I draw in 120/240 1Ø motor

  • #1
Where an AC induction motor can be wired for 120v or 240v, will the total amperage be the same no matter how you wire it?

The way I understand it is, since the 120V wiring has 1 conductor it is carrying all the amps. I realize the neutral plays a part but it does not have amperage flow. The 240v circuit has half the amperage flow per conductor, but since there's two conductors, isn't the total amps the same for the motor?

If this is not true then I don't understand a lot of things. Even if it is true I still do not understand a lot of things!

I'm just trying to figure out how a motor that can run on 120 or 240 can cost essentially the same amount of money to run, and the 240v wiring arrangement is drawing half the amps. I do understand that increasing volts decreases amps all things being equal, but when you wire a 120/240 motor for 240, it changes the way the internal windings are wired so it is not equal. How confused am I on this?
 

Answers and Replies

  • #2
Averagesupernova
Science Advisor
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In single phase both wires carry the same current. It doesn't matter if it is 120 volts or 240 volts.
 
  • #3
586
2
Where an AC induction motor can be wired for 120v or 240v, will the total amperage be the same no matter how you wire it?
Current at 240v will be half that required at 120v. Look up Ohm's Law.
 
  • #4
887
98
I'm just trying to figure out how a motor that can run on 120 or 240 can cost essentially the same amount of money to run, and the 240v wiring arrangement is drawing half the amps.
You do not pay for amps, you pay for watts. And watts = volts * amps.
 
  • #5
I guess I can't wrap my head around it somehow. I understand Ohm's law clearly. A motor wired for 120 pulls 10 amps (for example) through the one conductor (L1). The same motor wired for 240 pulls half the amps (5) but through both conductors (L1 and L2).

When I put my amprobe on each main supply conductor at the panel for instance, while the 120v load is running, I will see the 10 amps on one of the conductors (provided no other loads are switched on). When I run the same motor wired at 240V my amprobe reads 5 amps on each leg. That says to me the motor is still pulling 10 amps no matter how it's wired. :confused:
 
  • #6
vk6kro
Science Advisor
4,081
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The motor is drawing 5 amps on 240 volts.

The 5 amps that goes in one wire is the same 5 amps that is coming out on the other wire, just like the motor on 120 volts had 10 amps going in one wire and 10 amps out on the other wire, so it was drawing 10 amps.

You don't add them up, because you are just observing the same current in two different places.

Each way the motor was getting 1200 volt-amps.
 
  • #7
Averagesupernova
Science Advisor
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I guess I can't wrap my head around it somehow. I understand Ohm's law clearly. A motor wired for 120 pulls 10 amps (for example) through the one conductor (L1). The same motor wired for 240 pulls half the amps (5) but through both conductors (L1 and L2).

When I put my amprobe on each main supply conductor at the panel for instance, while the 120v load is running, I will see the 10 amps on one of the conductors (provided no other loads are switched on). When I run the same motor wired at 240V my amprobe reads 5 amps on each leg. That says to me the motor is still pulling 10 amps no matter how it's wired. :confused:

Reread my first post. BOTH wires carry the same current at either voltage. Do not interpret what I am saying as 120 volt and 240 volt current are the same. Other posters have pointed out that they are not, and they are correct. What makes you think only one wire will carry the current at 120 volts? The neutral has to carry the same current the hot carries.
 

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