Register to reply 
Question on Ohm's law and motorsby sharnrock
Tags: motors 
Share this thread: 
#1
Jun2610, 07:45 PM

P: 14

I am an electrician, and I recently got into some trouble on an issue. We were trouble shooting a fan motor that was rated at 240v and was only getting 208v. The more experienced electricians I was with said the amperage was going up because there wasn't enough voltage. This makes no sense to me. I get that watts=volts*amps, and that formula makes sense to them as to why the amperage is going up, but I remember in my apprenticeship class when you have a dual voltage motor you can wire the stators in either series or parallel to change the resistance and keep the output equal regardless of the voltage. So, V=IR makes sense to me.
Basically every electrician I talked to, and I talked to a lot of them, including a few masters and a contractor told me I was wrong. So, I guess my questions are (1)When voltage goes up does amperage go up? and (2)Regarding NEC 430.250, why do the full load amps seem to support their views. 


#2
Jun2610, 08:54 PM

Sci Advisor
P: 4,016

Your friends are talking about the effects of back EMF on the current drawn by a motor.
Here is a quote from a similar thread a few days ago: If you imagine a motor that is already turning, you have a coil rotating in a magnetic field, just like a generator. There is a voltage generated that opposes the current flowing into the coil from the supply. This causes a reduction in supply current as the motor turns. If you stall the motor, this generated voltage vanishes and the motor then draws a much larger current. This effect is called Back EMF or Counter EMF and is very important in motor behaviour. You can read more about it here: http://en.wikipedia.org/wiki/Counter_emf HOWEVER, the load on a fan depends on how fast it is turning and it is common practice to reduce the voltage on a fan to reduce its speed. This is done without any ill effects, except that the fan runs slower. This is because the fan gets less back EMF as it runs slower, but it also gets less load (because the fan has less wind resistance at the slower speed) so the current does not increase as it would with a constant, heavy load. 


#3
Jun2610, 09:40 PM

P: 3,014

An electric motor is not a resistor.



#4
Jun2610, 09:45 PM

P: 4,663

Question on Ohm's law and motors
If the motor is an induction motor designed for 240 V ac 60 Hz, its quasisynchronous RPM is ~1740 RPM. If you reduce the voltage to 208 V ac, it still wants to run at 1740 RPM (and deliver the same torque and power). To do this, it must draw more current.
Bob S 


#5
Jun2610, 09:57 PM

P: 14

My friends were definitely not talking about back emf. I believe there is a HUGE misconception in the trade about how electricity works and it just baffles me. I just got out of apprenticeship school and people that having been doing this work for 20 or 30 years don't seem to have a clue as to how stuff works. I was told when voltage goes down amperage goes up and that is regardless of the situation. Even something with a static resistance like a water heater element has the same effect.
for instance, 60w bulb rated for 120v draws .5 amps. They believe if you supply that light with 30v you will increase the amperage to 2 amps. So, I ask, "if you put 400v in the same bulb why does it explode or burn out?" They're answer is usually because the 400 volts blew it up. I try asking why #12awg is rated for 20 amps and not volts, and I just get blank stares. I just can't accept what they're telling me because none of them can explain it to me with any logical sense. That's why I'm here, because I figure I'll either come away with an explanation on why I'm wrong or find that I'm right, which might be worse. Because there are A LOT of people that feel I'm wrong. Anyway, thank you for your response. 


#6
Jun2610, 09:59 PM

P: 14




#7
Jun2610, 10:03 PM

P: 3,014




#8
Jun2610, 10:06 PM

P: 697

In any situation where the system is going to provide nearly constant power, even if the voltage changes a little, you can expect the current to go up when voltage goes down. This may not be always an exact relationship (I mean constant power), unless you have an actual feedback control loop that maintains constant power. 


#9
Jun2610, 10:09 PM

P: 14

This is the explanation I keep getting, and it just doesn't make sense. 


#10
Jun2610, 10:13 PM

P: 14




#11
Jun2610, 10:15 PM

P: 3,014




#12
Jun2610, 10:16 PM

P: 697

I think the answer is really dependent on the situation. Your way of looking at it has a reasonable basis; however, if you observe a system in which a small change in voltage results in an inverse change in current, you likely have a nearly constant power situation. Mechanical power is torque times rotational speed, so the motor may slow down and have more torque capability at lower voltage. The details matter and the situation is not always simple. 


#13
Jun2610, 10:25 PM

P: 14

wow, good information. So, you're saying if the voltage only migrates a little the amperage could actually go up? Now, would it, let's say, double if the voltage gets cut in half?



#14
Jun2610, 10:29 PM

P: 14




#15
Jun2610, 10:30 PM

P: 697




#16
Jun2610, 10:33 PM

P: 4,663

[added] When the voltage is reduced slightly, say 10%, the power factor usually increases. Bob S 


#17
Jun2610, 10:40 PM

P: 697




#18
Jun2610, 10:52 PM

P: 14

I tried an experiment with a dimmer and an amp probe on my bench grinder. What I found wasn't what I expected. I think dimmers are made a little different today, so I'm not sure if what I found was accurate. I was definitely able to make the motor run slower with less voltage, but the amperage stayed the same once it got up to whatever speed it could. I don't really have the right tools though for that kind of experiment.
for instance: 60v 1000rpm 2amps 90v 1200rpm 2amps 120v 1400rpm 2amps not actual results, but an idea of what I got. 


Register to reply 
Related Discussions  
Ohm's law question  Introductory Physics Homework  0  
Ohm's Law Question  Introductory Physics Homework  1  
Question about DC motors  Electrical Engineering  4  
Really dumb Ohm's Law question  Introductory Physics Homework  2  
Quick question about Ohm's Law  Introductory Physics Homework  3 