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stupid question

Discussion in 'Electric Vehicles' started by emanresu, Aug 14, 2007.

  1. emanresu

    emanresu Member

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    Q: Can the polarity of an electric motor be switched during operation to create a braking effect?


    thanks
     
  2. WarpedOne

    WarpedOne Supreme Premier

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    You COULD do that, but braking effect would be WAY to strong.

    It is enough to lower the input voltage a little and motor starts to generate the "reverse" current - starts braking and filling the batteries. More you lower the voltage, stronger the braking = regenerative braking.
     
  3. doug

    doug Administrator / Head Moderator

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    I assume you're talking about a DC motor, since in an AC motor, by definition, the "polarity" is constantly changing. The answer to your question is basically yes, though a more common practice is to short the leads of the DC motor together to get what's called dynamic braking.
     
  4. emanresu

    emanresu Member

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    How strong would the braking effect be? Comparable to disc brakes, or alot less?

    Could you stop a car alot faster by switching the polarity and/or applying disc brakes?

    Electric motors have alot of torque, maybe there could be a system if you slammed on the brakes it locks the tires with friction brakes and then spins the wheels in reverse to stop the car super fast.

    I dont know, but I'm surprised I havent heard more about this possibility.
     
  5. doug

    doug Administrator / Head Moderator

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    If your tires are spinning backwards while you're moving forwards, that's not going to stop you faster. This is because the friction between the tires and the road is now kinetic instead of static. If you want to stop quickly, you don't want your wheels to skid. This is the whole point of ABS.
     
  6. TEG

    TEG TMC Moderator

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    When you put energy into an electric motor you make it want to spin faster (create torque) for acceleration.
    If you want to slow down you can turn the motor into a generator but the current created has to go somewhere. If you simply shorted the wires it would get them very hot to the point of melting. Some emotor brake systems have a big resistor with a heat sink to take the excess current and turn it into heat. Regen braking systems take the extra current and use it to charge the batteries.
    If you simply reversed the polarity and tried to run the motor in reverse you would have a "fight" on your hands with both the batteries and the motor (now generator) trying to push current into each other. Unless you have something to collect or dissipate that current somewhere you are likely to heat up and burn something out. You could fry your motor controller, motor, batteries, wires, etc.

    Crude EVs only push current in one direction. They send battery power to the eMotor and then disconnect when not wanting acceleration. That way the eMotor "freewheels" and doesn't slow the car down or act as a generator when you let off the accelerator pedal. Those vehicles have to use friction brakes to slow down. The next step up would be a DC motor with a regen circuit to use the eMotor as a generator to slow down and then to recharge the batteries as needed.
    Tesla uses an AC induction motor which requires much more sophisticated electronics to control. AC induction motors tend to have longer lifespans, less weight, less cost and other benefits compared to permanent magnet motors. It is the way to go if you can come up with a motor controller sophisticated enough to do AC regen. Tesla has the best possible technology in that regard.
     
  7. emanresu

    emanresu Member

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    #7 emanresu, Aug 15, 2007
    Last edited: Aug 16, 2007
    It would stop the car faster. By the same logic your using it would be equally hard to push a car backwards with the wheels spinning as it would with its wheels locked.

    Spinning wheels generate friction which moves a car forward or backwards. If a car spins its wheels backwards during forward movement it will stop you faster since the wheels are be imparting static friction in addition to a negative directional force of the wheels spinning backwards.

    ABS is irrelavent. ABS is only activated during bad conditions such as snow or ice. During normal conditions, ABS will not stop you faster but slower since 'pumping' the brakes will create less frictional force than if the brakes are locked. ABS will not stop you faster than locking the brakes on snow or ice either, it will just allow you to maintain control and not skid out like you would if you locked the brakes on ice.

    Would it work? I mean if it could work that would be a huge +1 for the electric car. Electronic braking, no more replacing brake pads or rotors. Along with no more oil changes, no emissions tests, no engine maintenance, no pumping gas. Imagine that banner at some future electric car dealership. I hope eestor comes through!
     
  8. WarpedOne

    WarpedOne Supreme Premier

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    You have some serious misconceptions about basic physics. I suggest you reexamine your school books on friction, resistance, energy etc.
     
  9. doug

    doug Administrator / Head Moderator

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    #9 doug, Aug 16, 2007
    Last edited: Aug 16, 2007
    With regard to Dynamic Braking, one of course uses wires that can handle the power for the particular application. It is normal to use some kind of shunt resistor. The higher the resistance the slower the braking effect.

    With regard to the main thrust of this tread, however, I'm going to have to agree with WarpedOne.
     
  10. donauker

    donauker Member

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    Anyone that has done a burnout knows that a spinning tire has very little traction. And anyone that has even casually observed drag racing knows that the guy that spins his tires on takeoff stands the greatest chance of losing the race. Deceleration is no different then acceleration in this regard.
     
  11. TEG

    TEG TMC Moderator

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    Do keep in mind that burnouts do have their place in race. Drag racers do them to heat up their tires because a hot tire is stickier than a cold one.

    If you watch F1 or GrandPrix racing you will see that the cars zig-zag frantically on the pace lap to heat up their tires.

    Now, I wouldn't really recommend that a Tesla driver try to employ this trick... It wears your tires down rather quickly.

    Regarding electric braking... Some of those "hub motor" cars use regen and a "brake resistor" to handle slowing down without use of friction brakes. I have always hated the concept of wearing down pads and rotors to slow down. I think electric brakes are a good idea...
     
  12. WarpedOne

    WarpedOne Supreme Premier

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    AC means alternating current. AC motors runs on AC current which is constantly switching its direction i.e. polarity. You must be switching the polarity for the motor to run at all. If you do not do it, motors doesn't turn. Your suggestion doesn't make any sense.

    Go read TEG's post again.
     
  13. emanresu

    emanresu Member

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    OK, maybe polarity isnt the right word. If you switch the positive and negative terminals of an AC motor, it will reverse direction. EX: The roadster has 2 gears, the motor runs backwards for reverse. No reverse gear is needed.
     
  14. TEG

    TEG TMC Moderator

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    It isn't that simple. You can't simply "reverse the polarity" of the terminals on an AC motor like you can with a simple two wire DC motor.

    I think the Tesla motor is a 4 pole 3 phase AC induction motor, and it has multiple wires going from the controller (PEM) to the eMotor. The PEM firmware gets told to run the eMotor in reverse and it does some sort of negative torque calculation to make the magnetic fields pull the windings backwards from the usual direction. You can run AC motors or DC motors in reverse, but the way you control them to do this is very different.
     

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