3 Phase Motor Electronic Start Stop Circuit with Soft Start , No Contactor Relay Needed

Added: 2026-05-10

[00:00:02]Hello friends, welcome back. In this video, I will explain a simple electronic start-stop push button circuit for controlling a three-phase motor using SCRs, optocouplers, and power triacs. The most interesting feature of this design is that it can electronically latch the motor on and off without using a mechanical contactor relay. Now let us understand the circuit step by step. On the left side of the circuit, we can see the low voltage control section built around a BT169 SCR. This SCR works like an electronic memory latch. There are two push buttons here. The upper push button is the on switch, and the lower push button is the off switch. Initially, the SCR is switched off. Therefore, no current reaches the optocouplers, and all the triacs remain switched off. As a result, the three-phase motor also remains switched off. Now let us see what happens when the on push button is pressed. When the on button is pressed, a small trigger current reaches the gate of the BT169 SCR through the 1 kiloohm resistor. This instantly switches on the SCR. Once triggered, the SCR becomes latched permanently due to its holding current. Now current starts flowing through all the optocoupler input LEDs connected in parallel. As soon as the optocouplers activate, they begin triggering their respective power triacs. These power triacs then connect all the three AC phases to the motor. As a result, the three-phase motor starts rotating. The circuit uses MOC3061 optocouplers, which are zero cross optocouplers. This means the triacs are switched on only near the AC zero crossing points. Because of this feature, the circuit generates much lower electrical noise and reduced switching stress. The optocouplers also provide complete electrical isolation between the low voltage push button section and the dangerous high voltage three-phase AC mains. Now let us understand the role of the 360 ohm resistors. These resistors limit the gate current for the triacs and protect the optocoupler outputs. Next, we have the 39 ohm resistor and the 0.01 microfarad capacitor connected across each triac. This network is called a snubber network. Its function is to suppress voltage spikes and prevent false triggering of the triacs caused by the inductive motor load. Now, let us see how the off switch works. The off push button is a normally closed push-to-off switch connected in series with the SCR cathode return path. Under normal conditions, this switch remains closed, allowing the SCR holding current to flow continuously. However, when the off button is pressed, the cathode current path instantly opens. As soon as the SCR current drops below its holding current, the SCR switches off and loses its latch. This immediately cuts off current to all the optocoupler LEDs. The optocouplers stop triggering the triacs.

[00:02:58]All the AC phases disconnect from the motor and the motor stops rotating. Even after releasing the off button, the SCR remains switched off until the on button is pressed once again. One major advantage of this design is that the push buttons handle only low voltage low current signals, while the heavy motor current is controlled entirely through the triacs. This makes the system compact, silent, and free from relay contact wear and tear. However, for high-power industrial motors, the triacs must be mounted on proper heat sinks and their current ratings should be selected much higher than the motor current. Now, let us see how the above design can be upgraded with a soft start circuit stage using IC 7555, as shown in this diagram on your screens. The new section at the bottom is the 7555 PWM soft start circuit. Initially, the SCR is off, so the optocouplers and triacs remain off and the motor does not run. When the on push button is pressed, the BT169 SCR switches on and latches permanently. This powers the optocouplers and also activates the 7555 timer circuit. The 7555 is configured as an astable PWM generator. However, unlike a normal 555 circuit, pin five is connected to a high-value capacitor and a charging resistor. At startup, the pin five capacitor is fully discharged, so the PWM output at pin three begins with very narrow pulses. These narrow pulses chop the current flowing through the optocoupler LEDs, causing the triacs to switch on only briefly. As a result, the motor initially receives a low RMS voltage and starts slowly. Now, the pin five capacitor begins charging gradually through the resistor. As the pin five voltage rises slowly, the PWM pulses become wider and wider. This gradually increases the conduction time of the triacs and smoothly increases the motor voltage and speed. Finally, once the pin five capacitor becomes fully charged, the PWM output becomes almost continuous, allowing the triacs to conduct fully and the motor to run at full speed. Since the circuit uses MOC3061-0 cross optocouplers, the switching remains clean with lower electrical noise and reduced stress on the motor and triacs. The off push button interrupts the SCR holding current, which instantly switches off the entire system and stops the motor. So, that's it, friends. If you enjoyed this detailed explanation, do not forget to like, share, and subscribe for more electronics projects and circuit tutorials. Thanks for watching.