Can someone please tell me what exactly we are doing when we rewire a 120v induction motor to 240v. Are we adding in more stator wire that isn’t used in the 120v mode? Adding in a resistor? What?
The reason I ask, is because I can’t figure out why a motor rewired from 120v to 240v has the current rating dropped in half. For example I recently saw the nameplate data on a Jet table saw rated for 14 amps at 120v but only 7 amps at 240v.
I understand that at the same load the 240v motor runs at half the amperage as the 120v motor and cooler since the I squared R heat loss is one quarter that for the 120v motor. Since the motor runs cooler for the same load, and since heat is the major determinant of a motor’s rating, it seems to me we ought to be able to safely double the 240v motor’s load, ie HP rating.
The only thing that I can think of is that rewiring from 120v to 240v does not (for some reason) increase the pull out torque and so at over 7 amps load the 240v motor, while running cooler, is on the verge of stalling.
I used to be pretty well versed on motor theory, but that was years ago, and my knowledge base was in three phase stuff. To this day I barely understand how a capacitor start motor works, and the concept of rewiring a motor for a higher voltage was not on our radar screen. I just asked this question to two licensed electricians today and neither could give me a good answer.
Thanks,
Chris
Replies
> The reason I ask, is because I can't figure out why a motor rewired from 120v to 240v has the current rating dropped in half. For example I recently saw the nameplate data on a Jet table saw rated for 14 amps at 120v but only 7 amps at 240v.
Chris, there are two windings in there that are wired in parallel for a 120v feed. They are rewired in series for a 240v feed. Each winding will have the same voltage across it and the same current through it (for given RPM and load) in either case. The motor should do the same work. The only difference is the change in current through the wiring TO the motor (inc switch) and the voltage/current switched (inc back EMF). You need the wiring to handle this without allowing significant voltage drop or arcing.
> I understand that at the same load the 240v motor runs at half the amperage as the 120v motor and cooler since the I squared R heat loss is one quarter that for the 120v motor. Since the motor runs cooler for the same load, and since heat is the major determinant of a motor's rating, it seems to me we ought to be able to safely double the 240v motor's load, ie HP rating.
Gobbledygook to me and I've taken the theory (a while back). Seems to me that the output of that statement is based on bad input.
> I used to be pretty well versed on motor theory, but that was years ago, and my knowledge base was in three phase stuff. To this day I barely understand how a capacitor start motor works, and the concept of rewiring a motor for a higher voltage was not on our radar screen. I just asked this question to two licensed electricians today and neither could give me a good answer.
I posed that question to two fellow electrical engineers recently and neither could answer it either. When I provided the answer, they looked stumped at each other and said they'd 'think about it'. ;) I think they've been reading too many woodworking mags.
Andy
"Since the motor runs cooler for the same load..."
It does not. This seems to be a very prevalent misconception. The motor doesn't care one way or the other. Input power consumption is given by voltage × current. Double the voltage and halve the current, and the power consumption remains exactly the same. Likewise, the output power developed by the motor is also exactly the same. As Andy says, rewiring a motor in this way is simply a matter of changing from two windings in parallel (lower voltage, higher current) to the same two windings in series (higher voltage, lower current). See the attached sketch.
The only significant benefit of a 240V vs. 120V motor is in the facilities wiring. Since the current is cut in half, the wires themselves can be a smaller gauge (or, you can run more or larger motors from the same wiring). There can also be a cost benefit in the circuit breaker required, although that doesn't really have an effect until you get to very high currents.
-Steve
Thanks guys, I get it. It is very interesting that both the Dewalt rep (I have their hybrid table saw) and the Delta rep (I have their band saw) told me that I really should rewire their machines to 240v because they would "run a lot cooler". At the time, I did not know what rewiring actually did, so I had no reason to question either of them.
I can still do really basic circuit analysis, and it is clear that I squared R is the same in both cases, so converting to 240v does nothing to up the allowable load on the machine.
Thanks again,
Chris
Best use power = voltage * amperage, and the power is the same at 220 or 110
Its an inductive load, so I^2 R doesn't apply (that applies to the power of a resistive load). The resistance of the wiring is only a few ohms, so if not for the back EMF, etc., you'd throw a breaker. I^2 DOES apply to the resistive component of the load, however.
There is a bit of truth to the 'runs cooler' theory. Sort of. You run half the current through the motor windings, but the resistive component of the impedance is (roughly) doubled. Nonetheless, (1/2I^2) * 2R still gives you less in terms of resistive power disipation than I^2* R would have. However, as noted above, R is very small, and only a small part of the power disipation would be associated with the resistive component of the impedance.
If you halve current consumption, you halve the voltage drop on the wire feeding the motor, whatever that may be. A electric motor is designed to run at a particular voltage. At 120 volts, a 100' 14 gauge cable carrying 15 amps will drop 7.8 volts (http://genuinedealz.com/voltage-drop.html) and deliver only 93% of the desired voltage. The motor runs slower, generates less back EMF and gets warmer than if it were running at the design voltage.
At 240, the current and therefore voltage drop is half, but it delivers 98.3% of the design voltage. Runs faster, makes less heat.
Long story short, if you can run any machine at at 220 or 240, or whatever your flavor is do it. Its better.
"You run half the current through the motor windings..."
No, NO, NO!!! Whether the motor is wired for 120 V or 240 V, exactly the same current is running through the windings. The motor has two sets of windings. Each winding sees V volts and I amps (e.g., 120 V, 10 A), regardless of how the motor is wired. When the windings are wired in in parallel, the two together see V volts and 2I amps (120 V, 20 A). When the windings are wired in series, the two together see 2V volts and I amps (240 V, 10 A).
How many times do we have to explain it?
-Steve
Arg I did bugger the I^2 * R equation, you are right (sorry long day)Still the drop at the feed wire is directly proportional to the current and the motor would run less efficiently at 120 vs 240 as a result.of droop, unless its a very short run or a very big cable.
"Still the drop at the feed wire is directly proportional to the current and the motor would run less efficiently at 120 vs 240 as a result.of droop, unless its a very short run or a very big cable."
Right. Which is exactly what I and others said in previous messages in this thread.
-Steve
An induction motor does not run faster or slower due to voltage. The speed of an induction motor is fixed by the number of poles and the system frequency.
However, voltage drop has a big effect on motor torque. So running an induction motor undervoltage drastically reduces the torque. THEN when you load the motor heavily it tends to slow down and overheat.
> The only significant benefit of a 240V vs. 120V motor is in the
> facilities wiring. Since the current is cut in half, the wires
> themselves can be a smaller gauge (or, you can run more or larger
> motors from the same wiring).Which is another way of saying that the resistance is lower if you stay with the same size wires, so the motor will run cooler.George Patterson
Edited 12/5/2007 9:11 pm ET by grpphoto
Which is another way of saying that the resistance is lower if you stay with the same size wires, so the motor will run cooler
By what process did the wire change resistance?------------------------------------
It would indeed be a tragedy if the history of the human race proved to be nothing more than the story of an ape playing with a box of matches on a petrol dump. ~David Ormsby Gore
"Which is another way of saying that the resistance is lower if you stay with the same size wires, so the motor will run cooler."
Unless you're seriously overloading your facilities wiring, the difference is negligible. Let's look at a 1 HP motor as an example: The full-load efficiency is going to be around 85%. Of that 15% of inefficiency, copper losses make up about half, iron losses another third, and the rest is in bearing friction, cooling fans, etc. So, 1 HP = 750 W, and given the stated efficiency, we get a nominal current at 120 V of 7.4 A, along with 66 W for copper losses, and 44 W for iron losses.
Let's assume that the facilities wiring is AWG 12, and that the distance from the panel breaker to the motor is 50 ft. At 7.4 A, I get a voltage drop at the motor of almost exactly 2 V. So, our 120 V motor is actually running at 118 V, which means that the current has to increase accordingly, which results in an increase in copper losses of 2.3 W.
Now let's do the same calculation for 240 V. To weaken my own argument, I'm going to assume that the wiring is still AWG 12, even though it could be smaller, since the current is lower. The voltage drop in the lines is now 1 V, and the increase in copper losses in the motor is 0.6 W.
So, the difference is 1.7 W. But wait! There's more! What about the iron losses? Iron losses are proportional to voltage, so as the copper losses go up, the iron losses go down. The iron losses are reduced by 0.7 W in the 120 V case, and 0.2 W in the 240 V case.
So, after taking everything into account, the net difference in power consumption inside the motor is 1.2 W, not even enough to light a Christmas tree bulb.
In contrast, the difference in losses in the facilities wiring is substantially greater, about 11 W.
-Steve
> Let's look at a 1 HP motor as an example: The full-load efficiency is going to be around 85%.
Steve, while I would be nit-picking by saying that the efficiency is lower for this small motor and you didn't mention power factor, these are things lost on the greater crowd.
Its interesting to sit back and watch the responses, with some air of authority, discuss a subject that is beyond them. It would be like me describing the best way to sharpen a hand plane. ;)
I would leave this thread at "there's no appreciable benefit to rewiring" if not for the entertainment it provides. At least there is noone suggesting unsafe practices in this one.
Andy
"At least there is noone suggesting unsafe practices in this one."
Your 60 Hz-only motors will be perfectly happy running under full load on 50 Hz power.
There, feel better? ;-)
-Steve
"The reason I ask, is because I can't figure out why a motor rewired from 120v to 240v has the current rating dropped in half. For example I recently saw the nameplate data on a Jet table saw rated for 14 amps at 120v but only 7 amps at 240v. "
The current draw is the same. Think watts (volts X amps).
14 amps X 120 volts= 1680
7amps X 240 volts= 1680
> The current draw is the same. Think watts (volts X amps).
> 14 amps X 120 volts= 1680
> 7amps X 240 volts= 1680Its not that simple to calculate power for motors since they have power factors less than 1.0, but the power would be the same (lower than the numbers above) in both cases since the PF and inefficiencies are the same. Maybe Steve will draw another sketch... ;) Andy
The same amount of current runs through each winding whichever way they are connected. With 120V you get two sets operating at, lets say 10 amps each, for a total of 20 amps on the line. Hook them in series and both winding have 10 amps through them. Total line current 10 amps @ 240V.
Most reps are salespeople. I read such an outrageous statement by a rep on a company sponsored board I nearly fell over and the man spoke as if he were an expert.
Edited 12/4/2007 8:42 pm by polarsea1
"Since the motor runs cooler for the same load"
False info...it runs cooler on the higher voltage if running a longer cord. It will run cool on the lower voltage if you usea heavier cord. Obviously a heavier cord is more expesive considering the cost of copper.
> The reason I ask, is because I can't figure out why a motor rewired
> from 120v to 240v has the current rating dropped in half.
The simple answer is that it hasn't. You had a single 110 volt lead providing 14 amps. Now you have two 110 volt leads providing 7 amps each. The amount of current is the same.
George Patterson
Edited 12/5/2007 9:10 pm ET by grpphoto
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