Re: 3 phase motor as a generator ?
- From: vj@xxxxxxxxxxxxxxxxxxxxxxx
- Date: Thu, 10 Aug 2006 09:59:24 +0100
On Wed, 9 Aug 2006 17:50:44 +0100, "Andrew Mawson"
<andrew@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Can one use a 3 phase motor as a 3 phase generator?
A rotary phase converter gets a normal 3 phase motor spinning using
essentially 2 phases by using an artificially generated second phase
via a capacitor, and the motor generates the third phase.
So if a 3 phase motor has one phase mains energised and is then spun
up to synchronous speed will it generate the two other phases? (I
appreciate that a normal 3 phase generator would have an armature that
is a DC magnet spinning in the three stator coils).
AWEM
Not quite - it has to be beyond synchronous speed before it
it behaves as an induction generator.
The normal squirrel cage 3 phase induction motors work
because the rotor speed is slightly lower than the synchronous
speed of the rotating magnetic field. This generates large
currents in the rotor bars at this difference frequency and this
provides the motor output torque.
If it is mechanically driven to reach synchronous speed the
difference frequency falls to zero so that no current is induced
in the rotor bars. The machine is now passive - it neither
absorbs or generates power.
If the machine is mechanically driven ABOVE synchronous
speed large currents are again generated in the rotor bars but
this time the torque generated opposes the input drive. The
magnetic field from the rotor rotates at rotor rotation frequency
minus the difference frequency = synchronous frequency and this
generates power which is fed back into the supply. i.e although
the rotor speed is slightly above synchronous speed it feeds back
power at supply frequency.
If we plot shaft torque against shaft speed we get an "S"
curve lying on its back. Torque passes through zero at
synchronous speed and rises to a maximum at rotor speeds just
below and just above synchronous speed - absorbing power below -
generating power above.
Strictly speaking the above applies to a 3 phase machine
connected to a high power 3 phase supply. However, if only one
phase is connected, the machine will also act as a rotary 3 phase
converter. Three phase power will be available and power will be
fed back to the single energised phase.
Note - this only works because it is connected to a
relatively high power supply. This defines the synchronous speed
and permits a stable system.
Jim
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