39. The role of thermal relays
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 is the case for the reverse-braking and energy-consuming 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), etc.
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 an intermediate relay be used instead of a 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.
46. 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 from the top to the bottom, from left to right, and can be arranged horizontally or vertically.
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