Dynamic braking of wound motor polyphase induction motors

Claim 2

A circuit for effecting dynamic braking of a wound rotor polyphase induction motor having stator phase windings fed from a polyphase mains supply and a wound rotor, comprising rectifier means connected to the rotor so as to rectify the output of the rotor, a variable mark-space ratio pulse controller, switch means comprising first switching contactor means connected between the polyphase mains supply and the stator phase windings of the motor and operable in a first condition of the switch means to disconnect the stator phase windings from the mains supply, second switching contactor means connected between the rectifier means and the pulse controller and third switching contactor means connected between said pulse controller and said stator phase windings, said second and third contactor means being operable in said first condition of the switch means to connect the output of said rectifier means through said pulse controller to said stator phase windings, thereby to effect dynamic braking of the motor, the pulse controller being operable to control the level of direct current in said stator phase windings during such braking.

Claim 3

A circuit as claimed in claim 1, wherein in a second condition of the switch means the first contactor means are operable to connect said stator phase windings to said mains supply and the second contactor means are operable to connect said pulse controller across said rectified rotor output, whereby the pulse controller varies the rotor current and hence the effective resistance of the rotor thereby to vary the driving speed of the motor.

Claim 4

A circuit as claimed in claim 2, wherein there is provided a smoothing circuit connected to the rectifier means so as to smooth the rectified rotor output and comprising a reservoir capacitor which, when the switch means is in its first condition, is connected in a series path including said pulse controller and at least one of said stator phase windings, whereby, on operation of the switch means from its second to its first condition, the reservoir capacitor begins to discharge through said stator phase winding or windings thereby to establish an initial direct current through said stator phase winding or windings.

Claim 5

A circuit as claimed in claim 3, in which the smoothing circuit includes a first unidirectional current path connected across the rectifier means and the reservoir capacitor receives charging current from the rectifier means when the pulse controller is non-conducting, and a second unidirectional current path connected in a series path including the reservoir capacitor and the pulse controller and through which the reservoir capacitor tends to discharge when the pulse controller is conducting, the said current paths being connected to the switch means in such a manner that, during dynamic braking of the rotor, both the charging and discharging currents of the reservoir capacitor flow in the same direction through the stator phase winDing or windings.

Claim 6

A circuit as claimed in claim 4, in which the pulse controller comprises a main thyristor connected to the rectifier means in parallel with the reservoir capacitor and said first and second unidirectional current paths, a commutating capacitor connected, in series with an inductor and a second thyristor, across the main thyristor, and means for charging the commutating capacitor in such a direction that on firing the second thyristor into conduction the commutating capacitor reverse biases and so commutates the main thyristor, the commutating capacitor then being charged in the reverse direction from the rectified rotor output through the series path including the inductor and second thyristor and when fully charged reverse biasing the second thyristor to commutate it, and in which the first unidirectional current path of the smoothing circuit includes a semiconductor switching device adapted to be rendered conducting when the reverse charge on the commutating capacitor has reached a predetermined value sufficient to ensure that the second thyristor is commutated when the commutating capacitor is fully charged.

Claim 7

A circuit as claimed in claim 1 and adapted for use with a three-phase induction motor the stator phase windings of which are delta-connected, in which the switch means are adapted to connect two of the stator phase windings in parallel to the rectified rotor output and to short circuit the remaining phase winding.

Claim 8

A circuit as claimed in claim 1 in which there are provided sensing means for providing a first electrical signal dependent on the speed of the rotor, means for providing a first electrical signal dependent on the speed of the rotor, means for providing a second or reference signal, comparator means for comparing the magnitudes of the first and second signals and for providing an output signal dependent on the magnitudes of the first and second signals, the said output signal being supplied to the switch means to cause the switch means to enter its said first condition thereby to effect dynamic braking of the rotor when the comparison of the first and second signals indicates that the motor speed is greater than a predetermined value corresponding to the magnitude of the second or reference signal.

Claim 9

A circuit as claimed in claim 7, in which the comparator means is connected to the variable mark-space ratio pulse controller so as to vary, when the switch means is in its first condition, the mark-space ratio of the pulse controller in dependence on the magnitude of the output signal of the comparator means thereby to control the level of direct current in the stator phase winding.

Claim 10

A circuit as claimed in claim 8, in which the switch means are adapted in a second condition thereof to connect the stator phase windings to the mains supply and to connect the rectifier means to the pulse controller so that the rectified rotor output is supplied to the pulse controller, the pulse controller being adapted to afford variation of the effective resistance of the rotor thereby to vary the driving speed of the motor, and in which the comparator means are connected to the pulse controller so as to vary, when the switch means is in its second condition, the mark-space ratio of the pulse controller in dependence on the output signal of the comparator means thereby to control the driving speed of the motor.

Claim 11

A circuit as claimed in claim 7, in which manually operated control means are provided for varying the second or reference signal thereby to vary the said predetermined speed of the motor.

Claim 12

A control circuit for speed control of polyphase wound rotor induction motors comprising rectifier means for rectifying the electrical output of the rotor, sensing means for providing a first electrical signal dependent upon the velocity of the rotor, means for providing a second or reference signal, comparator means for comparing and providing an output signal dependent on the magnitudes of the first and second signals, a variable mark-space ratio pulse controller connected to the rectifier means and adapted upon variation of the mark space ratio thereof to vary the resistance of the rotor circuit, means for connecting the comparator output to the pulse controller thereby to vary the mark-space ratio of the pulse controller and switching means actuated, when the difference between the first and second signal is in a reverse sense with respect to that which causes the rotor to drive, to connect the rectified rotor output to the stator and disconnect the stator from the electrical supply thereby to effect dynamic braking of the rotor.

Claim 13

A control circuit for speed control of polyphase wound rotor induction motors comprising rectifier means for rectifying the electrical output of the rotor, sensing means for providing a first electrical signal dependent upon the velocity of the rotor, means for providing a second or reference signal, comparator means for comparing and providing an output signal dependent on the magnitudes of the first and second signals, a variable mark-space ratio pulse controller connected to the rectifier means and adapted upon variation of the mark space ratio thereof to vary the resistance of the rotor circuit, means for connecting the comparator output to the pulse controller thereby to vary the mark-space ratio of the pulse controller and switching means actuated, when the difference between the first and second signal is in a reverse sense with respect to that which causes the rotor to drive, to connect the rectified rotor output to the stator and disconnect the stator from the electrical supply thereby to effect dynamic braking of the rotor.

Claim 14

Control apparatus for controlling the speed of a wound rotor polyphase induction motor, comprising means for rectifying the output of the rotor, a variable mark-space ratio pulse controller comprising a main thyristor connected across the rectified rotor output, a commutating capacitor connected, in series with an inductor and a second thyristor, across the main thyristor, and means for charging the commutating capacitor from the rectified rotor output in such a direction that on firing the second thyristor into conduction the commutating capacitor reverse biases and so commutates the main thyristor, the commutating capacitor then being charged in the reverse direction through the inductive series path including the rectified rotor output and the second thyristor and when fully charged reverse biasing the second thyristor to commutate it, the apparatus further including a smoothing circuit comprising a reservoir capacitor arranged to receive charging current from the rectified rotor output through a first unidirectional current path when the main thyristor is commutated and to discharge through a second unidirectional current path and the main thyristor when the main thyristor is rendered conducting, the first unidirectional current path including a semiconductor switching device, and firing means operable to render conducting the semiconductor switching device when the charge on the commutating capacitor in the said reverse direction has reached a predetermined value sufficient to ensure that the second thyristor is commutated when the commutating capacitor is fully charged.

Claim 15

Control apparatus for controlling the speed of a wound rotor polyphase induction motor, comprising means for rectifying the output of the rotor, a variable mark-space ratio pulse controller comprising a main thyristor connected across the rectified rotor output, a commutating capacitor connected, in series with an inductor and a second thyristor, across the main thyristor, and means for charging the commutating capacitor from the rectified rotor output in such a direction that on firing the second thyristor into conduction the commutating capacitor reverse biases and so commutates the main thyristor, the commutating capacitor then being charged in the reverse direction through the inductive series path including the rectified rotor output and the second thyristor and when fully charged reverse biasing the second thyristor to commutate it, the apparatus further including a smoothing circuit comprising a reservoir capacitor arranged to receive charging current from the rectified rotor output through a first unidirectional current path when the main thyristor is commutated and to discharge through a second unidirectional current path and the main thyristor when the main thyristor is rendered conducting, the first unidirectional current path including a semiconductor switching device, and firing means operable to render conducting the semiconductor switching device when the charge on the commutating capacitor in the said reverse direction has reached a predetermined value sufficient to ensure that the second thyristor is commutated when the commutating capacitor is fully charged.

Claim 16

Control apparatus as claimed in claim 12, wherein the semiconductor switching device is a thyristor.

Claim 17

Control apparatus as claimed in claim 12, wherein the means for charging the commutating capacitor from the rectified rotor output comprises a unidirectional current path connecting the junction of the reservoir capacitor and the said first and second unidirectional current paths of the smoothing circuit to the series circuit including the commutating capacitor, inductor and second thyristor.

Claim 18

Control apparatus as claimed in claim 12, wherein the firing means for the semiconductor switching device is operable to render conducting said switching device a predetermined time after the main thyristor is commutated if the reverse charge on the commutating capacitor has not reached said predetermined value within said predetermined time.

Claim 19

Control apparatus as claimed in claim 12, wherein there is provided a firing circuit for said second thyristor, said firing circuit being connected to the commutating capacitor and operable to supply a firing pulse to said second thyristor to render it conducting when the forward charge on said commutating capacitor reaches a predetermined value.