Research on Aviation Motor and its Protector

Research on Aviation Motor and its Protector

HUANG,Yong

Aircraft Maintenance and Engineering Corporation（Chongqing Branch）401120

*Corresponding Author: HUANG Yong

Abstract: This paper introduces the different operating environments of aviation motors and civil motors, analyzes the common faults and protection principles of motors. For more serious open-phase faults and overload protection, this pater puts forward a protection circuit with 555 counter as the main component to prevent vibration, heat and even fire caused by motors unbalanced phrase.

Key words: motors; short circuit; open-phase; overload; 555 counter

Introduction

Broadly speaking, an electrical motor is a transformation device for electrical energy, including rotating motors and stationary motors. The rotating motor is an energy conversion device that makes the mutual conversion between electrical energy and mechanical energy based on the principle of electromagnetic induction; the stationary motor is an electromagnetic device that makes voltage changes based on the law of electromagnetic induction and the principle of magnetic potential balance, which is also called a transformer.

There is no essential difference in the basic working principles of aviation motors and motors used in other industries. They are mainly the used power supply equipment for power supply, power equipment for machinery to provide executive control, status detection, AC-DC conversion, signal transmission and so on.

Different from most civil motors, aviation motors are in a changing working environment. Due to changes in flight altitude and seasonal changes, the operating temperature range of motors on aircraft is about: -56°C ~ +50°C. Consequently, the rapid change in temperature may cause changes in the resistivity of conductive materials, the insulation materials cracking, bending and delaminating; the magnetic permeability of the magnetically conductive material becomes poor; the organic substance product becomes brittle to break; the overload capacity becomes poor; the strength and elasticity of the material decrease; the components produce stress due to coefficient linear expansion, all of which will directly affect the performance of the motors or produce mechanical failure. And the changing atmospheric pressure conditions will lead to insulator performance deterioration; the dielectric strength of the air will decrease, which is easy to cause breakdown and cause arc discharge. If the insulation distance is not enough, it may even cause corona phenomenon; thin air is difficult to dissipate heat, accelerating the aging rate of insulator materials, which is more unfavorable for the motor brush wear, commutation situation, spark suppression. If the motors work in a high humidity environment, the insulation resistance and breakdown voltage will be greatly reduced. In severe cases, the water absorption will cause leakage of electronic components and affect the normal operation of electronic circuits; it will cause corrosion of metal materials under the action of water vapor. In addition, ozone in the atmosphere will have an impact on the electrical contact; salt spray will destroy the insulation performance of the material and accelerate metal corrosion; particles in the sand and dust adhere to the insulation material will reduce the insulation performance of the material and cause leakage.

Under normal conditions, the motor can work for long periods of time. But the motor may run out of control under non-standard operating conditions. The consequences are unimaginable. It may cause damage to the motor, and seriously it may even cause equipment to catch fire. This paper mainly discusses the fault principle and protection circuit design of the three-phase asynchronous motors with high power, high heat generation and serious damage after failure in aircraft.

1 Motor Fault Analysis

The common faults of electric motors can be pided into two categories: symmetrical faults and asymmetrical faults. Symmetrical faults include overload, locked rotor and three-phase short circuit. Asymmetric faults include open-phase, reverse phase, inter-phrase short circuit, turn-to-turn short circuit and so on. It is characterized by negative sequence current and zero sequence current in the motor current.

1.1 Short-circuit protection

Short-circuit faults will occur when the line insulation in the motor control line is damaged, when the load is short-circuited, and when the stator winding is short-circuited. The instantaneous fault current generated by the short circuit is ten to dozens of times of the rated current. Electrical equipment or distribution lines may be damaged, arced, or even cause fire due to the strong electrodynamic force generated by the short circuit current. Short-circuit protection requires that the power supply be cut off in a very short time after the short-circuit fault. The common method is to connect a fuse or a low-voltage circuit breaker in series in the line. The operating current of the low-voltage circuit breaker is set to be 1.2 times of the starting current of the motor. However, because the instantaneous starting current of the motor is relatively large, usually 4 to 7 times of the rated current, the delay circuit can be used to effectively avoid the starting current of the motor and avoid malfunction when designing the protector.

1.2 Open-phase and

three-phase unbalance protection

Open-phase fault is an asymmetrical fault that can easily cause component damage, motor burnout and even fire. Therefore, the open phase protection of the motor is also very important. The judgment of the unbalance of the motor open phase is mainly based on the negative sequence current and the unbalance ratio of the three-phase current. Considering various errors and the maximum unbalance rate of the current during various open phase, it is more appropriate to take the maximum unbalance rate δ>30% as the basis for determining open-phrase and three-phase unbalanced faults.

1.3 Overload protection

If the motor is in an overload state for a long time, the motor windings will be overheated, and finally the insulation between the windings will be damaged. So it is not allowed to run the motor in overload for a long time. The thermal inertia of the motor makes it have a short-term overload capability. So the short-term overload is still normal operation. Only when the heat accumulation temperature reaches to the level of damage to the motor, it will be protected. The overload protection is not required to be affected by the short-term start-up rush current or short-circuit current of the motor and delay action, and thermal relays are usually used as overload protection components.

2 Protector Circuit Design

   Since short-circuit and overload faults in the circuit can be protected by using thermal relays and circuit breakers, the following mainly introduces the circuit design for open-phase and over-current protection.

2.1 555 counter circuit and its function

The 555 counter gets its name from the fact that the integrated circuit contains three 5kΩ resistors. It is a medium-scale integrated circuit that combines analog circuits and digital circuits. It has strong logic functions, is used flexibly, and can easily form a variety of logic function circuits. It occupies an important position in digital circuits and is generally valued by people. Its internal circuit consists of voltage pider, voltage comparator C1 and C2, basic RS trigger and discharge tube

TD . Its internal circuit is shown in Figure 1, and the truth table of each pin is shown in Table 1.

                      Figure 1  555 counter circuit

In the figure, pin 1 is the ground terminal GND, pin 2 VI2 is the low-level trigger terminal (also called trigger terminal), pin 3 is the output terminal V0, pin 4 D is the reset terminal, pin 5 VCO is the voltage control terminal, and pin 6 VI1 is the high-level trigger terminal (also called the threshold terminal), the pin 7 V ’0 is the discharge terminal, and the pin 8 VCC is the power supply terminal.

  Table 1 (Note: * indicates arbitrary level)

The truth table of the function pinout of 555 counter is shown in Table 1, where the pin 4D is the reset terminal. When the pin 4 is at low level, no matter what the state of other input terminals is, the output V0 is low. Only when pin 4 is at high level, the output state will be determined by the voltage of the low-level trigger terminal of pin 2 VI2 and the high-level trigger terminal of pin 6 VI1. Therefore, pin 4 should be connected to high level in normal operation.

When VI1 is at any level and VI2≤(1/3)VCC, the discharge transistor TD is turned off, and the output terminal is at high level. When VI1≥(2/3)VCC, VI2＞(1/3) VCC, the discharge transistor TD is turned on, and the output terminal Vo is at low level. When VI1＜(2/3)VCC, VI2＞(1/3) VCC, the circuit keeps the original state unchanged.

In summary, it can be concluded that when VI1 is at high level (VI1≥(2/3) VCC), the trigger is set to 0, and when VI1＜(2/3) VCC, there is no effect on the trigger. It can be seen that VI1 is at high level. It can be effectively set to 0, that is, high level is effective; when VI2 is at low level (VI2≤(1/3) VCC), the trigger is set to 1, and when VI2＞(1/3) VCC, there is no effect on the trigger. It can be seen that the low level of

VI2 can effectively play the role of setting 1. That is, the low level is effective.

2.2 Schematic diagram of protector circuit

The schematic diagram of the protector is shown in Figure 2, in which terminals 1 and 2 are connected in series to the current of any phase of the motor, terminals 3 and 4 are connected to the control line of the main contactor of the motor, and terminals 5 and 6 are the power input terminal of the protector.

Figure 2

When AC 220V or 115V is added to the primary side of the power transformer T1 (depending on the control voltage of the actual control circuit of the motor), the secondary voltage is about 10V, which is rectified by the bridge B1 and filtered by the capacitor C5 to obtain a voltage of about 13V. The DC voltage is sent to the voltage regulator tube DW through the current limiting resistor R5 for voltage regulation, and the output stable DC voltage VCC is about 12 V. When the motor is running, the current transformer LH converts the line current of the motor into the corresponding AC signal voltage, which is rectified by D1 and filtered by C1 as the input signal voltage of the 555 counter. When the motor is running normally, its line current is converted into a voltage that is not greater than (2/3) VCC through LH and W1, which is applied to the pin 6 of the 555 counter. And the DC voltage VCC is converted into a voltage that is less than (1/ 3) VCC, which is applied to pin 2 of the 555 counter. It can be seen from Table 1 that pin 4 is at a high level, and the level of pin 2 of the inverting input terminal of the 555 counter is less than (1/3) VCC. So the R-S trigger is set, the output pin 3 is at high level, there is no voltage difference between the two ends of the relay J1 coil, and its normally closed contacts 1 and 2 remain closed, so the motor runs normally.

When the motor is not connected to any other phase of the transformer, the current sensed by the LH input terminals 1 and 2 of the current transformer will increase to about 1.73 times

of the original working current, which will make the original input signal voltage of the pin 6 of the 555 counter increase not greater than (2/3) VCC to greater than (2/3) VCC, the input signal voltage of pin 2 increases to greater than (1/3) VCC. Under this situation, R-S trigger is reset, and pin 3 becomes low level, the relay J1 is energized and closed, and its normally closed contacts 1 and 2 are opened, so the motor contactor is de-energized and released, and the motor stops.

When the phase of the current transformer LH is disconnected in series, and there is no induced current on the secondary side of LH, and there is no voltage at both ends of C1; the pin 4 of the 555 counter is equivalent to add low level. From Table 1, it can be seen that pin 4 is the forced reset end of the R-S trigger. As long as pin 4 is added with low level, its output of pin 3 will be at low level. Thus J1 can still be energized and closed, the motor contactor is powered off, and the motor also stops.

When the motor is overloaded, the line current increases. When the current gets to the pre-set value (generally set to about 1.2 times of the rated current), after LH conversion, adding the value greater than (2/3)VCC to pin 6 of the 555 counter, meanwhile the voltage of pin 2 is greater than (1/3) VCC, its output pin 3 also outputs low level, J1 is energized, and the motor stops.

The function of C1 in the circuit is to delay and avoid the instantaneous current impact when the motor is started. The function of C3 is to avoid the jitter of the J1 contact, and the second is to make some very short-term interference with high current not cause J for maloperation. The button “TEST” can directly provide pin 4 of the reset terminal low level. In the circuit, the contact of J1 is used to connect the red and green light-emitting diodes with the buzzer to indicate the working state of the protector. The button “RESET” and J2 can make the sound of the buzzer disappear. From the above analysis, the protection circuit has two kinds of function: open-phase and overcurrent protection.

3 Conclusion

This paper introduces the operating environment and common faults analysis of aviation motors, and designs a protection circuit with 555 counter as the core for open phase and overload defects of the motors. Auxiliary functions such as delay protection, status indication and sound warning are also added to the circuit design, which has great practical value. In order to prevent overheating or even fire caused by the motor on the aircraft, it is recommended to strengthen the on-line protection of the asymmetrical operation of the important motor, enhance the monitoring and maintenance of the motor, keep the surrounding of the motor clean and without dust and combustibles, reduce the possibility of fire, and install on-line automatic detection and alarm device and accurately tell the pilot what happened, take timely countermeasures, train the crew and maintenance personnel, and improve the perception ability of electrical fire.

References

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