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Posted by on October 14, 2006, 9:07 am
Jake wrote:
> stu@aaronj.com wrote:
> >
> > "At the same time torque is being produced, the conductors are moving
> > in a magnetic field and generating a voltage. This voltage is in
> > opposition to the voltage that causes current flow through the
> > conductor and is referred to as a countervoltage or back EMF. The value
> > of current flowing through the ****armature**** is dependent upon the
> > DIFFERENCE between the applied voltage and the countervoltage."
>
> For at least the second or third time... induction AC motors don't have
> an armature... they have a rotor. They don't have brushes and are not
> 'commutated' like some synchronous AC or virtually all DC motors.
>
> Reliance is talking about DC motors in the above quote... and the rest
> of this balderdash I couldn't find in the Reliance material so I guess
> you made it up....
>
> Jake
I stand corrected on mistakenly calling a rotor an armature.
>From wikipedia:
Every motor is a generator. This is true, in a sense, even when it
functions as a motor. The emf that a motor generates is called the back
emf. The back emf increases with the speed, because of Faraday's law.
So, if the motor has no load, it turns very quickly and speeds up until
the back emf, plus the voltage drop due to losses, equal the supply
voltage. The back emf can be thought of as a 'regulator': it stops the
motor turning too quickly. When the motor is loaded, then the phase of
the voltage becomes closer to that of the current (it starts to look
resistive) and this apparent resistance gives a voltage. So the back
emf required is smaller, and the motor turns more slowly. (To add the
back emf, which is inductive, to the resistive component, you need to
add voltages that are out of phase. See AC circuits.)
>
> >
> >
> > The countervoltage or back EMF is opposite the applied voltage and it
> > therefore
> >
> > "bucks" or reduces it. For example: If the applied voltage is 220
> > VAC and the back
> >
> > EMF is 70 VAC, the motor appear to see only 150 VAC.
> >
> >
> > The result is the equivalent of increasing the total impedence and thus
> > reduces the current that
> >
> > would flow if there was no EMF.
> >
> > This is why an AC motor must spin to avoid overheating.
> >
> > An automatic thermal overload protector is usually built into HVAC fan
> > motors.
> >
> > Or, if you desire, you can always add a correctly chosen fuse to
> > protect your AC motor.
> >
> >
> >
> > http://www.reliance.com/mtr/mtrthrmn.htm
> >
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