Common misconceptions with brushless motors

Common misconceptions with 'hobbyist level' brushless motors

    1. A low Kv motor produces more torque than a high Kv motor

As is discussed in more detail in this post, a low Kv motor will produce more torque for a given current (higher torque constant) than an otherwise identical high Kv motor. However, since the total amount of copper used to wind the motors remains the same, both motors will produce an equal amount of torque for a given amount of input power (equal motor constant). Therefore, both motors will become equally hot for a given torque output and so their thermally limited torque density is the same.


    2. Motor current is equal to the current drawn from a power source

Motor controllers act as power converters. The take a relatively high voltage power source (e.g. a battery) and a moderate current and convert it to a relatively low voltage and high current for use by an electric motor. They can do this because the resistance, and therefore voltage drop, across a typical  'hobbyist level' brushless motor is generally very low (e.g. < 0.1 Ohm). Therefore, at a standstill (i.e. 0 RPM), the current supplied to the motor will be far higher than the current supplied to the motor controller. For example, a brushless motor with a winding resistance of 0.1 Ohm will dissipate 10W when supplied with 10 A (P = I^2R). This same motor would only draw ~0.417 A from a 24 V power source, assuming no losses in the motor controller and the motor is not rotating.
When a motor is rotating then you must also take into considering the voltage required to overcome the back EMF produced by the motor. Refer to this post for more details.

    3. A 44V (12s equivalent) motor running off a 22V (6s equivalent) power source will produce half as much torque
As is described in more detail in the point above, a 'hobbyist level' brushless motor will generally have a very low (e.g. < 0.1 Ohm) winding resistance. Therefore, only a small voltage is required to produce a large current in the motor windings. Since the torque output of a motor is roughly linear with the winding current the full torque output of the motor can be achieved at even modest voltages. Changing the voltage of your power source will, of course, impact the base speed (max RPM for a given supply voltage) of your motor.

Note that gimbal motors generally have a much higher winding resistance than other motor types and so the above statement may not apply.


   4. When plotting torque vs speed, hobbyist brushless motors have a 'constant power' and a 'constant torque' region

In normal use, the torque produced by a motor is approximately proportional to the current supplied to its windings. Therefore, so long as the same current can be maintained in the windings the motor will continue to produce the same torque regardless of its rotational speed. That is until the motor reaches its base speed. At a motors base speed, the back EMF generated by the motor starts to limit how much current can be supplied to the windings. Therefore the motor torque must be reduced to further increase its speed, reducing the motors power output in the process. This reduction in motor torque can continue until it falls to zero and the no-load speed of a motor for a given supply voltage is reached. For a far more detailed description see this post.


 The relationship may look quite different from the more familiar 'constant torque' and 'constant power' chart shown below.

Note however that a 'constant power' region cannot be achieved with 'hobbyist' grade brushless motor controllers. Operating a motor above its base speed at a constant power requires the use of motor control technique called field weakening, whereby the current vector is adjusted as the rotational speed is increased so as to counteract the generated back EMF. This has the same effect as lowering the field strength of the magnets seen by the stator. See this clip for an example of 'physical' field weakening.

To my knowledge, there are no 'hobbyist level' brushless motor controllers which support field weakening. Therefore all 'hobbyist level' brushless motors are operated with constant torque up to their base speed, and reduced power to their no-load speed.

Last updated: 12.05.19

2 comments:

  1. To nit-pick a little bit: While as far as I know there are no hobby controllers that field weaken, that's not quite the same as saying they all operate in the constant torque region. Even without field weakening you can operate above base speed in the voltage-limited region - you just won't be able to achieve maximum current, because of the voltage generated by back-emf and inductance.

    Anyway, good to find another motor enthusiast!

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  2. Hi Ben

    Feel free to nit-pick away. I’m aiming for everything on here to be as accurate as possible and so this helps a lot.

    From your comment it sounds like I don’t fully understand how things work when operating above base speed. Looks like I have some more reading to do. I will update this page once I have a better handle on the topic.

    Thanks!

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