US 6,175,220 B1Granted

Short-circuit protection for forward-phase-control AC power controller

AssigneePower Innovations, Inc.

InventorsRichard R. Billig, Steven B. Carlson

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Abstract

A thyristor-based forward phase control (FPC) system for controlling the power delivered to a load from an AC power source employs a short-circuit test cycle to protect the thyristor from damage. Upon activation of the FPC system, the thyristor is triggered at a phase less than one-twelfth cycle before a zero crossing so that, if there is a short, the resulting peak current will be insufficient to damage the thyristor but will exceed a predetermined threshold current corresponding to a repetitive overload current. If during the test cycle, the threshold current value is crossed, the FPC system does not trigger the thyristor again and does indicate that a short exists. The threshold current value is selected to correspond to the thyristor's steady-state load rating. If it is exceeded during normal (including warm-up and request-based) operation, steps can be taken to address an overload condition. Accordingly, the same circuitry used for general overload protection can be used as well for short-circuit protection.

Claims16

Dependent claims

A system for controlling AC power delivery to a load, said AC power defining an AC voltage, said AC voltage defining a series of AC voltage cycles, each of said AC voltage cycles having zero crossings, said system comprising:

an activation input for receiving an activation signal;
a switch for selectively coupling a load to AC power, said switch having a switch-control input, said switch in normal condition coupling said load to AC power in response to a coupling command received at said switch-control input;
threshold circuitry for detecting when a current-parameter value of a current parameter associated with a current through said load exceeds a threshold value, said threshold circuitry including a sensor for sensing said current parameter;
phase tracking means for determining the phase of said AC voltage;
and
a controller for controlling said switch by sending said coupling command to said switch-control input, said controller in response to receiving said activation signal leaving said load uncoupled from AC power during the first half of a voltage half cycle and then sending at least a first coupling command to said switch at a short-circuit-test phase angle within the second half of said half cycle, said controller being coupled to said phase tracking means, said controller determining whether or not a short-circuit criterion is met, said short-circuit criteria requiring that said current-parameter value exceed said threshold value during at least one short-circuit-test AC voltage cycle, said controller withholding coupling commands during succeeding voltage cycles if said short-circuit criterion is met.

3 more elements

Dependencies are inferred from the claim text.