JPS5826592A - Protecting system of variable speed motor from overloading - Google Patents

Abstract

PURPOSE:To protect a motor from overloading without fail by a method wherein the detecting signals of a motor and the set up signals of a function generator are compared and checked with each other converting the results of measurement into the protective operation signals at optical speed. CONSTITUTION:The current, speed and revolution are respectively detected by the input current detector 11, the output frequency detector 12 and the revolution detector 13 while the motor current I at the revolution N and the set up signals of function generator 14 are compared and checked by means of a comparator 15 assuming the results of measurement by the comparator 15 to be the protective signal at optional speed. Through these procedures, the load status of the motor 5 at that time may be automatically measured and if the load exceeds the dangerous value, a motor trip command can be output to protect the motor from overloading without fail.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an overload protection system for a variable speed motor when a load such as a blower, a pump, etc. is operated at variable speed. It is said that In FIG. 1, 1 is a switch, 2 is an instrument current transformer, and the signal detected here drives an overcurrent relay 3. 4 is a variable speed power supply device for the electric motor 5;
6 is an internal temperature detector of the electric motor 5, and 7 is a load driven by the electric motor, such as a blower, a pump, etc.

FIG. 2 is a characteristic diagram showing the allowable cooling characteristics and load characteristics of the motor as a function of speed. Here, a and a' are the load characteristics, b is the motor's allowable overcurrent characteristic curve, and A
is the rated current value of the motor.

In the case of driving the electric motor St- in FIG. It can be operated at any speed within the range. Here, the electric motor 5 is operated, and in the process, when the motor load current flows in excess of the rated current A shown in Fig. 2, the overload relay 3 shown in Fig. 1 is activated, and the motor 5 is stopped. If this continues for a long time, the motor coil temperature will rise, but as a protective measure at that time, if the temperature detected by the motor internal temperature detector 6 exceeds the allowable temperature of the motor, the power supply is cut off, and the electric motor 5 is stopped.

Therefore, conventional overload protection methods for variable speed motors were carried out by detecting overcurrent of the motor or detecting internal temperature of the motor, as in the case of general constant rotation motors. The overload protection method did not match the allowable cooling characteristics. In addition, it is difficult to install a motor internal temperature detector on all variable speed motors due to the detection principle, installation and other structure, or characteristics depending on the motor capacity. The disadvantage was that it was difficult to take appropriate overload protection measures.

The present invention has been made to eliminate the above-mentioned drawbacks, and therefore, the rotational speed and load current of a variable speed motor are continuously detected during starting or operation, and both detected data and previously known data are used. A variable speed system that ensures overload protection for the motor at any speed by comparing and checking the permissible cooling characteristics of the motor with a function generator that generates a set speed-current characteristic curve. The purpose is to provide an overload protection method for electric motors.

Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

In FIG. 3, the same parts as in FIG. 1 are designated by the same reference numerals. In FIG.
4 is an output frequency detector, 13 is a rotation speed detector directly connected to the electric motor 5, and 14 is a function generator that generates a pattern that simulates the allowable cooling characteristics of the electric motor, and a synthesis comparator 15 compares the data with the current value. It is something to do.

The operation of the present invention having such a configuration will be explained below. In FIG. 3, when the load 7 of the electric motor 5 is a blower, a pump, etc., its torque characteristic is generally expressed by the relationship TaN2 (where T is the load resistance torque and N is the rotational speed). Also, since the relationship between the motor current and the torque T is Tαl, the normal motor current I
The relationship between and the rotational speed N is expressed as IαN2. (See characteristic curve a in Figure 2.) However, the above relational expression applies when the conditions (temperature, pressure, etc.) of the raw materials (air, water, etc.) supplied to the load 7 are constant.
When these conditions change, or when an abnormality occurs in the load 7, the above relational expression does not necessarily hold true (and the characteristics change to the characteristic curve a' in Fig. 2, or some arbitrary change occurs). Abnormal torque may occur depending on the speed.On the other hand, the permissible cooling characteristic of a cooling fan directly attached to the shaft of the main motor, that is, a motor with a self-cuff fan, is shown in characteristic curve b in Figure 2. As ILαNn, C
In the present invention, considering the load under the above conditions and the characteristics of the motor, overload protection of the motor is performed based on the electrical characteristics of the motor itself. In FIG. 3, the current is detected by the motor input current detector 11, the speed is detected by the output frequency detector 12, or the rotational speed is detected by the rotational speed detector 613. The motor current I at the speed N is set by the function generator setting signal IT (IT≦I
If I>ITK by matching L with CEL)), the motor is tripped.

In addition, in the above embodiment, for convenience of explanation, an application example was described for driving loads such as blowers and pumps, but by arbitrarily changing the setting value of the function generator, it is possible to drive not only these square torque loads but also other loads. Needless to say, it can also be used for other loads. Of course, it can also be applied to restraint protection of electric motors.

In addition, by using the present overload protection device in conjunction with conventional protection means such as a filtering current relay 3 and a motor internal temperature detector 6, a further motor protection system can be established.

As described above, according to the present invention, the current and speed of a motor whose speed is controlled at variable speed are continuously detected, and this detected data is used as a parameter to match data. By matching with the reproduced function generator, the load condition of the motor at that time is automatically determined and a motor trip command is output when the load exceeds a dangerous value, so the coil heat associated with the motor capacity is reduced. Unlike protection methods based on the influence of time constants or simple overcurrent detection, it is possible to take protection measures that match the characteristics of variable speed motors, and it is remarkable that filtering load protection measures can be taken reliably and easily at any speed. There is an effect.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional overload protection circuit for a variable speed motor, and FIG. 2 is a diagram showing the relationship between speed and current torque during variable speed operation, and the allowable current characteristics of the motor corresponding to the speed. FIG. 3 is a block diagram of an overload protection circuit for a variable speed motor showing an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1...Switch, 2...Measurement current transformer, 3...Overcurrent relay, 4...~variable speed power supply device, 5...Electric motor,
6... Motor internal temperature detector, 7... Load, 11... Motor input current detector, 12... Output frequency detector of variable speed device, 13... Rotational speed attached to the motor Detector, 14...Function generator that simulates the allowable cooling characteristics of the motor, 15...-Synthesis comparator. Note that the same reference numerals in the figures indicate the same or equivalent parts. Agent Shin Kuzuno (1 other person)

Claims (1)

[Claims] A speed detection device detects the speed of the motor or a current corresponding to the speed, and a function generator outputs the relationship with the motor current along the rotational speed of the motor, and the output signal of the speed detection device and the An overload protection method for a variable speed motor, characterized in that a comparator compares and matches a setting signal of a function generator, and the result determined by the comparator is used as a protective operation signal at an arbitrary speed.