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 close to 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 direct current 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 of the contactor's normally closed contact be used?
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.
system
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 of the contact of the speed relay?
The speed of the general speed relay contact is about 140r/min, and the reset speed of the contact is 100r/min.
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