AC Drives and Soft Starter (6)

Motor Characteristics Using Soft Starters

Unlike the ac drive, the line current and motor current for a soft starter is always the same. During starting the current varies directly with the magnitude of the applied voltage. The motor torque varies as the square of either the applied voltage or current.
The most critical factor when evaluating a soft starter is the motor torque. Standard motors produce approximately 180 percent of the full load torque at starting. Therefore, a 25 percent reduction in voltage or current will result in the locked rotor torque equal to the full load torque (180%*(.75)2 = 101%). If the motor draws 600 percent of the full load current on starting, then the current in this example will reduce the normal 600 percent starting current to 450 percent of the full load current.

Table 1 below gives more examples of the effects of reducing the voltage or current on a motor’s locked rotor torque. This data is valid for soft start and series impedance starting. They do not apply to other types of reduced voltage starting such as autotransformer and wye-delta starting.
When applying soft starters, the same constraint as electromechanical reduced starters applies. That constraint is “will the motor be able to produce enough torque to get the load started with the current the soft starter is allowing to flow to the motor?”
Soft starters do have an advantage over conventional reduced voltage starting. They are able to adjust voltage, current, and, therefore, torque over a wide range instead of single or a few fixed values. This can be seen in Figure 10. When voltage or current is held to a constant value, the speed-torque curve labeled “Current Limit” is produced. This curve would move up or down depending on the current limit setting. The upper boundary of this adjustment is the “Full Voltage” curve.
The soft starter can also ramp the voltage from an adjustable initial value up to full voltage over an adjustable time frame. This is represented by the “Soft Start” curve. A stepless transition, which is designed to eliminate current/torque transients, is produced by this ramp.
The operating speed of the motor cannot be varied because the soft starter only adjusts the voltage to the motor and not the frequency. The frequency applied to the motor is always the line frequency. Because of this, the acceleration time is more dependent on the load than the ramp time.