47. What are the characteristics of reverse-phase braking and energy-consuming braking of three-phase AC motors?
When the power supply is reversely connected, the relative rotational speed of the rotating magnetic field of the rotor and the stator is nearly twice the synchronous speed of the motor, so the reverse braking current flowing through the rotor winding at this time is equivalent to twice the starting current at the start of the full voltage of the motor. Therefore, the reverse braking torque is large and the braking is rapid.
In the energy consumption braking, the control method for accessing the DC power is different, and there are two kinds of time principle control and speed principle control. Both methods require the addition of a DC power supply and a transformer, and the braking is slow.
48. In the “positive-reverse-stop” control circuit of the motor, the composite button has already played an interlocking role. Why should the interlocking contact of the contactor be used for interlocking?
Because when the contactor main contacts are "sold" together by a strong arc or the contactor mechanism fails, the armature is stuck in the pull-in state, and if the other contactor is activated, the power supply is short-circuited. When the contactor normally closed contacts are interlocked with each other, the occurrence of a short circuit accident in this case can be avoided.
49. What is self-locking control? Why is the contactor self-locking control line with undervoltage and voltage loss protection?
The self-locking circuit is an action of interlocking the output signal itself to maintain the output.
When the power supply voltage is too low, the contactor coil is de-energized, and the self-locking contact returns to disconnect the coil circuit. When the voltage rises again, the coil cannot be energized, which forms undervoltage and voltage loss protection.
50. What are the electrical schematic design methods? Which one is commonly used for simple machine control systems? Write the steps of the design.
There are two kinds of experience design and logic design. Commonly used is experience design. The design steps are: main circuit → control circuit → auxiliary circuit → interlock and protection → overall inspection → repeated modification and improvement.
51. What is the speed range when the contact of the speed relay is moving?
The speed of the general speed relay contact is about 140r/min, and the reset speed of the contact is 100r/min.
52. According to the principle of operation, what are the time relays?
The time relays are electromagnetic, air damped, electric and electronic.
53. Which types of low-voltage electrical appliances are classified according to their purposes?
1) Control electrical appliances 2) Power distribution electrical appliances 3) Execution electrical appliances 4) Communication low-voltage electrical appliances 5) Terminal electrical appliances
54, the principle of selection of time relay
When selecting, it can be selected from the aspects of delay length, delay precision, control circuit voltage level and current type, delay mode and contact form and quantity.
55, motor one-way reverse connection brake control circuit principle
Press SB2, KM1 absorbs and protects itself, the motor runs, and when the speed reaches 140 rpm, the KS contact closes. When SB1 is pressed, KM1 is powered off, KM2 is pulled in and reverse brake is applied. When the speed reaches 100 rpm or less, the KS contact is disconnected, the brake is over, and the motor stops rotating slowly.
56, the working principle of the jog control circuit
When SB is pressed, the KM coil is energized, the KM contact is closed, and the motor is rotated; when the SB is released, the KM coil is de-energized, the KM contact is disconnected, and the motor is stopped.
57, the working principle of the starting and self-protection control circuit
When SB2 is pressed, the KM1 coil is energized, KM is pulled in, the main electric shock is turned on to the motor power supply, and the motor is running; at the same time, the auxiliary contact is closed, the control loop is turned on, and is maintained. When SB2 is released, since the auxiliary contact has closed the control loop, the auxiliary loop continues to open the control loop and the motor continues to run. When SB1 is pressed, KM1 is powered off, the auxiliary contact is disconnected, the main contact is disconnected from the motor power supply, and the motor stops rotating slowly.
58. Working principle of multi-point control circuit
Press any button of SB2, SB4, SB6, the KM coil will be energized, KM will be sucked and self-protected, the motor will run; press any one of SB1, SB3, SB5, the KM coil will lose power, KM will When disconnected, the motor will slowly stop.
59. How the contact action works when the speed relay works counterclockwise
The shaft of the rotor is connected to the shaft of the controlled motor, and the stator is sleeved around the periphery of the rotor. When the motor is running, the rotor of the speed relay rotates with the motor shaft, the permanent magnet forms a rotating magnetic field, and the cage bar in the stator cuts the magnetic field to generate an induced electromotive force, forming an induced current, which generates electromagnetic torque under the action of the magnetic field, so that the stator Rotating in the direction of rotation of the rotor, but due to the return lever blocking, the stator can only rotate a certain angle with the direction of rotation of the rotor. When the stator is deflected to a certain angle, the normally closed contact is opened under the action of the lever 7, the normally open contact closure.
60, working principle of positive, stop, and reverse circuits
When the forward rotation starts, press the forward rotation start button SB2, the KM1 coil is energized and sucked and self-locked, the motor is started and rotated; when the reverse rotation is started, the reverse start button SB3 is pressed, and the KM2 coil is energized and combined with the self-locking The motor starts and rotates in reverse. In the control circuit, the normally closed auxiliary contacts of the KM1 and KM2 forward and reverse contactors are connected in series in the circuit of the counterpart coil to form a mutual control. If the forward start button SB2 is pressed, the motor has entered the forward running. After turning the motor to change, you must first press the stop button SBl and then press the reverse start button.
