A safety circuit for preventing or reducing phase control system malfunction

Claim 2

A safety circuit for preventing or reducing, when the safety circuit is in use, a malfunction occurring in a phase control system including a thyristor bridge comprising a plurality of thyristors supplied with N-phase power from an N-phase power supply, and a firing angle control circuit for generating thyristor firing pulses to control the firing angle of said thyristors upon comparing an analog firing control input signal, arriving at an input terminal of the firing angle of control circuit, with each/N-number of sawtooth signals, said safety circuit comprising: an input signal control circuit for connection to the analog firing control input signal input terminal of said firing angle control circuit, for controlling the level of said analog firing control input signal; and a firing circuit input reset circuit, to be controlled in synchronism with said thyristor firing pulses, for enabling said input signal control circuit for at least an interval of time equivalent to the commutation overlap time of said thyrisitors, whereby when the safety circuit is in use said analog firing control input signal will be suppressed or restricted by said input signal control circuit immediately after the generation of each of said thyristor firing pulses.

Claim 3

A safety circuit according to claim 1, which is such that said analog firing control input signal will be held at a constant level by said input signal control circuit immediately after the generation of each of said thyristor firing pulses.

Claim 4

A safety circuit according to claim 1 or 2, in which said input signal control circuit comprises a transistor which, when the safety circuit is in use, is rendered conductive when said input control circuit is enabled, whereby the input to said firing angle control circuit is shorted to ground to establish zero potential at said input.

Claim 5

A safety circuit according to any preceding claim, when in operable combination with a phase control system as defined in claim 1.

Claim 6

A combination according to claim 4, wherein the phase control system is a motor control system and comprises a closed loop for negatively feeding a load current signal back to the input side of the motor control system.

Claim 7

In a phase control system for controlling the firing angle of thyristors in a thyristor bridge to regulate a motor load connected to the load side of the thyristor bridge when the system is in use, and the system comprising a closed loop for negatively feeding a load current signal back to the input side of a motor control circuit, and a firing angle control circuit for generating thyristor firing pulses, said firing angle control circuit having an input terminal to receive a firing control input signal; there being a safety circuit for preventing or reducing a malfunction occurring in said phase control system, the safety circuit comprising: an input signal control circuit, connected to the input terminal of said firing angle control circuit, for controlling the level of said firing control input signal, and a firing circuit input reset circuit, controlled in synchronism with said thyristor firing pulses, for enabling said input signal control circuit for at least an interval of time equivalent to the commutation overlap time of said thyristors, whereby when the phase control system is in use said firing control input signal is held at a constant level by said input signal control circuit immediately after the generation of each of said thyristor firing pulses.

Claim 8

In a phase control system for controlling the firing angle of thyristors in a thyristor bridge to regulate a motor load connected to the load side of the thyristor bridge when the system is in use, and the system comprising a closed loop for negatively feeding a load current signal back to the input side of a motor control circuit, and a firing angle control circuit for generating thyristor firing pulses, said firing angle control circuit having an input terminal to receive a firing control input signal; there being a safety circuit for preventing or reducing a malfunction occurring in said phase control system, the safety circuit comprising: an input signal control circuit, connected to the input terminal of said firing angle control circuit, for controlling the level of said firing control input signal, and a firing circuit input reset circuit, controlled in synchronism with said thyristor firing pulses, for enabling said input signal control circuit for at least an interval of time equivalent to the commutation overlap time of said thyristors, whereby when the phase control system is in use said firing control input signal is held at a constant level by said input signal control circuit immediately after the generation of each of said thyristor firing pulses.

Claim 9

A system according to claim 6, in which said input signal control circuit comprises a transistor which, when the phase control system is in use, is rendered conductive when said input control circuit is enabled, whereby the input to said firing angle control circuit is shorted to ground to establish zero potential at said input.

Claim 10

A circuit or a system according to any preceding claim, wherein the firing circuit input reset circuit is arranged to be controlled by said thyristor firing pulses.