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RC motors, 3 phase..? Tell me about them.

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  • RC motors, 3 phase..? Tell me about them.

    Looking at my grandson's RC cars one of the motors has 3 leads which I assume is 3 phase.

    That same motor says max rpm is 100,000. That seems unrealistic, but what do I know about the current RC stuff.

    I see they still have the old standby, the RC10.

  • #2
    I may be way off here - but JT or someone will correct me - it's DC so no reason for phases - one wire is the + the other the - and the other the signal (I forget the correct name) it's brushless and that's what the third wire is for... to tell the comp. where the motors at for the proper signal...

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    • #3
      If three leads then they are are BLDC (brushless DC) motors they have a 3 conductors fed from a PWM controller.
      Only any two of the three windings are conducting at any given time, they represent a DC motor turned inside out.Hence BLDC.
      They are extremely high RPM.
      Most of the BLDC are out-runner motors.
      Max.

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      • #4
        brushed motors have their days counted. Even electric fork lifts have three phase motors. With the improvement of power electronics its easy to convert DC to AC and three phase is the easy way to get a motor to turn. 100K rpm is obtainable by high frequency switching. BLDC motors include magnets so that they don't have to "induce" a magnetic field on the rotor and makes the motors more compact and efficient.
        Helder Ferreira
        Setْbal, Portugal

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        • #5
          RC cars are stupidly powerful nowadays:
          https://www.youtube.com/watch?v=TyDvXA_0ZyA

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          • #6
            It is a three phase design, sensorless. 100,000 rpm is only attainable from an “in-runner” design. Poorly made ones will throw magnets, as the magnets spin on the shaft. Out-runner designs, the outside can rotates. It cannot throw magnets as centrifugal force holds them tighter to the can. Out-runners are now the most popular design for everything but cars. They have lower rpm’s but much more torque. They can spin bigger props for more efficiency. All of my airplanes and drones use them. I must have 15 of them.
            The electronic speed control is kind of like a VFD. It cycles the three legs in order to control rpm but with DC. They use back EMF to sense where the stator is. They have for the most part, along with LiPo battery packs, replaced IC engines in the hobby. They are simply amazing.

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            • #7
              Well there you have it sounds like RB knows exactly what it is - I thought i remember seeing ones that had two thick wires and one very thin one so did not think that would be used for power just a sensor wire for brushless control...

              RB my little fuego brushless has three wires all the same size - so is it a three phase ac or somehow alternating dc with a pulse?
              Last edited by A.K. Boomer; 03-12-2018, 10:29 AM.

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              • #8
                Originally posted by A.K. Boomer View Post
                .....

                RB my little fuego brushless has three wires all the same size - so is it a three phase ac or somehow alternating dc with a pulse?
                Those are the same thing, really, so the answer to the question is "yes". The DC battery voltage is converted to pulses by the electronics.
                1601

                Keep eye on ball.
                Hashim Khan

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                • #9
                  There is no, or very little significant difference in the design of a BLDC RC or 3ph AC synchronous motor. Just the way it is commutated
                  With BLDC there is only 2 windings energized at any given time, there are 3 pulses/ electrical rotation and the pulses are 120°
                  apart, and PWM switched for varying RPM. The pole count decides how many electrical revolutions/mechanical revolutions there are.
                  In the case of a VFD it produces a pseudo 3 phase signal with varying frequency and the sine waves itself is modulated.
                  You will see RC motors rated in Kv this is not Killo-volts but RPM/volt, (thousand rpm/volt).
                  Max.
                  Last edited by MaxHeadRoom; 03-12-2018, 11:14 AM.

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                  • #10
                    Almost all motors are AC, conventional brushed motors just use a mechanical inverter called a commutator.

                    Pure DC motors and generators exist, but they're rare in application. Michael Faraday demonstrated the first electric motor in 1821 and it was of this pure DC or Homopolar type.

                    Homopolar generators were used to produce very high currents at low voltages for refining metals and in electroplating.

                    Various terms are used to describe brushless motors and many of them are more for marketing than anything else. Assuming a permenent magnet motor, then one definition is that AC means it's driven by a sinusoidal waveform and DC means a trapezoidal waveform.
                    Paul Compton
                    www.morini-mania.co.uk
                    http://www.youtube.com/user/EVguru

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                    • #11
                      Originally posted by EVguru View Post
                      Various terms are used to describe brushless motors and many of them are more for marketing than anything else. Assuming a permenent magnet motor, then one definition is that AC means it's driven by a sinusoidal waveform and DC means a trapezoidal waveform.
                      I have been using BLDC servos for some decades now and can assure you they are DC fed, they are typically fed with a Square PWM type signal, the trapezoidal shape comes from the BEMF being sinusoidal in nature.
                      IOW. back feed them and they show a perfect 3phase generator display.
                      I have had cause to align many with the hall style encoder tracks now fitted to the incremental encoders on the motor, using a D.B. 'scope.
                      The AC 3ph servo is a virtually identical motor but the commutation is different due the power being true 3phase in nature.
                      Max.

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                      • #12
                        The answers are already in but you might like a little more on the ESC (Electronic Speed Control) that feeds them.

                        The output of the ESC uses six power MOSFETs arranged in a three way bridge pattern just the same way you'd see diodes in a car's alternator to convert the 3ph AC from the windings to DC output. But in this case the MOSFETs are used to switch the power from DC back to the signals needed for the motor.

                        Part of the "trick" to it all is that because the ends of all three windings can be connected to either positive or negative that we actually achieve a doubling of the battery voltage at the motor's leads by connecting each tap on a "Y" wound motor first to the positive and then later to the negative. This is another reason why these motors can be so powerful for their size and weight compared to the old brushed motors.

                        The early brushless motors for model use had position sensors. But all the motors right now for model hobby use are sensorless. There's a pretty good basic write up on how both styles work HERE . In particular the first few paragraphs of the sensored and sensorless operation portions start off with nice simple explanations before it gets into the nitty gritty and math.

                        Between the advances in the motors and batteries we are now at a point where for the same weight I can power a model with a brushless outrunner and it's actually lighter for the same weight than a glow engine in all but the most extreme examples of high power racing glow engines. This is based on not only the engine and motor but also the "stuff" that is needed to run it and fly the plane. So in a glow engine situation we have the engine, muffler (in almost all cases), fuel tank, fuel and the onboard battery pack needed to run the radio gear. On the other side we switch all that stuff for a motor, ESC and larger battery pack. Picking a motor, ESC and battery that provides me with a similar power level and flight time at the same sort of flight profile ends up being the same or less weight then the engine with other items. Usually less.

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                        • #13
                          Originally posted by MaxHeadRoom View Post
                          I have been using BLDC servos for some decades now and can assure you they are DC fed, they are typically fed with a Square PWM type signal, the trapezoidal shape comes from the BEMF being sinusoidal in nature.
                          IOW. back feed them and they show a perfect 3phase generator display.
                          I have had cause to align many with the hall style encoder tracks now fitted to the incremental encoders on the motor, using a D.B. 'scope.
                          The AC 3ph servo is a virtually identical motor but the commutation is different due the power being true 3phase in nature.
                          Max.
                          You both are talking about the same thing. Trapezoidal or sinusoidal waveform, made with (square)PWM modulation.
                          BLDC pole shape or whatever trick in mechanical dimensions causes the BEMF waveform to look more trapezoidal whereas sine wave optimized motor BEMF is closer to sine wave lookalike.
                          "Wrong" waveform also works but its not optimal.

                          Most rc toy motor controllers would output "true" 3-phase sinewave output with just a software update.
                          Last edited by MattiJ; 03-12-2018, 12:52 PM.

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                          • #14
                            You can run a stepper motor with low voltage 50 or 60Hz and a small capacitor just like a regular single phase motor.
                            Helder Ferreira
                            Setْbal, Portugal

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                            • #15
                              As for sensored brushless motors, those are from early on, and the sensors usually used a five-conductor ribbon cable along with the three power leads

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