Direct start of a three phase motor and inversion of rotation

The circuit is suitable for controlling an automatic gate. The opening takes place when pressing the button [Pa]; the closure takes place automatically, after a certain delay from the opening, delay which is set in the timer KT.
The completed closing or opening are detected by two limit switches, FCA and FCC.
During closing, if the opening button is pressed, the gate continue closing.
The intervention of the thermal relay or of the emergency button causes an immediate stop.

Power supply circuit:

Above all there is a triad of fuses that protects the motor and the rest of the circuit from high currents deriving from an insulation fault inside the motor, or from an incorrect wiring of the circuit.

After the fuses there are two contactors that feed the motor: one directly, and the other inverting two phases of the power supply so as to make it turn in the opposite direction. At the same time only one of these contactors can be closed, because the motor can not run in two different directions at the same time, and because there would be a short circuit between one phase and another.

Immediately before the motor terminals there is the thermal relay, this protects the motor from a possible fault, and in such case opens the power circuit and its auxiliary contacts.

The last component is a three-phase asynchronous motor with the windings connected in star (if it was connected in triangle, the power and control circuit would be unchanged).

Control circuit:

The gate opens when this conditions are verified:

  • the emergency button (Pe) is not pressed
  • the thermal relay (RT) is in its normal state
  • the gate is not already open (FCa)
  • the gate is not closing (Kc not active)
  • there is a press to the Pa button.

The closing occurs if:

  • the emergency button (Pe) is not pressed
  • the thermal relay (RT) is in its normal state
  • the gate is not already closed (FCc)
  • the gate is not open (Ka is not active)
  • the timed relay has reached its calibration value and then the contact T1 has closed

The arrest occurs if:

  • the emergency button is pressed or the thermal relay is activated

Signaling circuit:

  • LV indicates that the motor is stopped. It is On if both contactors are off.
  • LR indicates that the motor is running. It is On if one of the two contactors are energised.
  • LG indicates the intervention of the thermal relay. It is On if the thermal relay is activated.


This schematics and text I made for a high school report in 2003 (I was 17 years old). Translated to English in December 2017.
TDP Report – Giorgio Demurtas – 4Bea ITIS G. Marconi – Cagliari – 17 October 2003

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