Nilfisk-Advance SC900-28D Service Manual

Contents

15Service Manual – SC900 20 - Wheel System, TractionDrive Motor PWM FunctionThe drive motor is controlled from a Curtis 1228 controller, which isa pulse-width-modulation speed controller designed specifically forpermanent magnet DC motors Pulse-width-Modulation (PWM) is a formof motor speed control that alters the power to a motor by rapidly turningthe power on and off The ratio (also called “duty cycle”) between theOn and Off states determines how much power the motor receives Theshorter the “off-time” the closer to full power the motor will receive Thisswitching occurs so fast (15kHz for this controller) that the motor simplysees it as a reduction in power (voltage) instead of the rapid on/off PWMis a standard motor control technique because it is easier to turn powerall the way on and all the way off, than it is to vary the magnitude of thepower Varying the magnitude would create a lot of heat that would needto be dissipatedDrive ControllerThe primary purpose of the drive controller is, of course, to control the drive motor However, it also providessome ancillary control of the other functions of the machine, which will be discussed in greater detail in theapplicable chapters As a motor controller, it delivers PWM-varying power to the drive motor in both positiveand negative polarities for forward and reverse motor operationEven though the machine does not utilize an electromechanical brake system, the drive controller possessesthe output control for one This is used for ancillary machine control This is more aptly referred to as aBrake-Release versus a brake control Whenever the machine is not commanded to move, the brake outputsare inactive, and if a brake was present, it would be mechanically engaged The brake outputs are activeonly when the machine is commanded to move This feature is used to activate the scrub functions of themachine only when the machine is commanded to move (even if the speed is set to zero)The controller is powered directly from the batteries, but through a 30-amp circuit breaker The drivecontroller remains inactive (inhibited) until the KSI input receives positive battery power through the mainkey switch When KSI is active, the drive is ready to operate, and awaiting a throttle command to moveNote: The drive controller is configured for an inverted throttle input from typical. Instead of thethrottle being active-high, it is active-low. When the throttle input is at 5 volts, the throttle isoff. When the throttle input is at 0 volts, the drive is commanded for full throttle.In addition to the throttle being inverted, it is also limited to just two states: Off and Full Because thethrottle input is either off or full, the speed of the machine is controlled by the speed limit potentiometerinput This input ranges from 0 to 5 volts, with 0 volts equaling no speed, and 5 volts equaling maximumspeed The bias voltage of 0 and 5 volts for the potentiometer is provided by the speed controller from thePot-Lo and Pot-Hi outputs25% PWM Duty Cycle50% PWM Duty Cycle75% PWM Duty Cycle100% PWM Duty Cycle1-Cycle(15kHz)