Motors 101

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

Think "school science fair"


Electromagnetic field

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

What happens to the magnet?



Opposite poles attract

Similar poles repel

Turning force exerted on magnet



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



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?



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

More torque to keep going


Same problem again...

W field contribution weakens

Need to switch again soon...


The chase continues

Now, move (-) to V

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


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


Halfway around

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


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?



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


Switching the first state

Controller turns on transistor bases


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


Major components

Modeling the car parts


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!


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?


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


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



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 ...



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...