Application – Motors, especially ones attached with fly wheel, de-energized upon pressing the stop button will still rotate due to inertial energy stored in the rotors. This motor condition is quite dangerous in some application. Many person loss their fingers after they stop motor thinking that there is no longer any effect to the machine which the motor drives. See figure below (A) and (B).
Theory of Application
Consider Figure below (A). Rotor R due to the rotating north and south poles at poles A, B, C and D. The Rotating north and south poles A, B, C, and D rotates because of AC voltage is remove from lines L1 and L2, the rotating poles at poles stops rotating because of inertia, the rotor R remains rotating. The rotor may rotate few seconds or even minutes depending upon the load it carries.
(A) Rotor R will still rotate even if the AC voltage at lines L1 and L2 is removed.
(B) Rotor R come to stop immediately if the AC voltage at lines L1 and L2 is substituted by a DC voltage.
If after removing the AC voltage at lines L1 and L2 is immediately substituted by DC voltage, a steady (none rotating) north and south poles stays at poles A, B, C, and D. See figure above (B). These steady north and south poles at poles A, B, C, and D will lock the rotor R and will cause it to stop immediately.
Diode
A Diode is a device that allows the flow of current in one direction but blocks the current in the opposite direction. See Figure below (b) and (c).
(a) An actual and schematic diagram of a diode.
(b) A half wave rectifier where the diode is conducting
(c) A half wave rectifier where the diode is not conducting.
(d) A full wave bridge rectifier using 4 diodes.
Full Wave Bridge Rectifier
The full wave rectifier shown in figure above (d) converts the AC voltage at input L1 and L2 to DC voltage at the output load L.
Operation of full wave rectifier
During the positive half cycle, current flows in line L1, D1 down to load L, D3, and line L2. This give a positive current (downward Direction) in the load L. During the negative half cycle, current flows from D2 down to load L, D4, and line L1.This also give a positive current (downward direction) in the load L. Notice since the direction at the load L1 is always downward, the Ac input at lines L1 and L2 is converted to pulsating Dc output in the load L
Circuit Operation of a Dynamic Braking
Figures below (A) and (B) illustrate the power and control circuit of a “Dynamic Braking”. Note the contactor C has 2 normally closed auxiliary contacts C (11-12) and C (21-22).
Pressing the start push button will energize contactor C. Contact C (13-14) will closed to maintain the control circuit. Contacts C (1-2,3-4,5-6) will close to energize motor M to lines L1, L2, and L3. Contacts C (11-12) and C (21-22) will open to ensure that no DC current from the bridge rectifier can enter the motor windings at terminals T2 and T3 while the motor is operating under an AC source.
(A) Control circuit of the Dynamic Braking.
Pressing the stop push button will de-energize contactor C. Contactor C (14-15) will open and release the control circuit from maintained condition. Contact C (1-2,3-4,5-6) will open to de-energize the motor M. Contacts C (11-12) and C (21-23) will return to close position. At This condition the motor may still run some several rounds.
(B) Power circuit of the Dynamic Braking.
Using the dynamic brake (DB) push button for stopping the motor instead of the ordinary push button. If the DB push button is pressed instead of the stop push button, contactor C will also de-energized. Contact C (13-14) will open. Also contact C (1-2,3-4,5-6) will open to disconnect the motors for lines L1, L2 and L3. Contact (11-12) and C (21-23) will return to close position. If the DB push button is pressed further to short its contacts DB (3-4), the dynamic brake contactor DB is energized. Contact DB (11-12) will open to ensure the contactor C will not be energized. Contacts DB (1-2) and DB (3-4) will close to permit the DC current from the bridge rectifier B to enter the motor windings at terminals T2 and T3. This will bring about the sudden stopping of motor M.
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