Electricity: motive power systems – Induction motor systems
Reexamination Certificate
2001-02-26
2003-08-26
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Induction motor systems
C318S456000, C318S457000, C318S461000, C318S434000, C318S800000, C318S801000
Reexamination Certificate
active
06611125
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an inertia calculating method and an electric motor driver. More particularly, it relates to the inertia moment (inertia) calculating method and the electric motor driver at the time of executing the velocity control of an inductive electric motor.
2. Description of the Related Art
When executing the velocity control of an electric motor, the mechanical inertia becomes necessary as a control constant. As a prior art for measuring the inertia, JP-A-61-88780 has disclosed the following method: The acceleration and the deceleration are executed at the velocity-changing rates the absolute values of which are the same in the same velocity differences (their velocity width &Dgr;&ohgr;
r
). Then, the acceleration torque &tgr;
ac
and the deceleration torque &tgr;
d
are calculated from the respective torque proportion signals so as to calculate the inertia J from the integrated quantities of the respective torques during the acceleration and the deceleration. Hereinafter, the method will be explained in detail:
FIG. 4
is a diagram for illustrating the motor torque &tgr;
m
, the load torque &tgr;
L
, and the acceleration torque &tgr;
ac
in the case of performing the calculation of the inertia J in accordance with the prior art. In
FIG. 4
, letting the motor machine's angular velocity be abbreviated as &ohgr;, the relation holding between J and the torques is given by the equation (1):
J
ⅆ
ω
ⅆ
t
=
τ
m
-
τ
L
=
τ
a
c
(
1
)
The velocity difference &Dgr;&ohgr; caused by the acceleration at the acceleration time-period is equal to the velocity difference &Dgr;&ohgr; caused by the deceleration at the deceleration time-period. Integrating both sides of the equation (1) to determine J from the torques at the acceleration and the deceleration time-periods, J is given by the equation (2):
J
=
∫
ta2
ta1
(
τ
m
-
τ
L
)
ⅆ
t
Δω
=
∫
ta3
ta4
(
τ
m
-
τ
L
)
ⅆ
t
-
Δω
(
2
)
Determining once again J by averaging the above-described J values calculated at the acceleration and the deceleration time-periods, J is represented by the equation (3):
J
=
1
2
{
∫
ta2
ta1
(
τ
m
-
τ
L
)
ⅆ
t
Δω
+
∫
ta3
ta4
(
τ
m
-
τ
L
)
ⅆ
t
-
Δω
}
(
3
)
Here, since the acceleration and the deceleration are executed in the same velocity differences during the same time-periods, the integrated values of the load torque &tgr;
L
during the acceleration and the deceleration time-periods become equal to each other:
∫
ta2
ta1
τ
L
ⅆ
t
=
∫
ta3
ta4
τ
L
ⅆ
t
(
4
)
Accordingly, from the equations (3) and (4), J is determined from &tgr;
m
alone as is expressed by the equation (5):
J
=
∫
ta2
ta1
τ
m
ⅆ
t
-
∫
ta3
ta4
τ
m
ⅆ
t
2
Δω
(
5
)
Using a detected torque current I
qFB
, the value of &tgr;
m
can be calculated as is expressed by, e.g., the equation (6):
τ
m
=
3
(
P
2
)
M
L
2
·
MI
d
*
·
I
qFB
≡
Δ
0
·
I
qFB
(
6
)
where, P, M, L
2
, and I
d
* denotes the following, respectively: The motor pole number, the motor mutual inductance, summation of the motor mutual inductance and the motor secondary-side leakage inductance, and the magnetic field excitation current instruction. Based on the above-described explanation, J is calculated from the equations (5) and (6).
In this method, the cancellation of the load torques &tgr;
L
makes it possible to calculate the inertia J independently of the form of the load torque.
In the method disclosed in JP-A-61-88780, however, as will be pointed out below, the motor is in a danger of being transitioned into a regenerative state at the deceleration time-period. This regenerative state overcharges, e.g., a smoothing capacitor within an inverter, thereby damaging the capacitor.
In
FIG. 4
, the motor torque &tgr;
m
becomes the lowest at the deceleration-terminating time (t=t
a4
). At this time, the torque current I
q
also becomes its minimum. Assuming that the load torque &tgr;
L
is proportional to the square of the angular velocity &ohgr; (i.e., square load), I
q
is determined from the equations (1) and (6) as is expressed by the equation (7): Incidentally, the reference notations therein denote the following, respectively: &ohgr; the motor velocity, &ohgr;
0
the rated motor velocity, d&ohgr;/dt the velocity-changing rates (the acceleration and the deceleration rates), P, the motor pole number, M, the motor mutual inductance, L
2
the summation of the motor secondary-side leakage inductance and M, I
d
* the magnetic field excitation current instruction, J the mechanical inertia, and, I
q0
the rated motor torque current.
I
q
=
(
ω
ω
0
)
2
·
I
q0
+
1
3
(
P
2
)
M
L
2
·
MI
d
*
ⅆ
ω
ⅆ
t
J
=
(
ω
ω
0
)
2
·
I
q0
+
1
Δ
0
ⅆ
ω
ⅆ
t
J
(
Δ
0
=
3
(
P
2
)
M
L
2
·
MI
d
*
)
(
7
)
As a result, there exist some cases where the minimum value of I
q
(i.e., the equation (7)) becomes negative and thus the motor is transitioned into the regenerative state, because the deceleration rate d&ohgr;/dt is negative at the deceleration time-period. As seen from the equation (7), the condition under which the minimum value of I
q
becomes negative and the motor is transitioned into the regenerative state is the case where the deceleration is executed in a region of small &ohgr; (the load torque) and |d&ohgr;/dt|, i.e., the deceleration rate at that time, is large. consequently, in order to prevent the regenerative state from occurring at the deceleration time-period, it becomes absolutely required to reduce the deceleration rate (=the acceleration rate). In that occasion, however, the acceleration or the deceleration torque does not become larger enough as compared with the motor torque and the load torque components that become an error. This gives rise to an expectation that the inertia-identifying accuracy will become worse.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an inertia calculating method and an electric motor driver that are preferable for calculating the inertia and for driving an electric motor without causing the regeneration to occur and based on a configuration that is simpler as compared with the configuration in the prior art.
In order to accomplish the above-described object, in a driver including a non-regenerative type power converter and executing the velocity control of the electric motor with the use of a mechanical inertia constant, the non-regenerative type power converter being a converting apparatus for converting an alternating current from an alternating power supply into an alternating current of a variable voltage and a variable frequency, the non-regenerative type power converter including a forward converter for converting the alternating current from the alternating power supply into a direct current, a smoothing capacitor connected to a direct current circuit, and a backward converter for converting the direct current into the alternating current, when calculating the mechanical inertia, the mechanical inertia is calculated during only the motor acceleration time-period so that a voltage of the smoothing capacitor included in the non-regenerative type power converter will not exceed a predetermined value.
Also, when calculating the mechanical inertia, the accelerations are executed at a plurality of times at the mutually different velocity-changing rates, and the mechanical inertia is calculated from the integrated quantities of the respective torque proportion signals and the velocity-changing widths.
Also, in a driver including a power converter and executing the vel
Fujii Hiroshi
Nagata Koichiro
Okamatsu Shigetoshi
Okuyama Toshiaki
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Miller Patrick
Nappi Robert E.
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