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# AC Motor, on DC supply???

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#### pike

##### Member
Hi guys,

I am a bit curious about AC motors and DC motors. Would it be possible to run an AC motor off a DC current of the equivalent voltage ? And how do these motors differ physically???

Can someone explain what really happens in a 3 phase motor? My only guess about this is that each live-wire controls 120 degrees of the revolution or 1/3 of the revolution.

And i think this could be true but since that our house hold power is at 50 to 60 hertz, wouldn't that mean the turbines at the power station are spinning between 3000 and 3600 RPM??? (60 sec x hertz)

And lastly when they say 230v as-in like house hold power do they mean 230v PEAK or Root Mean Square (RMS)

Sorry guys about these wierd questions, i'm not game enough to play with my house hold power until i get some knowledge (we all had to start some where)

I am a bit curious about AC motors and DC motors. Would it be possible to run an AC motor off a DC current of the equivalent voltage ? And how do these motors differ physically???

No, not directly. You have to transform the dc into the proper ac and that's an expensive process, although it's done all the time with variable frequency drives in industry. Common dc motors have a commutator and brushes on the armature (rotor) that switches the dc field's polarity as the armature turns so that there's always the proper magnetic attraction/repulsion to cause it to turn. There are brushless dc motors available, but there's a lot of electronics with that package to make it work.

Can someone explain what really happens in a 3 phase motor? My only guess about this is that each live-wire controls 120 degrees of the revolution or 1/3 of the revolution.

3-phase motors have one phase on each winding, spaced 120° apart in the motor. As time progresses through the cycle, you'll find that these cycles produce a constantly changing magnetic field within the motor such that the field ratates within the stator, essentially pulling the rotor around. There's a bit more to it of course, with transformer action and such, but it makes for a very simple motor. The only moving parts in a 3-phase motor is the rotor and the bearings. A single-phase motor can get really messy with centrifugal switches, starting caps and such.

And i think this could be true but since that our house hold power is at 50 to 60 hertz, wouldn't that mean the turbines at the power station are spinning between 3000 and 3600 RPM??? (60 sec x hertz)

The power company has multiple windings on their generators so that they produce several cycles of power with each rotation of the generator shaft. It would be hard to imagine a turbine in a dam spinning at a speed like that, wouldn't it?

And lastly when they say 230v as-in like house hold power do they mean 230v PEAK or Root Mean Square (RMS)

The standard 120/240 systems are always referred to in RMS units. 120v RMS is nearly 170v peak or 340v peak-to-peak as seen on an oscilloscope.

Sorry guys about these wierd questions, i'm not game enough to play with my house hold power until i get some knowledge (we all had to start some where) Thanx in advance

No need to be sorry. The folks who lurk on this forum live to answer questions like this.

Dean

I am far from being the motor expert and Dean's response is great. One thing I'd add - it's my understanding that there are "universal" motors that will run on AC or DC. I don't think you can convert a motor to be universal. As far as I know the uses are limited to small appliances, fans, etc. I don't know how they work.

stevez said:
I am far from being the motor expert and Dean's response is great. One thing I'd add - it's my understanding that there are "universal" motors that will run on AC or DC. I don't think you can convert a motor to be universal. As far as I know the uses are limited to small appliances, fans, etc. I don't know how they work.

Motors used in things like mains powered electric drills and vacuum cleaners are usually AC/DC motors - which is why you can do cheap and easy speed control on them. I've never paid much attention to how they work, but they look like normal DC motors with brushes (but without permanent magnets) - I suspect the armature and field coil are wired in series (like a car starter motor), this makes the rotation direction independent of power supply polarity.

And the universal motor run half DC voltage as AC rating.

It is my understanding that AC/DC motors have finer laminations to reduce eddy currents in the iron.

Thanx Dean,

You deserve a medal!

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