,,

as far as your first point,,i thought i made it clear in the first post that i knew it was all messed up,,i didnt install it,,i was just there to deal with the current(pardon the pun) situation,,and needed to get through the week to a down day to do something permanent,,in the mean time,,i wanted to identify what was going on here

and again,,thanx

Fred Raud

 

[Leadfoot]

If I read your original post correctly, that contactor was in place with the drive that blew up. It was also operated by starting the drive and closing the contactor.

If that is correct, it is no wonder the drive and motor went bad.
First rule of motor contactors is close the contactor, enable the drive. Second rule of motor contactors is stop the drive and open the contactor.

The contactor should an aux switch, try wiring the N/O to the drive run contact if you insist on keeping contactor.

The drive is a UL rated thermal overload. It is far superior to any one attached to an across the line starter. Most software today actually trips before the damage is done.

BTW, I am in firm agreement that the contactor needs to get pulled out. Your drive will thank you.

 

[Fred Raud]

Please excuse my drawing here,,looks like i need to learn how to use my autocad,I wasnt goint to make this drawing being i tried to stay away from whats wrong with this installation(can i say it enough times that i did not install this here?),,but focus on the electro physics behind whats going on here,,but since theres alot of advice on what should be done to fix the problem,and it was brought up as a possible good lesson on how NOT to install an inverter,,so here goes,,

I have to draw up a two inverter set up in proposal form this morning(its 2 am right now and im doing some work on my own time to try to earn some respect to get some fun projects) and ill post up what i can when i get done in a few hours,,

Please keep in mind that this is the first time ive used paint to do something like this,,and i was in a hurry,,

Fred Raud

Note that they set the parameters in the inverter to recognize the MOL input,,then jumpered them off when done,,LOL

 

[Alan Case]

Real easy and quick fix. Connect the drive direct to the motor, throw the overload away, use the contactor (1 leg) to control the stop input of the drive. When you get more time replace the contactor with a relay.
Regards Alan Case

 

[DickDV]

You're ok, Fred. We realize you inherited this mess. And, trying to make it better is noble work. Hopefully, your superiors will recognize it and appreciate it.

My comments were intended, as always, to be helpful, not only to the original poster, but to anyone who happens by on the BBS and shows some interest. And, as I said before, and now even more so with your diagram, it is an excellent example of a learning opportunity.

 

[Fred Raud]

this diagram(though too big to put in post) is what i made up this morning for a two inverter system for two conveyors,,i welcome any critisism being i just emailed it to my boss for an ok to go forward with that on wednesday and hopefully will instill confidence enough in me to tackle some more fun stuff


Fred Raud

 

[kamenges]

I think Fred has a handle on this concept now. However, his confusion is quite common and I want to make sure it is addressed again.

Most if not all electric motors are NOT by their nature constant horsepower devices between zero and rated speed. Fred originally made a relatively common mistake. He said 'I have a 1HP motor, it is running at 25% its rated voltage, to get my rated horsepower I need 4 times the rated current'.

While his logic makes sense for a constant horsepower device, it falls apart in that motors are NOT constant horsepower devices from zero to rated speed. They are constant torque devices. As several people said through this post, a motor can only deliver its rated current for any serious period of time. Any more and the motor will overheat. This leads to the variable horsepower nature of motors from zero to base speed. A 1HP motor is only capable of 0.25HP continuous if it is run at 25% of base speed. That same motor is capable of 0.5HP at 50% base speed.

This is because motor losses are relatively independent of motor speed up to base speed, especially for three phase induction motors. So the thermal limt shows itself as a torque limit, not a horsepower limit.

Keith

 

[Fred Raud]

He said 'I have a 1HP motor, it is running at 25% its rated voltage, to get my rated horsepower I need 4 times the rated current'.

im not sure how much of a difference this makes,,but my concern was at 25% of its rated voltage ,,i was thinking it should be 4 times the current i would get at the rated voltage,,hp and rated current i wasnt concerned with,,

Fred Raud

 

[darrenj]

Fred..I guess it all boils down to this...

Do you understand why you have less amps at a lower frequency?..If not i am sure people like DickDV can break it down for you..or i could try and explain in Electitians terms why..(I understand why but dont understand the theory..I can explain it..It might not be 100% corredct but you will get the jist of it)..

Forget if you have contactors or overloads before or after the drive..these thing are only relevent when you grasp the concept of what the drive does..

Please dont take this as a "what you dont know" post...Beleive me i just got my head around it not to long ago..sometimes we can be overcome by the techno speak..

D

 

[kamenges]

Fred-

I extrapolated your thinking, maybe incorrectly. So here might be a good place to ask the question directly.

Why did you think you would get 4 times the current if you had 1/4 the voltage?
Keith

 

[darrenj]

Fred-

I extrapolated your thinking, maybe incorrectly. So here might be a good place to ask the question directly.

Why did you think you would get 4 times the current if you had 1/4 the voltage?
Keith

If i may be so bold...

As a sparky i was told that if the motor was run on less voltage the ampage goes up..eg a 10hp motor 110v will draw 5 amps at 240volts..so i see where fread came from..

D

 

[Fred Raud]

480vac 3phase motor that pulls 1.5 amps, will pull 3 amps when wired for 240vac,,,thats where i am stuck on that train of thought

Fred Raud

ps,,darrenj,,i was hessitant to even bring up the overloads,,contactors,,etc,,but brought it up so people could better follow my train of thought and maybe cut me off where i was going astray,,i guess it didnt work,,lol

 

[rsdoran]

480vac 3phase motor that pulls 1.5 amps, will pull 3 amps when wired for 240vac,,,thats where i am stuck on that train of thought

Fred Raud

ps,,darrenj,,i was hessitant to even bring up the overloads,,contactors,,etc,,but brought it up so people could better follow my train of thought and maybe cut me off where i was going astray,,i guess it didnt work,,lol

Fred you forgot one thing, a motor is an inductor and the impedance is a produt of frequency, therefore when you lower the frequency you will change the current or in the case of a VFD you will limit and/or control the current drawn to a specific value.

The above highlighted statement pertains to running at 60HZ.

 

[kamenges]

Originally posted by Fred Raud:

480vac 3phase motor that pulls 1.5 amps, will pull 3 amps when wired for 240vac

The thing to keep in mind is in the case you describe with a dual voltage motor you are rewiring it to get it to operate that way. You are changing the characteristics of the motor by changing its wiring. As Ron said, you are changing the inductance (I THINK by a factor of 4). This will lower the ultimate resistance to current flow and allow a greater current through the motor at a lower voltage.

Contrast this with a motor connected to a drive. You aren't rewiring the motor. You are just changing the voltage and frequency to make the motor operate at a different speed. So the fundamental basis of the motor for the given winding configuration don't change. For example, a 1HP 3-phase 460VAC motor will draw about 2.1 amps at rated load. For that winding configuration you can't run at over the 2.1 amps for any real period of time or the motor will overheat. It doesn't matter what speed it is running or what voltage it is seeing.

Also, as Ron said, the drive is chaning both the voltage and frequency at the same time. So the motor wired for 460 VAC and run at half speed will not be running at 230 VAC 60 Hz, but 230 VAC 30 Hz. Changing the voltage and frequency together maintains a voltage/impedence balance in the motor that keeps the current relatively constant regardless of speed.

Keith

 

[Fred Raud]

thank you all for your posts and help,,my work thinks im a bit batty for going so deep into this stuff,,but i think it pays off in the long run,,

i found some links that relate,,
http://www.peekspump.com/VFD.html
http://www.ecmweb.com/mag/electric_understanding_variable_speed_3/
[size=-1]www.integralcontrols.com/Single%20to%20Three%20Phase.doc

and an interesting article that may belong in that thread on the voltage glitching snake oil thread
http://www.pdma.com/VFDtest.html

Fred Raud


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