3 Phase Motor Soft Starter

Electromechanical reduced-voltage starters must make a transition from reduced voltage to full voltage at some point in the starting cycle. At this point there is normally a line current surge. The amount of surge depends on the type of transition used and the speed of the motor at the transition point. Figure 1 illustrates typical transition current curves for reduced-voltage starters.

Figure 1(a) Open-circuit transition versus closed-circuit transition.
Figure 1(b) Transition at low speed versus transition near full speed.

There are two methods of transition from reduced voltage to full voltage, namely, open-circuit transition and closed-circuit transition. Open transition means that the motor is actually disconnected from the line for a brief period of time when the transition takes place. With closed transition, the motor remains connected to the line during transition.

Open transition will produce a higher surge of current because the motor is momentarily disconnected from the line. Closed transition is preferred over open transition because it causes less electrical disturbance. The switching, however, is more expensive and complex.

3 Phase Motor Soft Starter

A 3 phase motor soft starter limit motor starting current and torque by ramping (gradually increasing) the voltage applied to the motor during the selected starting time. They are commonly used in operations requiring smooth starting and stopping of motors and driven machinery.

Figure 2 Soft start ramped-up voltage and current limiting.

Figure 2 illustrates typical transition voltage and current curves for soft starters. The time to full voltage can be adjustable, usually from 2 to 30 seconds.

As a result there is no large current surge when the controller is set up and correctly matched to the load. Current limiting is used when it is necessary to limit the maximum starting current and is usually adjustable from 200 to 400 percent of full-load amperes.

SCRs are often employed in solid-state soft starters to reduce the amount of voltage delivered to an AC motor on starting.

3 phase motor soft starter
Figure 3

Figure 3 shows a typical solidstate reduced-voltage control circuit made up of two contactors: a start contactor and a run contactor. The SCR is a unidirectional device in that it can conduct current in one direction only.

In this application bidirectional operation is obtained by connecting two SCRs in antiparallel (also known as reverse parallel). Using the antiparallel connection, with a suitable triggering circuit for each gate, both positive and negative halves of a sine wave may be controlled in conduction. The operation of the circuit can be summarized as follows:

  • When the motor is first started, the start contacts (C1) close and reduced voltage is applied to the motor through the antiparallel-connected SCRs.
  • Triggering of the SCRs is controlled by logic circuits that chop the applied sine-wave system power so that only a portion of the wave is applied to the motor.
  • The logic circuits can be programmed to respond to any of several sensors to control the voltage: internal time ramp, current sensor feedback, or tachometer feedback.
  • The voltage is increased until the SCR is being triggered at the zero crossing point and the motor is getting full line voltage.

At this point the run contacts (C2) close and the motor is connected directly across the line and runs with full power applied to the motor terminals.

3 Phase Motor Soft Starter

Figure 4 shows the wiring for a typical 3 phase motor soft start starter. The different standard modes of operation for this controller are:

Figure 4 Typical soft start starter.

Soft start: This method covers the most general applications. The motor is given an initial torque setting, which is user-adjustable. From the initial torque level, the output voltage to the motor is steplessly increased during the acceleration ramp time, which is useradjustable.

Selectable kick start: The kick start feature provides a boost at start-up to break away loads that may require a pulse of high torque to get started. It is intended to provide a current pulse, for a selected period of time.

Current limit start: This method provides current limit start and is used when it is necessary to limit the maximum starting current. The starting current is user-adjustable. The current limit starting time is user-adjustable.

Dual-ramp start: This starting method is useful on applications with varying loads, starting torque, and start time requirements. Dual-ramp start offers the user the ability to select between two separate start profiles with separately adjustable ramp times and initial torque settings.

Full-voltage start: This method is used in applications requiring across-the-line starting. The controller performs like a solid-state contactor. Full inrush current and locked-rotor torque are realized. This controller may be programmed to provide full-voltage start in which the output voltage to the motor reaches full voltage in ¼ second.

Linear speed acceleration: With this type of acceleration mode, a closed-loop feedback system maintains the motor acceleration at a constant rate. The required feedback signal is provided by a DC tachometer coupled to the motor.

Preset slow speed: This method can be used on applications that require a slow speed for positioning material. The preset slow speed can be set for either low, 7 percent of base speed, or high, 15 percent of base speed.

Soft Stop: The soft stop option can be used in applications requiring an extended stop time. The voltage ramp-down time is adjustable from 0 to 120 seconds. The load will stop when the voltage drops to a point where the load torque is greater than the motor torque.

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