34. What is the difference between short circuit protection and overload protection?
When the circuit is short-circuited, the circuit generates a large short-circuit current and electric power to damage the electrical equipment. Need to cut off the power quickly. Commonly used short circuit protection components are fuses and automatic switches.
The motor allows short-term overload, but long-term overload operation will cause the temperature rise of the winding 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 with a small current control.
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, why install a thermal relay since the fuse is installed? What is the difference between them?
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.
