JPH0328920B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an overcurrent and overload protection system for an induction motor having a square decreasing torque load.

[Conventional technology]

FIG. 1 is a circuit diagram showing an example of a conventional speed control device for an induction motor. In the figure, 1 is an induction motor, 2 is a three-phase bridge rectifier, 3 is a starting resistor, 4 is a transistor chopper, 5 is a secondary current detector, 7 is a speed setting device, 20 is a transistor ignition control device, It is.

In FIG. 1, if the transistor chopper 4 turns on, the three-phase rectifier bridge 2 will cause
The starting resistor 3 will be short-circuited. Chiyotsupa 4
When the motor turns off, a starting resistor 3 is connected to the secondary side of the induction motor 1. In this way, the current flowing through the secondary circuit of the induction motor 1 changes by turning on and off the transistor chopper 4, and therefore the current flowing through the secondary circuit of the induction motor 1 changes.
The rotation speed of changes.

The transistor ignition control device 20 takes in the speed command signal a set by the speed setting device 7 and the secondary current (torque current) value d detected by the current detector 5, and turns it on so that the two match. , sends an off firing command to the transistor chopper 7. As a result, the transistor chopper 4 turns on and off so that a secondary current (torque current) corresponding to the speed command signal a flows, and the motor speed also reaches the set speed.

Now, such an induction motor with a speed control device may have a quadratic decreasing torque load as a load. The square decreasing torque load is the torque/speed characteristic as shown in Figure 2, that is, the rotation speed is 100%.
If the torque is also 100% when the speed is 100
%, the torque gradually decreases at the rate of its square (for example, when the speed decreases)
When the torque reaches 50%, the torque gradually decreases to 25%), and such loads include pumps,
Examples include blowers, fans, etc.

In the graph of Fig. 2, the rotational speed on the horizontal axis is the command signal a of the set speed in the case of an induction motor with a speed control device as shown in Fig. 1, and the torque on the vertical axis is the quadratic It will be easily understood that it can be expressed by taking the detected value d of the current (torque current).

[Problem to be solved by the invention]

Now, conventional problems related to motor overload protection when an induction motor equipped with a speed control device as shown in Fig. 1 drives a load having a quadratic decreasing torque characteristic as shown in Fig. 2. This point will now be explained with reference to FIG.

FIG. 3 is a graph that redraws the torque/speed characteristics shown in FIG. 2 and also shows a conventional setting level L (secondary current 120%) for overload protection as an example.

In other words, the setting level L for overload protection of conventional induction motors is set to a constant value of 120% over the entire rotational speed range of the motor, and if the secondary current is 120%, then the secondary current exceeding this level is set to 120%. When this occurred, protection was provided by detecting this and issuing an alarm or stopping the motor.

As can be seen from Figure 3, when the motor is operating at 100% speed, an overload is determined if the secondary current increases by ΔL (=20%) from the rated current (100%) at that time. Ru. However, when the motor is operating at 50% speed, the increase in secondary current ΔL' required to be judged as overloaded is about five times the rated current (25%) at that time. reach

In other words, when the motor is operating at 50% rotational speed, unless a current of five times the rated current flows, it will not be judged as overloaded and will not be protected. Become.

If we consider that the motor is a self-ventilating motor that ventilates through the fan action of the rotor itself or by the blades attached to the rotor, at 100% rotation speed, the cooling capacity against heat generation is Although it is high, the cooling capacity decreases considerably when the rotation speed is 50%. Nevertheless, if the overload is not detected unless the secondary current reaches five times the rated value, the motor may burn out due to the temperature rise, or it may not be possible to withstand it. dielectric strength must be applied, which results in an unnecessary increase in cost.

The present invention has been made to solve the problems in the prior art as described above, and therefore, an object of the present invention is to provide insulation even when operating at a low rotational speed of 100% or less. Provides an overcurrent and overload protection method for induction motors that provides sufficient overload protection for loads with square-law decreasing torque load characteristics without incurring unnecessary cost increases due to proof stress, etc. It's about doing.

[Means to solve the problem]

In order to achieve the above object, in the present invention, as shown in FIG. 4, a setting level M for overload protection is provided substantially parallel to the square decreasing torque/speed characteristic b.

(Function) In this way, when the rotational speed is low and the cooling capacity is low, overload detection is performed even with a relatively low overcurrent, and it is possible to effectively protect the motor.

〔Example〕

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 5 is a circuit diagram showing one embodiment of the present invention. The configuration shown in FIG. 5 includes a block S shown by a broken line in the induction motor speed control device shown in FIG.
This corresponds to the configuration with the addition of .

In FIG. 5, 8 is an inverting amplifier, 9 is an analog multiplier, 10 is a variable resistor, 11 is an adder, and 12
13 is a variable resistor, 13 is a voltage comparator, and 21 is an overcurrent detector. The above components constitute an overcurrent detection system.

In addition, 14 is an inverting amplifier, 15 is a variable resistor, 16 is an adder, 17 is an integrator, 18 is a variable resistor, 19 is a voltage comparator, and 22 is an overload detector, and the above components detect overload. A system is constructed.

Next, the circuit operation will be explained with reference to FIGS. 4 and 5. The set speed command signal a is inverted and amplified in the inverting amplifier 8, and then in the multiplier 9 as (a
×a), and the resulting signal b=a ² (see characteristic b in FIG. 4) is then multiplied by the adder 11 to the value ΔB
As a result, the signal M (characteristic M in Fig. 4) is added.
reference) is created.

This signal M is converted by the variable resistor 12 into a value e proportional to it. This value e and the torque current d are compared in the voltage comparator 13, and when d exceeds e, the voltage comparator 13 outputs a signal f, and the overcurrent detector 21 detects an overcurrent in the induction motor 1. Detect that this is occurring.

On the other hand, the signal M from the adder 11 is inverted and amplified by the inverting amplifier 14, and then converted by the variable resistor 15 to a value g proportional to the signal M. Next, in the adder 16, the calculation (d-g) is performed, and the resulting difference signal h=(=d-g)
is integrated in an integrator 17. Integral result i
is compared with a certain set value j set in the variable resistor 18 in the voltage comparator 19, and when i exceeds j, an output signal k is generated from the voltage comparator 19, and the overload detector 22 detects the inductive It is detected that the electric motor 1 is in an overload state.

In the induction motor 1, the overcurrent detector 21
The overload detector 22 detects an overcurrent condition if it occurs even momentarily, and the overload detector 22 detects this when the overcurrent condition continues for a certain period or more. This seems clear from the above explanation.

As explained above, according to the present invention, even when the rotational speed is at a low speed of 100% or less, an induction motor can effectively respond to a load having a square-law decreasing torque load characteristic. There is an advantage that overcurrent protection and overload protection can be provided.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an example of a conventional induction motor speed control device, Fig. 2 is a graph showing the torque/speed characteristics of a square decreasing torque load, and Fig. 3 is a graph showing the torque/speed characteristics shown in Fig. 2. In addition to the torque/speed characteristics shown in Figure 2, Figure 4 is a graph showing the setting level L for conventional overload protection after redrawing the characteristics. FIG. 5 is a graph showing the setting level M and a circuit diagram showing an embodiment of the present invention. Description of symbols, 1... Induction motor, 2... Three-phase bridge rectifier, 3... Starting resistor, 4... Transistor chopper, 5... Current detector, 7... Speed setting device, 8, 14... Inverting amplifier, 9... Analog multiplier, 10, 12, 15, 18... Variable resistor, 1
1, 16... Adder, 13, 19... Voltage comparator, 17... Integrator, 20... Transistor firing control device, 21... Overcurrent detector, 22... Overload detector.

Claims (1)

[Scope of Claims] 1. In a speed control device for an induction motor, means for creating a torque value in accordance with the square decreasing torque/speed characteristics of a load using a speed command value for speed control; In addition to comprising means for detecting the motor current as an actual value, by creating a first set value proportional to the created torque value and comparing the set value with the detected motor current. a first means for detecting the presence or absence of an overcurrent, a second set value proportional to the created torque value, and an integral for the amount by which the detected motor current exceeds the second set value; and a second means for detecting the presence or absence of an overload based on whether the integral result exceeds a certain limit; and one or both of the following means. , overload protection method.