JPS64920B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for protecting an induction motor, and particularly to a method for calculating the resistance value and temperature rise of a secondary resistor.

Appropriate overload protection devices are required to effectively utilize the capacity of induction motors. Induction motor overload protection is generally provided by thermal relays. Thermal relays have large variations in settings, unmatched temperature rise characteristics between the thermal element and motor, and unreliable operation 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 a motor, a thermal relay is insufficient, and in some cases a temperature sensor is embedded in the motor to directly measure and monitor the internal temperature of the motor. ing. In this case, appropriate overload protection is possible, but there are problems such as the reliability of the temperature sensor itself, the sensor placement being subject to restrictions on motor manufacturing, and the need for sensor wiring.

The present invention uses electrical quantities in the operating state of the motor that can be easily measured, such as the primary terminal voltage, primary input current, and motor rotational speed of the induction motor, as well as constants unique to the motor, to calculate changes in the state of the motor as the temperature of the motor increases. The purpose of this invention is to provide a method for protecting an induction motor that 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, the rotor temperature can be detected by monitoring the interrelationship between the primary terminal voltage, the primary input current, and the 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).

Rt=Rt ₀ {1+α(t-t ₀ )} ...(1) where Rt: resistance value at t (℃) Rt ₀ : resistance value at ₀ (℃) α: resistance value of metal material at t ₀ (℃) Temperature Coefficient of Resistance Therefore, the temperature of a metal material can be determined by the following two equations by knowing its resistance value.

t=t ₀ +(Rt−Rt ₀ )/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. Below, various quantities on the secondary side of the motor are converted to values on the primary side. During operation of the induction motor, the resistance value r ₂ of the secondary conductor in its steady state is given by the following equation (3).

r ₂ = L ₂ /M・ωsΦ ₂ /i ₂ ...(3) where L ₂ : Secondary conductor self-inductance M : Mutual inductance between primary and secondary windings ωs : Slip angular velocity Φ ₂ : Secondary conductor Number of magnetic fluxes linked to i ₂ : Secondary current If secondary magnetic flux Φ ₂ is constant, slip angular velocity ωs
and i ₂ are proportional, and the secondary resistance r ₂ is included as its proportionality constant. Therefore, by detecting ωs and i ₂ and observing the change in their ratio, the secondary resistance r ₂ can be determined. ωs is the difference between the angular velocity ω ₁ of the power supply frequency and the rotational angular velocity ωr of the motor, and ωs = ω ₁ - ωr
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 = √ ² ₁ - ² ₀ ...(4) In equation (4), i ₀ is the exciting current component of the primary current i ₁ that gives the secondary magnetic flux Φ ₂ , and is almost the primary current at no load. More precisely, it is the value obtained by subtracting the iron loss current and the current component equivalent to the no-load area loss.

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

Φ ₂ = 1/ω ₁・M/L ₁ {√ ² ₁ ―( _{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 ₁ v ₁ : Primary terminal voltage In equation (5), the value within the square root is v ² ₁ ≫(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 ωs=ω ₁ −ωr 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 ωr,
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 approximately constant, so the calculation formula for r ₂ can be further simplified as shown in the following equation (10).

Here, K is a constant determined by the power supply and motor, and as explained above, the terminal voltage of the motor v ₁ ,
From the input current i ₁ and rotational speed ωr, find the resistance value of the secondary conductor in the operating state using equation (8) or the simplified equations (9) and (10), and find the rotor temperature from equation (2). be able to.

FIG. 1 is a block diagram showing an embodiment of the present invention. In Figure 1, 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 rotational speed; 4A and 4B are current transformers for converting the motor input current; 5 is a power supply frequency ω ₁ 6 is a voltage detector to detect the power supply voltage v ₁ , 7 is a current detector to detect the primary current i ₁ from the current transformer output, 8 is from the pulse transmitter 9 is a setter for constant i ₀ in equation (8) for r ₂ , 10 is a setter for constant a, and 11 is a setter for constant b 12 is a setting device for the reference temperature t ₀ , 13 is a setting device for the secondary resistance value r ₂₀ at the reference temperature t ₀ , 14 is a setting device for the fixed value temperature coefficient α of the resistance of the secondary conductor material, 15 is a setting device for the temperature coefficient α of the resistance of the secondary conductor material; Secondary resistance r ₂
(8) r ₂ calculator for calculating by formula, 16 is (2)
a t calculator that calculates the rotor temperature t according to the formula t=t ₀ + (r ₂ − r ₂₀ )/α based on the basic formula; 17 is a display for displaying the rotor temperature t; 18 is a setter for the maximum allowable rotor temperature value t ^* , and 19 is a setter for t ^*
This is a comparator that generates an overload signal "OL" when t>t ^* .

r ₂ in the operating state is determined by the r ₂ calculator 15 using the measured values ω ₁ , v ₁ , i ₁ , ωr from each detector and the set values i ₀ , a, b from each setter as inputs. .
Furthermore, this calculation result r ₂ and the setting value from each setting device
The rotor temperature t is determined by the t calculator 16 using t ₀ , r ₂₀ , and α as input, and the rotor temperature can be displayed on the display 17 for monitoring. Also, when t is t ^* , an overload signal is generated, and from that signal an overload alarm is generated.
An overload indication can be given, and in some cases, the signal can be used to directly cut off the power to the motor.

If i ₂ r ₁ is small with respect to v ₁ , r ₂ operator 1
Equation (9) can be used as the arithmetic expression in step 5, and the b setter 11 is not required, and the arithmetic unit 15 is also simplified.

FIG. 2 is a configuration diagram showing an embodiment of the present invention when the power frequency and power voltage are constant, such as an induction motor operated by a commercial power source. Items with the same numbers as in Figure 1 are the same parts, and 20
The constant ω ₁ in equation (10) for r ₂ is a setter, and 21 is also a setter for the constant K. In this case, since the power supply frequency and power supply voltage are constant, a frequency detector and a voltage detector are not required, and the configuration is simplified. Furthermore, the r ₂ arithmetic unit 15 only needs to have the simplified arithmetic function shown in equation (10).

Also, if there is no need to monitor the rotor temperature and only the overload signal is required, there is no need to perform calculations to obtain the temperature, and the objective can be achieved by monitoring the secondary resistance _r2. . Since the secondary resistance value is uniquely determined by the rotor temperature, it is only necessary to compare it with r ₂ ^* at the maximum allowable value of the rotor temperature. In ^other _{words ,} as _shown in _FIG ^.
3, an overload signal "OL" can be generated when r ₂ >r ₂ ^* . In this case 16
Since the temperature calculator and the necessary constant setting device are not required, the configuration becomes very simple.

In a region where the input current i ₁ is small, there is a possibility that the accuracy of the r ₂ calculation will decrease due to the setting deviation of i ₀ and the detection accuracy of i ₁ . Additionally, an overload condition usually occurs when the input current continues to exceed the rated value, so judgment is made based on the calculation result r ₂ or t, provided that i ₁ is greater than the rated value. , it is more practical to judge overload, and thereby it is possible to prevent malfunctions based on calculation accuracy.

Therefore, in FIG. 4, the output of the current detector 7
The comparator 25 compares i ₁ and the set value i ₁ ^* from the setting device 24 for setting the rated current value i ₁ ^* , and determines that i ₁ >i ₁ ^*
When , the comparator output establishes the AND gate 26, and based on the signal D of t>t ^* or r ₂ > r ₂ ^* , it is determined that there is an overload, and an overload signal OL is generated.

As described above, according to the present invention, the rotor temperature can be monitored without embedding a temperature sensor in the motor, and the motor can be protected against overload by focusing on the rotor temperature. Using commonly used current detectors and speed detectors, a relatively simple arithmetic circuit has high setting accuracy, and highly reliable monitoring and protection is possible, making it possible to effectively push the overload capacity of induction motors to their limits. It can be used for.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing one embodiment of the present invention, and FIG.
The figure is a block diagram showing an embodiment of the present invention when the power supply voltage and the power supply frequency are constant, Figure 3 is a block diagram when an overload signal is obtained by changing the secondary resistance value, and Figure 4 is a diagram showing the configuration when the overload signal is obtained by changing the secondary resistance value. FIG. 3 is a configuration diagram when an overload signal is obtained under conditions of a rated value or higher. 1; 3-phase power supply, 2; 3-phase induction motor, 3; pulse transmitter, 4A, 4B; current transformer, 5; frequency detector, 6; voltage detector, 7; current detector, 8; speed detector , 9; i ₀ setting device, 10; a setting device, 11;
b setting device, 12; t ₀ setting device, 13; r ₂₀ setting device, 1
4; α setting device, 15; r ₂ setting device, 16; t calculator, 17; display device, 18; t ^* setting device, 19, 2
3, 25; Comparator, 20; ω ₁ setter, 21; K
Setting device, 22; r ₂ ^* setting device, 24; i ₁ ^* setting device, 2
6; And gate.

Claims (1)

[Claims] 1. The angular velocity of the power supply frequency is ω ₁ , the rotational angular velocity of the induction motor is ωr, the terminal voltage is V ₁ , and the exciting current component is
i ₀ , input current i ₁ , primary winding self-inductance of induction motor L ₁ , secondary winding self-inductance L ₂ , primary winding resistance r ₁ , secondary winding conductor resistance temperature coefficient α , the secondary resistance value at reference temperature t ₀ is r ₂₀ , and the secondary resistance value r ₂ of the induction motor is Further, the temperature t of the secondary winding at this time is determined by the calculation t = t ₀ + (r ₂ − r ₂₀ )/α, and this calculation value t and the preset value A method for protecting an induction motor, characterized in that an overload condition of the induction motor is determined based on whether the calculated value exceeds the set value by comparing the calculated value with a maximum allowable rotor temperature value t ^* . .