Figure 9.1 Three-phase reversing starter
A three-phase reversing starter consists of two contactors enclosed in the same cabinet. As seen in the power circuit, the contacts (F) of the forward contractor, when closed, connect L1, L2, and L3 to motor terminals T1,T2, AND T3, respectively. The contacts (R) of the reverse contactor, when closed, connect L1 to motor terminal T3 and connect L3 to motor terminal T1, causing the motor run in the opposite direction. Mechanical and electrical interlocks are used to prevent the forward and reverse contactors from being activated at the same time, which would cause a short circuit.
(b) Electrical Pushbutton Interlock
(c) Electrical Auxiliary Contact Interlock
Figure 9.2 Reversing starter mechanical and electrical interlocks
With the mechanical interlock, the first coil to close moves a lever to a position that prevents the other coil from closing its contacts when it is energized. Electrical pushbutton interlocks use double-contact ( NO and NC) pushbuttons. When the forward pushbutton is pressed, the NC contacts open the reverse-coil circuit. There is no need to press the STOP button before changing the direction of rotation. If the forward button is pressed while the motor is running in the reverse, direction, the reverse control circuit is deenergized and the forward contactor is energized and held closed. The reversal of a dc motor can be accomplished in two ways :
- Reversing the direction of the armature current ad leaving the field current the same
- Reversing the direction of the field current and leaving the armature current the sameca
Most DC motors are reversed by sitching the direction of current flow through the armature. The switching action generally takes place in the armature because the armature has a much lower inductance than the field. The lower inductance causes less arcing of the switching contacts when the motor reverse its direction.
Source :
Source :
Petruzella, Frank. (1995). Industrial Electronics. Mc Graw Hill.251-253
