Electrical Braking (Regenerative and Plugging

Bypass Circuit Without Shunt Field Activated

bypass contactor closed, the logics starts to monitor drive motor current. When the drive motor current decreases to a preset limit, the logics turns the shunt field OFF. With the shunt field OFF, both armature current and motor speed will increase. If the drive motor current increases above the preset limit (e.g., climbing a ramp or hill), the logics turns the shunt field back ON. This gives maximum torque to climb ramps or steep grades.

Electrical braking permits the lift truck operator to

Electrical Braking Regenerative Circuit (Power Transistors ON)

slow, stop and change the direction of travel electrically without having to use the service brake. This action allows for smooth shuttle operation while saving wear on the service brakes.

Whether the truck is in plugging or regenerative mode depends on where the current generated by the drive motor is routed. Regenerative braking applies when armature generated current is forced to flow backwards through the battery. This system has much better characteristics than previous “plugging only” systems. The braking energy is used to charge the battery momentarily, reduce heat build-up in the drive motor and extend motor brush life.

Plugging applies when armature generated current takes a more direct path back to the armature without passing through the battery.

Electrical braking begins when the operator selects a direction opposite the lift truck’s motion, while keeping the accelerator depressed. The logics will deactivate the regen and bypass contactors (if activated), turn the drive shunt field ON (if it was OFF) and change the direction contactor to the new direction selected. The logics will pulse the drive power transistors at a slow rate.

When the drive power transistors (T1D, T2D and T3D) are ON with the drive shunt field energized and the drive motor turning opposite the direction selected, the drive motor begins to act like a generator. Battery current through the drive shunt field maintains the magnetic field necessary for the motor rotation to generate a voltage across the armature.

The generated voltage produces a current which flows from the armature, through the direction contact tips, current sensor, regen diode D6D, power transistors, motor series field, direction contact tips and back through the armature. This generated current energizes the drive motor series field, which tries to turn the armature opposite the rotation caused by truck motion. This brakes the truck electrically. The logics controls the power transistor pulse rate to keep the current below a preset level as monitored by the current sensor. Refer to the Testing And Adjusting section of this manual for proper setup procedures.

Regenerative Circuit (Power Transistors OFF)

When current peaks at its preset value, the logics will turn OFF the power transistors. The generated current will flow from the drive motor field and armature, through the current sensor, D6D, line fuse, line contactor tips, battery, flyback diode D4D back to the drive motor field. This is the charging cycle for the battery. When the current decreases, the logics will turn ON the power transistors, and the cycle starts again.

As electrical braking slows the truck, the motor generates less and less current. The logics makes up for the decreasing generated current by increasing the pulse rate of the power transistors.

Plugging Circuit (Power Transistors ON)

When the speed of the truck slows to less than 4 kph (2.5 mph), the drive motor is no longer turning fast enough to generate enough current to charge the battery. The logics monitors this condition through the current sensor and reactivates the regen contactor. To prevent contactor tip arcing, the electrical braking of the truck is paused while the regen contactor tips close.

The generated current can now flow through D5D. This causes wire #1 at logics P1-20, P2-19 and P2-20 to become a more positive voltagethan sensing wire #51 at P2-5 due to voltage drop across diode D5D. The plugging mode of electrical braking controls the last few feet of the stopping distance and is not adjustable.

The power transistors continue to pulse at a slow rate to decrease the truck speed to zero. When the power transistors are ON, battery current flows through the power transistors, drive motor field, direction contactor tips, current sensor and the regen contactor tips back to battery negative. The generated current will flow from the armature through plugging diode D5D, back to the armature.