What Am I Not Understanding?

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I just completed training in school for two years to become an electrician. However one thing I came across is really confusing me, and I hope one of you guys can clear things up because nobody else I know can. If you have a 120 volt motor operating at 60 amps, this means 60 amps will be running down one 120 volt leg. However the same motor 240 volt operates at 30 amps, this will have 30 amps running down two 120 volt legs, correct? If this is the case wouldn?t both legs equal out to 60 amps still? What confuses me if this was the case, then power would increase with voltage. So what am I not understanding here?
 

augie47

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Location
Tennessee
Occupation
State Electrical Inspector (Retired)
The flaw in the thinking may be with " then power would increase with voltage".
Think of Power (wattage) as a constant...
A motor that requires 60 amps at 120 volt uses 7200 watts (P=E X I )
A motor that requires 7200 watts on 240 volts would pull 30 amps... ( I = P/E )
 
No, I don't believe that power increases with voltage. It appears that way and that is my confusion. My question is, why is there 30 amps running down both 120 volt legs in this 240 volt motor, which would make it equal 60 amps still? I know there is something I am not understanding.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Try this....
Current flows from the source and back to the source.
In the cases you state, you have two wires..the current flows out thru one then back thru the other (an extreme simplification)...
The 60 amps it takes to produce 7200 watts @ 120 volts is flowing thru one line and back thru the other..a ammeter would show 60 amps on both lines..
It only takes 30 amps at 240 volt to produce the same effect...30 out on 1 line, the same 30 back on the other. In each case you are seeing the same current flow in each line.. it is not the additive of both...

Some one else may give you a better explaination...
 

handy10

Senior Member
Another way of saying what Augie said is that current is not flowing down each wire. It is flowing down on one wire and flowing back on the other; this is because the two voltages are 180-deg out of phase. On the 120 volt circuit, the 60 amps returns on the neutral so if you add the current on the neutral and the hot you would have 120 amps.
 

realolman

Senior Member
TRANSFORmer.JPG This is a ( not so good ) drawing of a transformer. Anything you connect will be eventually connected to the transformer. The primary side here is shown as being hooked to 2400 volts ... It could be much higher... or in some cases it could be lower.

The secondary side is where you connect your loads... in your example a motor.

There is a difference in potential between the two wires connected to the ends of the secondary winding of 240volts ...there is a difference of potential of 120 volts between either outside wire and the center tapped wire... usually the center tapped wire will also be grounded and is referred to as the neutral.

The difference in potential is caused by invisible waves (lines of flux) radiating from the primary winding because current flows through it. Because the current is alternating, it is continually rising and falling and reversing direction...the lines of flux cut across the windings of the secondary side ... the ratios of the windings determine the difference in the voltage from the primary side.

Any current flowing in a wire connected to any of the terminals on the transformer also has to be flowing in a wire connected to another terminal. Otherwise there won't BE any current flowing.

Appliances, heating elements,light fixtures, motors, or whatever loads you connect are engineered and constructed so that they can only "draw" a certain amount of current. In AC circuits their "impedance" ( resistance to A.C. current flow ) will only allow a certain amount of current to flow.

The apparent power is the amount of current flowing in the circuit multiplied by the voltage to which the load is connected.

In the scope of this discussion I think it would be acceptable to say that the watts of the circuit is the current multiplied by the voltage.

So the current is limited by design to a particular level, when connected to the voltage for which the load was designed.
 

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richxtlc

Senior Member
Location
Tampa Florida
In a 120 volt motor the windings are connected in parallel wye or delta with 120 volts across the parallel coils and 30 amps flowing in each coil of the parallel circuit. In the 240 volt motor, the coils are connected in series wye or delta with 120 volts across each coil and 30 amps flowing in each one. The power is the same in both motors as stated above.
 

kwired

Electron manager
Location
NE Nebraska
I just completed training in school for two years to become an electrician. However one thing I came across is really confusing me, and I hope one of you guys can clear things up because nobody else I know can. If you have a 120 volt motor operating at 60 amps, this means 60 amps will be running down one 120 volt leg. However the same motor 240 volt operates at 30 amps, this will have 30 amps running down two 120 volt legs, correct? If this is the case wouldn?t both legs equal out to 60 amps still? What confuses me if this was the case, then power would increase with voltage. So what am I not understanding here?

With the 120 volt motor you have 60 amps running down 1 line and 60 amps returning on the neutral. Voltage between 1 line and the neutral is 120 volts.

With the 240 volt motor you have 30 amps 30 amps running down line 1 and 30 amps returning on line 2. Voltage between line 1 and line 2 is 240 volts. Neutral is not required for this circuit.

If you have a motor that can operate at 120 volts or 240 volts the motor will have have taps in the windings made accessible for you to change the configuration of how the windings are connected. Windings will be divided into equal segments rated for 120 volts per segment. If operating at 120 volts, each segment actually has 30 amps of current flowing through it, but they are connected in parallel with each other and the total current of both adds up to 60 amps.

Now reconfigure the same motor for 240 volts and the two segments are connected in series, and 240 volts is applied to each end of the whole series. Each segment still sees a current of 30 amps, since the segments are equal half of the applied voltage drops acrosss each segment. So each segment still sees 120 volts applied across it.
 

kwired

Electron manager
Location
NE Nebraska
In a 120 volt motor the windings are connected in parallel wye or delta with 120 volts across the parallel coils and 30 amps flowing in each coil of the parallel circuit. In the 240 volt motor, the coils are connected in series wye or delta with 120 volts across each coil and 30 amps flowing in each one. The power is the same in both motors as stated above.

Op needs to understand simple single phase concepts before you start bringing three phase terminology into the picture.
 

SG-1

Senior Member
I just completed training in school for two years to become an electrician. However one thing I came across is really confusing me, and I hope one of you guys can clear things up because nobody else I know can. If you have a 120 volt motor operating at 60 amps, this means 60 amps will be running down one 120 volt leg. However the same motor 240 volt operates at 30 amps, this will have 30 amps running down two 120 volt legs, correct? If this is the case wouldn?t both legs equal out to 60 amps still? What confuses me if this was the case, then power would increase with voltage. So what am I not understanding here?

It might help to think like a clamp-on ammeter. With the 240 volt motor you clamp the meter around one ungrounded conductor and it reads 30Amps. If you clamp it around the other ungrounded conductor it reads the same 30Amps returning to the source. To verify it is the same 30Amps clamp the meter around both conductors & it reads zero not 60. Works the same way with 120 volts & 60Amps.

I used to have a hard time with this myself. I wanted to add when I needed to subtract. In my case it was just my blondness asserting itself.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
I am not sure if you have an answer to your question but the amount of wattage used is the same in 240v as it is in 120V wiring. 240v is more efficient as you can use smaller conductors and it is easier to distribute the load in single phase systems. In a sense you can say that 60 amps on 120V is the same as 30 amps on 240V as the wattage used is the same as Gus pointed out. In either case you basically are using 7200 watts.
 

charlie b

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Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
It is a good idea, early in your career, to forever forgo the notion that you add amps on the two (or three) legs of a circuit to get the ?total amps.? It is simply not true. If you have a balanced, three phase circuit that has 30 amps flowing on each leg, the ?total amps? is 30, not 90. You simply do not add amps that way. As others have already said, the amps flowing out on one wire will flow back on another wire, so it is the same amps. If you had a $10 bill in your left hand, and you transferred it to your right hand, would that mean you now have $20? No, because it is the same money, and it is simply moving from one place to another.

Welcome to the forum.
 

kwired

Electron manager
Location
NE Nebraska
It is a good idea, early in your career, to forever forgo the notion that you add amps on the two (or three) legs of a circuit to get the ?total amps.? It is simply not true. If you have a balanced, three phase circuit that has 30 amps flowing on each leg, the ?total amps? is 30, not 90. You simply do not add amps that way. As others have already said, the amps flowing out on one wire will flow back on another wire, so it is the same amps. If you had a $10 bill in your left hand, and you transferred it to your right hand, would that mean you now have $20? No, because it is the same money, and it is simply moving from one place to another.

Welcome to the forum.

If you will transfer a $10 bill from your left hand to my right hand I will let you know if that analogy is accurate. You may need to use a $100 bill for me to understand better:D
 
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