The Soft Starter enables a selective reduction in starting current associated with a continuous and surge-free increase in torque. By reducing the motor voltage, the soft starter can also be used to control a soft stop of the three-phase motor.
The reduction of the motor voltage works on the principle of phase control. For this purpose, two thyristors, also called SCR (silicon controlled rectifier or semiconductor controlled rectifier) are connected in anti-parallel for each phase - one SCR for the positive half-wave and the second for the negative half-wave.
The conducting period is determined by the ignition angle α of the thyristor and thus the value of the effective voltage can be controlled.
After the start time tStart has expired, the thyristors are fully turned on, that means that the full sine half-waves are applied to the motor. The maximum value of the ramp (=> Top Of Ramp TOR) is achieved.
As you see the thyristors are only active during the run-up or the run-down phase. Once the start-up phase is completed, they can be bypassed by mechanical contacts for static continuous operation. Due to the significantly lower contact resistance of the mechanical switching contacts, the power loss at the soft starter can be reduced.
The parameters Start voltage V-Start and Ramp time t-Start determine the ramp-up time of the drive, which means the linear increase up to full line voltage VPhase.
Two-phase or Three-phase soft start control?
The Two-Phase-controlled soft starter is the cheaper alternative used for smaller three-phase motors. Because only two phases are phase-controlled, the resulting rotating field is elliptical during starting and braking. Normally this soft starter is equipped with internal bypass contacts. With the three settings time Start, Voltage-Start and time-Stop the handling is easy. However, it is only suitable for star (inline) circuit.
The Three-Phase controlled soft-starter generates a pure circular rotating field and is mainly used for bigger engines. Within the control circuit, it can fulfill more demanding tasks. Both types of circuit - star and delta - are possible. It is normally equipped with current limiting and motor protection function. The bypass contacts are often not internally installed, but the bridging of the thyristors with completed startup is realized via a control output for switching an external contactor.
Work order: Agitador
A three-phase motor for an agitator is started up using a soft starter. The characteristics of this motor used can be found in the following technology scheme:
Further information on the motor: IStart / IN = 7.8 TStart / TN = 2.8
1. Set the parameters for the starting voltage and the ramp time on the soft starter according to the manufacturer's table:
2. Calculate the nominal torque of the motor from the motor nameplate!
3. For operation with a soft starter: Calculate the starting torque with the starting voltage to be set and check whether a breakaway torque of 40 Nm can be overcome.
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