How to Start an AC Motor
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- Category: News
- Published on Tuesday, 21 November 2023 11:00
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Starting a 3-phase motor presents several problems, well known by designers and users of this type of equipment. 
Direct starting an AC motor will cause inrush current frequently higher than 5 times full-load current (FLA); the resulting high torque may produce mechanical stress on the equipment connected; and, last but not least, users may incur penalties from their Utility, due to consequences of the temporary high inrush current on the network.
The following is a compact and simplified exposition of the methods used to start a three-phase electric motors. The choice of these methods depends on the need to minimize the effects mentioned above – effects which are, in turn, depending also on the power rating of the motor.
The simplest method, which is commonly used for small 3-phase motors, where the transient effects above are manageable or can be neglected, is called “Direct Online Starting”. It involves a simple 3-phase contactor with overload relay set to the max. current. This is a low-cost solution, employed by most users of small power motors with no special requirements. Since torque can reach 200% of the rated value at startup, typical problems which may occur are slipping or breaking of mechanical couplings (belts, gears).
A second method for starting a motor, while reducing the effects of high current and torque, consists in introducing a device which reduces the voltage applied to the motor, either by means of manual intervention, or automatically. Manual reduction of voltage is typically obtained by one of these two methods:
Auto-transformer start: during the starting sequence, the motor is connected to one (or to a sequence) of intermediate taps of an auto-transformers, providing lower voltage levels which are sufficient for starting the motor, but generate lower current (and torque). When the motor has reached nominal speed, the full voltage is applied. This system requires more contactors and an auto-transformer of appropriate size, resulting in more cost.
Star-Delta start: this method requires a motor where extremities of each winding are separate and accessible (6 terminals instead of three). In this case, using a set of contactors (main, star and delta contactors) or even a star-delta switch, the voltage between phase and neutral of the network is initially applied to the windings (that is, approx.1/3 of direct online start); when speed has reached a determined value, the voltage between phases is applied to each winding. The main disadvantage is the reduction of the initial torque. This system is relatively economical and simple to install and operate.
Soft Starter: this method requires a special electronic device, a configurable “Soft Starter”, which offers ramping up the voltage according to need, control speed and torque both at start-up and stop, selecting constant torque mode, and other. On the other hand, this method uses a relatively expensive equipment, which also generates heat and harmonic on the network. An additional bypass contactor (contacts closing in parallel with the soft starter when rated speed is attained) may be required.
The third option involves use of an AC drive (Variable Frequency Drive): this device provides speed and torque control during start-up and stop, minimizing inrush current. It is, however, a more complex and expensive method, (AC drives for large motors are generally very costly), which produces heat and introduces harmonic in the line (a filter may be required).
In conclusion, the selection of the method employed to start a three-phase motor depends on several factors such as power of the motor, system requirements in terms of acceptable inrush current and torque at start, operation of auxiliary equipment to minimize such effects. Consultation with a qualified technician is important to assist with such choice.
