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The Job of the Electric Motor/Generators

The Prius has two electric motor/generators located in a housing attached to the side of the petrol engine, in the same place you would normally find the transmission in a non-hybrid FWD car. Inside this same housing is the powersplit device. In place of a conventional transmission with different gears providing different torque ratios, the Prius has the power split device that has only one gear - top gear. So it is solely the job of the electric motor to provide all the torque provided by 1st gear in a non-hybrid car.

The reason the electric motor can do this is because electric motors produce torque all the way through their rev range and in fact produce more torque at lower revs, unlike the internal combustion engine, which only produces torque at high revs. So the electric motor always pulls the car away from a stand still and the petrol engine starts to come in around the same time a non-hybrid car would change to 2nd gear. As the Prius speeds up the revs increase in the petrol engine, increasing the torque output and therefore demanding less torque (power) from the electric motor. As the Prius reaches motorway speeds, the Prius will normally be driving solely on the petrol engine. However the Prius will favour the electric motor whenever possible, so on a downhill gradient for instance, where the power requirement is minimal, the Prius will drive solely on the electric engine.

When a large amount of extra power is required, for instance when accelerating hard to over- take another vehicle, the electric engine will come back in over the top of the petrol engine. At times like this when the power demand is at it's greatest, the electric motor and the petrol engine will both be putting power though the powersplit device at the same time.

As the Prius has no transmission it is also solely the job of the electric motor to provide reverse gear. The electric motor does this by doing something that can't be done with an internal combustion engine, simply running the electric motor in the other direction.

When an Electric motor becomes a Generator

In conventional non-hybrid cars when the driver takes their foot off the accelerator the car will slow down against the resistance of the petrol engine still turning. When the driver takes their foot off the accelerator in a Prius, rather than the car slowing against the resistance of the petrol engine, the car slows against the resistance of the electric motor turning.

Also, when the driver of a Prius puts their foot on the brake, rather than the car slowing down using friction brakes, the Prius will send more resistance through the powersplit device, thus slowing the car more whilst at the same time spinning up the generator faster and therefore increasing the amount of electricity that is generated. This is because in an electric motor if the rotor is spun by an outside force, an electric current flows in each set of windings in turn, thus generating electricity that can be used to charge a battery or power the another motor.

Thus, the same device can be a motor or a generator depending on whether current is pushed into the windings to attract the rotor magnet or drawn out when something else spins the rotor around. The only time the friction brakes are used in the Prius is when the driver really stands on the brakes or at the final part when the car is coming to a complete stand still. Therefore the brake pads tend to last up to 3 times as long compared to a non-hybrid car.

Construction and Operation of the Electric Motor/Generators

Both the electric motors in the Prius are very similar in construction, but different in size. They are both synchronous, AC, permanent magnet brushless electric motors, which are very flexible and efficient. The rotor (the part that spins with the shaft) contains powerful permanent magnets. If you put these magnets on your refrigerator, you'd need tools to get them off again! There are no coils of wire in the rotor and no electrical connections to it. This increases reliability over DC motors which have windings in the rotors. All windings are in the stator (the part of the motor/generator that stays still around the outside of the spinning rotor). The control electronics passes an alternating current through these windings to turn the rotor. This current must be "synchronous" with the rotor's movement. This means that the current must pass first in one direction and then in the other at the precise time that each magnet embedded in the rotor passes the winding. A position sensor on the shaft tells the control electronics where the rotor is and how fast it is spinning.

The stator of MG2 has 48 "teeth", that is 48 metal protrusions towards the stator. Each winding passes around several teeth, in the "slots" between them. The windings for each of the three phases are overlapped. These windings produce 8 magnetic poles which rotate more smoothly than other electric motors that use less. The rotor also has eight poles, with eight permanent magnets embedded in it.

Operation of the Electric Motor

Current is passed through the windings first in one direction then in the opposite direction. This generates a north and then south magnetic pole. Current is passed through each set of windings in a circular manner with multiple windings that sit opposite each other getting current at the same time. First current is passed through one winding and then to the next winding. As the current is passed through the winding, around the stator poles, the rotor pole is "pushed" around by magnetic repulsion. The next rotor pole is "pulled" by magnetic attraction behind the first. The control electronics senses the rotor position and keeps the stator poles magnetized so as to both push and pull the rotor around at the same time.

As the rotor pole passes by a stator pole, the winding current changes direction (from north to south) and the stator pole briefly has no magnetization. It changes from having opposite magnetization to the approaching pole, pulling it closer, to having the same magnetization, thus pushing it away. The time at which a winding reaches peak current and the stator pole reaches its most intense magnetization, the rotor "magnet" is then exactly adjacent to it. The like pole of this magnet is toward the counter clockwise direction, and is pushed away. The unlike pole is toward the clockwise direction, and is pulled onward. To change the direction the motor runs in, it would only be necessary to change the timing of the current pulses so that at the peaks of stator's magnetization the like pole of the adjacent stator magnet is in the clockwise direction and the unlike pole is in the counter clockwise direction. The timing of current in the windings relative to the rotor position will always be as explained above. That is, current will peak in a winding when exactly between rotor poles. If this did not happen, the motor would operate less efficiently. If the Prius require less torque from the electric motor, the Prius computer will tell the control electronics to pass less current though the windings. The rotor poles are never allowed to "catch up" to the rotating magnetic field when motor operation is required.

The Clean Green Car Company would like to thank Graham Davies for his contribution to the material of this page.


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