4000 Switch Machine - Alstom

SWITCH MACHINE CONTROLThe GM4000 uses a brushless DC motor and amplifier to drive the gear train. A brushless DCmotor contains a permanent magnet armature that rotates and drives the 5:1 gearbox. The motorcontains three field coils that surround the armature. The amplifier energizes and de-energizesthe three field coils in a circular fashion. This creates a rotating magnetic field that pulls thearmature around.There are three hall sensors that monitor the armature and provide position information to theamplifier. This information is used to control the circuitry that powers the three field coils.Together, the hall sensors and the amplifier regulate the motor’s RPM. The motor’s RPM isindependent of the switch machine’s load. This implies that as the switch machine’s loadvaries, the switch machine’s throw time remains constant (the switch machine doesn’t slowdown as the load increases).The amplifier also monitors the motor’s current. Motor current is directly proportional to motortorque. As the switch machine sees an increased load such as point pressure or an obstruction,the motor sees an increased load. The amplifier senses the increased load on the motor andsupplies more current to the motor thus increasing the motor torque. This additional motortorque increases the switch machine’s output force up to 4000 pounds.Fail Safe FeaturesThe universal switch machine amplifier is fail safe. Fail Safe, as used in the signaling contextmeans that no failure will cause the system to be less safe than it would be if the failure had notoccurred. That is, a failure will leave the system at least as safe as it was before the occurrenceof the failure. One of the fail safe properties is that the motor cannot turn on at the wrong time.For example, if the motor uses a transistor as a power switch and the transistor shorts out due toa surge, the motor cannot be allowed to turn on. The same holds true for the control lines on themotor controller. If there are transistors or transistor junctions used as switches on the controllines and one of them shorts out, the motor again cannot be allowed to run. Any failure mode ofany part of the circuitry cannot cause the motor to operate. This includes transistor junctionsshorting, opening or shorting to ground. A failure, or combination of failures, cannot cause orpermit the motor to operate without the presence of a proper control signal (i.e. switch call).The amplifier consists of two functional blocks. The first is the MOTOR DRIVER. There is onedriver that switches power to each motor coil. If any or all of the drivers short out, the field coilassociated with the damaged driver could be energized but it would not be de-energized at theappropriate time. The result: the motor would stop running.P2444, Rev. Jan/07 2-2ALSTOM Signaling Inc.