Sabtu, 14 Agustus 2010
Motor Starting Techniques
The iron and copper wires in large motors need to become magnetized before running at full speed. The inrush current required to start the motor to create the necessary magnetic fields can be as high as 7–11 times the full load current of the motor. Therefore, when large motors start, they often cause low-voltage conditions from voltage drop on the conductors from high-current flows. Utilities normally adopt guidelines or policies for starting large motors. When starting a motor exceeds the utility requirement for voltage dip or flicker (usually set around 3–7%), then special motor starting techniques are usually required.
There are several methods for reducing voltage dip and flicker. Reduced voltage motor starting equipment (i.e., soft starting), such as capacitors, transformers, special winding connections, and other control devices, are commonly used in motor circuitry to reduce the inrush current requirements of large motors during start-up conditions.
The three most common means of providing soft starting or reduced voltage starters on large motors are the following:
1. Resistance is temporarily placed in series with the motor starter breaker contacts or contactor to cause reduced current to flow into the motor when started. This approach can reduce the inrush current to less than five times full load current. Once the motor comes up to full speed, the resistors are shorted out, leaving solid conductors serving the motor power requirements.
2. Wye–delta connection changeover in the motor windings is another very effective way to reduce inrush current. The motor windings are first connected in wye, where the applied voltage is only line to ground; then the motor windings are connected in delta for full voltage and output power.
3. Auto-transformers are sometimes used to apply a reduced voltage to the terminals when started and then switched out to full voltage after the motor reaches full speed. This scheme can be used with motors that do not have external access to the internal windings.