Motor Circuit Diagrams

Circuit Diagram Of Soft Starters For Induction Motors

Soft starter actually an electronic device that has a command on the starting of an electronic motor with the help of managing the voltage applied. Now let’s find what we need for a starter for any motor. Then we shall move towards the circuit diagram of soft starters for induction motors.
Circuit Diagram Of Soft Starters For Induction Motors

An alternative-current motor is capable of self starting because of the relationship between the flux of  Spinning field of magnet & the winding flux of rotor, that makes a high LRA stand for Lock Rotor Amps as force is increased. Due to this high current is drawn by the stator & the motor arrive at the full speed by time, some huge quantity of current is wast. The wasted current is larger than the current rate of current & this can heat the motor up, finally destroying or damaging it. In order to happen this,we need  some different kind of motor starter. A soft starter is shown above in the previous image. Later we’ll take a look at the circuit diagram of soft starters for induction motors also working of soft starters for 3-phase  induction motors.

We can start motor in three ways

  • By using of all voltage load at different time spam: Line starting on directly
  • By using of voltage reduce moderately : Soft starter  & Star Delta Starter
  • By using part of start winding: Auto-transformer starter

Defining Soft Starting

Now let us pay attention to soft starting.

Technically a soft starter is a device that reduces the applied torque to the motor. It is normally made up of SSD(State Solid Device),SSD are that devices in which electron flow through semi-conductor the example of SSD device is thyristors use to manage the supply voltage’s application to that motor. The device that we are using is starter. Now let see who its work the give the high voltage at high speed turn and motor will be start. So, the fact that applied forced have  same ratio then the current starting square, which in succession have same ratio then the voltage applied. Thus the current & the torque can be manage by decrease in voltage at the time when motor is started.

There are two type of ways to control a soft starter:

Open Control: We apply some voltage with the time in start, regardless of the spinning of the motor or the drawing current. 2 Silicon Controlled Rectifier are attached one by one for each phase & they are conducted at the beginning & at interval of 180 degrees the half wave cycles (for the Silicon Controlled Rectifier conducts). That interval can decrease moderately with time till the ramping up of applied voltage to the  voltage fully supply, this ramp is called Time Ramp Voltage System. This way is irrelevant as it does not control the acceleration of motor.

Closed Loop Control: the behavior of all motor when the start the must drop some speed of motor and current & the voltage applied at starting can be manage due to the need to get the required response. All the things are happening are monitored at each phase like current & if it overreaches a certain point sets, the interval voltage ramp is then bring.

the basic rule of soft starter is by managing and monitoring  the conduction angle of the Silicon Controlled Rectifier the application of voltage supply can be manage.

2 A basic needs of soft starter’s components

  • Control power switching: (Silicon Controlled Rectifier) SCR’ are used in phase controlling because the are used in every part of that cycle. Here there are there phases motor, two Silicon Controlled Rectifier are attached one by one in each phases. The switching devices have to be rated at least 3X more than the voltage line.
  • Control Logic:Used Microcontrollers or PIC(Proportional-Integral-Derivative) controller or rule to control the application of gate voltage to the Silicon Controlled Rectifier, i.e. to control the firing angle of Silicon Controlled Rectifier to build the Silicon Controlled Rectifier conduct at the required part of the supply voltage cycle.

Working Module of Electric Soft Start System for a there phase induction motor

These are some components that the system consist:

  • two Silicon Controlled Rectifier Back to Back, i.e. total six Silicon Controlled Rectifiers.

Logic circuitry control in the form of 2 comparators – LM339 & LM324 used to generate the ramp voltage & the level & an opto isolator for managing the gate voltage’s application to the every Silicon Controlled Rectifier in each phases.


Circuit Diagram Of Soft Starters For Induction Motors
Circuit Diagram Of Soft Starters For Induction Motors

The comparator LM324 is used to generate the voltage level, the non changing terminal is attached with the help of a fixed source of voltage & non changing terminal is attached using a capacitor interfaced to an NPN transistor’s collector. The charging & discharging of capacitor forces.The result of comparator to change properly & level of voltage to vary from high – low that output voltage of level is applied to non changing terminal of  some other comparator LM339 inverting terminal of which is attached by means of ramp-voltage that ramp voltage is generated by means of a comparator – LM339 that actually match the pulsating direct curren-voltage that is gives by its changing – terminal to the pure direct curren – voltage at its non – changing terminal & produce (0V) zero voltage reference signal that changed to a ramp-signal by the discharging & charging of an electrolytic capacitor.

The third comparator- LM339 generates the large width pulse signals for each voltage high-level, which is decreased moderately as the voltage level is reduced. The signal is generated & used to the Opto isolator that gives gate pulses to the Silicon Controlled Rectifier. When the level of voltage is decrease, they increases the width of pulse of the Opto isolator & more the pulse-width, lesser will be the delay & moderately the Silicon Controlled Rectifier is fired without any time. Due to this we can manage the time interval between the delay or pulse between the pules application, the launching  angle of Silicon Controlled Rectifier is managed & controlled & the application manage the supply current, thus managing the motor output Force.

The process as whole is an open-loop control system where the time of application of gate firing pulses to each Silicon Controlled Rectifier is overcomed, based on the how earlier the ramp-voltage decreases from the level-voltage.

Read Also>>  Circuit Diagram Of DC Motor Speed Control Using Microcontroller 

Benefit of Soft Start

Now we’ve gotten known how soft start system works on an electronic. Now let see why we used soft start and why soft start is better then other ways.

  • Efficiency-Improved: The smartness of soft starter system using solid state switches is more because of the low on voltage state.
  • Startup-Controlled: At starting of motor current can be manage by easily changing the voltage start & they ensures starting of the motor without any problem.
  • Controlled-acceleration: easily manage the acceleration of motor.

Low-Cost & size: that’s only possible by the use of solid state switches.



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