Recuperative braking (with giving up the energy into main)

AM mechanical characteristics in braking modes

AM can operate at the same braking modes as DCM:

- recuperative braking;

- counter switching braking;

- dynamic braking.

Fundamentally all these modes can be applied both to AM with cage rotor and to AM with phase rotor.

It can be realized at a speed that is higher than synchronous: and respectively at a negative sliding ().

In motor mode rotation magnetic field intersects the conductors of stator and rotor windings in one direction and that`s why the electromotive forces (emf) of stator and rotor are coincide by phase.

In generator mode the conductors of stator winding are intersected by rotating magnetic field in previous direction, and the rotor conductors is in opposite direction (in motor mode the motor lagged from stator field, and in generator vice versa the rotor field leads the stator one). So, stator emf doesn`t change its sign, and the rotor emf is change its sign into opposite one. It is more complicated with rotor current. Its active component changes its sign into opposite, as in generator mode the motor moment changes the sign into opposite one. Reactive component of rotor current doesn`t change its sign, because the machine in motor and in generator modes consumes reactive current from the mains for creation and conducting of rotational magnetic field through the machine magnetic wire.

Recuperative braking can be realized when lifting the load. Under the action of this load AM rotor can accelerate and reach a speed value . AM passes into generator mode and begins to create a braking moment at this.

AM mechanical characteristic in braking mode with giving up the energy into main is shown on a figure. Here the part of characteristic placed in 2^nd quadrant corresponds to this type of braking.

Figure 3.40 – Characteristic of recuperative braking.

Since the rotor in motor mode lagged during rotation of stator field (positive sliding), and at (mechanical speed of a rotor and stator field) the speeds are the same, so in a mode of recuperative braking during rotation the rotor leads the speed of stator field, it means that sliding will be negative and place in these limits .

As it was mentioned above in this mode the maximal machine moment is higher than the maximal moment of motor mode (by module)

.

At realization of recuperative braking it should not forget that the active static mechanism moment wouldn`t exceed the maximal moment by its value, which AM can make АД in generator mode.

The highest braking moment can be achieved (at minimal exceeding of rotor speed under) on neutral characteristic 1 (). With resistance increasing in armature circuit stiffness of mechanical characteristic is decrease.

This is at active static moment of resistance (at the expense of potential energy). Recuperative braking is also possible at reactive static moment of resistance (at the expense of kinetic energy).

At presence of reactive static moment on the AM shaft recuperative braking is possible in some cases of speed regulation, when AM passing to the lower speeds from a higher intersects the point. For example, multispeed (double-speed) AM that operates at a higher speed when switching into lower field speed, rotor speed will be higher than this synchronous speed (look at the figure).

Figure 3.41 – Characteristics of recuperative braking at double-speed AM operation.

Characteristic section BCD corresponds to a mode of recuperative braking of double-speed AM. - synchronous speed of higher speed degree (with a lower pole pairs number), - synchronous speed of lower speed degree (with a higher pole pairs number ). During polarityof stator winding, if the motor operates with resistive moment point А will be operation point of a motor. If at AM operation at a point A stator winding will change the polarity intosynchronous speed will abruptly deceases from to , and rotor will continue to rotate with a speed due to accumulated energy (kinetic). Situation arises, so the conditions of recuperative braking will executed, AM will pass to operation at a point B and its speed will decease due to braking moment. Due to decreasing of kinetic energy braking moment will decease (to synchronous speed - point D). Since AM doesn`t make the moment at this point, so due to resistance moment AM speed will decrease and in motor mode to the point E – that is steady state AM operation mode with a new (less) speed.

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