JPS6349475B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for protecting an induction motor, and more particularly to a device for detecting overload of a motor based on changes in speed characteristics of the motor due to temperature rise.

Appropriate overload protection devices are required to effectively utilize the capacity of induction motors. Overload protection for induction motors is generally provided by thermal relays. Thermal relays have large variations in settings, and their operation is unreliable due to unmatched temperature rise characteristics between the thermal element and the motor, or due to differences in ambient temperature. Therefore, there are many problems such as setting too low and not being able to fully utilize the motor's ability, or setting too high and burning out the motor.

Therefore, in order to provide appropriate overload protection for motors, thermal relays are insufficient, and in some cases a temperature sensor is embedded in the motor to directly measure and monitor the internal temperature of the motor. There is. In this case, appropriate overload protection is possible, but there are problems such as the reliability of the temperature sensor itself, the sensor placement, which is subject to restrictions on motor manufacturing, and the need for sensor wiring.

The present invention develops the speed characteristics of the motor as the temperature of the motor increases, based on the electrical quantities in the operating state of the motor that can be easily measured, such as the primary terminal voltage of the induction motor, the primary input current, and the motor rotational speed, as well as the constants specific to the motor. The present invention provides a protection device for an induction motor that detects changes in the motor and thereby protects the motor from overload.

Induction motors generate secondary copper loss under load, and the heat increases the temperature of the secondary conductor.
Its resistance value also changes. Since the characteristics of the induction motor change significantly when the secondary resistance value changes, temperature increases can be detected by monitoring the interrelationships among the primary terminal voltage, primary input current, and motor rotation speed.

Generally, the secondary conductor of an induction motor is made of copper or aluminum. The resistance values of these metal materials change depending on the temperature, and the relationship can be expressed by the following equation (1).

R _t = R _t0 {1+α(t-t ₀ )} ...(1) Here, R _t : Resistance value at t (℃) R _t0 : Resistance value at t ₀ (℃) α : t ₀ of metal material Constant mass resistance temperature coefficient in (°C) Therefore, the temperature of a metal material can be determined by the following equation (2) by knowing its resistance value.

t=t ₀ +(R _t −R _t0 )/α (2) That is, the rotor temperature of the induction motor can be determined by knowing the resistance value of the secondary conductor. The resistance value of the secondary conductor in the operating state of the induction motor can be determined from constants specific to the motor and various input and output quantities of the motor, as described below. Hereinafter, various quantities on the motor secondary side are converted to primary side values. When an induction motor is operating, in its steady state, the resistance value of the secondary conductor r ₂
is given by the following equation (3).

r ₂ =L ₂ /M・ω _s Φ ₂ /i ₂ ...(3) Here, L ₂ : Secondary conductor self-inductance M : Mutual inductance between primary and secondary windings ω _s : Slip angular velocity Φ ₂ : Number of magnetic fluxes interlinking with the secondary conductor i ₂ : Secondary current If the secondary magnetic flux Φ ₂ is constant, the slip angular velocity ω _s
and i ₂ are proportional, and the secondary resistance r ₂ is included as its proportionality constant. Therefore, by detecting ω _s and i ₂ and observing the change in their ratio, the secondary resistance r ₂ can be determined. ω _s is the difference between the angular velocity ω ₁ of the power supply frequency and the rotational angular velocity ω _r of the motor, and ω _s = ω ₁ − ω _r
is given by Although i ₂ cannot be directly detected, it can be determined from the primary current i ₁ using the following equation (4).

i ₂ = √ ₁ ² − ₀ ² ...(4) In equation (4), i ₀ is the exciting current component of the primary current i ₁ that provides the secondary magnetic flux Φ ₂ , and is approximately the primary current value at no load. To be more precise, it is obtained by subtracting the iron loss current and the current component equivalent to the no-load mechanical loss.

The secondary magnetic flux Φ ₂ of the induction motor is strictly influenced by the load current i ₂ . Considering this influence, Φ ₂ is given by the following equation (5).

Φ ₂ = 1/ω ₁・M/L ₁・{√ ₁ ² −( _{1 0} − ₁ ′ _1
2 ) ² −r ₁ i ₂ } (5) Here, r ₁ : Primary winding resistance L ₁ : Primary winding self-inductance l′ ₁ :=L ₁ L ₂ −M ² /L ₂ v ₁ : Primary terminal Voltage In equation (5), the square root is v ₁ ² ≫ (r ₁ i ₀ −
ω ₁ l′ ₁ i ₂ ) ² , so if we consider it as v ² , Φ ₂ ≒ 1/ω ₁・M/L ₁ (v ₁ − r ₁ i ₂ ) ……(6). Furthermore, by substituting equations (4) and (6) and ω _s =ω ₁ −ω _r into equation (3), the following equation (7) is obtained.

Equation (7) is an approximate equation for determining the secondary resistance value. Therefore, the power supply frequency ω ₁ , the motor terminal voltage v ₁ ,
By measuring the input current i ₁ and rotational speed ω _r ,
The secondary resistance value r ₂ can be determined by calculation.

L ₁ , L ₂ , and r ₁ are constants specific to the motor, so L ₂ /L ₁
If we set = a, r ₁ = b becomes. The value of r ₁ also changes depending on the temperature on the motor stator side, but in order to simplify equation (8),
If r ₁ is ignored, b=0 and the following equation (9) is obtained.

When the induction motor is operated with a general commercial power source, both v ₁ and ω ₁ are substantially constant, so the calculation formula for r ₂ can be further simplified as shown in the following equation (10).

Here, K: Constant determined by the power supply and motor As explained above, the terminal voltage of the motor v ₁ ,
From the input current i ₁ and the rotational speed ω _r , find the resistance value of the secondary conductor in the operating state using equation (8) or the simplified equations (9) and (10), and calculate the rotor temperature from equation (2). You can ask for it.

Equation (10) shows the relationship between the primary current i ₁ and the rotational speed ω _r using the secondary resistance value r ₂ as a parameter. When this is illustrated, the load characteristics of the motor as shown in FIG. 1 are obtained. In Figure 1, r ₂ (t ₀ ), r ₂
(t ₁ ) and r ₂ (t ₂ ) are the rotor temperatures t ₀ (℃) and t ₁ , respectively.
(℃), is the secondary resistance value at t ₂ (℃), and indicates the load characteristics of the motor at that temperature.

In the no-load state, the primary current is only the excitation current component and i ₁ =i ₀ . Further, the motor rotation speed becomes a synchronous speed, and ω _r =ω ₁ . As the load increases,
As i ₁ increases, the slip increases and the rotational speed decreases.

The degree of this fall differs depending on the secondary resistance value. If t ₂ (℃)＞t ₁ (℃)＞t ₀ (℃), then r ₂ (t ₂ )＞r ₂
( _t1 )
> r ₂ (t ₀ ), and the larger the secondary resistance value, that is, the higher the rotor temperature, the greater the degree of fall in the rotational speed.

Now, in Fig. 1, if t ₂ (℃) is the maximum allowable temperature of the rotor, when the relationship between the primary current i ₁ measured in the operating state and the rotational speed ω _r is in the shaded area, the rotation is The child temperature exceeds t ₂ (°C) and can be seen as an overload condition.

FIG. 2 is a configuration diagram showing an embodiment of the present invention. In Figure 2, 1 is a 3-phase AC power supply, 2 is a 3-phase AC power supply, and 2 is a 3-phase AC power supply.
A phase induction motor, 3 is a pulse transmitter that is mechanically coupled to the motor and generates a pulse train with a frequency proportional to the rotation speed, 4A and 4B are current transformers for converting the motor input current, and 5 is from the pulse transmitter. 6 is a primary current signal _{i 1 from} the current transformer output.
7 is a function generator, 8 and 9 are comparators, and 10 is an AND gate.

The function generator 7 inputs i ₁ and outputs a rotational speed reference value ω _r * with a secondary resistance value corresponding to the maximum allowable temperature of the rotor as a parameter. That is, the load characteristic corresponding to r ₂ (t ₂ ) in FIG. 1 is stored. A comparator 8 compares the output ω _r * of the function generator and the output ω _r of the speed detector. As a result of the comparison, if ω _r >ω _r *, the rotor temperature is within the allowable temperature, and if ω _r <ω _r *, the rotor temperature is over the allowable temperature. Therefore, comparator 8
The output of becomes a logic signal “1” when ω _r <ω _r *,
This becomes an overload signal seen from the rotor temperature.

In a region where the input current i ₁ is small, the calculation accuracy decreases due to the setting deviation of i ₀ and the detection accuracy of i ₁ . In addition, an overload condition usually occurs when the input current is 100% or more, so if i ₁ is 100% or more, ω _r <ω _r * It is more practical to judge overload by making a judgment as follows, and it is possible to prevent malfunctions based on calculation accuracy.

i ₁ * is 100% of the input current of the induction motor (rated value)
This is a set value, and the comparator 9 compares it with the signal i ₁ from the current detector. When i ₁ > i ₁ *, the output of the comparator 9 becomes a logic signal "1", and the 10 AND gate is established. , generates an overload signal when the output of the comparison path 8 is "1". This overload signal can be used to issue an overload alarm, display an overload display, or directly cut off the power to the induction motor to protect the motor.

As explained above, according to the present invention, overload protection of an induction motor can be performed by focusing on the rotor temperature without embedding a temperature sensor in the motor. Using a commonly used current detector and speed detector, it is possible to achieve highly accurate and reliable overload protection with a relatively simple calculation circuit, effectively utilizing the overload capacity of the motor to its limit. be able to.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing changes in the load characteristics of an induction motor using a secondary resistance value as a parameter, and FIG. 2 is a configuration diagram showing an embodiment of the present invention. 1: 3-phase power supply, 2: 3-phase induction motor, 3: Pulse transmitter, 4A, 4B: Current transformer, 5: Speed detector, 6: Current detector, 7: Function generator, 8, 9:
Comparator, 10: AND gate.

Claims (1)

[Claims] 1 current detection means for detecting the primary current of the induction motor; means for inputting the output signal of the current detection means and outputting a rotational speed signal corresponding to the maximum allowable temperature of the rotor of the induction motor; a comparison means for outputting an overload signal by comparing output signals of the means and a rotation speed detection means for detecting the rotation speed of the induction motor; and a comparison means for causing the comparison means to respond when the primary current of the induction motor is equal to or higher than a rated current. A protection device for an induction motor, comprising means.