Dynamic braking

Counter switching braking

It is a braking that is more widely applied on a practice. Since counter switching is a mode where actuating mechanism rotates a motor in opposite direction to the acting of motor moment, so AM rotor must coercively rotates in opposite direction. Such mode, as for DCM IE, can be obtained at load motive moment (active Mc) that is higher then a starting moment.

It is known that AM starting current is: , that is at (at a 1^st starting moment) current is substantial, and at it is more higher, and a moment vice versa will decrease as a result of a great value of inductive resistance (inductive resistance rises due to substantial value of rotor current frequency at starting)

.

For moment increasing and current decreasing (is AM with phase rotor) it should to connect substantial active resistance into rotor circuit.

If to brake by natural characteristic 1, then braking moment will even decreases not increases at substantial sliding (negative speeds), part CDF of characteristic.

Figure3.42 – Characteristic of counter switching braking with active.

If AM in motor mode operates on this characteristic, for example at a point A with resistance moment Mc and substantial active resistance is connected into a rotor circuit (AM with a phase rotor), then AM speed begins to decrease. Beginning from the point B AM will pass into counter switching mode (part EDL of characteristic) into a 4^th quadrant, and at sliding S (speed module) increasing braking moment as it seen on a figure increases vise versa. Steady state mode will occur at a point D (when increasing braking moment on 2^nd characteristic will not be equal to mechanism resistance moment Mc.

Starting moment are changed at maximal moment value, by 1^st characteristic it is , and by 2^nd characteristic it is .

Counter switching mode can be obtained at reactive static moment of resistance (at accumulated kinetic energy) by the AM reversing on the run (switching of two phases on the run, i.e. changing of phase sequence into opposite one). It will cause instantaneous changing of field rotation, and the rotor will continue to rotate in previous direction due to reactive static moment (accumulated kinetic energy).

If AM in motor mode operates at natural characteristic 1 with resistance moment Mc and constant speed ωном at a point A for realization of counter braking it should to change two stator phases on running and connect active resistance into a rotor circuit – motor will pass to a point B from the point A, or to connect additional resistance, then the motor will pass to the point F and change two stator phases on running.The motor will pass to the point K from the point F.

Figure 3.43 – Characteristic of counter switching braking with reactive.

When changing two stator phases magnetic field will change the direction of rotation onto opposite instantaneously, and rotor under moment resistance action will continue to rotate in previous direction due to accumulated energy (kinetic) – point B or K. The speed will decrease under the action of braking moment (section BKC of characteristic 2`). At a point C, when rotor speed is equal to zero, it should to deenergize the AM. If not to do it AM will pass into motor mode of opposite direction (section CD) – spontaneous reverse of actuating mechanism, that can bring to emergency or accident.

If the reverse of producing mechanism is the target, then the AM is mot deenergized at a point C, and at point D it should to disconnect additional resistance form the rotor circuit. AM operation will pass at 1` characteristic and the motor will rotate at steady state speed ωном at a point E if the resistance moment Mc didn`t change comparatively with the motor mode of previous direction.

AM dynamic braking can be realized in two ways:

- dynamic braking with independent excitation;

- dynamic braking with self excitation.

Дата добавления: 2014-01-05