In the speed regulation of asynchronous motor, the speed control method of changing the power frequency.
32. Principle of energy consumption braking of three-phase asynchronous motor
The energy consumption braking is that when the motor stops cutting the three-phase power of the stator winding, the stator winding is connected to the DC power source to generate a static magnetic field, and the braking induced current interacts with the static magnetic field to generate a braking torque for braking.
33. Working principle of reverse connection braking of three-phase asynchronous motor
The reverse braking is to change the phase sequence of the three-phase power of the stator winding when the motor is stopped, so that the rotating magnetic field of the stator winding is reversed, and the rotor is quickly stopped by the braking torque opposite to the rotating direction.
34. What is the difference between short circuit protection and overload protection?
When a short circuit occurs, the circuit generates a large short-circuit current and electric power to damage the electrical equipment. Need to cut off the power quickly. Common short circuit protection components are fuses and automatic switches.
The motor allows short-term overload, but long-term overload operation will cause the winding temperature rise to exceed the allowable value, and also protect the motor from power failure. A commonly used overload protection component is a thermal relay.
35. When the motor starts, the current is very large. Why does the thermal relay not work?
Since the thermal element of the thermal relay has thermal inertia, it will not deform quickly, the current is large when the motor starts, and the starting time is short, and the large current is not enough to cause the contact of the hot element to cause the contact action.
36. Under what conditions can an intermediate relay be used instead of an AC contactor?
The number of contacts is the same, the rated voltage of the coil is the same, and it can be replaced when the current is controlled by a small current.
37. What are the common relays according to the principle of action?
Electromagnetic, magnetoelectric, inductive, electric, photoelectric, piezoelectric, time and temperature (thermal) relays.
38. In the main circuit of the motor, since the fuse is installed, why should I install a thermal relay? What is the difference?
The fuse can only be used as a short-circuit protection and cannot be used as an overload protection; the thermal relay can only be used as an overload protection and cannot be used as a short-circuit protection. Therefore, it is necessary to install both in the main circuit.
39, the role of thermal relay
The thermal relay is an electrical appliance that works by the principle of the thermal effect of the current. It is mainly used for the overload protection of the motor, the phase failure protection and the control of the heating state of other electrical equipment.
40. What kinds of rated work systems are there?
Rated working system: 8-hour working system, long-term working system, short-time working system, intermittent working system.
41. What are the conditions for the reverse braking and energy braking of the three-phase AC motor?
Reverse brake is suitable for small-capacity motors up to 10 kW that do not brake frequently. The energy brake is suitable for motors with large capacity that require smooth, accurate and frequent braking.
42. What are the commonly used master switches?
Control buttons, travel switches, proximity switches, universal transfer switches, master controllers and other main electrical appliances (such as foot switches, reverse switches, emergency switches, toggle switches, indicator lights).
43. What is the basis for electrical control analysis?
According to the equipment manual, electrical control schematic diagram, total wiring diagram of electrical equipment, electrical component layout and wiring diagram.
44. Which types of relays are classified according to the nature and working principle of the input signal?
According to the nature of the input signal: voltage, current, time, temperature, speed, pressure, etc.
According to the working principle: electromagnetic, inductive, electric, thermal, electronic and so on.
45. What is the difference between the intermediate relay and the contactor? Under what conditions can the intermediate relay be used instead of the contactor?
The main contact of the contactor has a large capacity and is mainly used for the main circuit; the number of intermediate relay contacts is large, and is mainly used for the control loop. When the circuit current is small (less than 5A), an intermediate relay can be used instead of the contactor.
