Nice article! I just have a couple comments: First, these aren't induction motors ... induction motors don't have magnets on the rotor. Induction motors are used in more industrial applications. These motors go by a lot of different names - sometimes just "brushless," or BLDC or BL - but probably the most accurate/descriptive name would be PMSM - permanent magnet synchronous motor.
Second, in the section on timing you mention magnets moving past coils creates a current in the coils and that current creates a voltage. This is actually backwards - a moving magnet in a coil creates an emf (voltage) which can cause current to flow (but doesn't always). You can prove this by taking a motor that isn't connected to an ESC and hooking up a multimeter or o-scope across 2 of the leads of a motor and spinning the rotor. You will measure a voltage but there will be no current flowing because the leads aren't connected to anything. The physical law behind this is Faraday's law of Induction. Faraday's law, when applied to brushless motors, basically says that the back-emf is proportional to the rate of change of he flux. The rate of change of flux has to do with the speed of the motor - the faster the motor spins, the higher the rate of change. You can see this on an o-scope as well ... turn the shaft of the motor at 500 RPM and at 1000 RPM and the back-emf will be greater at 1000 RPM.
Third, the angle that the ESC measures is not really virtual. It's a real angle. Motor engineers make a distinction between mechanical angle and electrical angle. Electrical angle = (Mechanical angle) x (pole pairs). 1 mechanical revolution = 360 mechanical degrees. 1 electrical revolution = 360 electrical degrees. 1 mechanical revolution is how far the rotor has to turn in order for it to come back to its mechanical starting position. 1 electrical revolution is how far the rotor has to turn in order for it to come back to its electrical starting position. What this means is that if the starting position is the center of one of the north pole magnets, then 1 electrical revolution is when the shaft rotates far enough so that the next north pole is centered on that starting position.
I would make the argument that these are indeed a type of induction motor, though they are not AC induction motors. They still use the principles of induction to function, though the mechanism is slightly different from the AC induction motors used in industrial applications. The loaded start ability of the AC induction motor being the biggest difference.
On the second point, you're absolutely correct, I inverted my terminology. That's what you get when you're writing too late at night. :D Fixed it. The external drive method that you are describing is actually the most reliable way of measuring the kV of a motor. The byproduct of the BEMF signal being lower at lower revolutions means it is very difficult to start a brushless DC motor spinning due to the difficulty in getting an accurate read on the BEMF signal. Kind of a chicken before the egg issue.
On the third point, the electrical degrees are still not literal degrees of rotation, it depends on the number of poles as to the actual degrees of rotation that is produced by 360 degrees of electrical rotation. In our case with a typical 14 pole brushless motor for instance, as you mentioned 1 electrical revolution is 360 electrical degrees, but that is not literal degrees it is only the pole to pole, so it works out to about 51.43 literal degrees of rotation per 360 electrical degrees. On a 12 pole motor such as an 11XX class, that would would work out to 60 degrees of physical rotation. In that way it is a virtual number, more referring to the way it would be plotted on a graph as a sine wave than actual rotational degrees. On a related note if we switch from a 3 phase to a different number of phases, the degrees per phase of the BEMF signal would change accordingly, it may be less than 60 degrees per phase. That means in some motors the timing advance may be measured differently. You don't typically see that arrangement in brushless motors, unless you're looking at steppers, and they don't use the BEMF wave in their typical application.
Eric:
Sep 23, 2017 at 02:16 AM
Nice article! I just have a couple comments: First, these aren't induction motors ... induction motors don't have magnets on the rotor. Induction motors are used in more industrial applications. These motors go by a lot of different names - sometimes just "brushless," or BLDC or BL - but probably the most accurate/descriptive name would be PMSM - permanent magnet synchronous motor.
Second, in the section on timing you mention magnets moving past coils creates a current in the coils and that current creates a voltage. This is actually backwards - a moving magnet in a coil creates an emf (voltage) which can cause current to flow (but doesn't always). You can prove this by taking a motor that isn't connected to an ESC and hooking up a multimeter or o-scope across 2 of the leads of a motor and spinning the rotor. You will measure a voltage but there will be no current flowing because the leads aren't connected to anything. The physical law behind this is Faraday's law of Induction. Faraday's law, when applied to brushless motors, basically says that the back-emf is proportional to the rate of change of he flux. The rate of change of flux has to do with the speed of the motor - the faster the motor spins, the higher the rate of change. You can see this on an o-scope as well ... turn the shaft of the motor at 500 RPM and at 1000 RPM and the back-emf will be greater at 1000 RPM.
Third, the angle that the ESC measures is not really virtual. It's a real angle. Motor engineers make a distinction between mechanical angle and electrical angle. Electrical angle = (Mechanical angle) x (pole pairs). 1 mechanical revolution = 360 mechanical degrees. 1 electrical revolution = 360 electrical degrees. 1 mechanical revolution is how far the rotor has to turn in order for it to come back to its mechanical starting position. 1 electrical revolution is how far the rotor has to turn in order for it to come back to its electrical starting position. What this means is that if the starting position is the center of one of the north pole magnets, then 1 electrical revolution is when the shaft rotates far enough so that the next north pole is centered on that starting position.
quadmcfly:
Sep 25, 2017 at 04:38 PM
I would make the argument that these are indeed a type of induction motor, though they are not AC induction motors. They still use the principles of induction to function, though the mechanism is slightly different from the AC induction motors used in industrial applications. The loaded start ability of the AC induction motor being the biggest difference.
On the second point, you're absolutely correct, I inverted my terminology. That's what you get when you're writing too late at night. :D Fixed it. The external drive method that you are describing is actually the most reliable way of measuring the kV of a motor. The byproduct of the BEMF signal being lower at lower revolutions means it is very difficult to start a brushless DC motor spinning due to the difficulty in getting an accurate read on the BEMF signal. Kind of a chicken before the egg issue.
On the third point, the electrical degrees are still not literal degrees of rotation, it depends on the number of poles as to the actual degrees of rotation that is produced by 360 degrees of electrical rotation. In our case with a typical 14 pole brushless motor for instance, as you mentioned 1 electrical revolution is 360 electrical degrees, but that is not literal degrees it is only the pole to pole, so it works out to about 51.43 literal degrees of rotation per 360 electrical degrees. On a 12 pole motor such as an 11XX class, that would would work out to 60 degrees of physical rotation. In that way it is a virtual number, more referring to the way it would be plotted on a graph as a sine wave than actual rotational degrees. On a related note if we switch from a 3 phase to a different number of phases, the degrees per phase of the BEMF signal would change accordingly, it may be less than 60 degrees per phase. That means in some motors the timing advance may be measured differently. You don't typically see that arrangement in brushless motors, unless you're looking at steppers, and they don't use the BEMF wave in their typical application.