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2.3.3. POWER CONTROL
It is necessary to supply, from a single phase mains supply (230 V - 50 Hz or 120 V - 60 Hz),
three variable tensions with amplitude and with frequency, dephased by 2π/3.
The voltage is rectified then filtered by C to supply the constant voltage +E. The value of the
capacitor C is high.
For a 230V – 50 Hz mains supply
The 6 transistors act as switches (open or closed).
These transistors commute at a fixed frequency (8 kHz).
To affect the voltage at T we act upon the commands of the transistors a and b, b always being
the opposite to a (a = closed, b = open). By changing the cyclic ratio (closed/open) we obtain a
mean sinusoidal value so we are able to vary the amplitude and frequency.
The pilot frequency must be elevated compared to the frequency of the generated sinusoidal
voltage. The latter is around 400 Hz at 24000 r.p.m. (f pilot / f sin = 10 at max. speed).
In the same way, the voltages at points R and S will be sinusoidal between OV and +E and
dephased by 2π/3 between them.
These 3 voltages are simple voltages varying between OV and +E. They are partly sinusoidal
and partly on a baseline situated at +E/2. Thus the O of the sinusoidal voltage corresponds to 50
% of the cyclic ratio.
The voltages applied to the rotor phases are the compound voltages VR-VS,VS-VT, VT-VR and
will vary between +E and –E (0-(+E) = -E).
Thus the voltages with amplitude close to that of the mains are reproduced at the motor
terminals.
Speed synchronisation is directly proportional to the frequency of the voltage supplied to the
motor.
Ns = fx 60 (for a 2pole motor).
The motor rotates at a speed N below the speed Ns.
Ns – N = SLIPPAGE (the slippage varies between 1 and 3 %).
Ns
Service Manual 11290099
E
A
+
C
B
0
E = 230 √2 = 325 V
C = 1000 µF / 400 V
R
T
page 13 of 44
S
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