# 240 volt motor on 120 volts?

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#### linwue

##### Member
The company I work for has a floor buffer that has a 120v cord on it,a dual voltage motor120/240.They ran it for awhile and told me it wasn't running properly,I checked the motor leads and it was wired for 240v.
just wondering if it would damage the motor to run it 120v when wired for 240v

#### charlie b

##### Moderator
Staff member
Motors tend to be "constant power" devices. That is, the poduct of voltage times current will be approximately the same, no matter what voltage level you give to the motor. If you cut the voltage in half, you will double the current drawn by the motor. Take a look at the values in Table 430.248, and compare the same HP motor at two different voltage levels, and you will see what I mean. Thus, a motor wired for 240V that is powered by 120V is likely to be damaged, even destroyed, by excessive heat that results from excessive current.

#### linwue

##### Member
thanks

thanks

Thanks Charlie B. that's what I thought,I just needed some positive reinforcement

#### mattsilkwood

##### Senior Member
saw a carpenter wire his 120v air compresor into 240 over a weekend. i got back on monday and he asked me to look at it. when i asked him what happened he said i dont know that thing never ran so good for about 5 minutes.

then i wired the painters airless into wild leg once. luckily i caught it before any damage took place. he wanted me to leave it he said man i can paint the bar joists from right here on the ground.:grin:

#### cjnickjr

##### Member
charlie b said:
Motors tend to be "constant power" devices. That is, the poduct of voltage times current will be approximately the same, no matter what voltage level you give to the motor. If you cut the voltage in half, you will double the current drawn by the motor. Take a look at the values in Table 430.248, and compare the same HP motor at two different voltage levels, and you will see what I mean. Thus, a motor wired for 240V that is powered by 120V is likely to be damaged, even destroyed, by excessive heat that results from excessive current.
You lost me Charlie. I must be missing something, I don't get an under voltage damaging a motor connected for the higher voltage. Guess I need to hit the books again.

#### robbietan

##### Senior Member
the motors I have seen would overheat from excess current if ran below 10% of their rated voltage. and this is 50% below its rated voltage.

#### wbenitez1128

##### New member
The reason the motor overheats is because it is forced to draw more current to make itself work. Table 430.248 clearly exemplifies that. For eg. a 1HP single phase motor's full load current will reach 16 amps at 120V vs 8 amps at 240V. This is what charlie b is saying (p=vi) whereas the p or volt-amps will need to be the same at any voltage. In our 1HP case we need to reach 1920VA.

#### grich

##### Senior Member
mattsilkwood said:
saw a carpenter wire his 120v air compresor into 240 over a weekend. i got back on monday and he asked me to look at it. when i asked him what happened he said i dont know that thing never ran so good for about 5 minutes.

then i wired the painters airless into wild leg once. luckily i caught it before any damage took place. he wanted me to leave it he said man i can paint the bar joists from right here on the ground.:grin:

During construction of our building 25 years ago, the receptacles on one floor weren't hot yet, but the lighting was. A drywaller decided to hang his own outlet out of a switch box by tying to the side of the switch, and to the conduit for his neutral. The color of the switch wires was orange...you can guess what happened to his screw gun.

#### tdj1000

##### New member
charlie b said:
Motors tend to be "constant power" devices. That is, the poduct of voltage times current will be approximately the same, no matter what voltage level you give to the motor. If you cut the voltage in half, you will double the current drawn by the motor. Take a look at the values in Table 430.248, and compare the same HP motor at two different voltage levels, and you will see what I mean. Thus, a motor wired for 240V that is powered by 120V is likely to be damaged, even destroyed, by excessive heat that results from excessive current.

Charlie, I have to disagree with you on this one...

Motors tend to be constant power devices when wired correctly, ie 120 on a 120 circuit and 240 on a 240 circuit. If you plug in a 240 motor into a 120 circuit it will draw half the current and in turn, half the power.

The table in 430.248 is correct IF the motor was wired for the correct current.

Another way to look at this is to measure the resistance of the motor coil. Say the motor is wired for 240 volts and has a resistance of 40 ohms. Run this at 240 volts you have 240/40 = 6 amps (V=I*R). Leave the motor wiring the same, plug it into 120, you have 120/40 = 3 amps. Don't believe me? Try it yourself with a 240 volt motor, a 240 and 120 circuit, and your clamp-meter.

#### donselectric

##### Senior Member
over or under voltage does damage maybe not instantly but will rob years
of life from motors

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