The speed of a three-phase AC electric motor is defined in rotations per minute (rpm). For example, if the motor's declared speed is 1500 rpm, that means that when the motor is running in the stationary mode of operation it has 1500 rotations per minute in a given direction. The direction of the rotation of the motor can be clockwise or vice versa. If we send three phases R, S and T on the poles of the motor, then the motor will be running in one given direction. To change the direction of rotation of the motor we need to change two phases of the motor. So, if the voltage poles of the motor are A, B, and C, and if we provide phase R to pole A, phase S to pole B and phase T to pole C, than the rotation of the motor will be in one given direction, which we call forward direction of rotation (for example, clockwise direction). Now, if we change (switch) any two phases of the motor, we will change the direction of the rotation. If we provide phase R to pole C, phase S to pole B and phase T to pole A (switch the phases of poles A and C), the rotation of the motor will be opposite from the previous. For this purpose, we need two contactors. The connection of the energy contacts of the contactors is shown on Picture 1.
Picture 1: Forward / Reverse connection of 3-phase AC motor
The schematic of this energy circuit is classic. All three phases R, S and T are protected with fuses e1, e2 and e3. Usually, these fuses are slow, and the nominal current for which they break off the circuit is chosen according to the nominal current of the motor for the given supply voltage. After fuses, the phases are send through the proper connection of the energy contacts of contactors c1 and c2, which will provide forward/reverse rotation of the motor. In this circuit, the motor is also protected with overcurrent protection e4 (thermal overload relay or motor starter protector). The energy contacts of the contactors, 1-2, 3-4 and 5-6 are normally open, and if the voltage is provided on the contactors coils (a-b), the energy contacts are closed and the current is passing through them. In order to achieve a forward/reverse rotation of the motor, the energy contacts are connected as follow: when contactor c1 is ON and c2 is OFF, the phase R is sent to the first pole of the motor through the energy contact 2-1 of the contactor c1, the phase S is sent to the second pole of the motor through contact 4-3, and the phase T is sent through the contact 6-5 to the third pole of the motor. In this case, the motor is running in forward rotation. When contactor c1 is OFF and c2 is ON, the phases R and T switched their places, so now, phase R is sent to the third pole of the motor through the contact 2-1 of the contactor c2, the phase S is sent to the second pole through the contact 4-3 (remains the same), and the phase T is sent to the first pole through the contact 6-5. So, in this case, we have switched the first and third phase on the motor, thus the motor is running in reverse rotation. Here is good to mention that contactors c1 and c2 need to operate in some kind of blockade mode, which means when the contactor c1 is ON, the c2 must be OFF, and vice versa, when the c2 is ON, the c1 must be OFF. This way, we prevent the short-circuiting of two phases. The control circuit for this energy system can be implemented in different ways. On Picture 2 is shown one configuration of the control circuit.
Picture 2: Control circuit for forward / reverse start of 3-phase AC motor
The control circuit is very simple and it provides basic functionality and protection for the forward/reverse connection of 3-phase AC motor. First, the fuse e5 provides over-current protection of the control circuit. Here, the control circuit has working voltage of 230 V AC (50 Hz), which means that the contactor coils are 230 V AC operated. In case of use of the DC voltage operating contactors (for example, contactor with 24 V DC coil voltage), the control circuit should remains the same, but the only change will be power supply voltage for the circuit (instead of phase R there will be + 24 V DC voltage, and instead of null N there will be 0 V DC or ground). Depending on power supply voltage (current) for operating with contactor coils, the proper fuse e5 should be chosen. After fuse e5, the control signal is passed through the NC contact from the motor overcurrent protection e4. This contact is closed when the protection relay is off, which means that the current that motor consumes is not above the maximum. If the motor start to consume current which is greater than its nominal current and it is above the value that is set on the motor protector, then the protection relay is activated, and the contact e4 in control circuit will be open. Thus, the control circuit will be interrupted, so, the motor is out of power and it is protected from over-current. After the contact from protection e4, the control signal is passed through the switch b1. This switch is for starting the motor. Actually, the motor can be started with one of the key switches b2 or b3, but first, the switch b1 must be in position I (ON). The rest of the control circuit, is the most interesting and important part. The coils of the contactors c1 and c2 are combined with their NO/NC contacts and key switches b2 and b3 in the way that motor can be run only in forward or only in reverse direction of rotation. In this part of the control circuit is implemented the bockade between the contactors c1 and c2. The first line of this circuit provides operating voltage to the coil of the contactor c1 (a-b). The line starts with parallel connection of the key switch b2 and the normally open contact of c1 (13-14). Then the line continues with the normally closed contact of the contactor c2 (11-12) and ends with the coil of the contactor c1. The second line provides operating voltage to the coil of the contactor c2 (a-b). This line starts with parallel connection of the key switch b3 and the normally open contact of c2 (13-14). Then the line continues with the normally closed contact of the contactor c1 (11-12) and ends with the coil of the contactor c2.
Now let's see how this control circuit works. The protection relay is set to a given maximum current for the motor and the contact is closed, so, the control voltage is provided to the switch b1, which is in position 0 (off). If we turn on the switch (position I), we provide the control voltage to the parallel lines with coils of c1 and c2, which are both off at this moment. Now, we can start the motor in forward or in reverse direction of rotation. To start the motor in forward direction we should press the key switch b2. For this short time interval when b2 is pressed, the voltage is provided to the coil of c1, the c1 is turn on, and it will remains on through its own NO contact (13-14), which is now closed because c1 is ON. Now, if we want to change the direction of the motor from forward to reverse, before pressing the key switch b3 we should turn off the motor with turning the switch b1 into position 0, and then put back in position I. Otherwise, if we don't do Off -> On operation with switch b1 first, then pressing the key switch b3 during the motor is running in forward direction of rotation will change nothing in energy circuit, because the contactor c2 is blocked with the NC contact (11-12) from the contactor c1, which in this case is open because c1 is ON. The same principle of operation applies in the opposite situation when the motor is running in reverse direction.
Finally, the only bad thing about this control circuit is that every time when we want to change the direction of rotation of the motor we must first turn it off, and then turn it on again and start with the proper direction. This can be solved with adding the extra logic in the control circuit, but that is another discussion. Here, the point was to implement correct system for forward/reverse start of 3-phase AC motor, which will be functional and simple. Of course, things can always be improved.
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