4QD have, for many years, supplied motor speed controllers to the Greenpower battery-powered electric car competition and our controllers are generally highly efficient. We don't have efficiency figures for other controllers, but our design team do not like wasting power, so they are likely to be as high in efficiency as any controllers you can get. The event has been won more than once using our controllers.
We have written this page to assist would be-entrants in choosing a controller for the event.
There is a lot of information including the rules, tips etc at the official Greenpower www site.
The information here is based on the rules for the Formula 24 event as published in May 2005.
Motor and batteries are fixed. So the electrical energy choices are simply:
The rules say Forms of additional free natural power can be used. This clearly encourages the use of solar panels to keep the batteries topped up. But let's examine possible sources of free energy. These could include
Even if you have a freewheel (so cannot use regenerative braking) it does no harm and a controller with regenerative braking is usually slightly more efficient than one without (for reasons, you will have to get quite technical). So you should chose a controller with regenerative braking.
This means building an efficient design. Efficiency must be maximised in the following areas of the design:
Remember also that if you are (as you should) using regenerative braking, you should also consider the efficiency of the gears when used in reverse, during braking, to feed energy back to the controller.
Such motor performance curves show only the performance at one voltage, i.e. without a speed controller. However for this sort of contest you need to have as much power output as possible at maximum efficiency: that requires running the motor at the highest allowable voltage, i.e. 24v. That is with the speed controller at maximum! Efficiency curves for other voltages are not quoted, but one can expect the same sort of shape of curve - at low motor currents efficiency drops off because of brush pressure and other mechanical friction. At high currents motor heating causes high losses. Looking at the graph - motor efficiency is maximum at a load of 1.5 amps.
The motor's efficiency is a maximum 72% at around 1800 rpm, falling fast at slower speeds, either unloaded or under heavy loads. So you need to keep the motor speed around 1500-1800 rpm as far as is possible. This requires a variable speed mechanical drive, or at least a very good range of gear ratios. An electrical speed controller's purpose is to vary the speed by altering te applied voltage, so will may not be able to maintain the motor near its maximum efficiency and power output.
Even with very good mechanical gearing, if you connect a motor through a simple switch to a battery a huge current flows. Thus huge current causes heating proportional to the square of the current. Power dissipated is I2R, where R is the resistance of motor, wiring and batteries. Accelerating the motor voltage in a controlled fashion via a speed controller will reduce this surge current.
The speed controller can also give you regenerative braking which increases efficiency.
The motor current should not rise much above around 50 amps: at 50 amps the motor efficiency is already down to 55%. So suitable controllers include: