A comprehensive introduction to the asynchronous motor starting system
Asynchronous motor starting system
a. Introduction
When the motor is started, the power supply generates a large inrush current, especially if a faulty power cord is used.
This current will cause a voltage drop that affects the load operation. This pressure drop can have a significant impact on lighting equipment.
In order to eliminate this effect, some departments have banned the use of motors with direct start systems and powers exceeding a given range. For information on this, see the DistributionBT1999/2000 product catalog on pages K34 and K39 and the pressure drop table allowed by the NFC15-100 standard.
The starting system can be divided into several types depending on the specifications of the motor and the load.
When selecting a starting system, the user needs to consider the electrical and mechanical requirements as well as the economic benefits, while also taking into account the load of the drive.
b. Main starting mode
Direct start
This is the simplest starting mode and the stator is connected directly to the power supply (Figure 1). The motor starts up according to its own characteristics.
After the motor is energized, its operation is similar to the secondary side of the transformer, with the secondary side formed by a shorted ultra low resistance rotor cage. There is a high induced current in the rotor that causes the current in the power supply to peak:
Current at start-up = 5 to 8 times rated current.
The average starting torque is:
T (starting torque) = 0.5 to 1.5 times the rated torque T.
Although direct starting has some advantages (simple equipment, high starting torque, fast starting, low cost), it is only suitable for the following occasions:
The power of the motor is lower than that of the power supply, which limits the interference generated by the inrush current. The machine to be driven does not need to be accelerated step by step, or there is a damping device that limits the vibration at start-up. It is required to reach the rated speed immediately. The starting torque may be relatively high and will not affect the machine's operation or the load being driven.
