Soft starting and stopping is not a function of the remote control but a feature of some Motor Control Units (MCUs). There are 2 main classes of MCUs: electro-mechanical types, which rely on power switching relays, and semi-conductor types, which use PowerFETs (Field Effect Transistors) to switch motor currents. These are the salient characteristics of both types:
- Low cost, relays cost R50 or less,
- Availablity, every Midas stocks them
- Can be fitted by anyone, and without special tools,
- Motor runaways are rare if properly protected,
- No repair can cost more than 4 relays,
- Easily controlled by switches, pushbuttons, and other relays (such as with remote controls)
- They switch between 0 and 100% power, leading to jerky movements,
- They favour the skilled operator, particularly when tandem axles are driven,
- They don’t allow Creep Mode,
- The smallest increments are 10 – 20mm of movement.
- Ganged relays required for currents greater than 30A,
- Relay contacts can weld together if not protected against over-current, leading to motor runaway.
- Volume manufacture is even cheaper than relay systems,
- PWM (Pulse Width Modulation) allows seamless power delivery between 0 and 100%,
- PWM allows Creep Mode where minute movements are possible,
- PWM allows differential power supply so motors can be driven at different speeds. Great for smooth turns.
- PWM allows a progressive ramp-up of power (Soft Start) and similar ramp-down,
- Differential power makes tandem axles easy for unskilled operators,
- Impossible to troubleshoot or repair without know-how, special instruments, and schematics (easily withheld by manufacturer),
- Not suitable for direct switch operation, totally dependent on dedicated remote control unit,
- Vulnerable to failure anywhere between remote button and PowerFET, water damage is a major concern,
- Failure often results in motor runaway, so emergency nearby cut-out switch is essential,
- Users are subject to extortionate repair bills,
A ‘soft start’ only available on PWM solid state MCUs. When any motor is started the motor is supplied with a series of power pulses which lengthen over a period of 2 to 2.5 seconds until they stop being pulses and join to form a continuous supply. This is the same feature that allows creeping forward – tap any motor button momentarily it never gets past sending the shortest power pulses to the motor, which causes it to turn a minimal amount only.
When turning a double axle caravan, and the correct mode is set up in the control unit the outer wheels receive full power while the inner are given only half. This makes the caravan perform a slightly more elegant turn than with a relay driven MCU, where the operator makes a turn by applying full power to all 4 wheels but periodically drops power to either the left or the right pair to achieve the same result, though a bit more jerkily, and at a tiny fraction of the overall cost of the imported movers.
The reason power is pulsed rather the battery voltage to the motor being lowered is that a huge amount of heat would be generated in the series resistance required to lower the voltage by that method. By pulsing the 12V power the motor is given the task of evening out the pulses into a lower average current level which brings down the motor’s torque and makes it turn more slowly. The motor doesn’t mind much and the power pulses are prevent from returning to the battery, where they could upset or damage other loads, by a large filter on the PC board.