AN1362 STMicroelectronics, AN1362 Datasheet - Page 2

• Manufacturer Part Number: AN1362
• Description: SWITCHED RELUCTANCE MOTOR CONTROL BY ST FIVE
• Manufacturer: STMicroelectronics

AN1362
Generally, the equation between stator poles Ns and rotor poles Nr is the following:
where q is the number of phases.
Due to the pole saliency, the magnetic path is different during the rotor motion since the reluctance is
minimal when stator and rotor poles are aligned and maximum when poles are not aligned. Formally, the
inductance can be written as follows:
with N number of coils and rotation angle of the motor shaft.
The equation above and the reluctance profile are responsible for the inductance profile shown in figure
2, which is measured on the motor used in this application: on x-axis the rotor mechanical position (in
degree), on y-axis the measure of inductance (mH).
Figure 2. INDUCTANCE PROFILE
Energy and inductance considerations [1][3] lead to the following definition of the motor torque expression
:
From the equation above, one may argue that, in order to obtain a positive torque, the phase coil needs
current only during the rising edge of the inductance profile.
The synchronization of the phase polarization with the rotor poles position measured through the Hall
sensors is the main task of the electronic driver.
There are different ways to arrange the sensors: one of the simplest is to arrange the sensor position in
order to obtain the necessary information about the rising/falling edge of 'L' for each phase.
Fig.3 shows a typical sensor arrangement for a 3-phase 6/4 SRM.
Since the phase coil is a big inductance with a low series resistance, the current rising time is strongly
related to the L/R constant time of the circuit. At a very high rotational speed, this rising current time is
comparable to duration of the L-rising edge. This does not allow the current to rise at suitable levels to
deliver the expected torque. In this case, a temporal coils energizing advance (prephase injection) is
introduced in the phases in correspondance with the L-rising profile beginning.
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1 2
1 0
8
6
4
2
0
T
Nr
em
L
=
=
Ns
=
1
-- - i
2


------------ -
2
N
Ns
------ -
q
2
dL
------ -
d


P h a s e 1
P h a s e 2
P h a s e 3
E n c o d e r # 1
E n c o d e r # 2
E n c o d e r # 3