JPH0531392B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a speed control method for an induction machine capable of controlling secondary excitation, and is particularly suitable for variable speed driving of a machine that requires a relatively large starting torque. This invention relates to a preferred speed control method.

[Conventional technology]

Cerbium devices have been used for variable speed drive of pumps and fans. This connects a forward converter (AC-DC converter) to the secondary winding of a wound induction machine, and then converts the secondary power into AC power via a DC reactor and an inverse converter (DC-AC converter). It is a device that regenerates energy. In this device, the capacity of the inverter increases in proportion to the size of the speed control range, and since the inverter operates with a firing delay angle of α≒90° in the low slip region, the reactive current is reduced. This has the disadvantage that the power factor of the entire system also decreases. To avoid this, the variable speed range of this type of equipment is usually set to 60 to 100 of the rated speed.
%, and the speed range from 0 to 60% was operated by connecting a starting resistor to the secondary winding.

Figure 2 is a starting resistance control circuit diagram, where P is the primary winding installed on the stator, S is the secondary winding installed on the rotor, and the secondary winding S is connected to a variable coil via a slip ring B. A starting resistor R is connected. In this way, the resistance value kr ₂ of the secondary circuit is determined by the sum of the resistance value r ₂ of the secondary winding S and the resistance value R of the starting resistor R (r ₂ + R
= kr ₂ ), and at the time of starting, R, that is, k, is kept at the maximum to obtain a large starting torque, and then by decreasing R as the speed increases, the torque-speed curve is shaped like a known proportional transition. By principle, the third
As shown in the figure, the curve moves to the right as →→. At this time, when the resistance value R is changed stepwise, the current changes in a sawtooth pattern, and the torque changes as shown in _a1 →b→c→d→e→f in FIG. In order to prevent this torque discontinuity from occurring, the starting resistor R needs to be continuously variable. Furthermore, there is a problem in that the torque fluctuates discontinuously when switching from operation using the starting resistor R to operation using the Serbius device.

Further, when the starting resistor R is used for speed control, it is necessary to use one with a capacity that can withstand the temperature rise due to the amount of heat generated. From these viewpoints, a large-scale device such as a water resistor is usually used as a starting resistor, but this is contrary to space saving and maintenance efficiency.

Incidentally, related speed control methods of this type include, for example, Japanese Utility Model Application Publication No. 56-175000 and Japanese Patent Application Laid-Open No.
60-82082, JP-A-60-156297, and the like.

[Problem that the invention seeks to solve]

As mentioned above, in the conventional speed control method, the entire speed range is controlled only by the Cervius device, and a large current capacity inverter is required to obtain a large starting torque, and the power factor decreases. If you try to smoothly control the entire speed range using only resistors, you will need a continuously variable resistor with a large power capacity, which will make the device larger. When control was performed using a Cervius device, there was a problem that torque would become discontinuous if the switching timing between the two control regions was incorrect.

Therefore, an object of the present invention is to provide a speed control for an induction machine that can obtain a large starting torque using a relatively small-capacity inverter and perform smooth torque speed control with a high power factor over its entire speed range. The purpose is to provide a method.

[Means for solving problems]

In order to achieve this object, the present invention provides a forward converter for converting the secondary voltage of a doubly exciting induction machine into direct current, and a self-extinguishing element for short-circuiting the output circuit of the forward converter. A secondary chopper for secondary current control, a capacitor connected in parallel with the secondary chopper via a diode, and an inverter that converts the secondary power of the induction machine obtained between the terminals of the capacitor into alternating current. In the speed control method for an induction machine, the speed is controlled by regenerating the secondary power of the induction machine into an AC power supply, wherein the induction machine is fixed to the secondary side of the induction machine in parallel with the forward converter via a switching element. A resistor with a resistance value is connected, and when the rotational speed is less than a predetermined value, the switching element is closed and the resistor is connected to the secondary side of the induction machine, and while current is flowing through the resistor, the secondary side is simultaneously Next, the secondary current is controlled by the chopper and the secondary power is regenerated into the AC power source, and when it is larger than a predetermined value, the switching element is opened and the resistor is disconnected, and all the secondary power of the induction machine is transferred to the AC power source. It is characterized by regeneration.

[Effect]

In the low speed region where the rotational speed is below a predetermined value,
By constantly controlling the secondary current with a secondary chopper while passing current through the resistor, large starting torque can be obtained without increasing the capacity of the inverter, and torque can be increased by using a resistor with a fixed resistance value. can be controlled continuously and smooth speed control can be performed. In addition, in a high-speed region where the rotational speed is higher than a predetermined value, high efficiency can be obtained by separating the resistor and regenerating all the secondary power to the AC power source, excluding the Joule loss caused by the resistor.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG.

First, the configuration of the main circuit will be explained. 1 is a wound induction motor (hereinafter referred to as IM), 2 is a forward converter that converts the secondary voltage of IM1 to DC, 3 and 6 are transistors, 4 is a diode for blocking reverse current, and 5 is a smoothing capacitor. be. Transistors 3 and 6 are GTO (gate turn off thyristor) which are self-extinguishing elements.
An element such as may also be used. Note that the transistor 3 constitutes a secondary chopper, and the transistor 6 constitutes a regenerative chopper. 7 is a freewheeling diode, 8 is a thyristor inverter for regenerating the secondary power of IM1 to an AC power source, and 9 is a transformer. 26 is a resistor connected to each phase of the secondary winding of IM1 in parallel with forward converter 2. 27 is a circuit opening/closing element for disconnecting the resistor 26 from the circuit, and is composed of a semiconductor switch made of a semiconductor element, a contactor, or the like. Note that the opening/closing element 27 is IM1
When starting (when the rotation speed is below the specified value), the circuit is closed,
The circuit is configured to be open during steady operation (when the rotational speed is above a predetermined value). Also, 23 is a power line, 2
4 is an electric wire connecting the inverter 8 to the transformer 9.

Next, the detection section and control circuit section will be explained.
10 is a speed detector directly connected to the IM 1, 11 is a speed command circuit, and 12 is a speed regulator that amplifies the deviation between the signal of the speed detector 10 and the speed command signal and outputs a current command signal i*. 13 is a current detector for detecting the output current i _d of the forward converter 2, and 14 is a comparison between the current command signal i* sent from the speed regulator 12 and the current detection signal obtained from the current detector 13. A comparator with a hysteresis characteristic outputs an on/off control signal for the transistor 3, and an amplifier 15 supplies a base current to the transistor 3. A control section for controlling the voltage of the capacitor 5 to be constant is constructed using almost the same method as the aforementioned control circuit section. That is, 16 is a voltage command circuit, 17 is a voltage detector, 18 is a voltage regulator, 19
2 is a current detector, 20 is a comparator with a hysteresis characteristic, and 21 is an amplifier for driving the base of the transistor 6. 22 is a circuit for controlling the firing of the inverter 8 at a constant firing phase.

The basic operation in the speed control method of this embodiment is as follows. According to the output signal i* of the speed regulator 12, the transistor 3 (secondary chopper) is turned on and off to control the output current id of the forward converter 2, thereby increasing the secondary current i ₂ of IM1. ,
Furthermore, speed control is performed by controlling the torque which is in a proportional relationship with this. By the way, while the transistor 3 is off, the DC current charges the capacitor 5 via the diode 4. At this time, the secondary power of IM1 is
The signal is sent to a capacitor 5 and converted into a direct current with a substantially constant voltage. Next, the transistor 6 (regenerative chopper) is turned on and off so that the voltage of the capacitor 5 becomes constant. The current flowing during the on period of the transistor 6 reaches the inverter 8, and as a result,
The secondary power of IM1 is regenerated into an AC power source via an inverter 8 and a transformer 9.

Transistor 3, diode 4, capacitor 5
The circuit formed by the transistor 6 and the transistor 6 has the function of converting the secondary voltage that fluctuates due to the slip, that is, the output voltage of the forward converter 2, into a constant voltage DC.
Here, if circuit loss is ignored, the power transmitted to the inverter 8 side will be equal to the secondary power of IM1. Since secondary power is expressed as the product of secondary power and secondary voltage, now considering that the secondary current is proportional to the square of the rotation speed and the secondary voltage is inversely proportional to the rotation speed, the secondary power The value of shows the maximum value when the slip is 1/3, and the value is 1/6 of the motor output. Therefore, for a load such as a pump or fan where the load torque changes as the square of the rotation speed, the power handled by the inverter 8 can be at most 1/6 of the motor output.

This secondary power is supplied to the transistor 3 as described above.
, 6, etc., the current of transistor 6 (regenerative chopper) and inverter 8 is converted into a constant voltage direct current, even at the maximum, the current of forward converter 2 rated output current (IM1 (proportional to current)
The capacity of the inverter 8 and the transformer 9 can be reduced to about 1/6 of the motor capacity.

Furthermore, since the inverter 8 is controlled with a constant firing phase, the power factor is always maintained at a constant high value (0.7 to 0.8) regardless of slip. Especially near the rated speed (slip is almost zero), the current of the inverter 8 is almost zero (the secondary power is almost zero), so there is no generation of reactive power from the inverter 8, and the entire system is As a result, high power factor and high efficiency operation can be achieved. Therefore, the capacity of the inverter 8 and the system power factor are independent of the speed control range, so the speed range can be set over the entire range from 0 to 100%.

However, when the inverter capacity is set to 1/6 of the motor capacity as described above, the torque (secondary current) at zero rotational speed (slip = 1) is limited to about 1/6 of the rated value. This is usually sufficient for pumps, fans, etc., but in cases where the load torque at startup is large, this will result in insufficient torque. If the capacity of the inverter 8 is increased, this torque shortage can be resolved, but at the same time, the current capacity of the diodes 4, 7, capacitor 5, transistor 6, and transformer 9 must also be increased, which is uneconomical.

The present invention solves these drawbacks in the following manner.

Generally, the torque of an induction machine is proportional to the secondary input P ₂₀ (synchronous watt). Further, this secondary input _P20 is divided into a mechanical output M and a secondary electrical output _P2 depending on the value of the slip s of the rotating machine. That is, M = (1-s) P ₂₀ ... (1) P ₂ = s・P ₂₀ ... (2) Also, the relationship between torque T and secondary electrical output P ₂ is
It is shown by the following formula.

P ₂ =ωs/p·T...(3) Here, ωs: slip angular frequency, p: number of pole pairs. That is, P ₂ is determined according to the relationship between torque T and slip s. _P2 is divided into a part that is regenerated into the AC power supply by the operation of the transistor 3 and the inverter 8, etc., and a part that is consumed by the resistor 26 connected in parallel with the forward converter 2. Power consumed 3Ri ² _R from P ₂ to inverter 8
If it is determined that the remaining power is obtained by subtracting the maximum regenerative power P _r , the following equation holds true.

3Ri ² _R = P ₂ −P _r ...(4) Here, R: resistance value of one phase of resistor 26,
i _R : Resistor current.

From this, the resistance value R is R=P ₂ −P _r /3i ² / _R (5). In addition, when using the resistor 26 with a fixed resistance value, R is determined taking into consideration the case where equation (4) is maximum in the operating range where the switching element 27 is closed.

The resistor 26 having such a resistance value is connected as described above, and the transistor 3 connects the forward converter 2.
By controlling the output current of the inverter 8, the required torque can be obtained while suppressing the regenerated power of the inverter 8 within the allowable upper limit value of the inverter 8. Furthermore, since the output current i _d of the forward converter 2 is controlled in proportion to the current command signal i* according to the speed deviation from the speed regulator 12, the rotation speed is adjusted to match the speed command value. controlled.

FIG. 4 is a diagram showing these relationships as a slip versus secondary current curve of an induction machine. Since the secondary current is proportional to the torque of the induction machine, the slip-torque characteristics can be understood from this diagram. In the figure, curve I ₂ shows the operating characteristics of only the control circuit without a starting resistor. In this case, as can be seen from the figure, the value of the starting torque at slip s=1 is very small. If the load torque at startup is small, such as with a normal pump or fan, this is sufficient to start operation, but if the load torque becomes large for some reason, operation becomes impossible. As a way to increase the starting torque to deal with this,
Earlier we talked about how to connect resistors. The curve I′ ₂ is
This shows the slip-secondary current (torque) characteristics due to the connection of a secondary resistor.As shown in Figure 3, by appropriately selecting the value of the additional resistor,
The proportional progression allows the starting torque to be increased. In the normal operation method, sufficient starting torque is obtained using an additional resistor, the motor is operated from point a on curve I' ₂ to the intersection point M with curve I ₂ , and controlled operation is performed by disconnecting the resistor from point M. As can be seen from the figure, _the curve _I'2 shows a drooping characteristic, and the torque is in a decreasing direction until it reaches the M point. Therefore, even if we ideally disconnect the additional resistor at the slip s _M at point M, the curve
A variation point occurs between I ₂ and the right upward direction, making it impossible to obtain smooth acceleration characteristics. Furthermore, disconnection of the resistor is delayed due to the inertia of the induction machine, etc.
If this is done at point M', the torque will rise all at once to point P, resulting in large torque fluctuations.

Curve I″ ₂ shows the slip-secondary current characteristics according to the speed control method of the present invention, and
By connecting this, sufficient starting torque is obtained, and at the same time, smooth acceleration characteristics are obtained by combined control with chopper control by the transistor 3. In other words, by sharing the hatched part in the figure with the control circuit, the starting torque at point a is secured and the regenerated power is suppressed to within the upper limit of the allowable value of the inverter 8, which is not possible with the conventional method. It is possible to obtain smooth acceleration characteristics.

As described above, with the control according to the present invention, sufficient torque can be obtained even in the low speed rotation range from the time of starting (slip s = 1), but if the additional resistor 26 is left connected as is, Joule loss occurs in the resistor 26, causing a decrease in efficiency. Therefore, it is necessary to disconnect the additional resistor 26 at an appropriate point to obtain higher efficiency. This is done by opening/closing element 2.
7, and operates when a predetermined slip (rotational speed) is reached. From this point on, all secondary power is regenerated into AC power by the inverter 8.

As described above, in this embodiment, a large starting torque can be obtained while keeping the capacity of the inverse converter 8 to a minimum, and the output current of the forward converter 2 is constantly controlled by the secondary chopper by the transistor 3. Therefore, the torque can be continuously controlled, and by using a fixed resistance type resistor 26, the torque can be controlled to a desired value, and smooth speed control can be performed. Further, the step change in torque when the resistor is turned on and off, as seen in the case of a simple secondary resistance start, can be alleviated in accordance with the action of the speed regulator 12. That is, when the motor current changes as the resistor 26 turns on and off, the torque changes and the speed also changes, so the current command signal i*, which is the output signal of the speed regulator 12, changes, and as a result, the motor current changes. Changes in torque are suppressed within a short period of time, torque fluctuations are alleviated, and smooth operation can be achieved.

Note that the resistor 26 may be connected not only to a three-phase star connection as shown in FIG. It can also be provided at the output end of the DC side. This has the effect of reducing the number of resistors 26 to one.

Further, the electric motor is not limited to a wound induction machine, but any induction machine having a secondary excitation function may be used. For example, it is also possible to apply it to the brushless squirrel cage induction machine described in Japanese Patent Application Laid-Open No. 59-129588. That is, the first winding wound around the stator core is used as the primary winding of the wound induction machine, the second winding is used as the secondary winding, and the second winding is By connecting the shown control device, the speed control method of the present invention can be applied. In this case, there is an advantage that brushless variable speed control is possible.

FIG. 5 is a circuit diagram showing a control device for implementing a speed control method according to another embodiment of the present invention;
Current detector 13 in the control device shown in FIG.
Instead, a secondary current detector 13' of an induction machine and a diode rectifier 28 are provided. In this embodiment, by controlling the current as follows, it is possible to almost completely eliminate current fluctuations in the induction machine secondary current when the resistor is turned on and off, which remains in the previous embodiment.

That is, instead of detecting the output current i _d of the forward converter 2, the secondary current i ₂ of the IM 1 is detected by the current detector 13', it is converted to direct current by the diode rectifier 28, and then the secondary current i 2 of the IM 1 is detected by the current detector 13'. In addition to 14, feedback control is performed. This has the effect that smoother variable speed control can be performed.

In the above embodiment, the current command signal and the detection signal from the current detector are compared using a comparator with hysteresis characteristics to control the secondary chopper on and off. is added to a P-I control current regulator, its output signal and a carrier wave signal that determines the on/off frequency of the chopper are added to a comparator, and the output pulse signal of this comparator controls the chopper on/off. Similar control can be performed even if

〔Effect of the invention〕

As described above, according to the present invention, the starting torque of the induction machine can be increased and the torque can be continuously controlled.
It is possible to obtain smooth speed control characteristics with a high power factor in the entire range from startup (slip = 1) to maximum speed while keeping the capacity of the inverter to a minimum, and it is also possible to obtain smooth speed control characteristics with a high power factor in the entire range from startup (slip = 1) to maximum speed. In the area, all secondary power is regenerated to the AC power supply, so it can be operated with high efficiency.Moreover, the secondary current is controlled by the resistor and regeneration to the AC power supply, so the capacity of the resistor is small. It can be controlled using an inexpensive fixed resistance value resistor.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a control device for carrying out a speed control method for an induction machine according to an embodiment of the present invention, FIG. 2 is a circuit diagram explaining a resistance starting method, and FIG.
The figure is a characteristic curve diagram for explaining the increase in starting torque due to proportional transition, Figure 4 is a slip-secondary current characteristic curve diagram, and Figure 5 is a speed control method for an induction machine according to another embodiment of the present invention. FIG. 2 is a circuit diagram showing a control device for implementing the above. 1... Wound induction motor (IM), 2... Forward converter, 3... Transistor, 4... Diode, 5
... Capacitor, 6 ... Transistor, 8 ... Inverter, 26 ... Resistor, 27 ... Circuit switching element.

Claims (1)

[Claims] 1. A secondary current control system consisting of an induction machine capable of secondary excitation, a forward converter that converts the secondary voltage of the induction machine into direct current, and a self-extinguishing element that short-circuits the output circuit of the forward converter. comprising a secondary chopper, a capacitor connected in parallel with the secondary chopper via a diode, and an inverter that converts the secondary power of the induction machine obtained between the terminals of the capacitor into alternating current,
In the speed control method of the induction machine, which controls the speed by regenerating the secondary power of the induction machine into an AC power source, a resistor with a fixed resistance value is provided on the secondary side of the induction machine in parallel with the forward converter through a switching element. When the rotational speed is below a predetermined value, the opening/closing element is closed and the resistor is connected to the secondary side of the induction machine, and while current is flowing through the resistor, the secondary chopper simultaneously Controlling the secondary current to regenerate the secondary power to the AC power source, and when it is larger than a predetermined value, open the switching element to disconnect the resistor and regenerate all the secondary power of the induction machine to the AC power source. A speed control method for an induction machine characterized by: