In North America, many single phase motors motors in the range of 1 hp to 2 hp canbe rewired to run at either 120 volts or 240 volts (or 115 vs 230 volts, itdepends on what voltage is assumed "nominal").Such motors will typically have six leads coming out of the motor to the wiringbox, or some of the connections may be screw terminals.The best way to change the voltage on a motor is to follow the wiring diagram onthe label. But sometimes when you open up a motor, there"s just six wiresand no diagram! This happened to be the case for the 1.5 hp motoron my old table saw.20 years ago I wired it to 240 volts, but I wanted to switch it back to 120 volts.for where I moved it to.
You are watching: 230 volt single phase motor wiring
Internally, the motor has two 120 volt windings, which are in serieswhen the motor is wired for 240 volts (left, at left).When switching it to 120 volts, the two windings are reconfigured to be in parallel.It would be easier to connect A to C, and then connect power to B.But this will switch the polarity of the winding between A and B, which means windingA-B would be fighting winding B-C. If you then plugged it in like that, themotor would draw about 100 amperes, but it wouldn"t run. If the circuit breakerdidn"t pop right away, the motor would start to smoke within ten seconds.
But it"s not that simple: There is also a starter winding
But it"s actually more complicated than illustrated above. The motor also hasa starter winding, which is in series with a starter switch and starter capacitor(see red outline at left). The starter winding is only active while the motoris spinning up to speed.If the starter winding and capacitor also needed to be reconfigured for voltagechanges, the wiring would be quite a nightmare!So instead, the starter winding in these motors is always a 120 volt winding,and the motors two 120 volt windings are used as an autotransformer to make the 120volts for the starter winding. Reconfiguring between 240 and 120 volts is donethe same way, but the starter winding stays connected to one of the windings.If you don"t have a wiring diagram, and the motor is currently wired for 240 volts,you can identify point "B" by the fact that it isn"tconnected to either power lead. Using an ohm meter, check whichof the three wires from B lead to the power lead with just one wireattached to it. That"s the one you need to disconnect and connect to C.And the winding end at A needs to be brought to B.
After working this out, I realized, 20 years ago, I moved the starter capacitor mount onthis motor so it wouldn"t protrude above the table saw table when the blade istilted 45 degrees. And in moving the capacitor, I ended up mounting it right overthe motor"s label plate, which also shows the wiring.So removing the capacitor cover, I could see the label, complete with wiring diagramand I was able to check my work before I plugged it in.
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Working out the wring for a 120/240 volt single phase motor without at wiring diagramSuppose you have a mystery single phase induction motor, 1750 rpm or 3500 rpm(or very close to those RPMs). There are six leads, or wires coming out of it.How do you wire it up?On some motors, it will be six connections, but some of them may be screw terminalsin the wiring instead of wire leads. I"ll just call them leads.If the motor has screw posts for attaching the leads to,it will typically have an additional screw post for connecting wires together in240 volt operation, but the screw post doesn"t connect to anything in the motor.Using an ohm meter, find a pair of leads that has less than 5 ohms between them.The reading should not change as you hold the meter on those. Label thesewires 1 and 2. 1 and 2 shouldn"t have any conductivity to any other leads coming out.Now find another pair of wires with the same resistance as 1 and 2 between them,Label these 3 and 4. 1-2 and 3-4 are the main windings.
The remaining two leads should connect to the starting capacitor, starting switch,and straiting winding in series (with the motor not running, the starting switchwill be closed).Label these remaining leads 5 and 6.If you measure the resistance between 5 and 6, you should see thereading on your meter continually increasing (set your meter to something otherthan the lowest ohm range). If youswap the meter probes between 5 and 6, the resistance reading will be lower again,but again go up. You are measuring resistance across the capacitor, and as it"charges up" from the meter applying current to measure,the resistance reading will go up.For 120 volt operation, you need to either connect1,3,5 to one power lead and 2,4,6 to the other OR 1,4,5 and 2,3,6. But which one??If you get it wrong, you will blow the circuit breaker or destroy the motor.Basically, if winding 1-2 is opposing winding 3-4, very bad things happen.You can, for a short time, run the motor off 120 volts using just one of the 120 volt winding.So just leave leads 3 and 4 disconnected. Connect one power lead to 1,5, the otherto 2,6, and plug it in to 120 volts. The motor should run.Unplug the motor, now add lead 3 to 1 and 5 (1,3,5 and one of the power leadsall together), and leave just 2,6 to the other power lead.Plug in the motor, while it runs, measure the voltage between the remaining unconnected lead 4 and the other lead supplying power, connected to leads 2,6).If the voltage is less than 10 volts, then youcan connect lead 2,4,6 together. Your motor is now wired for 120 volts.If the reading is over 200 volts, then you need to swap leads 3 and 4.Re-label lead 3 as 4, and 4 as 3, then repeat the step above and make sure the differencereading is less than 10 volts.To reverse the motor, swap leads 5 and 6 (the ones that go to the starter winding)To wire the motor for 240 volts, connect lead 1 to one power leadconnect leads 2,3,5 together (without connecting them to either power lead)Connect the other power lead to 4,6.If the motor has screw posts in the wiring box, there will be an extra screw post,not connected to anything, for connecting leads 2,3,5 together.And as before, to reverse the motor, swap leads 5 and 6If this doesn"t work for you, it"s possible that the motor is not a dual voltagesingle phase motor, or there is something wrong with it. Feel free to email me.I probably won"t be able to help you, but it"s useful to know whereyou run into problems. That way, if a lot of folks get hung up on the sameproblem, I might be able to add some notes about that.And if you blow up a motor or it catches fire, don"t blame me!