Polyphase AC Induction Motors
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Polyphase means more than one phase AC is the prevailing mode of electrical distribution Single-phase 208V to neutral from a three-phase transformer on the pole is the most prevalent form found in your home and office The phase voltage that comes from the pole is 240V These are widely available in nearly every city in the industrialized world If one phase is good, then three phases are better, right Well, usually Stationary threephase electric induction motors are inherently self-starting and highly efficient, and electricity is conveniently available Three-phase AC connected to the stator windings of a three-phase AC induction motor produces currents that look like those shown at the top of Figure 6-8 they are of the same amplitude, but 120 degrees out of phase with one another As in a DC motor, power and torque are also a function of current in an induction motor Because the current is equal to the voltage divided by the motor reactance, at any given voltage, current is a function of stator, rotor, and magnetizing reactances that change as a function of frequency The top of Figure 6-9 shows this at a glance The characteristic induction motor torque to slip graph, shown in Figure 6-9 for both its motor and generator operating regions, offers insight into induction motor operation If an induction motor is started at no load, it quickly comes up to a speed that might only be a fraction of 1 percent less than its synchronous speed When a load is applied, speed decreases, thereby increasing slip; an increased torque is generated to
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Polyphase A C motor operation summarized
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meet the load up to the area of full load torque, and far beyond it up to the maximum torque point (a maximum torque of 350 percent rated torque is typical) Speed and torque are relatively easy to handle and determine in an induction motor So are reversing and regenerative braking If you reverse the phase sequence of its stator supply (that is, reverse one of the windings), the rotating magnetic field of the stator is reversed, and the motor develops negative torque and goes into generator action, quickly bringing the motor to a stop and reversing direction (see Figure 6-10) Regenerative braking action pumping power back into the source is readily accomplished with induction motors How much regenerative braking you apply creates braking (moves the steady state induction motor operating point down the
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Figure 6-9 frequency
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Polyphase A C motor s unique speed, torque, and slip characteristics ver sus voltage and
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AC electric motor (Cour tesy of Zero Emission V ehicles of Australia)
torque-slip curve), and generates power (instantaneously runs with negative slip in the generator region, supplying power back to the source)
Wound-Rotor Induction Motors
You might also encounter the wound-rotor induction motor in your EV power source quest A wound rotor s windings are brought out through slip rings conductive rings on the rotor s shaft through brushes (analogous to DC motor construction) to an external resistance in series with each winding The difference is that slip rings are continuous with no commutator slots The advantage of the wound-rotor induction motor over the squirrel cage induction motor is that resistance control can be used to vary both the motor s speed and its torque characteristics Increasing the resistance causes maximum torque to be developed at successively higher values Along the way the wound rotor has better starting characteristics and more flexible speed control What you give up is efficiency at an increased complexity and cost
Today s Best EV Motor Solution
Different motor types in their numerous variations give you an almost unlimited bag of tricks in terms of solutions, but you have to figure out which one to use If someone tells you there is only one motor solution for a given application, ask another person Their not-so-surprising and different answer ought to convince you that there are no blackand-white motor solutions only shades of gray You can probably find three or four good motors for any need and you should look for and ask the questions that uncover these alternate solutions On any given motor solutions list, you might choose the motor with the lowest price because that is your most important criteria Another person might choose the motor with the lightest weight Still another might choose the motor with the best startup torque Regenerative braking might be most important to another person, and so on Just remember shades of gray Following along with this theme, the solution recommended here is not the solution, it is just a solution that happened to work best in this case However, for economy conversions, DC series wound has been noted to be the best approach Other motors are getting harder to find and not every controller on the market can drive the motor For efficiency and for regenerative braking (which buys about 25 percent range in city driving), AC motors are the best way to go