JPH0669307B2 - Induction motor control method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a control method for improving the efficiency of an induction motor drive system fed from a power converter.

[Conventional technology]

In an induction motor drive system that controls the torque or electric power of the induction motor by the power converter or controls the rotation speed based on the result, in general, the magnetic flux of the motor is controlled to be a substantially constant value for ease of control. To be done. Further, since the motor voltage is substantially proportional to the product of the magnetic flux and the frequency, when the frequency exceeds a predetermined frequency, the magnetic flux is reduced in inverse proportion to the frequency or the rotation speed corresponding to the frequency for the purpose of keeping the motor voltage constant. Is common.

[Problems to be solved by the invention]

However, such control has the following problems in terms of efficiency of the electric motor and the power converter.
That is, in the control as described above, for example, in order to establish a predetermined magnetic flux in the electric motor regardless of the magnitude of the desired torque,
A relatively large exciting current always flows, but this causes copper loss (resistance loss) and iron loss of the electric motor, and also causes loss in the power converter due to the exciting current and the voltage corresponding to the exciting current. However, when the desired torque is extremely small, a sufficient exciting current is not always necessary, and therefore such control causes unnecessary loss.
In particular, in a limited power supply situation such as a system having a power supply device of its own such as a ship or a vehicle and driving its own propulsion motor with the same power supply, improving the efficiency of the device is an important issue.

Therefore, the present invention improves the efficiency of an induction motor drive system that is fed by a power converter by controlling the voltage, current, etc. of the motor so that the generated loss is minimized according to the load condition. With the goal.

[Means for solving problems]

The current of the induction motor is decomposed into a current (magnetizing current) component I _M that contributes to magnetic flux generation and a current (torque current) component I _T that is orthogonal to I _M and contributes to torque generation in a vector manner, to obtain a desired torque or a desired power. as maintain a predetermined relationship the ratio of I _T and L _M is given as a function of frequency regardless of the voltage of the motor, by controlling the input voltage of the current or power converters, combination loss of the motor and the power converter Is the minimum.

[Action]

Obtains a value of a like combination loss of the motor and the power converter is minimized expressed as a function of the ratio a between the torque current component I _T magnetization (magnetic flux) current component I _M, the motor voltage based on this, the current Perform predetermined control such as control.

〔Example〕

 First, the principle of the present invention will be described.

When the main loss of the induction motor drive system fed through the power converter is classified according to the factors, an element proportional to the square product of the current as represented by resistance loss and a voltage like the snubber circuit loss of the power converter. And a factor proportional to frequency such as a switching loss and a snubber circuit loss of a semiconductor for a power converter. Here, since the element proportional to the frequency is restricted by the rotation speed of the electric motor and the like, it is excluded from the improvement of the device efficiency. Therefore, if the current component that contributes to the magnetic flux generation of the induction motor is I _M and the current component that is orthogonal to I _M in vector terms and that contributes to torque is I _T , the magnitude of the current will be And the loss proportional to the squared product of the current is I _M ² + I _T ²
Proportional to. (If the excitation reactance and _{L M L M I M)} the motor voltage frequency ω and the flux a product of, i.e. .omega.L _M
It is approximately proportional to I _M. If controlling the input voltage of the converter in response to the motor voltage, the loss is proportional to the product of the voltage is proportional to ^{2 (ωL} _M I _M). Therefore, the generated loss P is given by the following equation: P = K _I (I _M ² + I _T ² ) + K _V (ωL _M I _M ) ² ...
Meanwhile _{... (1) (K I,} K V is a proportional constant), the torque T of the motor is proportional to the product of flux and I _M, is given by the following equation.

T = K _T L _M I _M I _T ...... (2) (K _T is a proportional constant) where a = I _T / I _M , and Eq. (1) is a function of T given by Eq. (2). When expressed, it becomes the following formula.

The above equation is differentiated by a in order to find the condition that the above equation becomes the minimum with respect to the variable a.

Under the condition of dp / da = 0, if a and a ₀ where P takes an extreme value, the following equation is obtained.

The polarity of a corresponds to the polarity of T, which represents the direction of torque. When T is positive, a ₀ is a positive value, and when T is negative, a _0.
Is a negative value. If a> 0, then dp when a <a ₀
/ Da <0, P monotonically decreases with respect to a, and when a> a ₀ , dp / da> 0, P monotonically increases with respect to a, and thus a = a _0.
Is a condition for minimizing P. When a <0,
It is similar to the case of> 0.

FIG. 4 is an example showing the relationship between a and P when ω is a constant value. P increases as the torque increases, but the loss P generated is always the minimum when a = a _0. I understand. FIG. 5 is an example showing the relationship between ω and a ₀ , where a ₀ is a function that monotonically increases with respect to the frequency ω. Torque T when a = a ₀
Is the following equation from equation (2).

6 and 7 show I _M and I _{T with} respect to T at this time.
Where L _M and I _T are functions proportional to the square root of T. Further, the proportional coefficient is from the relationship of FIG.
For I _M, it decreases as ω increases, while for I _T, it increases as ω increases. In particular, when the ω is 0, or loss dependent on the voltage almost negligible a I _{M =} I _T. Furthermore, approximately the constant omega, may be substantially constant ratio of I _T and I _M.

The principle of the present invention is shown in FIGS. 6 and 7 on condition that the ratio of I _T and I _M is a ₀ expressed by the equation (5), one example of which is shown in FIG. (6) in relation to formula, by controlling the I _M and I _T also corresponds to the desired torque, in which incurred loss is running the device in operating conditions is minimized. Incidentally, I _M it is desirable to limit the maximum value depending on the frequency. That is, according to the basic principle as described above, I _M and I _T may be controlled in accordance with the target values of the torque in accordance with the relationships shown in FIGS. 6 and 7, but the excitation intagance L _M generally has a saturation characteristic. And when exceeding a predetermined I _M , the magnetic flux is maintained at a substantially constant value even when a current larger than that is supplied. That is, the current is that it does not act effectively, I _M from the viewpoint of efficiency improvement is not that good to limit so as not to exceed a predetermined value. Further, when I _M is extremely small, a problem may occur due to stability of control, and it may be preferable to limit I _M so as not to become smaller than a predetermined value. Furthermore, since the voltage of the motor is approximately proportional to the product of the frequency and the magnetic flux, the voltage of the motor does not exceed the maximum voltage value that can be output by the power converter. The I _M corresponding to the magnetic flux should be limited so as not to exceed a predetermined value.

FIG. 1 shows the maximum efficiency with respect to the torque target value T ^* .
I _M, respective target values of I _T I _{M ^*,} in block diagram showing one embodiment of a calculation circuit for calculating the I _T ^*, is for carrying out the invention. In the figure, 1A is an I _M calculator for calculating a target value of I _M that minimizes the loss, and (5)
First equation to従Tsuta calculation formula and (6), or obtains a first target value I _M ^** of I _M by referring to the table of data stored in the memory circuit. 2 the maximum value calculator for obtaining the maximum value I _M max that acceptable in I _M corresponding to the frequency or rotational speed, 3 as a target value of I _M does not exceed I _M max, also less than a predetermined value in limiter to limit as that does not, the output is the final target value I _M ^* of I _M. 4 has a division function is _{I T} calculator for calculating a target value _I ^{T *} of ^{T *} and _I ^{M *} follow from equation (6) connexion _{I T.}

On the other hand, considering the transient state in which the magnitude of I _M changes, the magnetic flux is given by the first-order lag system of I _M. FIG. 2 shows another embodiment of the arithmetic circuit in consideration of the transient state of magnetic flux. In the figure, 1B is the first target value φ of the magnetic flux that minimizes the loss.
A magnetic flux calculator that calculates ^** , and the function generator of Fig. 1
1A to consists of arithmetic circuit memory circuit corresponding or thereto, calculates a function multiplied by L _M. Reference _numeral 2 is a maximum calculator for calculating the maximum value of the magnetic flux φ _{max in accordance} with the frequency or the number of revolutions, and 3 is a limiter for limiting the target value of the magnetic flux so that it does not exceed φ _max and does not become smaller than a predetermined value. Then, the output becomes the target value φ ^{* of the} magnetic flux. 4 has a division function, Of _{I T} calculator for calculating a target value _I ^{T *} of _{I T} from the relation,
Reference numeral 5 is an I _M calculator having a differential element with respect to φ ^* and having a coefficient inversely proportional to L _M in a steady state.

When I _M ^* and I _T ^* are obtained as described above, the voltage, current and slip frequency of the corresponding motor can be obtained from the known relational expressions in the known slip frequency control and vector control, and the desired Torque control is achieved. Here, as an example of slip frequency control using a voltage source inverter, FIG. 3 shows a main circuit and a control block.

In the figure, 6 is a variable DC power supply, 7 is a voltage type inverter, 8 is an induction motor, and 9 is a speed detector. 10 is a slip frequency calculator for calculating the slip frequency omega _Sl of the induction motor, determining the Supporting connexion omega _Sl in the following equation (7).

(K _Sl is a constant proportional to the secondary resistance value) 11 is an adder, which calculates the output frequency of the inverter by adding ω _Sl and the rotation speed ω ₂ . 12 I _M ^* and I _T ^* was vector addition, the target value of the magnitude of the motor current the current command calculator for obtaining the I ^*, 14 is a I detected by the current detector 13 I ^*
Is a current regulator that controls the voltage for increasing the deviation of the voltage to obtain a desired current. It should be noted that on / off of the voltage source inverter may be controlled by known pulse width control in order to control the current.

In the above, the control target is the torque of the induction motor, but even when the generated power of the induction motor is set to a target value, for example, the generated power is proportional to the product of the torque and the number of revolutions. It can be applied to achieve control with minimal loss.

〔The invention's effect〕

According to the present invention, the ratio between the magnetic flux current component and the torque current component of the induction motor is controlled so as to maintain a predetermined relationship given as a function of frequency regardless of the desired torque. It is possible to minimize the combined loss of the container.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of an arithmetic circuit used to carry out the present invention, and FIG. 2 is a block diagram showing another example of an arithmetic circuit used to carry out the present invention.
FIG. 3 is a configuration diagram showing an application example of the present invention, FIG. 4 is a graph showing the relationship between the current ratio a and the loss P with the torque T as a parameter, and FIG. 5 is a graph showing the frequency ω and the current ratio a ₀ . FIG. 6 is a graph showing the relationship, and FIG. 6 is a torque T with the frequency ω as a parameter
And the magnetic flux current component I _M , FIG. 7 shows the torque T and the torque current component I with the frequency ω as a parameter.
It is a graph which shows the relationship with _T. Description of symbols 1A, 5 ... I _M calculator, 2 ... Maximum value calculator, 3 ... Limiter, 4 ... _IT calculator, 6 ... Variable DC power supply, 7 ... Voltage source inverter, 8 ... Induction motor (IM), 9 Speed detector, 10 Sliding frequency calculator, 11 Adder, 12
…… Current command calculator, 13 …… Current detector, 14 …… Current regulator.

Claims (3)

[Claims] 1. A drive system for an induction motor for controlling the torque of an induction motor fed through a power converter, comprising: a first current component I _M that contributes a current of the induction motor to generation of magnetic flux; decomposing the current component and the second current component I _T contributes to vectorially perpendicular to torque generation, the, to the product of the flux and the second current component of the motor dependent upon the first current component desired torque Under the condition of corresponding, in order to minimize the motor-generated loss P represented by the following equation (a), the ratio of the second current component I _T and the first current component I _M is set to a. The control method for the induction motor is characterized in that the ratio a is controlled so as to satisfy the following equation (b) given by the function of the frequency of the motor. Note P = K ₁ (I _M ² + I _T ² ) + K _v (ωL _M I _M ) ² ...
(A) a = ± [{K ₁ + K _V (ωL _M ) ² / K _I ] ^1/2 ...
(B) where K ₁ and K _V are proportional constants, ω is the motor frequency, and L
_M is the excitation reactance 2. The method for controlling an induction motor according to claim 1, wherein the ratio a is controlled to 1 when ω is 0 or the motor loss depending on the voltage can be ignored. Induction motor control method. 3. A method of controlling an induction motor according to claim 1 or 2, wherein the magnetic flux of the motor is smaller than a first predetermined value related to frequency or rotation speed,
Is larger than the predetermined value of
A method for controlling an induction motor, characterized in that the method includes a method for limiting a current component of the induction motor.