EcoSpeed Motors
The EcoSpeed EMtnD and EMD units use high speed brushless DC motors. High speed motors produce more power per unit weight than lower speed motors such as are used in hub drive systems. Brushless means that there are no mechanical switches inside the motor to wear out, so maintenance is eliminated and reliability is much higher.
1000 Watt Motor
We rate our standard large motor at 1000 Watts.. Notice the "we rate" statement. It's a common misconception that electric motors just produce a fixed amount of power based on some property of the motor, such as its size. Actually, electric motors are just like gas engines in that they can be, in effect, tuned to produce different amounts of power. We won't go into all the fine details, but the power a motor can supply is mostly set by the controller. So we set our Velociraptor controller to drive our large motor to the maximum legal e-bike power in Oregon, where we're based.
For the large motor, that's actually a conservative power setting. We've pushed them well over 1500 Watts in tests without any difficulty though, as with a highly tuned internal combustion engine, past a certain power level long term reliability starts to decline. We know these motors will last almost forever at 1000 Watts. At 1500 Watts, we're not as certain.
Electric motors operate on the principal of using the magnetic field that surrounds any wire carrying electrical current to attract either another current carrying wire or a permanent magnet. In the latter case, it is referred to as a permanent magnet (PM) motor. If it then uses direct current (DC) as its source of power, it is a PM DC motor.
In a motor, the current carrying wire is wound tightly around a metal frame to concentrate the strength of the magnetic field and so is called a "winding". Of course, if two magnets, or a winding and a magnet, are attracting each other, they will move as close together as they can and then stop. To get the continuous motion that characterizes a motor more than one winding is needed. The current is then switched off when a magnet moves close to a winding and at the same time the current in another winding further away is switched on thus continuing the motion. This switching process is called commutation.
If the commutation is done mechanically, by switches (brushes) activated by the rotating motor, it is a brush PM DC motor. If the commutation is done electronically via motor shaft position sensing and power transistors, then it is a brushless PM DC motor, usually shortened to just brushless DC motor. The brushless DC motor is the more expensive design but has the advantage of not having mechanical brushes that wear out and need periodic replacement.
All motors used for transportation need some means of varying speed and power output under user control, analogous to the carburetor or fuel injection on a gasoline engine. This function is provided by the motor speed controller. For a brushless motor, the speed controller and the commutation electronics are integrated and the whole assembly is simply called the controller. For a brush motor, the controller is simpler because the commutation is mechanical. The controller takes commands from an input device that gives a it signal, in the form of a voltage, telling it how fast to spin or, in more sophisticated designs like or Velociraptor, how much power to put out. That device, mounted on the handlebar of an electric bike is called, appropriately enough, the "throttle".
The controller varies motor power using a technique called "pulse width modulation" (PWM), which is just an engineer's way of saying that it turns the current on and off really fast, but leaves it on longer when more power is needed. Listen carefully to a running motor and you can hear the change in pitch as the PWM "duty cycle" is varied by the controller.
Grab the output shaft of a PM DC motor with pliers (make sure it's a small one!) and turn on the power. You will instantly feel the twisting force on the shaft as the energized windings try to attract the permanent magnets inside. If this was one of EcoSpeed's motors you would feel several pounds (kg) of force trying to pull the pliers out of your hands. As long as you hold the shaft stationary, it is exerting a force but since there's no shaft motion, there's no power output. There's plenty of electrical power going into the motor though. So, the
efficiency which is defined as mechanical output power divided by electrical input power is at zero percent.
Allow the shaft to turn slighly and it's now producing mechanical power. Mechanical power is just twisting force (torque) mulitplied by shaft rotational speed (rpm). As the shaft slowly turns the the torque is about the same as when stationary but the motor is now producing a small amount of power. Efficiency is still very low, only a few percent, because electrical input power is the same.
Now release the shaft and let the motor spin free. If you've taken a physics course, you may recall that not only does a current in a wire create a magnetic field, but a wire
moving through a magnetic field creates a current in the wire.
As the windings pass through the fields of the permanent magnets, a current is induced which opposes the current that the motor controller is supplying. This induced current increases linearly with rotational speed and at a certain speed the controller can no longer supply enough current to increase the motor speed any further.
This is the peak speed of the motor. At this speed the motor is supplying only enough torque to overcome bearing drag and air resistance. There is none available to drive a load, so even though there's very little electrical power going in, there's also no mechanical power coming out. So, efficiency is again zero percent. In between the two extremes power and efficiency vary from zero to a peak and then back to zero again.
If there were no controller, motor torque and winding current would be limited only by the winding resistance of the motor and the reverse current. Current would peak at zero rpm and decrease linearly until the motor reached its peak rpm. If you plotted current and torque against motor rpm, similar to Fig. 1, you would have a straight diagonal line from the upper left of the plot to the lower right. Such a motor would have tremendous starting torque but would be very inefficient. Efficiency wouldn't go above 50% until half the peak motor speed and wouldn't peak until about 80% of peak speed. Peak power would be produced at 50% efficiency, i.e. half the electrical power input would be wasted.
To improve efficiency, all motor controllers are designed to limit the peak current that the motor can draw. By limiting current below a certain speed, efficiency is increased. Such a current limited motor can go above 70% efficiency well before half of peak rpm. Fig. 1 shows current plotted against rpm for a conventional current limited motor. A curve such as this would be representative of one of our motors operated by a conventional controller.
Fig. 2. shows the shape of the current vs RPM curve for a motor using variable current limiting. The current is
decreased with decreasing rpm below a certain point. This type of current limiting allows even higher efficiencies at the expense of somewhat lower low speed torque and also protects the controller from high stall currents for increased reliability. Because the EcoSpeed Mid-Drive system uses gears to boost torque, it can take advantage of the efficiency of this type of current limiting without losing significant performance. What this type of limiting does is increase average efficiency without affecting peak power, which occurs in the "Current limited by motor" portion of the plot. The EcoSpeed Velociraptor controller can take advantage of this type of current limiting.
