Direct start of 3-phase motor

Three-phase asynchronous electric motor with small power (for example, from 0.25 up to 4 kW) can be put in operation with direct start, since the energy it consumes is not very large. The current during the start is about 5 to 10 times greater than the nominal current. The circuit for direct start of the 3-phase electric motor is shown on the Picture 1. On this schematic the energy part from the circuit is drawn with the bold lines.

Picture 1: Direct start of 3-phase asynchronous electric motor


Every phase provided to the motor is protected with fuse (e1, e2, e3). The fuses are chosen appropriately for the nominal current In of the motor, and they can withstand current peaks 10 x In or 100 x In for short time intervals. Also, the fuses can be fast or slow, depending on the protection we need to provide for the motor. After fuses, every line is connected to the energy contact of the contactor, which is actually the switch for turning on/off the phases connected to the motor. The R phase is connected to the motor through the fuse e1 and the energy contact 2-1 from the contactor, the S phase through the fuse e2 and the energy contact 4-3, and the T phase through the fuse e3 and the energy contact 6-5. The energy contacts from the contactor c1 are normally open, which means that when there is no voltage on the contactor's coil a-b these contacts are opened, and when the voltage is provided to the a-b, the contacts are closed. The contactor shown on Picture 1 operates with 220 V AC, also, there are contactors which operates with 24 V DC, in that case we should have additional power supply of 24 V DC in order to operate with the contactor. As we can see from the circuit, the contactor c1, also has and two auxiliary contacts, the 11-12 contact which is normally closed (NC) and the 13-14 contact which is normally opened (NO). These contacts are useful for alert signals. If we connect the power supply for the signal lamp through the 13-14 contact, then the lamp will switch on when the motor m1 is running (the contactor c1 is ON). For example, we can use both auxiliary contacts and connect one red light signal lamp through the 11-12 contact which will be switched on when the motor m1 is not running, and we can connect another green light signal lamp through the 13-14 contact which will be switched on when the motor m1 is running. The contactor c1 is turned ON/OFF with the switch b1, which provide 220 V AC for the contactor c1 protected with the fuse e4 which is chosen appropriately for the contactor's coil. So, when we put the switch b1 in position I, we start the motor m1, and when we put the b1 in position 0, we stop the motor m1.