Electricity: motive power systems – Constant motor current – load and/or torque control
Reexamination Certificate
2003-02-24
2004-12-07
Duda, Rina (Department: 2837)
Electricity: motive power systems
Constant motor current, load and/or torque control
C318S700000, C318S801000, C318S807000, C318S814000
Reexamination Certificate
active
06828744
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a motor and, more particularly, to an apparatus and method for controlling torque of a motor.
2. Description of the Background Art
In general, a conventional apparatus and method for controlling a torque of a motor controls a torque of a motor in such a manner that a voltage value obtained by multiplying a ratio between a rated voltage simply supplied to a motor scalarly and a rated frequency by a speed instruction value expressed as a frequency and a voltage drop value according to a resistance of a stator of the motor are added, and a voltage corresponding to the added value is outputted to the motor to thereby control a torque of the motor.
The voltage drop value is obtained by multiplying a stator resistance value of the motor and a value of a current flowing at the stator resistance. However, with this control method, motor starting is not easy according to an operation form such as a speed of the motor or a load capacity applied to the motor.
The apparatus and method for controlling a torque of a motor in accordance with the conventional art will now be described with reference to
FIGS. 1
to
3
C.
FIG. 1
illustrates an equivalent circuit of a general induction motor.
As shown in
FIG. 1
, the equivalent circuit of the induction motor includes a resistance (rs) and inductors (XIs, Xm) constituting a voltage loop for an input voltage Vas; and inductors (XIr, Xm) and a resistance (rr/S) constituting a voltage loop for an output voltage (Vas/S).
That is, when a speed of the induction motor is controlled by making a radio between the input voltage (Vas) and a frequency (F) to be constant, the resistance (rs) has a constant value because it is a parameter irrespective of the frequency, whereas inductance of the inductors (Xm, XIs) is varied in proportion to the frequency. At this time, the relation between the inductance and the frequency is defined by the following equation (1):
XIs
=2
&pgr;×F×IIs,Xm
=2
&pgr;×F×Im
(1)
wherein IIs and Im are values of current flowing at each inductor (XIs, Xm), and ‘F’ is a frequency.
As noted in equation (1), when the ratio between voltage and frequency is made constant and the frequency is lowered down, a voltage (Eas) applied to the inductor (Xm) is reduced and an excitation current flowing at the inductor (Xm) is also reduced, so that an output torque of the induction motor is reduced. The output torque (T) of the induction motor is defined by the following equation (2):
T=k×I
m
×I
t
(2)
wherein ‘k’ is a constant, I
m
is an excitation current value, and I
t
is a torque component current value generating a torque.
In case of driving the induction motor by using the defined output torque (T), a torque should be suitably controlled according to conditions of a load applied to the induction motor.
For instance, when an elevator is full with passengers in a stop state, if a motor is initiated for an ascending operation, a torque boost voltage should be additionally applied to the motor in order to output more torque than the torque as shown in equation (2) because much torque is necessary at the initial stage of starting. In this respect, however, if the torque boost voltage is increased more than necessary, an over excitation current is generated to damage the motor and the inverter electrically connected to the motor may be damaged due to the overcurrent.
On the other hand, if the torque boost voltage is in short supply, the output torque of the motor is not enough to initiate the motor and ascent the elevator.
At this time, in case that a standard motor is operated by an inverter, since a voltage (V) is varied in proportion to the variation of the output frequency (F), a voltage drop is much increased in a low frequency domain and a torque generated from the motor is very small compared to a torque of a commercial electric power supply. Therefore, a voltage for compensating the shortage of the output torque of the motor by increasing voltage suitable to the voltage drop in the low frequency domain is called the torque boost voltage.
The motor torque control apparatus in accordance with the conventional art will now be described with reference to FIG.
2
.
As shown in
FIG. 2
, the conventional motor torque control apparatus includes: an angular velocity calculation unit
1
for calculating a radian frequency (We) on the basis of a goal frequency (in other words an output frequency) (F*) corresponding to a speed instruction value; a V/F converter
2
for receiving the previously stored goal frequency (F*) from a memory unit or from user's setting, converting the goal frequency (F*) into a voltage instruction value (V*) according to predetermined ratio (V/F) between an input frequency (F) and an input voltage (V) and outputting it; a voltage drop calculation unit
3
for receiving a current (Ias) flowing on a stator winding (referred to as ‘stator’) of the motor
6
from the previously stored memory unit or from a user's setting, and multiplying the inputted current value (Ias) and a stator resistance value (Rs) of the motor
6
to calculate a voltage drop value; an adder
4
for adding the voltage instruction value (V*) from the V/F converter
2
and the voltage drop value from the voltage drop calculation unit
3
and outputting it; and an inverter
5
for controlling an operation of the motor
6
according to the a radian frequency (We) and a voltage generated from the adder
4
.
That is, the conventional motor torque control apparatus is constructed such that the voltage obtained by converting the goal frequency (F*) into the voltage instruction value (V*) by means of the V/F converter
2
is compensated with a voltage drop component according to the stator resistance of the motor
6
and the compensated voltage is applied to the inverter
5
, thereby controlling the torque of the motor
6
.
FIGS. 3A
to
3
C are graphs showing a V/F profile that a boosted instructing voltage is outputted to the motor according to a load applied to the motor and an operation direction in accordance with the conventional art.
For instance, if a rated voltage of a motor is 220V and a rated frequency is 50 Herz, a voltage-to-frequency ratio is 220/60, which is about 3.7. the conventional motor torque control apparatus supplies a instructing voltage to the motor through an inverter (in more detail, a pulse width modulator and switching devices) according to the voltage-to-frequency ratio.
FIG. 3A
is a graph of a voltage-to-frequency profile applied to a conveyer or a small freight car driven by electricity, showing comparison between a torque boost quantity and an output voltage according to a certain voltage-to-frequency ratio. The conveyer or the electric-driven freight car refers to a load making a forward movement or a reverse movement in a horizontal direction, and when it moves forwardly or backwardly, the same torque is necessary. Thus, the conventional motor torque control apparatus outputs a instructing voltage according to a predetermined voltage-to-frequency ratio until it reaches a rated frequency of the motor equally in the forward movement and reverse movement of the load, which is the profile as shown by dotted lines in FIG.
3
A.
However, because much torque is required in the initial starting (that is, in case of the low frequency with a low speed), actually, a instructing voltage like a profile as shown by a solid line in
FIG. 3A
is boosted and outputted, and when the motor is rotated with a frequency close to the rated frequency, that is, when the speed of the load approaches the rated speed, the boost voltage becomes small, and then when the motor reaches the rated frequency, the boost voltage becomes zero.
FIGS. 3B and 3C
are graphs of a voltage-to-frequency profile for an ascending/descending load such as the elevator or a hoist, showing comparison between a torque boost quantity and an output voltage according to a predetermined voltage-to-frequency rat
Duda Rina
Greenblum & Bernstein P.L.C.
LG Industrial Systems Co. Ltd.
Miller Patrick
LandOfFree
Motor torque control apparatus and method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Motor torque control apparatus and method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Motor torque control apparatus and method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3317989