AN2357 Freescale Semiconductor / Motorola, AN2357 Datasheet - Page 16

• Manufacturer Part Number: AN2357
• Description: Sine Voltage Powered 3-Phase Permanent Magnet Motor with Hall Sensor
• Manufacturer: Freescale Semiconductor / Motorola

AN2357/D
Voltage Ripple
Cancellation
Field Weakening
Calculation
16
Sine Voltage Powered 3-Phase Permanent Magnet Motor with Hall Sensor
The voltage ripple cancellation recalculates the absolute required voltage into
a relative voltage, so that output voltage amplitude does not depend on the dc-
bus voltage.
where:
The function has two main purposes:
A field weakening of the motor may be required to reach the full speed range.
The PM motor can be field weakened by increasing the angle between motor
voltage and rotor flux. The basic control keeps the angle at 90°, as can be seen
in
also for setting the optimal working point of a motor. One goal of finding a
working point where the motor has maximum efficiency, can be simplified as
task of minimizing resistance losses. In this case, the reactive power has to be
minimized by the correct control angle.
is higher, depends on motor parameters and load of the motor. The
dependence of reactive power relative to control angle of motor used is shown
in
(T1, T2, and T3) and two speeds (ω1 and ω2), where T1<T2<T3 and ω1<ω2.
In the graph the ω is substituted by w. In
control angle depends on both the speed and the torque.
Figure
Figure
1. Change in dc-bus voltage does not effect torque limitation algorithm
2. Jump in dc-bus voltage does not cause transient speed deviation
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U
U
U
req
rel
DC
14. The measured reactive power is shown for three different torques
2. Change of control angle is not only useful for field weakening, but
U
is relative voltage going to sine wave generator [-]
is absolute voltage required by speed controller [V]
is dc-bus voltage [V]
rel
Figure 13. Control with Minimal Reactive Power
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=
U
--------- -
U
U
DC
req
s
j
ω
.L
s
.I
R
s
I
s
s
E
I
s
δ
Ψ
s
Figure 13
Figure 14
shows that the optimal angle
we can see that the optimal
Ψ
Ψ
r
Equation 12
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