Can a 240 Volt motor be run from 120 Volt power? - Page 1
Forum section: electronics / beginners

author: e2e8
author_rank: Newbie
timestamp: January 12, 2013, 05:19:50 am
content: Can a 240 volt 1/3 hp induction blower motor be run from 120 volt mains?Thanks

author: SeanB
author_rank: Super Contributor
timestamp: January 12, 2013, 05:51:43 am
content: Yes, but it will have a much lower power output, probably maximum of 1/10 hp power delivery.

author: johnwa
author_rank: Frequent Contributor
timestamp: January 12, 2013, 11:03:32 am
content: If you try running it directly from 120V, keep an eye on it to make sure it doesn't overheat. You really should keep the voltage to frequency ratio the same as what it was designed for. So, if it was a 240V/50Hz motor, then the 120V supply needs to be at 25Hz (which is probably not practical).Otherwise, the motor speed will still be reasonably close to the original synchronous speed, the blower will still absorb nearly the same amount of mechanical power, and the motor will draw excess current and overheat.

author: SamD
author_rank: Contributor
timestamp: January 12, 2013, 03:31:29 pm
content: Is this a hypothetical question or are you trying to use what's available? Look at the nameplate to determine if the motor is multi-voltage; typically you'll see something like "120/240 VAC" indicating the motor can be wired for either V. It will require rearranging the wiring under the cover plate (where the hookup is) but this is straightforward.As for running an induction motor at half-voltage with no changes, there's gonna be smoke, sooner than later.

author: e2e8
author_rank: Newbie
timestamp: January 12, 2013, 05:01:39 pm
content: This is not a hypothetical question. The motor says for voltage 208/240. I am a bit confused about the responses though. Is is going to produce less power which the first answer suggests or is it going to draw excess current and overheat(it is thermally protected)?

author: ivan747
author_rank: Super Contributor
timestamp: January 12, 2013, 05:22:29 pm
content: Itakes some sense because the energy is dissipated as heat instead of being transmited mechanically. *maybe* the excess current has something to do with the inductance, but my theory in inductors is rougth.

author: IanB
author_rank: Super Contributor
timestamp: January 12, 2013, 05:38:24 pm
content: Quote from: e2e8 on January 12, 2013, 05:01:39 pmIn the normal way a device needs to be operated within the specification on the rating plate. If a motor is designed for 240 V it is really not going to operate satisfactorily at 120 V. The difference is too great.

author: SeanB
author_rank: Super Contributor
timestamp: January 12, 2013, 05:51:50 pm
content: If it is a 208/240 motor you need to plug it into a drier socket with 2 hot wires to run it from the 208v the drier uses.

author: IanB
author_rank: Super Contributor
timestamp: January 12, 2013, 06:53:00 pm
content: Quote from: SeanB on January 12, 2013, 05:51:50 pmI'm curious about this comment--in which country or countries does a drier socket provide 208 V?Of course it doesn't help that e2e8 has not told us which country they are in? Please tell us, it may help.Here in the USA a drier socket has the following four connections:L1: hot 120 VL2: hot 120 VN: neutralG: groundL1 & L2 are in anti-phase, so that L1-L2 provides 240 V.If you are in the USA therefore, and you have a 240 V single phase motor, a 240 V supply can be found in most homes. If there is no unused drier socket nearby an electrician can most likely provide a 240 V socket in a garage or workshop.

author: SeanB
author_rank: Super Contributor
timestamp: January 12, 2013, 07:11:58 pm
content: 208V is the phase difference on a 3 phase 115V supply. Not often seen in the USA, but common enough. Normally USA has a split phase, unless you have a 3 phase supply where they give 3 115V phases and 4 wires.

author: e2e8
author_rank: Newbie
timestamp: January 12, 2013, 08:00:19 pm
content: Thanks for all the replays so far. I do have a dryer socket might be able to plug into. It is a nema 10-30 so it has hot, hot and neutral, 240 V.In case I couldn't make use of that 240V, I had wanted to know what I should expect from 120 V.

author: SamD
author_rank: Contributor
timestamp: January 12, 2013, 11:03:28 pm
content: Have a look at this:http://www.metalwebnews.com/howto/elec-mtr/elec-mtr.htmlIt provides a good, fundamental discussion of electric motors and their various, related issues.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 13, 2013, 06:28:41 pm
content: Quote from: IanB on January 12, 2013, 06:53:00 pmIn the US this is rare for individual houses but does happen in large apartment buildings that get 3 phase power distribution.

author: e2e8
author_rank: Newbie
timestamp: January 15, 2013, 12:06:19 am
content: I ran the motor on 120 Volts for a little while. It ran okay. It appeared to be running slower and/or with less torque but it did not seem to get excessively hot. I did not measure the current but I think is was not drawing more than the 2.7 A it would draw with 240 Volts.I have a follow up question: The 240 Volt outlet has 3 prongs: hot-hot-neutral. The motor has 3 connections: hot, hot and chassis connector for ground. Is it acceptable to connect the 240 neutral to chassis ground of the motor?

author: IanB
author_rank: Super Contributor
timestamp: January 15, 2013, 12:32:09 am
content: Quote from: e2e8 on January 15, 2013, 12:06:19 amIt's not ideal to do this as there could be some voltage present on the neutral wire, making the motor chassis unsafe to touch. Ideally you would run a ground wire from a grounded outlet nearby and attach this to the motor chassis instead.

author: SamD
author_rank: Contributor
timestamp: January 15, 2013, 05:59:35 am
content: "I have a follow up question: The 240 Volt outlet has 3 prongs: hot-hot-neutral. The motor has 3 connections: hot, hot and chassis connector for ground. Is it acceptable to connect the 240 neutral to chassis ground of the motor?"Ironically, the so-called neutral in a typical three-wire 240VAC residential installation is, in reality, ground. The older NEC code allowed the use of three wire installs even though many appliances (stove, dryer, etc.) used both 240 & 110- the 110 being used for things like timer motors, clocks, lights, etc..The flaw in this was the lack of a true grounding conductor; with the center prong used as the neutral leg for 120, it now became a current carrying conductor. At some time in the 80's-90's, the NEC revised the code to a four-wire install for any 240 circuit supporting both 120 & 240. Thus, the outlet would have four prongs- hot, hot, neutral & ground.Which gets us back to your situation: if you're trying to run the motor from your three-wire 240VAC outlet- but running it on 120 using the ground as neutral, you're still short one wire- the ground- or more specifically, you're short a neutral wire since you already have a ground wire (being used as neutral).This brings up the question: if you're using a 240VAC outlet, why not run the motor at rated voltage?

author: e2e8
author_rank: Newbie
timestamp: January 15, 2013, 06:20:45 am
content: Sorry for the confusion Sam. I was running the motor at 120 from a regular outlet and will run it at 240 from this outlet. So since there will be no current in this neutral wire I will use it as the chassis ground. thanks

author: lordulrich
author_rank: Newbie
timestamp: January 15, 2013, 03:52:14 pm
content: Do not connect the chassis ground to the ac mains neutral. While the nuteral is connected to ground code only allows a single point connection to ground from neutral, which happens at the breaker box. It is possible that you have the wire available for a 4 prong plug, as 3wireI pluss ground to cable is more available than 3 wire without ground. In which case it just requires some rewiring at each end and a new plug.

author: IanB
author_rank: Super Contributor
timestamp: January 15, 2013, 04:08:06 pm
content: Quote from: e2e8 on January 15, 2013, 06:20:45 amIt is not advisable to do this, even though the risk of dangerous voltages on the neutral wire is low. Do what I suggested above, get a ground wire from an outlet with a ground pin and use that to ground the motor frame.

author: elmo
author_rank: Contributor
timestamp: January 15, 2013, 06:38:38 pm
content: Quote from: e2e8 on January 15, 2013, 12:06:19 amThe current drawn by a motor is dependent on the load you put on it. If you tested it without load the difference in current draw maybe small, but with load it will be greater.The current rating on the motors nameplate is the nominal load current, the maximum continuous current drawn by the motor under load.

author: SamD
author_rank: Contributor
timestamp: January 15, 2013, 09:20:51 pm
content: Quote from: IanB on January 15, 2013, 04:08:06 pmIn a correctly wired, residential, three wire 240VAC branch circuit, the three wires equate to hot, hot & ground- NOT NEUTRAL. This is an all too common misconception brought about by years of using this ground wire as a "neutral" on appliances using both 120 & 240. Check the NEC or review any current wiring manual if uncertain about this.

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2013, 05:06:54 pm
content: If there's no 240V supply available, you could use an auto transformer to get 240V from 120V. A 350VA unit should do and won't be very heavy (about 1.6kg or 31/2lbs).

author: IanB
author_rank: Super Contributor
timestamp: January 17, 2013, 05:29:07 pm
content: Quote from: SamD on January 15, 2013, 09:20:51 pmThis is not a misconception, it is accurate in the case under discussion here: an old style three pole electric dryer socket (NEMA 10-30). In the NEMA 10-30 electric dryer socket the three wires are hot, hot and neutral--there is no ground connection. This socket is no longer allowed in new installations but it is often found in older premises.

author: SamD
author_rank: Contributor
timestamp: January 18, 2013, 02:18:24 am
content: Quote from: IanB on January 17, 2013, 05:29:07 pmNote my statement- correctly wired- as would be the wiring for a typical 240VAC water heater or other 240 device. I will concede your point, Ian, that the NEMA 10-30 socket identifies the three poles as H-H-N. My comment regarding misconception is due to the use (prior to '96, I believe) of 6-2 & 10-2 when running branch circuits I.E., using the bare ground wire as a conductor (neutral) with the NEMA 10-30 socket config. People unfamiliar with mains wiring thus confuse this with being ground- since the bare wire in typical cabling signifies ground.That said, I think we're both on the same page but, perhaps, seeing things from a different perspective. The thing to do in this instance, I think, is swapping out the existing socket for a NEMA 6 series which identifies the poles as H-H-G, eliminating the confusion regarding N-G.For the purpose of discussion, I'll call the wire in question "the center wire". With this in mind, if there current floating on the center wire, there's a problem at the panel or- worse- the branch circuit has been compromised somewhere between the panel and outlet; this is making the assumption an incorrectly wired sub-panel isn't involved.It's tricky when talking about grounding things to anything other than the panel ground. Water pipes, etc. are no longer considered acceptable due to the use of non-conductive plumbing materials- PEX, dielectric couplings, etc. In light of this, I believe using the "center wire" is the best bet for a safe grounding path rather than a separate wire to an uncertain grounding point.A clarification on my earlier statement: the use of "three wire circuit" is incorrect- it would be a two wire 240VAC branch circuit- the ground is not counted as a conductor- hence, 6-2, 10-2 etc.