Why Does The Motor Need To Take Step-down Starting Measures?
When the three-phase motor starts, the starting current is very large, which can reach 4~7 times of the rated current. The large starting current will cause a large voltage drop on the line in a short time, which not only affects the starting of the motor itself, but also It will affect the normal operation of other motors and electrical equipment on the same line.
For this reason, for large-capacity motors and frequent starts and stops, in order to limit the starting current, a step-down starting must be adopted. The so-called step-down start is to reduce the voltage applied to the stator winding of the motor when the motor starts, and then restore the voltage applied to the stator winding to the normal value after the motor turns up. Since the current is proportional to the voltage, reducing the starting voltage reduces the starting current. However, since the electromagnetic torque is proportional to the square of the motor stator winding terminal voltage, the starting torque of the motor is correspondingly reduced when the motor is started with reduced voltage, so the reduced voltage starting is suitable for starting under no-load or light load.
Reasons why high-power motors should be started with reduced voltage:
1. When the motor is started, the machine cannot withstand the impact torque of full pressure starting;
2. When the motor starts, its terminal voltage cannot meet the specification requirements;
3. When the motor starts, it will affect the normal operation of other loads.
Therefore, to reduce the voltage to start, if the above three effects will not occur when the motor starts, it can be started directly regardless of the power.
An electric motor is a device that converts electrical energy into mechanical energy. It uses the energized coil (that is, the stator winding) to generate a rotating magnetic field and acts on the rotor (such as a squirrel-cage closed aluminum frame) to form a magneto-electric rotational torque. Motors are divided into DC motors and AC motors according to different power sources. Most of the motors in the power system are AC motors, which can be synchronous motors or asynchronous motors (the stator magnetic field speed of the motor and the rotor rotation speed do not maintain a synchronous speed). The motor is mainly composed of a stator and a rotor, and the direction of the energized wire in the magnetic field is related to the direction of the current and the direction of the magnetic field line (magnetic field direction). The working principle of the motor is that the force of the magnetic field on the current causes the motor to rotate.
