795F AC Off-Highway Truck Electric: Electric Drive System Operation:

DANGER: The Power Train Electric Drive System will contain hazardous voltage levels during machine operation and for a short period of time after engine shutdown.
Do not remove any covers that will expose energized high voltage electrical components while the engine is operating.
Any type of maintenance on the following components can only be performed after the Power Train Electrical System Service Shutdown procedure has been followed:

High voltage compartments in the inverter cabinet
The rear axle housing that contains the electric drive traction motors
The generator
The retarding resistor grid, the grid blower motor and the grid system cabling
The excitation field regulator
The high voltage cables and connection enclosures

Failure to follow these instructions could result in personal injury or death.

DANGER:

In order to avoid the buildup of hazardous live voltages on the exposed surfaces, all grounding wires and grounding straps must be properly connected at all times during engine operation.
Any disconnected grounding wires, including the grounding wires for all high voltage components and the grounding strap for the inverter cabinet must be properly reconnected before the engine is started.Failure to follow these instructions could result in personal injury or death.

The electric drive train system operates at high DC voltage levels and at high AC voltage levels. The system is designed to keep these voltages isolated from the truck frame and isolated in protected areas where any incidental contact cannot occur.

After reading this section of the manual, the user should be familiar with the operation of the drive train system. The user should be familiar with the components that are used in the system. This section will enable the user to understand the procedures and practices that must be followed before performing any maintenance procedures or troubleshooting procedures.

These procedures are located in the Troubleshooting, "Electrical Shutdown and Voltage Discharge" section of this manual. Read and become familiar with the procedures. The procedures must be followed completely before performing maintenance procedures on the high voltage electrical system on the truck.

System Operation Overview

The 795F AC Off Highway Truck is equipped with an electric drive train system. The electric drive train system eliminates the need for a mechanical transmission and rear differential.

In the FORWARD speed range, the electric drive train system enables travel speeds of 1.6 km/h (1.0 mph) to 64.4 km/h (40.0 mph) without shifting.

In the REVERSE speed range, the electric drive train system enables travel speeds of 1.6 km/h (1.0 mph) to 12.9 km/h (8.0 mph).


A simplified diagram of how an AC variable frequency drive system works

The electric drive train system is an AC variable frequency variable voltage drive system. The system is designed to provide variable levels of three phase AC voltages to each of the electric traction motors. Electronic control of the traction motor supply voltage enables the electric drive train system to meet the travel demands of the machine operator.

The Inverter Cabinet houses most of the controls and components that are used to create and control the AC voltage supply for the traction motors.

Electronic control of the electric drive train system is provided by three Electronic Control Modules (ECM). The three electronic control modules are the Drivetrain ECM, the Motor 1 ECM and the Motor 2 ECM.

The Drivetrain ECM is located in the cab front ECM compartment. The two motor control modules are located in the Inverter Cabinet.

High speed operational communication between the three control modules is conducted on the CAN B Data Link circuits and the CAN A Data Link Circuits.

The Drivetrain ECM is the system priority control module. The Drivetrain ECM monitors the operational status of the complete system. The Drivetrain ECM receives the input command signals from the operator controls and monitors the status of various system components. The ECM coordinates the operation of the electric drive system with the operation of the other machine systems.

Based on the status of the system and the demands on the system, the Drivetrain ECM issues control commands to each motor ECM. Each motor control ECM provides the control capabilities to carry out the commands.

The main function of the Motor 1 ECM is to monitor and control the operation of the left-hand Traction Motor 1.

The main function of the Motor 2 ECM is to monitor and control the operation of the right-hand Traction Motor 2.

Each motor control ECM also provides other system functions that are discussed later in this section.

System Startup

The Drivetrain ECM will monitor and control the operation of the Generator. The Generator will be controlled to maintain the level of the DC Power Bus at a voltage that will meet the system loads. In order to enable the Generator to provide the most efficient operation, the Drivetrain ECM will send engine speed requests to the Engine ECM.

At Engine startup, the Drivetrain ECM will perform a "Drivetrain System Test". This test will verify that the operation of the Generator is correct. The test will also determine if the operation of the DC Power Bus is correct.

Once the Engine speed has stabilized at 700 rpm after startup, the Drivetrain ECM will command the EFR to send 2 amps of current to the Generator exciter winding. Then, the Drivetrain ECM will monitor the system for expected voltage and current. If the voltage or current is not what is expected, the ECM will activate one of the following four Events:

E1311, Level 3 - Low DC Power Bus Voltage Detected During Drivetrain System Test
E1312, Level 3 - High Generator Phase Current Detected During Drivetrain System Test
E1313, Level 3 - High DC Power Bus Current Detected During Drivetrain System Test
E2140, Level 2 - power train Limited Due to System Fault

Refer to the Troubleshooting, "Event Code List" for more information on the system test Events.

The Drivetrain System Test will take approximately 5 seconds to complete. If the shift control lever is moved to a travel position during the test, the test will be aborted. Normal machine operation will then be enabled if no system faults are activated.

Once the system test is completed and no abnormal conditions are detected, the Drivetrain ECM will command the Generator Excitation Field Regulator (EFR) to boost the Generator AC voltage outputs to 1000 VDC on the DC bus.

The Generator output voltage is rectified to create the DC Power Bus voltage. Depending on the system requirements, the Drivetrain ECM will try to maintain the bus voltage in a range of between 1000 VDC and 2700 VDC. When the truck is in the travel mode, the ECM will attempt to maintain the voltage of the DC Power Bus at approximately 2700 VDC. The maximum amperage that can be supplied by the DC Power Bus is 1050 amps.

The Drivetrain ECM controls the voltage on the DC Power bus using several different system components.

To raise the DC Power Bus voltage level, the ECM will command the EFR to send more current to the generator excitation winding. The result will be an increase of the Generator output voltage.

During machine operation, the stored energy in the DC Power Bus Capacitor and other component capacitors help to maintain the bus level. When immediate heavy system power demands are required, the system capacitance will help to provide extra power until the Drivetrain ECM can increase Generator output.

Some system functions require the voltage of the DC bus to be briefly decreased. In addition to using the EFR to decrease Generator output, the Drivetrain ECM will use the Chopper Module to decrease the DC Power Bus voltage.

The Drivetrain ECM will command the Motor 1 ECM to activate the negative side power transistor in the Chopper Module. The activation frequency will determine how much the bus voltage is decreased. The chopper operation enables quicker control than is possible by changing the output of the Generator. For this function, the duty cycle or duration of activation will be limited since the grid cooling fan will not be turned ON.

For more information on how the Drivetrain ECM controls the operation of the Generator, refer to the Systems Operation, "Electric Power Generation" section of this manual.

At startup, the Drivetrain ECM will also energize the current drive circuit for the Electric Drive Cooling Fan Solenoid. This action will start the operation of the system hydraulic cooling fan.

At Engine low idle, the fan speed is approximately 1800 rpm. Once the Engine speed reaches 1300 rpm or greater, the Drivetrain ECM will send the maximum current output to the solenoid. Maximum solenoid current will cause the fan to operate at the maximum 3400 rpm.

The fan draws outside air into vents in the front of the Inverter Cabinet. The air flows through heat exchangers on top of each phase module. The air flow dissipates the heat that is generated by the operation of the power transistors in the phase module. The cooling air is then channeled through ducts into the Generator and back to the axle housing in order to cool the traction motors.

At startup, the Drivetrain ECM will use a sinking driver circuit to ground an enable circuit to each motor control ECM.

Each motor control ECM will perform system checks in order to determine the status of the components that are being controlled. If no fault conditions are present, each ECM will use a sinking driver circuit to ground an enable acknowledge circuit. The circuit is connected to the Drivetrain ECM. the grounded circuits indicate that each motor control ECM has received the grounded enable signal from the Drivetrain ECM and is ready for operation. The enable and the feedback circuits will remain grounded at all times during system operation. If one of the grounded signals is interrupted, the Drivetrain ECM will disable drive train operation.

If a fault condition is detected, an Event code or a diagnostic code will be activated. If the fault condition is severe, drive train operation may be disabled.

All of these initial checks, startups, and enabling of the controls takes place in a short time. While the initialization is occurring, the traction motors are in a zero power state.

Travel Mode Control

Once the operator moves the shift control lever out of the PARK position, the Drivetrain ECM will issue commands to the Motor 1 ECM and the Motor 2 ECM to begin control of the traction motors. A small amount of current is sent to the traction motors in order to "flux" or energize the stator. The flux ensures that the motors are ready for a quick response when required.

The Drivetrain ECM receives a PWM input from the throttle pedal position sensor. When the operator depresses the pedal, the Drivetrain ECM will command the Engine ECM to increase the Engine speed to approximately 1300 rpm. The increased Engine speed enables an increase in the power output from the Generator.

As the accelerator pedal is further depressed, the Drivetrain ECM will command the Engine ECM to increase the Engine speed to around 1800 rpm. The ECM will command an increase in the output of the Generator in order to boost the DC Power Bus voltage level to approximately 2700 VDC.

As the accelerator pedal is depressed, the Drivetrain ECM will send torque commands over the CAN B Data Link circuits to the two motor control ECMs. The Drivetrain ECM will base these torque commands on several factors. The power that is on the DC Power Bus, the position of the shift control lever, the position of the accelerator pedal and the position of the retarding lever will all affect the Drivetrain ECM response.

When each Motor ECM receives these torque commands from the Drive Train ECM, the two ECMs will use six power transistor "pairs" in three phase modules to provide the AC power to the traction motors.

Each transistor pair consists of two power transistors that are connected in parallel. The parallel connected transistors provide more current handling capability in order to handle the high current loads that are required for motor operation.

Each transistor pair is turned on by one ECM gate driver circuit and provides one status feedback input circuit back to the controlling ECM.

At startup or during operation, if the controlling ECM does not detect the expected transistor status feedback signal, the ECM will activate a level 3 Event. The Drivetrain ECM will disable drive train operation.

Three transistor pairs are used to switch the positive side DC Power Bus voltage. Three transistor pairs are used to switch the negative side DC voltage. Each of the positive side pairs are also labeled as Power Transistor 1. Each of the negative side pairs are also labeled as Power Transistor 2.

One positive side switching pair of transistors and one negative side switching pair are used to create one of the 3 AC phases for the motor.

The gate drivers for each of the transistor pairs require a voltage pulse to turn ON (close) the transistors. Each motor control ECM will use driver output circuits to send the voltage pulses to the internal electronic gate drivers for each transistor pair.

Each motor control ECM is paired with an interface module. In order to provide isolation between the high voltage phase module circuits and the low voltage driver circuits, the ECM transistor driver circuits are sent through an interface module. The interface modules convert the voltage sources to a light pulse that is sent to the phase modules through fiber optic circuits.

In the phase module, the light pulse is converted back to a voltage pulse. The gate driver voltage pulses will switch the power transistors ON when the voltage pulse is high. The power transistors will be turned OFF when the pulse is absent or low.

The ECM will activate the gate driver outputs for the three pairs of positive switching transistors and the three pairs of negative switching transistors at a high frequency sequence that will create the three phases of PWM output voltage. The traction motors detect an RMS voltage from the PWM pulses that resembles an AC sine wave (Refer to illustration 1).

For each motor phase, when the motor control ECM modulates the output PWM pulses to increase the time that the pulse width is high, the AC voltage of each phase will increase.

For each motor phase, when the motor control ECM increases the transistor switching frequency, the current for each phase will increase. Higher frequency and current results in greater motor speed and greater torque output.

When a request to change the direction of machine travel is received from the Drivetrain ECM, each motor control ECM will electronically switch the two of the three phases for each traction motor. Switching the two phases will result in the motors reversing direction of rotation.

Note: The truck can be operated without a functioning Steer Cylinder Position Sensor. If the truck is operated in this way, the Drivetrain ECM will not adjust the speed of the rear tires during a turn. The result will be an increase in the turning radius and greater wear of the rear tires.

The high degree of variable control ensures that the requested motor speeds and torque requirements can meet the travel requirements of the truck.

Traction Control System

The Traction Control System (TCS) is controlled by the Drivetrain ECM. The TCS function is active when the shift control lever is in a travel position. The ECM will suspend TCS operation when the service brakes are used.

The TCS is a software function. No operator controls are used to control the function. The function is activated by the Drivetrain ECM when required. An indicator on the cab display is illuminated when the function is active.

The TCS function enables the Drivetrain ECM to limit the slippage of the rear wheels during travel. Under slippery conditions, the Drivetrain ECM will coordinate the blending of electric dynamic braking with the mechanical braking of the front wheels by the Brake ECM. This function is designed to limit slippage on downhill slopes.

The ECM monitors the inputs from the following components in order to control the TCS function:

The position of the shift control lever as reported by the Chassis ECM.
The signals from the four wheel speed sensors.
The signal from the service brake pressure sensor as reported by the Brake ECM.
The signal from the steer cylinder position sensor.

When the Drivetrain ECM detects slippage of the rear wheels, the ECM will adjust the torque commands to the motor controls. More torque will be applied to the rear wheel that is slipping the least as the torque to the slipping wheel is decreased.

When the truck is driven into a turn, the Drivetrain ECM will use the PWM input signal from the steer cylinder position sensor to determine the angle of the front wheels. The ECM will calculate the change in the rotation speed for the traction motors that is needed as the rear wheels travel through the radius of the turn. The Drivetrain ECM will command each motor control ECM to adjust the speed of the motors accordingly. This rear wheel speed adjustment eliminates wheel skid as the rear wheels travel around the radius of the turn. The speed adjustment allows greater turning performance and will greatly reduce the wear on the rear tires. If the signal from the cylinder position sensor is lost, the truck can still be operated. However, the TCS will not function during a turn.

Retarding Function

The dynamic retarding function can be activated at any machine speed. When the retarding function is activated, each motor control ECM will greatly reduce the current that is sent to the traction motors. The Drivetrain ECM will calculate the speed of the truck and the amount of retarding power that is needed to meet the operator request.

When dynamic retarding is requested, the Drivetrain ECM will reduce the output of the Generator. The ECM will use the Chopper Module to reduce the voltage of the DC Power bus to less than 1500 VDC before the retarding contactors are closed. Reducing the voltage before the contactors close will reduce wear on the contactor tips. The ECM may decrease the voltage of the bus as low as 1000 VDC depending on the retarding request. However, the bus voltage will not be allowed below this threshold.

Once the contactors close, the Drivetrain ECM will raise the voltage level of the DC bus proportionally up to approximately 2500 VDC depending on the amount of retarding power that is required.

The retarding contactors are always closed during retarding. When the retarding contactors close, electrical current will be directed through contactor grid 2 resistor elements. The current that is directed through the grid will supply operating DC voltage to the Retarding Grid Blower Inverter (RGBI) through circuits that are tapped off the grid elements. The RGBI will create three phases of AC voltage for the grid blower motor in order to dissipate the large amount of heat that is created by the resistor elements.

The contactor grid 2 resistor elements are located closer to the fan since the contactors provide the first 50 percent of the rated grid retarding power.

The electrical energy that is created by the traction motors is dissipated as heat by the grid resistor elements. The result is a resistive load being placed on the traction motors which slows the motor speed and the wheel speeds.

With little current supplied to the traction motors, the rotation of the traction motors will cause a voltage to be generated at levels that will reverse the current flow back to the DC power bus. The current will pass through diodes that are connected across each of the phase module power transistors. More rotational speed of the rear wheels will produce more current which requires more retarding power that must be dissipated in the grid resistor elements.

For retarding demands that are greater than 50 percent, the Chopper Module is used to channel more of the current that is produced by the traction motors through the chopper grid 1 resistor elements. The Chopper Module provides the upper 50 percent of retarding power that can be utilized.

When the Chopper Module negative side power transistors are gated ON (closed), the negative (-) DC bus is shorted to the positive (+) DC bus through the chopper grid 1 resistor elements. The positive side power transistors are not used in the Chopper Module.

The Motor 1 ECM will adjust the duty cycle of the chopper to control the amount of current and voltage that directed through the chopper grid elements.

A Chopper Module 100 percent duty cycle (negative side transistors held ON) will result in the full rated retarding power (4750 kw) being directed through the grid.

When the dynamic retarding function is no longer requested, the Drivetrain ECM must briefly reduce the DC bus voltage to less than 1500 VDC before the retarding contactors can be opened.

The retarding contactors and the Chopper Module are used in the same manner during the Engine Performance Test. For this test, the contactors and the Chopper Module are used to build up a resistive load on the Generator in order to load the Engine. The test will determine if the Engine will operate at the full load rated output.

For braking on low traction downhill surfaces, a blended retarding function is used. The front brake retarding can be activated by the operator. For this function, the Brake ECM and the Drivetrain ECM traction control system coordinate the amount of front mechanical brake retarding and the amount of rear wheel electric retarding that will be applied.

When the function is active, the blend of retarding will be one third front wheel mechanical brake retarding and two thirds rear wheel dynamic retarding. This blend of retarding provides slip control on low traction surfaces. Blended retarding should not be used on dry or high traction surfaces as unnecessary wear of the front brakes and tires can occur.

Crowbar

In the electric drive train system, the Crowbar is used as an over voltage control device. When the Crowbar is activated, the positive (+) DC Power Bus is shorted to the negative (-) DC Power Bus through the chopper grid 1 resistor elements. This results in an immediate discharge of the bus.

If the voltage of the DC Power Bus reaches 3000 VDC, a level 2 Event for high bus voltage will be activated by each motor control ECM. If the voltage level of the DC Power Bus reaches 3100 VDC, a level 3 Event for high bus voltage will be activated by each motor control ECM. The level 3 Event will also result in the Interface module 2 turning ON the Crowbar in order to discharge the bus immediately. When the Crowbar is activated, drive train operation will be disabled.

See You Soon!!!

MARYGAR