Method for determining the position of a rotor of a polyphase motor
The present disclosure relates to a method for determining the position of a rotor of a polyphase motor, e.g., the rotor angle, particularly at standstill. The method can include: applying voltage to phases of the motor, measuring a current in the phases, and determining the rotor position based on the measured current. According to the present disclosure, determination of the rotor position can be based on current values measured during a period where zero voltage is applied to the respective phases.
1. A method for determining a position of a rotor of a polyphase motor via a motor control system connected for driving the motor, the method comprising:
via the motor control system:
applying, before startup, symmetrical voltage pulse patterns to phases of the motor to keep the rotor stationary;
measuring a plurality of current samples in each phase between the voltage pulses;
generating a harmonic model from the measured current samples, the harmonic model defining a motor characteristic and a rotor position;
determining the rotor position and a rotor polarity from the harmonic model based on the measured current, wherein determination of the rotor position is based solely on current values measured in each phase during a period between the voltage pulses where zero voltage is applied to the phase; and
controlling a switching frequency of an inverter of the motor based on the rotor position determined from the measured current.
2. The method according to claim 1 , wherein determination of the rotor position is based on identification of time constants.
3. The method according to claim 1 , wherein determination of the rotor position is based on a model of stator current dynamics expressed by an equation in stator coordinates (α, β) as follows:
di
s
dt
=
-
R
s
(
L
s
)
-
1
i
s
with
i s : stator current,
R s : stator resistance,
L S : stator inductance.
4. The method according to claim 1 , wherein, for each zero-voltage period, a first resulting current vector is determined from a first set of current values, and a second resulting current vector is determined from a second set of current values, wherein a variation between the first and second resulting current vectors is expressed as a variation vector, depending on the rotor position and an excitation angle, times an amount of the first resulting current vector, wherein the variation vector is a function of the excitation angle by a reduced number of harmonic functions, and wherein a rotor angle position is estimated by determining coefficient vectors of the reduced number of harmonic functions.
5. The method according to claim 1 , comprising:
adjusting a pulse width modulation pattern of applied voltage pulses after startup to obtain a substantially persistent excitation of the rotor.
6. The method according to claim 1 , wherein voltage is not considered when determining the rotor position.
7. The method according to claim 4 , wherein the variation between the first and second resulting current vectors is modelled by a function as follows:
Δ i k+1 i =Δa i (θ r ,θ i )· i k i with
Δi k+1 i : variation between first and second resulting current vectors projected in a stator current axis,
Δa i (θ r , θ i : variation vector,
θ r : rotor angle, corresponding to rotor position in stator coordinates (α, β), and
θ i : excitation angle in stator coordinates (α, β),
i k i : amount of the first resulting current vector at time instant k.
8. The method according to claim 7 , wherein the variation vector is a reduced number of harmonic functions as follows:
Δ
a
i
(
θ
r
,
θ
i
)
=
∑
v
[
a
v
i
(
θ
r
)
cos
(
v
θ
i
)
+
b
v
i
(
θ
r
)
sin
(
v
θ
i
)
]
with
a v i (θ r ), b v i (θ r ): coefficient vectors.
9. The method according to claim 1 , wherein the applying and measuring steps comprise:
applying a first voltage pulse to the phases of the motor, wherein the voltage pulse is followed by a first zero-voltage period where zero voltage is applied;
measuring a first current value from the phases at a first time instant within the first zero-voltage period;
measuring a second current value from the phases within the first zero-voltage period at a second time instant following the first time instant;
applying a second voltage pulse to the phases of the motor, wherein the voltage pulse is followed by a second zero-voltage period where zero voltage is applied;
measuring a first current value from the phases at a first time instant within the second zero-voltage period; and
measuring a second current value from the phases within the second zero-voltage period at a second time instant following the first time instant.
10. The method according to claim 9 , wherein a plurality of voltage pulses is applied to the phases of the motor, each voltage pulse being followed by a zero-voltage period where zero voltage is applied, and wherein a first and second current value is measured within each of the zero-voltage periods at a first time instant and a second time instant respectively.
11. The method according to claim 10 , wherein at least a quantity of 6 voltage pulses are applied.
12. The method according to claim 9 , wherein a resulting excitation angle of a voltage pulse differs from a resulting excitation angle of a preceding voltage pulse.
13. The method according to claim 12 , wherein the resulting excitation angles of the voltage pulses are chosen such that an overall torque applied to the rotor by the plurality of voltage pulses is zero.
14. The method according to claim 9 , wherein a first set of current values is measured from the phases at a plurality of time instants within the first zero-voltage period, and a second set of current values is measured from the phases within the first zero-voltage period at a plurality of time instants shifted by a constant time shift.
15. The method according to claim 14 , wherein at least a quantity of 24 voltage pulses are applied.
16. The method according to claim 1 , wherein the voltage is applied to the phases of the motor by a pulse width modulation inverter, and wherein a pulse width modulation frequency at standstill of the motor is reduced compared to a frequency at high rotating speeds.
17. The method according to claim 16 , wherein determination of the rotor position is based on identification of time constants.
18. The method according to claim 16 , wherein the pulse width modulation frequency is reduced by at least 80%.
19. The method according to claim 16 , wherein the pulse width modulation frequency is reduced by at least 50%.
20. The method according to claim 19 , wherein the pulse width modulation frequency is reduced such that a sampling frequency used for measuring the current in the motor phases is at least 20 times higher than the pulse width modulation frequency.
21. The method according to claim 19 , wherein the pulse width modulation frequency is reduced such that a sampling frequency used for measuring the current in the motor phases is at least 10 times higher than the pulse width modulation frequency.