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Presentation at APEC 2008 by Steven Schultz

9895 Views 21 Replies 10 Participants Last post by  tdjvolt
I recently attended the 2008 Applied Power Electronics Conference. Steven Schultz gave a nice presentation that included some details about the Chevy Volt.

Here's a bio of Mr. Schultz:
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The easiest choice of driving electric motor is direct-would AC/DC motor such as the one found in KitchenAid mixer. It is easy to control its speed and torque simply by varying supply voltage (AC or DC). Also, it is easy to switch its rotational direction. However, since it has coil windings in its rotor it requires commutator and brushes, which are subject to wear and need periodical servicing. "Brushless" motors for automotive propulsion purpose use a permanently magnetized rotor and a stator, which generates moving magnetic field around the rotor. By changing the strength of the magnetic field and its moving speed you can control the speed and torque of the motor. For this purpose you need an electronic circuit to control the energy and frequency going into the stator coils. To control the energy efficiently I think automakers are using PWM or Pulse Width Modulation, which controls duty time of the electrical pulse. To control the frequency you need a variable frequency oscillator. This electronic control system is called VVVF (Variable Voltage, Variable Frequency). The permanent magnet rotor also allows the motor to be short, flat and high-torque.

There is one potential future problem casting a shadow on this "brushless" motor...
China is the major supplier of permanent magnet and also is the major producer or importer of rare metals (such as cobalt) needed to produce strong permanent magnet. It even discourages export of rare metals it produces by imposing export duty on them. China
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The direct-wound motor is the kind you make at elementary school science class. Because of the rotor commutator it runs on AC or DC in the same fashion. Its rotational direction can be reversed by simply switching the stator coil terminals (a nice feature for automotive use). You control its speed and torque by changing the voltage. Speed actually is load-dependant. On a very light load, the power/load equilibrium may make the speed so high that motor destructs itself. Torque (current flow) is highest when the motor is not running, which could cause the motor to fry. Speed of induction AC motor is frequency-dependant. Varying voltage does not change its speed (to a point). This is the reason why you need a variable frequency generator for automotive use. You use PWM to control torque (therefore, speed) of DC motor rather than rheostat because a resistor in series means waste of energy. PWM for AC motor controls the torque while the frequency determines the speed.
Since nobody has responded to your post yet, let me try to explain...

The direct-wound motor can be use AC or DC, but not in the same fashion...
It rotates by exactly the same principle.

In a DC motor brushes, you can't control rotational direction by switching the stator coil terminals...
Yes, you can. Take apart an electric screwdriver and look inside. If a motor has a permanent magnet stator you cannot change its rotational direction unless you use DC and switch its polarity.

In some AC motor, current is Torque, not speed... In any motor the amount of current is directly related to torque simply because the strength of magnetism is directly related to the amount of current.

...It can be on AC or DC motor, but it does not appy all motor...
Yes, it does. You can fry any motor by supplying too much current especially when it is not rotating.

...You never loses electron...
Yes, electron never loses. But, you loose energy as heat (current*current*resistance). Therefore, to control torque you change the duty time of the on/off pulse rather than putting a resistor in series.

...In AC motor, it a huge waste when it get a high speed...
No, an AC motor does not waste energy as it increases speed. Speed of AC synchronous motor (the kind used for EV's) is determined by the number of poles and the frequency of power supply. To increase its speed you increase the frequency. Its torque (power) is still controlled by the integrated amount of current that is controlled by PWM. In the VVVF design a complex combination of current (voltage) and frequency is needed.
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