What are the basic rules for drawing electrical schematics?
(1) The electrical schematic diagram is generally divided into two parts: the main circuit and the auxiliary circuit.
(2) Each electrical component shall adopt the unified graphic symbols and text symbols of the national standard.
(3) The position of the conductive parts of each electrical component shall be arranged according to the principle of easy reading and analysis, and different parts of the same electrical component may not be drawn together.
(4) The contacts of all electrical components are drawn in an open and closed state when there is no power or no external force.
(5) The connection points of the cross wires with direct electrical connections shall be indicated by black dots.
(6) The electrical components should generally be arranged in order of motion from top to bottom, from left to right, and can be arranged horizontally or vertically.
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 circuit 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 the simple machine control system? Write the design steps.
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.
