RE: 3 phase car alternators

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Leigh Copp" <Leigh.Copp@xxxxxxxxxxx>

Right you are, the stationary winding is always the stator in an 
induction machine (and a synchronous machine as well), and the 
rotating winding is still the rotor in an induction (squirrel cage, 
wound rotor, etc). , and a synchronous machine (I had actually 
forgotten that last point on the synch machine - so thanks!).

The original statement however was that to change the frequency (for 
a given rotational speed) of an automotive alternator, one would have 
to effectively change the number of poles on the stator. This much is true.

There was a reply to the effect that the rotor poles would have to be 
changed also.

My point was that it is not necessary to rewind the rotor to alter 
the frequency, only the stator. The rotor can effectively have one 
pole pair (North and South) from the DC excitation current.

I will agree howver that in any case it is much too much like work!

An addition to the last remark:

The torque in an induction machine is controlled by the air gap flux 
which is related to V/Hz.

The motor current can be broken down (via the Park Transform) into 
orthogonal components. The quadrature component of the current 
(called Iq) produces torque, and the direct component (Id) produces 
the magnetizing flux.

Our automotive alternator is a synchronous machine however, so things 
are much easier. The stator magnetic field always rotates at the 
sycnchronous speed, which will be whatever speed we drive the rotor 
at. The rotor lags (when motoring) or leads (when generating) by the 
torque angle (delta). There is no slip. Once we over excite the field 
(rotor) however, we don't get any more voltage, we merely get a sink 
for lagging VARs. The machine becomes a synchronous condenser actually.

Yes the frequency vs. speed is most definitely controlled by the 
number of poles. One of the original posts suggested rewinding the 
stator to place more windings in parallel. This is exactly how 
multi-speed motors work (without the benefit of VFDs). So the machine 
frequency -can- be changed, it just isn't easy.

Leigh



-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Tue 17/10/2006 8:34 PM
To: tesla@xxxxxxxxxx
Cc:
Subject: RE: 3 phase car alternators



	Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
	
	At 01:43 PM 10/17/2006, you wrote:
	>Original poster: "Leigh Copp" <Leigh.Copp@xxxxxxxxxxx>
	>
	>Jim,
	>
	>As I understand things, an automotive alternator (like the 990 MW units
	>at nearby Darlington Nuclear) does not have a rotor per se, but a
	>rotating
	>-field- winding, that is excited with DC through slip rings (as opposed
	>to brushes and a commutator). The magnitude of the DC current through
	>the field determines the magnitude and angle of the AC voltage that is
	>generated.
	
	Exactly.. the thing that rotates is the rotor.  The thing around it
	is the stator.  Even in an induction motor (which has no windings on
	the rotor), it's still called a rotor.
	
	But yes, in alternator usage, there's a DC field on the rotor and the
	AC comes off the stator.
	
	For that matter, an induction motor can work as a generator, if you
	provide some DC excitation in one of the windings to get it going.
	It's a variant of the "rotary phase converter". Once it's spinning,
	if the rotation rate is higher than the frequency applied to the
	windings (i.e. negative slip) you pull electrical power out of the motor.
	(Modern AC 4 quadrant drives do this quite nicely)
	
	
	>Are you thinking of a generator, which has a commutator (and is
	>basically a DC machine)?
	>
	>The alternator can be rewound to change the output frequency vs. machine
	>speed characteristic (or "k"). There are some efficiency tradeoffs as
	>well however.
	
	frequency vs speed would be fixed by the number of poles, I should
	think.  Voltage vs speed can be changed by changing winding
	properties. and current vs torque can be changed by changing field strength.