Controlling a three-phase motor using Silicon Controlled Rectifiers (SCRs), also known as Thyristors, is a common method for applications where smooth speed control is required. SCRs are semiconductor devices that can control the flow of current in a circuit, making them suitable for motor control.
Here's a basic outline of how you can control a three-phase motor using SCRs:
- Motor and Power Supply: Select an appropriate three-phase motor for your application and ensure you have a suitable three-phase power supply.
- SCRs: You will need six SCRs, two for each phase. SCRs are unidirectional devices, so you will need to use two SCRs in an anti-parallel configuration (back-to-back) to control each phase.
- Gate Control: To control the firing of the SCRs, you need to provide proper gate control signals. Gate pulses are generally generated using phase-locked loop (PLL) circuits or microcontrollers.
- Phase Angle Control: The speed of the motor can be controlled by adjusting the firing angle of the SCRs. By changing the delay between the zero-crossing of the supply voltage and firing the SCR, you can control the average voltage supplied to the motor and, therefore, its speed.
- Triggering Circuit: Create a triggering circuit for each SCR. The triggering circuit is responsible for generating a pulse to turn on the SCR when needed based on the desired speed.
- Protection Circuitry: Implement protection measures to safeguard the motor and the SCRs from faults, overloads, and overcurrent conditions. This may involve using fuses, current limiters, and thermal sensors.
- Control Interface: Depending on your application, you might need a control interface to set the desired motor speed. This could be a potentiometer, a keypad, or any other suitable input method.
- Testing and Calibration: Once the circuit is assembled, thoroughly test the system and calibrate it to ensure proper functionality and smooth motor speed control.
It's important to note that designing a three-phase motor control circuit using SCRs can be complex and may require expertise in power electronics. Additionally, SCRs generate harmonics in the supply current, so appropriate filtering might be necessary to meet regulatory standards and avoid issues with other connected equipment.
For simpler motor control applications, using modern motor control techniques such as Variable Frequency Drives (VFDs) might be more practical and efficient. VFDs can provide smoother and more precise control over motor speed and torque, with additional features like built-in protections, fault diagnostics, and energy-saving capabilities.
In this method the speed of the squirrel cage induction motor control by varying the stator voltage, which is possible with the help of three sets of SCRs connected back to back(parallel but opposite in direction ) with each phase of input supply as shown in the figure below.
The stator voltage can be reduced by delaying triggering pulses of the SCR, if we delay these pulses the voltage may be reduced from the rated i/p voltage, which causes to decrease in the speed of the motor. On increasing the triggering pulses of the gate terminal the action will be vice versa to the previous case. In this method, there are more losses are produced in the voltage and power due to the change in power factor, to recover these losses in power a special SCR is used called gate turn of SCR (GTOs), which forces the current to flow almost in phase with the voltage.
“The gate terminals are shorted and a common control pulse is used to control the speed of the motor”