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I know that putting 240V through an appliance rated at only 120V would almost certainly damage it.

But what if I connect an appliance rated for 240V into a 120V AC socket? Apart from degradation in performance, would there be any damage to the appliance itself?

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Not much happens at all :-) about half what you expect, at best. You probably won't damage it, but best not to have to find out. –  EJP Jul 13 at 8:24
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What's the -1 for? Any comments for it? –  shortstheory Jul 13 at 9:04
    
Maybe because it is off topic here @ EE. DIY would be a better fit. Warning: Do not mess with mains voltage, it really hurts! –  Turbo J Jul 13 at 10:44
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A fan motor rated for 240V may not start running and the lack of cooling air flow may cause components (including the fan motor itself) to overheat. –  jippie Jul 13 at 10:49
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+1 to offset the stupid and pointless -1. A reasonable question. –  Russell McMahon Jul 13 at 17:07

5 Answers 5

up vote 7 down vote accepted

Probably nothing serious would happen as has been mentioned already but there is a possibility of a potential serious situation. Consider an appliance only intended for operation on 240V AC but is able to work from (say) 200V to 250V. To do so might mean it uses a switch-mode power supply to regulate the internal DC voltages. Let's say it required 100 watts internally, maybe some form of audio amplifier.

At 250 volts AC it would draw 0.4 amps plus 10 % more for inefficiencies - that's a current of 440 mA. At 200 volts AC it would draw 550mA. At 100 V ac it would try and draw a current of nearly an amp if it were able.

The point is that it will try and draw more current at a lower AC voltage and this could blow an internal fuse or damage the switching transistor - the average current may only be 1 amp but the switching current might be 10 amps. Also, at a lower voltage (with the increase in current) the reservoir capacitor after the bridge rectifier will be struggling to maintain low ripple and between cycles the dc voltage before the switching element may sag to only 50 volts - this means a higher instantaneous current draw on a cyclic basis and possibly more damage to the regulating switching transistor.

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There could be damage. With half the RMS AC voltage, that is, half the force pushing charge through the device, we might expect half the current flow. If the device acts like a simple resistor, that's exactly true. That means 1/4 of the normal amount of power is used by the device.

If the device has capacitive or inductive reactance, and has nonlinear effects, then no. Still, with no specific device as subject discussion, we may as well assume one quarter the power usage.

If that power is primarily working a motor, then the motor will be spinning slower. (duh.) Some motors depend on spinning at a high speed to keep themselves cool. If it's not spinning fast enough, maybe it won't keep itself as cool. But at (probably) 1/4 the power, it's not getting as hot, either. Will friction or load keep the motor from spinning at all?

Whether the cooling effect is diminished in the same proportion as the motor heating, depends on the actual type of appliance, the load the motor is pushing, the presence of voltage regulating circuits, and for all I know, the appliance's astrological birth chart.

That's just considering basic motor physics. The range of parts and physical phenomena in a generic unspecified household appliance is vast, and so it is not possible to rule out some other way that half-voltage input could cause damage.

Short answer: without further info, it's guesswork, but the range of guesses must include the possibility of damage.

There is only one way to find out, assuming you can make the plug fit the socket...

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Thanks! "Plug fitting the socket" shouldn't be too hard with all the international adapters out there :P –  shortstheory Jul 13 at 9:06
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The rotational speed of an induction motor depends primarily on the frequency of the input voltage. The voltage itself changes how speed and torque relate. A lightly loaded induction motor will likely spin faster at 120V 60Hz than at 240V 50Hz. A heavily loaded motor will likely just stall instead. –  ntoskrnl Jul 13 at 10:18
    
Why are you so specific on motors while OP was quite generic? –  Vladimir Cravero Jul 18 at 6:49

In a linear situation (an electric blanket, for example), the power will just be reduced to 25%.

Switching power supplies such as PC power supplies (the kind with a slide switch to select the input voltage) will attempt to produce the required output power with the available voltage, and unless some kind of under voltage lockout or thermal protection kicks in could be damaged - the power devices will get much hotter than normal.

The most susceptible to damage are appliances like small refrigerators that require enough motor torque to get past the compressor torque humps. With low voltage in (such as a brown-out), the compressor can stall (reducing the motor back-EMF to zero) and thus draw a much higher current than usual, all of it converted to heat. As a bonus, any cooling fans will not operate at full efficiency, if at all.

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If you have a 240 VAC motor which is running at 90% efficiency and is putting out one horsepower into a load, then since one horsepower is 746 watts, it'll be consuming about 75 watts in order to deliver that 746 watts to the load.

That's about 820 watts, total, and since the input to the motor is 240VAC, it'll be drawing about 3.4 amps once it gets up to speed.

When it's just starting, however, it can easily draw ten times that current and dissipate it in the stator's winding resistance, so that power would be 240V * 34A = 8160 watts, and the stator's winding resistance would be 240V / 34A ~ 7 ohms.

Now, if you were to connect 120V to the motor and the static load on the shaft was high enough to keep the rotor from turning, then that 120V would see only the stator's 7 ohm winding resistance, and it would cause the statpr's winding to dissipate: P = E² / R = 120V² / 7R = 2057 watts!

Then, since the motor was designed, ostensibly, to rise a fixed temperature above ambient with 75 watts, steady state, being dissipated in the stator's winding, 2057 watts in it would certainly cause some damage after even a short time...

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Many electronic devices nowadays are rated for any voltage between 100 and 240V, for precisely this reason.

However, anecdotally, there are situations where too low a voltage can cause damage. A few years ago, I owned a particularly cheap phone, which came with a particularly cheap charger, which was only rated for 230V. When running on 115V, I found that the charger would not charge the phone, and indeed appeared to discharge it.

Given the low quality of both the phone and the charger, I suspect the charger simply transformed and rectified the voltage, which would mean the voltage dropped from 4.2V to 2.1V - too low to charge a Li-ion battery.

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