It is often necessary to control two or more motors from only one control point, making the temptation to connect the motors in parallel really irresistible.
Unfortunately, the tubular motors integrate a capacitor with the up and down limit switches and that is why the series M tubular motors cannot be connected in parallel.
As Fig.1 clearly illustrates, after the switch is closed to the right, the electrical current flows through the F2 and F3 limit switches in the two motors that run normally.
You can easily see that the electrical current that passes through the capacitor causes a voltage drop of about 40 V (depending on the type of motor and the supplied torque), at its ends; consequently, the voltage under the F1.1 and F2.1 limit switches gets a value of about 190 V compared to the neutral. All installers who feed motors without the test lead know this very well. Indeed, by connecting the two wires (i.e. the blue and brown ones) directly to the socket, installers will get a shock from the third wire which is often naked.
This occurs because each limit switch is closed during the reverse running and, consequently, it keeps the conductor it is connected to under tension.
Let’s suppose that the left motor stops as it reaches the point memorized by the F1.2 limit switch, which opens itself while the right motor keeps running.
Now, as shown in Fig. 2, limit switch F2.1 feeds the left motor in the opposite direction by means of limit switch F1.1 whose voltage, reduced to about 190 V, is still able to restart the motor in the opposite direction.
As soon as it starts, the left motor keeps away from limit switch F1.2 ,which closes again and gives 230 voltage to the coil that it is connected to. 230 V voltage prevails on 190 V voltage of the other side and this causes the reversing of the motor rotation again until limit switch F1.2 reopens.
The left motor will continue to reverse the rotation with short intervals, until the right motor has reached limit switch F2.2.
These continuous changes of rotation cause irreparable damage to themotors.
(The diagram of Fig. 3 proposes the galvanic isolation of the two motors made by a double zero central commutator with 6 terminals.)
As shown in the figure, limit switch F2.1 cannot feed the left motor because the connection is interrupted by the switch. After opening limit switch F1.2, the left motor stops definitely.
Instead of the double commutator, the catalogue offers
the DK-CX2 and DK-CP1 modules, which control more motors together while isolating them one from another.