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Motors 101



Rudimentary motor layout
    Three wire coils
    Bar magnet spinning on a pivot

Think "school science fair"


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Electromagnetic field



Energize one coil
    Current creates magnetic field
    Field aligns along coil axis

What happens to the magnet?


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Torque!



Opposite poles attract

Similar poles repel

Turning force exerted on magnet


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Connections



Common center is internal only
    So we don't really have access to it

Attach the orange wires
    Different labeling schemes:
    U/V/W ; A/B/C ; ...

Now we energize two coils at once
    Doubles force w/ same current


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Oops...



Now, the V coil is useless
    Still have contribution from the W coil
    But V coil wants to hold the red back

What do we do now?


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Next!



Answer: switch coils
    Move (+) from V to U
    New field builds from new direction

More torque to keep going


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Same problem again...



W field contribution weakens

Need to switch again soon...


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The chase continues



Now, move (-) to V

Maintains rotating field, high torque
    Always about 90 degrees to magnet


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Switched or sine?



If we're on sinewave line power instead ...
    Currents build fields smoothly
    All three coils carry current, except at zero-crossing
    Currents into and out of motor add up to zero

Alternate switching scheme
    Fire all three coils when appropriate
    Current splits equally through U and W
    Two half-strength north/green fields for a while


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Halfway around



Exact opposite of where we started
    V and W now reversed
    U is about to swing (-)


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Drive power



Waveform options
    Commercial power is already AC

Switching method can "fake" a sinewave
    Close enough to work well, anyway
    Magnetic field still rotates

How do we get all that from a DC battery?


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Inverters



Common 3-phase inverter circuit
    This shows up everywhere in hybrids and EVs
    And even a few conventional vehicles now

Turns DC into AC for motor drive


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Switching the first state



Controller turns on transistor bases


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It's revolutionary



Six switching states
    [Read down, not across!]

One changed connection per transition

To reverse, just swap any two leads
    (or reverse commutation steps backward from step 4)
    Motor controllers do this easily


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Major components



Modeling the car parts


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Familiar circuits?



Take away the transistors...
    It all works in reverse, too
    Spin the magnet, generate currents

Typical 3-phase rectifying bridge
    Almost every alternator contains this

Unregulated generating capability
    No controllable field winding here
    Magnets don't turn off

Don't dinghy-tow the car!


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Things to avoid



Switching both legs releases magic smoke

Need protective driver circuitry
    Most inverters guard against this in hardware
    Provision for minimum "dead time"

Halfway-on states are bad too
    But what about sinewave drive?
    How do we regulate motor power?


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Pulse width modulation



A method for current control

Resistors get HOT, right?
    Transistors act like resistors if driven linearly
    Partially on --> high voltage, high current, poof

All-on or all-off is more efficient
    Off: no current --> low wattage
    On: no (or little) voltage --> low wattage

On-time ratio yields average current
    Switching rate is fast -- 10 or 20 KHz or more
    Winding inductance smooths out pulses
    Diodes smooth out turn-off spikes
    Spike current sent into rails, helps charge cap back up

Helps avoid transistor breakdown

Variable duty cycle "fakes" a better sinewave


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Motor architectures



More poles for more torque, smoothness
    Like two of our original motors in one housing
    4 poles: 2 electrical revolutions / revolution
    Coils could be series or parallel

Real-life cars ...
    Insight: 12 poles, 6 r/r
    Prius: 8 poles, 4 r/r
    Civic: ??? Let's figure it out from RPM

Real-life motors are more "closed"
    Few stray external magnetic fields


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Commutation



aka, "Where the heck are we"

No more mechanical brushes
    We likely need some position feedback

Simple Hall-effect or inductive sensors
    Small magnets attached to rotor shaft
    Outputs trigger drive electronics

Frequently used in smaller motors, fans
    And the Insight IMA ...


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Commutation



Reluctance-driven quadrature sensor
    Tamagawa-Seiki "singlsyn" type
    Two outputs change relative amplitude and phase
    Faster and finer feedback than Hall switches

Used in the Prius
    Referred to as the "resolver"
    This explains the 6-wire position-sensor connectors
    Driver/translator chips built into hybrid ECU

Completes feedback loop for electric drive

Armed with all this, class may continue...