Tech Talk #01

Spotlight on: Soft Starter

Focused: Basic Operating Modes & Parameterization

Soft starter starts three-phase induction motor by gradually increasing the voltage, reducing both the starting torque and inrush current. This is done through a phase angle control method, where thyristors initially allow a portion of the voltage to pass and then ramp it up based on a predetermined start time.

Once the motor reaches full voltage, the internal bypass contacts of the soft starter automatically engage, allowing the power semiconductors to be bypassed. At this point, the motor operates normally.

Soft starters also facilitate a soft stop. In this process, full voltage is initially allowed through the thyristors. As the stop progresses, the timing of the firing signal is delayed, gradually reducing the voltage until it reaches the designated end voltage, based on the set ramp-down time. Finally, the voltage is completely removed, bringing the motor to a stop.

Parameterization

Before operating a soft starter, five key parameters must be configured:

1. U Potentiometer: This sets the starting voltage (initial voltage) for the motor, directly influencing the starting torque. A lower starting voltage results in a softer start and reduced starting current, while ensuring that the voltage is sufficient for a smooth motor startup upon receiving the start command.
2. t Potentiometer (Ramp Up): This defines the duration for ramping up the motor voltage from the set starting voltage to the mains voltage. The ramp-up time affects the motor's acceleration torque, which drives the load during this phase.
3. Ie Potentiometer: This is used to set the motor's rated operational current according to the mains voltage and connection type (wye-delta). The electronic overload protection refers to this value when active.
4. xIe Potentiometer: This sets the maximum allowable starting current as a factor of the rated motor current (Ie). For example, if Ie is set to 100 A and xIe to 5, the current limit becomes 500 A. Once this threshold is reached, the soft starter controls the motor voltage to prevent exceeding the limit.
5. t Potentiometer (Ramp Down): This determines how long power is supplied to the motor after the ON command is deactivated. The voltage ramp function reduces the motor torque during this period, allowing for a smooth application stop. Setting this potentiometer to 0 results in an immediate stop without a voltage ramp.
6. Configuring Motor Overload Protection (The CLASS potentiometer): It allows you to select the desired trip class (options: 10, 15, or 20). This setting determines the maximum time allowed for the device to trip when exposed to 7.2 times the rated motor current, starting from a cold state (conforms to IEC 60947 standards for motor protection). The tripping characteristics represent this time as a function of the tripping current.