Recuperative braking mode

Static characteristics of DC independent excitation motor in braking modes

Operation modes of electric motors

In electric drive, except motor operation mode, there are exist braking operation modes of electric motors. There are three types of braking electric mode:

- braking with giving up the energy into the main (recuperative brake);

- counter switching brake;

- electrodynamic brake.

In all these modes electric drive operates as generator and the difference is how generator emf is directed respectively to supply voltage.

At recuperative brake machine emf (as in motor mode) is directed opposite to supply voltage, however it becomes more than this voltage, thus predetermine the change of the current direction at transformation motor mode into brake one – opposite.

In counter switching braking mode machine emf and voltage supply are acting coordinated (in one direction) and current is predetermined by the sum of machine emf and supply voltage.

In electrodynamic braking mode armature circuit of machine is disconnected from the main and closed on external resistance, i. e. machine voltage is absent, and machine itself is a generator, operating on insulated load.

Transition to this mode will occur, when from executive mechanism on the motor shaft in motor mode are acting such moments, which coincides with direction of the moment, made by the motor. In consequence of that motor speed will increase and become more, than speed of ideal open circuit, and motor emf at this become more, than voltage, applied to the motor (supply voltage). Here motor will operate as generator in parallel with the main, to which the motor gives an electrical energy (energy recuperation). Current at this condition changes its direction into opposite one. It is obvious from the equation, knowing from course of electric machines:

(3.16)

where , , .

The current will change the sign into opposite one, as well as the moment, because

.

Under influence of this moment the speed will increase and electric state of machine will have the next view:

Motor mode	Ideal open circuit mode	Recuperative bra- king mode

In recuperative braking mode the motor, transforming mechanical energy, applied to the motor shaft from executive mechanism, into electrical one, produces braking generator moment. Electric drive achieves steady-state speed, when increased motor moment wouldn`t be equal to motive moment of executive mechanism.

In consequence of that the transition of motor mode to braking one is occurred without changing of motor parameters and powering scheme, but only for the reason of applying additional moment on motor shaft, equation of mechanical characteristic stays the same, as for motor mode.

An area of mechanical characteristic, placed in the second quadrant will correspond to recuperative brake, i. e. at

and

Increasing of armature circuit resistance increase the characteristic steepness (decrease the stiffness), as for motor mode, and increasing of braking speed

() with increased resistance will correspond to the same value of braking moment Mb.

Thereby, graphically mechanical characteristics of braking mode are the continuation of correspondent mechanical characteristics of motor mode in the second quadrant.

Such type of brake is possible, for example, in drive with load-lifting mechanisms at load lowering or in transport mechanisms at the motion on the slope.

Recuperative brake is extremely economical, because it is accompanied by giving up the energy into the main. The motor transforms into this energy the mechanical energy on the shaft, and this energy the motor receives from executive mechanism.

But brake by this method is essentially limited, because not in all drives and not in all cases can exists the condition

.

It should be taken into account, that the greater will be the armature circuit resistance the higher will be the braking speed at the same braking moment.

Дата добавления: 2014-01-05