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07-23-2014, 03:48 PM | #1 (permalink) |
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Electric motor voltage generation
I have a few questions about electric motors.
When you spin an electric motor to produce electricity, what voltage does it come out as? Or how is this determined? I am assuming the amps produced changes based on how fast it is spun. What about AC vs. DC motors? Does an AC motor produce AC currents? How do AC vs. DC Motors compare? Would an AC motor be a possible application in our hobby? Sorry, I realize these are a wide range of questions and my curiousity always gets the best of me but I do not fully understand electric motors or AC vs. DC motors in terms of benefits and disadvantages of each.
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07-28-2014, 12:40 PM | #2 (permalink) |
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When using a standard hobby brushless motor as a generator, just reverse the kV calculation. The voltage is RPM/kV. However, know that the voltage coming out is 3-phase, and needs to be rectified to get DC out of it.
As for all your AC vs. DC questions, you should know that a hobby brushless motor is basically what industry would call a permanent magnet AC motor. Our ESC's are actually creating crude 3-phase AC current from DC supply. Most common AC motors will not produce any current when spun. This is because both the rotor and stator rely on electric current to generate the magnetic field. With any electricity, they create no field, and can't generate electricity. I believe you can use them as a generator, but you need a battery to get the stator field started, after which point the whole thing will generate.
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07-28-2014, 04:06 PM | #3 (permalink) |
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Ah, that makes sense. So our motors are AC motors, we use the ESC to convert the DC power to an AC signal?
I have a ton to learn still on motors! Interesting how the AC Motors without magnets need to be "jump started' to get going. I know the Tesla uses an AC motor with no magnets and is still used for regeneration when slowing down. What are the benefits of a Permanent Magnet motor vs. a motor without magnets?
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07-29-2014, 09:34 AM | #4 (permalink) |
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Yeah, I think technically, our ESC's aren't making 3-phase AC power, because what they are doing is making 3-phase square waves. The industrial controllers I use, use PWM to create something like a sin wave as 3-phase AC truly is. Think standard inverter vs. a "true sine wave" inverter.
I think one of the main reasons not to use permanent magnets in large industrial motors, is mostly the cost of those large magnets?
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01-18-2015, 01:24 PM | #5 (permalink) |
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It's not just the cost of magnets... for an industrial system there is benefit in being able to change the current in the windings that replace the magnets.
Altering the current in these windings changes the magnetic field strength and allows a number of "games" to be played. For instance you could change the Kv value whilst the motor/generator was running. This would facilitate a speed control process without having to use a truly enormous ESC. [It wouldn't really be an ESC in practice but it would be electronics in the power path.] |
01-19-2015, 09:59 AM | #6 (permalink) |
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There are engineering tradeoffs which lead to permanent magnet motors having better power density at small sizes and AC induction motors having better power density at large sizes. The cutover point is somewhere between 50kW and 200kW. That's why Tesla uses AC induction motors while a Prius uses permanent magnet motors.
Also, large magnets are expensive, so if you don't care much about size and weight (typically you don't for a "large industrial motor") then AC induction is a clear choice. |
01-19-2015, 10:48 AM | #7 (permalink) | |
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Quote:
I was always curious why the prius used permanent magnet motors. Seems an AC induction would be much less expensive.
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01-19-2015, 12:49 PM | #8 (permalink) | |
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Quote:
Another big trade-off is that you can moderately easily spin AC induction motors to insane speeds, because you effectively control the field strength of the rotor, so you get to pick the kV of the motor. That's one reason they're a good choice for Tesla, which wants to have a single-speed car that can go 130 MPH, or for Mission Motors, where I helped design a single-speed motorcycle that could go 150 MPH. With a permanent magnet motor, driving the motor faster than voltage * kV is difficult (not impossible, but inefficient and challenging to control well). In a Prius, there's a crazy setup with two motors and an engine all connected to a four-way differential that lets their controller keep the engine and all the motors at reasonable speeds regardless of road speed - it's very clever. |
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