Motor drive circuit system

Abstract

A motor drive circuit for driving a motor having two or more phases, the motor drive circuit comprising: a motor bridge having a bridge arm for each phase of the motor, the bridge arm comprising an upper switch and a lower switches which can be opened and closed during normal operation to modulate the voltage applied to the respective phases in response to a drive signal from the motor control circuit, at least one solid state phase isolation relay, SSPIR, arranged in series to the respective phases of said motor Phases are connected in the electrical path of the corresponding bridge arm, the SSPIR is closed in normal operation to allow current to flow in that phase, and remains open in fault mode of operation to prevent current flow in that phase. The motor drive circuit includes monitoring means for monitoring current waveforms in the phases to provide an indication, at least in a fault mode of operation, when it is safe to disconnect the SSPIR without causing damage to the SSPIR due to avalanche mode a damaged output of the SSPIR, and a control circuit which, during normal operation, applies a voltage to each device sufficient to keep said SSPIR closed and respond to the output of said monitoring device in said fault mode of operation, This reduces the voltage applied to each SSPIR to a level that causes the SSPIR to turn off at a safe time.

Description

technical field [0001] The present invention relates to improvements in motor drive circuitry, particularly, but not exclusively, for electric power steering assemblies. Background technique [0002] Electric power steering systems are of a known type in which an electric motor applies an assisting torque to a part of the steering device, thereby assisting the driver of the vehicle in turning the steering wheel. The motor can typically act on the steering column through a reduction gearbox or can act on the steering rack. As the steering wheel is turned, a measurement of the torque the driver is applying to the steering is passed to the processor, which generates a torque demand signal which in turn is used to control the motors to produce the required assist torque . Applying assist torque in the same sense as driver applied torque reduces the force required to turn the steering wheel. [0003] Motors, such as multi-phase permanent magnet motors, may be star-connected or...

AI Technical Summary

Problems solved by technology

This current may be high enough that any subsequent attempt to disconnect the SSPIR will send it into avalanche mode where more power is dissipated than the SSPIR is rated for, causing catastrophic failure

[0014] Attempts have been made in the past to overcome this limitation by providing a snubber circuit that absorbs energy during the SSPIR's disconnection, which would otherwise result in damage to the SSPIR due to excessive power dissipation in the SSPIR

However, providing a snubber circuit itself can create problems with the snubber circuit not being able to handle sudden changes in current, and in any case, introducing additional components in the snubber adds cost and leads to more potential points of failure

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