Question about stepdown transformers and amperage

[grrl]

A friend of mine has a food processor running through a 220-110 transformer. An engineer told him that the processor will work well that way, but eventually will burn out because the amperage is too high. I'm not sure what he means by that though.

Isn't the amperage of a transformer merely the maximum it is able to produce? Plus, amperage is a function of resistance and can't be 'pumped' like voltage, correct?

 

[Heisenberg]

That doesn't make a whole lot of sense. The processor should only pull as much current as it needs. As long as the voltage is right, I don't see how it could be damaged. Usually, voltage, current, and resistance are related by V=IR. However, transformers are non-Ohmic, so that relation doesn't hold for them.

 

[grrl]

Originally posted by: Heisenberg
That doesn't make a whole lot of sense. The processor should only pull as much current as it needs. As long as the voltage is right, I don't see how it could be damaged. Usually, voltage, current, and resistance are related by V=IR. However, transformers are non-Ohmic, so that relation doesn't hold for them.

That's what I think, but what if the frequency is 50 Hz instead of 60 Hz?

 

[Heisenberg]

Originally posted by: grrl

Originally posted by: Heisenberg
That doesn't make a whole lot of sense. The processor should only pull as much current as it needs. As long as the voltage is right, I don't see how it could be damaged. Usually, voltage, current, and resistance are related by V=IR. However, transformers are non-Ohmic, so that relation doesn't hold for them.

That's what I think, but what if the frequency is 50 Hz instead of 60 Hz?

I suppose if the frequency is off, that could cause a problem. But if the transformer is designed to step down 220 to 110 for the US, the output frequency should be 60 Hz. What exactly did the engineer guy say?

 

[PowerEngineer]

I'm not sure I really understand the situation.

If the food processor is designed for 110-120 volts, then it should be okay being fed by the 220-110 transformer -- assuming that the transformer is actually being supplied with 220 volts. It's not clear to me why you'd want to do this instead of just plugging it directly into a regular 110 volt outlet.

Basic transformer theory says that the voltages on the two windings are related by the ratio of the turns in the two windings. The more turns in the winding, the higher the voltage. For instance, in your 220-110 transformer, the number of turns in the 220 volt winding is twice the number of turns in the 110 volt winding. The currents in the two windings are also related by their turns ratios, but this time it's an inverse relationship. The more windings you have, the less current you get. In your 220-110 transformer will have twice as much current in the 110 volt winding as it does in the 220 volt winding.

If you think about this a bit, it makes sense. The power you're supplying into the transformer should be roughly equal (losses are small) to the power you're drawing out of the other winding, which means that the product of voltage times amps should be equal on both sides. This is true if the voltage on the food processor side is half of the supply voltage while the current is twice that of the supply current.

If the supply voltage is actually less than 220 volts, then the voltage supplied to the food processor will be less than 110 volts (i.e. half the supply voltage). Since motors tend to act like "constant power" devices, the food processor will draw more current if the voltage it sees is lower. Higher currents mean more losses inside the motor, which means more heating, which ages the insulation more rapidly and leads to quicker failure.

Hope at least some of this helps.

 

[grrl]

My friend can't just use a 110 outlet because he's in Korea where 220 is now the standard.

I can't say exactly what the engineer said because I'm getting it second hand and my friend didn't really understand except that he said the amperage was the problem.

 

[Heisenberg]

Originally posted by: grrl
My friend can't just use a 110 outlet because he's in Korea where 220 is now the standard.

I can't say exactly what the engineer said because I'm getting it second hand and my friend didn't really understand except that he said the amperage was the problem.

Oh...this guy's in Korea. I have no idea what voltage/frequency/number of phases their power uses. All I can say is that if the transformer is designed to step down 220 to 110 for that power system, there shouldn't be a problem. Otherwise, like PowerEngineer said, if it's not stepping down to the correct voltage/frequency it could cause problems like drawing too much current.

 

[grrl]

Korea uses 220 60Hz.

What I assume is my friend has just been given bad information.

 

[PowerEngineer]

Okay, your friend is using a 110v 60hz food processor in Korea, which is a nominal 220v 50hz service?

The transformer will take care of the voltage problem, but there is still a slight problem with the frequency change. Motors designed to run at lower frequencies require more iron to avoid saturating their cores as the flux changes more slowly. (That's why aircraft electrical systems run at 400hz to keep requiremenst for iron cores as small as possible.) Saturation also leads to higher motor currents, higher losses, higher temperatures, and shorter motor life. Under the circumstances, not something your friend probably needs to worry too much about.

The disadvantage of needing more iron in lower frequency systems is offset by their lower hysteresis losses in the iron (i.e. higher effeciency). Too bad we couldn't have all agreed to a common standard...


If Korea is 60 Hz, then I don't see any problem

 

[FrankSchwab]

Okay, so I think what I hear is:
your Friend has a Food Processor designed for the US (110v, 60Hz). They are in Korea (220V, 50 Hz). They have a stepdown transformer.

There are several things going on here.
The first is, the transformer will not convert the 50Hz power to 60 Hz. So, the food processor will be running at 110 V, 50 Hz. This may cause the food processor to draw more current, and get hotter, eventually burning it out.

The second is, the transformer may not be rated for the amount of current the food processor pulls through it. This will work for awhile, but the transformer will get excessively hot, eventually burning it out.

The third is, the transformer may have been designed for 60 Hz, not 50 Hz. Running it at 50 Hz. may cause the transformer to dissipate more heat within it's iron core, causing it to heat up and burn out sooner than normal. At 50 Hz, you need more iron in the core to keep it from saturating for a given magnetic field (n.b. this is why many aircraft run with 400 Hz. power - the transformers are even smaller).

/frank

Edit:
Damn power engineers :disgust: Forget everything I said about 50 Hz problems. The only real problem you might have is number 2.

 

[sgtroyer]

When I read that "the amperage is too high," what I assume he means is that the current draw of the motor is higher than the transformer can supply. Which may or may not be true, depends on the rating of the transformer. If it's too small, something will eventually give.

 

[dkozloski]

It's quite likely that a food processor would have a universal motor in which case the frequency of the current is irrelevant. In fact the unit would appear to have more power if a bridge rectifier were placed in series with the transformer and the processor so it would operate on DC which would eliminate all hysteresis losses. If the motor has brushes the afore mentioned assertion is correct. If not forget it. A bridge rectifier placed in series with most power tools can do wonders for performance.

 

[grrl]

Thanks for the replies.

From what I've read (and can remember) Korea, unlike China and India along with others, has 220/60. Not 220/50. Unless that is wrong, then I suspect the engineer is simply pushing more Korean myths, like leaving a fan running all night in your bedroom can cause you to suffocate.

 

[Mday]

Originally posted by: grrl
Thanks for the replies.

From what I've read (and can remember) Korea, unlike China and India along with others, has 220/60. Not 220/50. Unless that is wrong, then I suspect the engineer is simply pushing more Korean myths, like leaving a fan running all night in your bedroom can cause you to suffocate.

what are the power ratings of hte food processor ,and the transformer.

i'll say this, for a simple motor, the frequency doesnt matter. for a transformer, depends on how it was designed.

 

[ReiAyanami]

Isn't the amperage of a transformer merely the maximum it is able to produce?

that's exactly what the decepticons want you to think

more than meets the eye

 

[grrl]

Originally posted by: Mday


what are the power ratings of hte food processor ,and the transformer.

i'll say this, for a simple motor, the frequency doesnt matter. for a transformer, depends on how it was designed.

I don't know offhand, I'd have to ask. All I know is the rating on the transformer is about twice what the processor draws.