Olaf wrote:
> "TheSnoMan" <admin@snoman.com> wrote in message
> news:CW4Bf.4161$rH5.3472@newsread2.news.atl.earthlink.net...
>
>>Al Bundy wrote:
>>
>>>John Horner wrote:
>>>
>>>
>>>>Al Bundy wrote:
>>>>
>>>>
>>>>>Nomen Nescio wrote:
>>>>>
>>>>>"Electric motors develop maximum torque at zero rpm......."
>>>>>
>>>>>Is that so?
>>>>>Unfortunately Nomen, you are a poser, just cutting and pasting things
>>>>>you read. That leaves you making error after error in how things are or
>>>>>could be.
>>>>>
>>>>
>>>>
>>>>Well Mr. Bundy, I am afraid that Nomen is correct, at least for
>>>>permanent magnet electric motors. Have a look at:
>>>>
>>>>http://claymore.engineer.gvsu.edu/~j...-71.html#54931
>>>>
>>>>John
>>>
>>>
>>>Well John, you are wrong because you took a blanket statement and
>>>qualified it to suit your answer. Again, nice pasting job though.
>>>
>>
>>
>>There are a LOT of varibles in electric motor design that can effect at
>>what RPM peak effort is achieved. Generally though with traction type
>>motors used to power electric cars and such, they achive maximum torque at
>>zero or very low RPMs to get the load moving.
>>
>>--
>>
>>-----------------
>>www.thesnoman.com
>
>
> I think I may see where Mr. Bundy is saying. Without having read any of the
> links provided, it makes sense to me that at zero RPM the motor is putting
> out zero torque. Once any torque great enough to make the motor turn is
> applied, then the RPM is no longer zero.
What batshit!
If electric motors put out *zero* torque at zero RPM, they'd never begin
to rotate at all. Internal combustion engines DO have zero torque at
zero RPM, which is exactly why you need an electric motor to start them!
How much torque an electric motor does put out vs. RPM depends a lot on
the design of the motor.
DC and AC/DC commutator motors put out their maximum torque at 0 RPM
(but will burn out if held at 0 RPM because only one winding on the
armature is carrying the full load). An example of a DC commutator motor
is the starter motor in a car, or the traction motors in older
locomotives. An AC/DC commutator motor is the type used in vacuum
cleaners, hand power tools, and blenders.
AC induction motors put out their peak torque at a few percent less than
their free running maximum RPM. As you lug them down, torque goes up at
first, but lug them too far and torque begins to decline again, but it
never drops to zero even at zero RPM. An example of an AC induction
motor is a fan motor, AC blower motor, AC compressor motor, or shop air
compressor motor. The most commonly used AC motors in the world.
AC Synchronous motors put out their maximum torque at the synchronous
RPM. They're used in heavy industry because they can be set to run at
leading power factor to compensate for induction motors that run at a
lagging power factor- save's the industry money overall.
Variable-frequency drive motors are induction or synchronous motors
driven by a variable frequency AC source, so that you can make them put
out peak torque at any RPM you want. Modern AC locomotive traction
motors are variable-frequency drive motors.
But regardless of the type, ANY self-starting electric motor puts out
SOME torque at 0 RPM. An example of a non-self-starting type would be a
synchrounous motor without any start/damper windings... but that's a
laboratory curiousity as all real-world electric motors have a provision
to give them self-starting torque.
Linear Mode
