These Solid State Switches are phase controlled in a similar manner to a light dimmer, in that they are turned on for a part of each cycle. The average voltage is controlled by varying the conduction angle of the switches. Increasing the conduction angle will increase the average output voltage. Controlling the average output voltage by means of solid state switches has a number of advantages. One of the major advantages being the vast improvement in efficiency relative to the primary resistance starter, due to its low on-state voltage of the solid state switches.
Smooth Start of a Single Phase Induction Motor Block Diagram
The required Hardware Components of this project include Op-amp, Opto-isolators, SCRs, Voltage Regulator, Transformer, Diodes, LED, Resistors, Capacitors, Lamp Smooth Start of a Single Phase Induction Motor.
The main aim of Smooth Start of an Induction Motor is to protect the induction motor with a supply of low voltage at the start and then gradually increasing the voltage so that the motor can be operated smoothly. Due to the supply of abrupt voltage at the start, the motor may get damaged and sometimes the windings can also get burnt. Hence the project aims at giving a smooth start to the motor by using the concept of SCR firing angle control.
The 230V AC power supply is first stepped down to 12V AC using a step-down transformer. This is then converted to DC using a bridge rectifier. The AC ripples are filtered out by using a capacitor and given to the input pin of voltage regulator 7812. At output pin of this regulator, we get a constant 12V DC which is used for different ICs in this project.
- The single-phase induction machine is the most frequently used motor for refrigerators, washing machines, clocks, drills, compressors, pumps, and so forth.
- The single-phase motor stator has a laminated iron core with two windings arranged perpendicularly.
- One is the main and
- The other is the auxiliary winding or starting winding
- These “single-phase” motors are truly two-phase machines. The motor uses a squirrel cage rotor, which has a laminated iron core with slots. Aluminum bars are molded on the slots and short-circuited at both ends with a ring.
- A soft starter is another form of reduced voltage starter for A.C. induction motors.
- The soft starter is similar to a primary resistance or primary reactance starter in that it is in series with the supply to the motor.
- The current into the starter equals the current out.
- The soft starter employs solid state devices to control the current flow and therefore the voltage applied to the motor.
- Soft starters are connected in series with the line voltage applied to the motor or can be connected inside the delta loop of a delta connected motor, controlling the voltage applied to each winding.
- Voltage control is achieved by means of solid-state A.C. switches in series with one or more phases.
Silicon Controlled Rectifier
- A Silicon Controlled Rectifier (or Semiconductor Controlled Rectifier) is a four-layer solid state device that controls current flow
- An SCR can be seen as a conventional rectifier controlled by a gate signal
- It is a 4-layered 3-terminal device
- When the gate to cathode voltage exceeds a certain threshold, the device turns ‘on’ and conducts current
- The operation of an SCR can be understood in terms of a pair of tightly coupled Bipolar Junction Transistors
- SCR has three states:
- Reverse blocking mode, forward blocking mode, and forward conducting mode
- Op-amps & comparators look very similar
- But a comparator gives a logic output indicating the relative potentials on its two inputs
- An op-amp amplifies the differential voltage between its two inputs – and is designed always to be used in the closed-loop application
- Potential dividers are connected to the inverting and noninverting inputs of the op-amp to give some voltage at these terminals.
- The supply voltage is given to +V and –V is connected to ground.
- The output of this comparator will be logic high (i.e., supply voltage) if the non-inverting terminal input is greater than the inverting terminal input of the comparator.
- If the inverting terminal input is greater than the non-inverting terminal input then the output of the comparator will be logic low (i.e., GND).
Optocoupler is a 6 pin IC. It is a combination of 1 LED and a DIAC. Pin 5 is not generally used and when light falls on the DIAC then it switches ON the DIAC. When the logic zero is given as input to the LED then the light doesn’t fall on DIAC so the DIAC is off meant the current does not flow through the DIAC. When logic 1 is given as input to the LED then light emitted by LED falls on DIAC so it starts conducting i.e., now there will be current flowing through the DIAC.
Working Project Of Induction Motor
The proposed system is intended to give protection to an induction motor, by applying voltage slowly from low to high for the level start. Induction motor throughout the first starting situation takes up much better current than its capability.
This directs to motor directly catching the speed resulting in high mechanical pressure and also sourcing some injury to the motor windings. In some cases, the windings may also get damage. Smooth start to the motor is based on the principle of SCR firing angle control.
This project includes two anti-parallels SCR’s which are connected in series with an induction motor to the supply. At the time of beginning, the firing angle is greatly delayed by received delayed pulses from an op-amp (operational amplifier). This operates in comparator mode which contrasts zero voltage pulses to expand a saw-tooth voltage.
Further, this waveform is evaluated against a changing voltage by charging & discharging of the capacitor to extend delayed pulses for activating the SCRs. A lamp is offered in this project instead of an induction motor for display purpose.
Further this project can be improved by using 6-SCRs, two connected in a back-to-back for each phase.