CIRCUIT DESIGN
BLDC Motor Control
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Overview
The page is dedicated to how to control a brushless DC (BLDC) motor in an embedded system. This includes control methods for both trapezoidal and sinusoidal wound BLDC motors. There are many different ways to control a BLDC motor, from simple halleffect based switching, to complex encoder based fieldorientated control with spacevector modulation (if you have no idea what these mean, don’t worry, read on).
Acronyms And Terminology
Subscript  Parameter  Units (metric)  Units (imperial) 

\(T_C\)  Continuous Torque  Nm  ozin 
\(T_{PK}\)  Peak Torque  Nm  ozin 
\(T_{CS}\)  Continuous Stall Torque  Nm  ozin 
\(T_F\)  Friction Torque  Nm  ozin 
\(I_C\)  Continuous Current  A  A 
\(I_{PK}\)  Peak Current  A  A 
\(N_{nl}\)  NoLoad Speed  rad/s  rpm 
\(P_T\)  Rated Power  W  W 
\(V_T\)  Terminal Voltage  V  V 
\(P_I\)  Input Power  W  W 
\(P_O\)  Output Power  W  W 
\(K_T\)  Torque Constant  Nm/A  ozin/A 
\(K_E\)  Back EMF Constant  V/rad/s  V/krpm 
\(K_M\)  Motor Constant  Nm/sqrt(W)  ozin/sqrt(W) 
\(J_R\)  Rotational Inertia  gramcm^2  ozins^2 
Motors
Important Parameters:
 Torque Constant: An important parameter which relates the torque to the DC current draw. All other things being equal, higherquality motors will have a higher torque constant.
 Rated Speed: Most are rated in the range of 30006000rpm. The motor can be driven at slightly higher speeds than those rated by a technique that is called ‘flux weakening’
 Rated Current
 Cogging torque  This is the maximum torque the motor has when it is not being driven. This causes the rotation of the axle to feel ‘bumpy’ if rotated by hand when it’s not being driven. For smooth operation at low speeds (< 100rpm), the cogging torque has to be small (or nothing at all). You shouldn’t be able to feel this ‘bumpy’ phenomenon on motors with small cogging torques.
 Mechanical Time Constant: The time for the motor to spin up to full speed at rated voltage. Normally between 410ms. High performance motors can have a time constant as low as 1ms to speed up to 30,000rpm!
 Electrical Time Constant
 Winding Inductance: High precision motors typically have lower inductance. The winding inductance has the useful property of limiting the rate of change of current through the motor, and is what allows you to use PWM to produce smooth current. The lower the inductance, the higher the PWM needed to the same current ripple.
 Number of pole pairs. The number of pole pairs is equal to the number of electrical revolutions per mechanical revolution. Typical BLDC motors have a number of pole pairs varying from 1 to 5. If the motors datasheet instead states the number of poles, this is just 2x num. pole pairs. Note that if the motor has more than 1 pole pair, you cannot orientate the motor to a known mechanical position without some type of feedback (halleffect, encoder).
The 3 phases of a BLDC motor are usually labelled either A, B, C or U, V and W.
Cogging torque is due to the variation in airgap permanence or reluctance of the stator teeth and slots above the magnets as the motor rotates.
Ripple torque is the torque produces by the interaction between the stator and rotor MMF. Ripple torque is mainly due to fluctuations in the field distribution.
The motor windings can be wound to either give trapezoidal or sinusoidal feedback, which relates to the control methods mentioned further down on this page. As quoted by Shiyoung Lee, Ph.D., A COMPARISON STUDY OF THE COMMUTATION METHODS FOR THE THREEPHASE PERMANENT MAGNET BRUSHLESS DC MOTOR),
The simulation results verify that mismatch of the backEMF waveform and commutation method produces ripple rich torque. Therefore, the BLDC motor and trapezoidal(sixstep) commutation and the PMSM and sinusoidal commutation are the most desirable combination to produce minimum torque ripple.
So it pays to get the right motor for the right job!
Trapezoidal vs. Sinusoidal
There are two standard ways of winding the coils. One is to produce a trapezoidal shaped BEMF, the other produces a sinusoidal shapes BEMF. Sinusoidal motors have lower torque ripple (less vibration, mechanical stress e.t.c e.t.c), but suffers from higher switching losses and greater drive complexity than a trapezoidal motor. For this reason trapezoidal motors are very common.
In a sinusoidal motor, current travels through all three windings at any point, while in a trapezoidal motor, current only flows through 2 of the 3 windings. Sinusoidal motors come under a number of names, including PMSM (permanent magnet synchronous motor), an AC servo motor, or BLAC (brushless AC, a term used by Atmel). Trapezoidal designed motors go under the name BLDC (brushless DC).
Positional Sensing
Because there are no brushes to switch the current in the windings (commutation), the position of the rotor relative to the stator needs to be known so that the current can be switched externally. There a three main methods for detecting where the rotor is:
 HallEffect Sensors
 An Encoder
 Zerocrossing (aka sensorless)
HallEffect sensors output a voltage relative the magnetic field strength (or more technically, the magnetic flux density). Three of them are spaced 120° apart from each other and designed so that their output voltage changes rapidly as the phase coils require switching. This can make the switching electronics easy to implement. Be careful, some halleffect sensors can be opendrain, even though the motor’s datasheet suggest that the output is fully driven!
The encoder method uses, well, an encoder attached to the axle to determine rotor position. This is more complex than the halleffect method as the encoder output requires decoding. The encoder is typically of the incremental quadrature type, which requires a counter to count the pulses and a phase detection logic to determine the direction. This feedback method can also suffer from glitches which causes the encoder count to drift from the correct value. The PSoC microcontroller has a very nice quadrature decoding component with builtin glitch filtering.
Zerocrossing has become popular in recent years due to the fact it requires no sensors, making it cheap to implement. It is the method of measuring the voltage of the floating winding during operation (1 winding is always undriven), to determine the position of the rotor. One disadvantage of this method it does not work below a minimum speed (because the voltage is too small).
Trapezoidal Control
A common example of a trapezoidal (or block) commutation cycle of a BLDC motor with hallsensor feedback is shown in the below table. This will drive the motor in one particular direction. This can be used to form a LUT for quick hardware/software commutation.
Commutation Table  

Sensor Output  Driver Input  
HS1  HS2  HS3  A  B  C 
0  0  1  X  HI  LO 
0  1  1  HI  X  LO 
0  1  0  HI  LO  X 
1  1  0  X  LO  HI 
1  0  0  LO  X  HI 
1  0  1  LO  HI  X 
where H1, H2, and H3 are the halleffect sensors, and A, B, and C are the motor phases of a 3phase starconnected BLDC motor. Note that the either the highside, lowside, or both driver inputs can be modulated with a PWM signal to provide speed control.
The schematic below shows the hardware used in a PSoC 5 microcontroller to perform halleffect based trapezoidal commutation. With the correct mux selects, this control method can run completely from hardware and be completely processor independent (when running at a constant dutycycle).
Sinusoidal Control
The benefits:
 Smoother operation/less torque ripple than trapezoidal
 Greater efficiency/less heat dissipation
 Able to run the motor at slower speeds
The disadvantages:
 Complex control
 Embedded firmware uses more flash/ROM space
 More processing power used
 Slightly lower maximum torque (although thirdharmonic injection can reduce this)
 Halleffect feedback becomes insufficient at low speed with varying loads, and optical encoders are preferred
 Back EMF feedback is not possible
Sinusoidal control (also known as voltageoverfrequency control) is more complex than trapezoidal techniques, but offers smoother operation and better control at slow speeds.
Lookup tables (LUTs) are recommended over using the sin()
function due to speed issues. The sin()
function in C is computationally intensive and can easily create delays that effect the performance of the control algorithm. The implementation of the sin()
is platform dependent, but for example, using the GCC compiler on a PSoC 5 CortexM3 processor, calculating the sin()
function three times (once for each phase), took approximately 24,000 clock cycles. With a processor running at 48MHz, this is about a 500us delay. Considering a 4pole BLDC motor spinning at 6000rpm takes about 830us to move between commutation states. In each commutation cycle you want at least 6bit resolution (64 PWM changes), and as you can see, the delay in calculating sin()
is far too large.
With a LUT that stores floats, and a small amount of float multiplication (no divide) on a embedded processor that does not have floating point hardware support (such as the ARM CortexM3), you could expect the lookup and assign process to take around 5001000 clock cycles (maybe 20us at 48MHz). This is a big reduction over using the sin()
function!
Phase displacement. Sinusoidal control requires three PWM signal’s, preferably dualoutput with adjustable deadtime for synchronous switching.
Example LUT Code
The following code is an example of how to create a LUT in the C programming language. This function calculates the LUT values at runtime, using the sin()
function. In space constrained applications it may pay to precalculate the values and save them directly into flash. More help with the C language can be found
here.


ThirdHarmonic Injection
With a pure phaseneutral sinusoidal drive, the maximum phasetophase voltage is only roughly \(0.86V_{bus}\)
. This can be improved by ‘injecting’ the sine wave with the third harmonic. The first thing you may think is, won’t this disrupt my nice and smooth sine wave control? Well, no, because as it happens, the third harmonic is inphase with every winding (which are 120 apart), and since it is applied to every winding, the phasetophase waveform does not change. It does however flatten the phaseneutral waveform, making the PWM become undermodulated. You can then scale this back up to fullmodulation, and gives approximately a 16% phasetophase voltage increase.
To implement thirdharmonic injection, all you have to do is add the thirdharmonic to the sinewave LUT. The thirdharmonic has an amplitude that is 1/6 of that of the fundamental (your original sine wave). You’ll notice that the maximum value in the LUT has decreased. At this point, scale up all the values in the LUT so they use the fullrange again.
Sensor Field Orientated Control (FOC)
The benefits:
 You can control motor parameters which relate directly to it’s physical behaviour (the variables make sense to a human)
 Angle can be determined from phase currents (usually using a sliding state estimator), no encoder needed
The disadvantages:
 Greater control complexity than trapezoidal or sinusoidal
 Requires fast processor to execute necessary maths
 Requires phase current to be measured (usually with lowside current sense resistors and an ADC)
 Requires the tuning of three PID loops (usually)
Sensor field orientated control (also called vector control) is a permanent magnet motor control method that allows one to regulated both the speed and torque independently from each other. However, this form of control is more computationally intensive than trapezoidal or sinusoidal. Part of the complexity is due to the need to be able to measure both the current going through the windings, while both trapezoidal and sinusoidal only requires positional feedback. Clark (aka alphabeta) and Park transforms (aka dq) have to be calculated with the phase currents.
The Clark Transformation (alphabeta)
The Clark transformation (also called the \(\alpha \beta\)
transformation, and occasionally called the Concordia transformation, but I don’t know why!) is the projection of three separate sinusoidal phase values onto a stationary 2D axis.
The unsimplified Clark transformation equation is shown below:
$$ I_{\alpha\beta\gamma} = TI_{abc} = \frac{2}{3} \begin{bmatrix} 1 & \frac{1}{2} & \frac{1}{2} \\ 0 & \frac{\sqrt{3}}{2} & \frac{\sqrt{3}}{2} \\ \frac{1}{2} & \frac{1}{2} & \frac{1}{2} \end{bmatrix} \begin{bmatrix} I_a \\ I_b \\ I_c \end{bmatrix} $$
where:
\(I_a\) = current in motor winding A
\(I_b\) = current in motor winding B
\(I_c\) = current in motor winding C
\(I_{\alpha\beta\gamma}\) = the Clark transformed currents
We are fortunate that when using a starconnected BLDC motor (most are!), \(I_c\)
is 0, so that we can simplify the equation to:
$$ I_{\alpha\beta} = TI_{ab} = \begin{bmatrix} 1 & 0 \\ \frac{1}{\sqrt{3}} & \frac{2}{\sqrt{3}} \end{bmatrix} \begin{bmatrix} I_a \\ I_b \end{bmatrix}$$
If you want the code to do the Clark Transformation (written in C++, and designed for embedded applications), check out the GitHub repository CppClarkTransformation.
The Park Transformation (dq)
Park transformation is a projection of three separate sinusoidal phase values onto a rotating 2D axis. The rotating axis (d, q) of the Park transformation rotates at the same speed as the rotor. When the projection occurs, the currents \(I_d\)
and \(I_q\)
remain constant (when the motor is at steadystate). It just so happens that \(I_d\)
controls the magnetizing flux, while \(I_q\)
controls the torque, and since both parameters are separate, we can control each individually!
The Park transformation equation is shown below:
where:
\(I_a\) = current in motor winding A
\(I_b\) = current in motor winding B
\(I_c\) = current in motor winding C
\(I_{dqo}\) = the Park transformed currents
If you want the code to do the Park Transformation (written in C++, and designed for embedded applications), check out the GitHub repository CppParkTransformation.
The Control Loop
The following picture shows the control architecture for a PMSM motor controlled with a PSoC microcontroller.
It is standard practice to set \(I_q\)
to some value depending on the torque/speed required, while keeping \(I_d\)
zero. This is because \(I_d\)
does nothing to help make the motor spin, and just wastes electrical energy as heat. However, there is a technique called flux weakening, and this is done by making \(I_d\)
negative. It will allow the motor to spin faster than it’s rated speed, in a zone called ‘constant power’. I have had good experiences at using this to squeeze more RPM out of BLDC motors that weren’t requiring much torque. A good method is to make \(I_d\)
proportional to \(I_q\)
(but always negative, no matter what sign \(I_q\)
is, which essentially gives you a fixed drive is angle which is over 90. You can use this equation to work out the proportion of \(I_q\)
that \(I_d\)
has to be for a certain angle.
$$r = \tan (\theta)$$
where:
\(r\) = ratio (\(I_d = rI_q\))
\(\theta\) = drive angle
FOC typically requires three PID loops, one mediumspeed loop for controlling the velocity and two highspeed loops for controlling \(I_d\)
and \(I_q\)
.
The are two methods to measure the phase currents. The first involves just one current sense resistor on the DC return path from the motor, while the second requires three currentsense resistors, one on each leg of the threephase bridge controlling the motor.
The dynamic equations for FOC linking voltages, currents and torques are:
$$ v_{q} = r_{s}i_{q} + L_{q}\frac{di_{q}}{dt}  w_{e}L_{d}i_{d} + w_{e}\lambda_{f} \\ \\ v_{d} = r_{s}i_{d} + L_{d}\frac{di_{d}}{dt}  w_{e}L_{q}i_{q} \\ \\ T_{m} = \frac{3}{2}\frac{P}{2}[\lambda_{f}i_{q} + (L_{d}  L_{q})i_{d}i_{q}] \\ \\ $$
During constant flux operation (which is normal operation, all the flux is created by the permanent magnets), \(I_d\)
becomes 0. This simplifies the torque equation into that similar to a brushed DC motor as:
Equations
$$ \text{Field Orientated Control:} \\ \\ V_{abc} = RI_{abc} + \frac{d}{dt}\Phi_{abc} \text{LenzFaraday model}\\ \\ R = \text{statoric resistance} V_{abc} = \begin{bmatrix}v_a & v_b & v_c \end{bmatrix}^T = \text{statoric voltages,} \\ \\ I_{abc} = \begin{bmatrix} i_a & i_b & i_c \end{bmatrix}^T = \text{statoric currents,} \\ \\ \Phi_{abc} = \begin{bmatrix} \Phi_a & \Phi_b & \Phi_c \end{bmatrix} = \text{global fields} \\ \\ $$
PWM Frequency
So what PWM frequency do I choose? The PWM frequency is a tradeoff between torque ripple and switching losses. Most controllers use a frequency between 1020kHz. If you want to reduce the audible noise from the motor, try using a frequency between 1720kHz (outside the audible range of most adults).
Deadtime
Deadtime is only important if you are going synchronous rectification (i.e. switching on the MOSFET rather than letting the body diode conduct when current is flowing through it in the reverse direction). In this case, during the offtime of the PWM signal, the complementary (on the same leg of the bridge) MOSFET(s) to those that are being switched are turned on. This allows the reverse current that would be usually be flowing through the inherent body diodes to instead flow through the MOSFET, resulting in lower heat losses (MOSFETs have a lower on ‘resistance’ than diodes). Deadtime between turning on the leg MOSFETs off and the other on is needed to prevent shootthrough.
Note that if the dutycycle of the PWM causes either the on or offtime to be less than the deadtime, it will appear as if the PWM stops working. This will happen at it’s two duty cycle extremes (at the top and bottom of the sine wave). Don’t be alarmed, this is normal and totally acceptable behaviour.
OpenLoop Feedback
Fully openloop control (no halleffect, encoder, or BEMF feedback) can be used in applications where torque ripple and efficiency are not important. The load has to be also relatively constant for this to work.
ClosedLoop Voltage Feedback
Closedloop voltage control is when feedback is used to commutate the currents, and the PWM duty cycle is varied from 0100% to control the speed of the motor (but with no velocity control). With this method, you have full control of the motors speed from 0100%, but the motor’s speed may vary as the load changes. As the load is increased, the current will increase, and the rpm of the motor will drop. It is called ‘voltage’ control because you can set the PWM duty cycle, which determines the voltage across the windings.
ClosedLoop Velocity Feedback
Closedloop velocity control involves controlling the rpm of the motor to a desired speed. The maximum currents must be taken into consideration when doing constant velocity control, as they may get very large if the motor stalls.
Feedback Can Come From:
 Halleffect sensors (o.k.)
 Optical encoder (better, especially at slow speeds)
 Back E.M.F (good, but not at slow speeds)
Control Methods:
 PID PWM Control (soft or hardchopping)
ClosedLoop Current Feedback
Closedloop current control will give you a constant output torque. This means that the motor will slow down when the load is increased, and speed up when it is decreased.
Feedback Can Come From:
 Lowside current sense resistors and singlesided ADC
 Highside current sense resistors and a differential ADC
Control Methods:
 PID PWM Control
Sliding Mode Observer
Used to provide estimates on the position and speed of the rotor when not using encoder or hallbased feedback.
Controller Bandwidth
Controller bandwidth is an important term which is used to define the update rate or speed of a particular control section for a motor. It is usually referred to when talking about the “Current Controller Bandwidth”, a control loop that measures the phase currents of a brushless motor and updates the control variable accordingly (usually with a PID loop). This is commonly used with FOC control and SVM. Slow current controller bandwidths are around 300Hz, while faster bandwidths are in the 1050kHz range (which updates the duty at the same speed as the PWM itself!). Higher current controller bandwidths are needed for lower torque ripple and slower motion (150rpm).
Code Execution Time
The execution time of the code which controls the PWM duty cycle is critical, especially when using sinusoidal or sensor field orientated control. Here are the best ways of making sure the code runs quickly
 Do not use the
sin()
function, instead use a LUT  Do not use any division operators (instead, convert to a double, and multiply by the inverse, if inverse is already known or can be computed more quickly than a normal division) e.g.
number1*(1/number2)
 Using fixedpoint arithmetic rather than floats or doubles.
I have an opensource fixedpoint library designed for running on embedded systems.
 Try to minimise function calls. Use the inline parameter if possible
 Precalculate any maths that can be done beforehand
 Make sure the compiler optimises for speed, not space
See the C Programming page for more help on this subject.
BLDC Maths
Some useful equations…
$$ \text{Basic Motor Control:} \\ \\ v_{rot} (Hz) = \frac{rpm}{60} \\ \\ \text{Commutation Cycles Per Second} = \frac{v_{rot}}{6} \\ \\ \text{num. electrical cycles per mechanical cycle} = \text{num. pole pairs} = \frac{\text{num. poles}}{2}\\ \\ \\ \\ \text{Sinusoidal Control:} \\ \\ V_{phaseneutral} = \frac{V_{bus}}{2} \\ \\ V_{phasephase} = \frac{\sqrt{3}}{2}*V_{bus} \\ \\ V_{thirdharmonic} = \frac{1}{6}V_{fundamental} \\ \\ $$
Flux = Magnetic field lines per unit area
Related ICs
FSB50825US  Fairchild Smart Power Module (SPM)
 Package: SPM23
 Shootthrough Protection: No
 Onboard Logic: None (microcontroller required)
Standard Enclosures
The NEMA17 and NEMA23 are two common sizes that BLDC motors come in. This designation defines the mounting hole pattern for the motor.
The NEMA17 mounting hole dimensions are shown below. The dimensions are in millimeters.
The NEMA23 mounting hole dimensions are shown below. The dimensions are in inches.
External Resources
One of the best documents that I’ve seen about BLDC motor theory is James Robert Mevey’s “Sensorless Field Orientated Control Of Brushless Permanent Magnet Synchronous Motors”. Click [here](/docs/James Mevey  Sensorless Field Orientated Control Of Brushless Permanent Magnet Synchronous Motors.pdf) to download a local version.
The second best is MicroChips AN885  Brushless DC (BLDC) Motor Fundamentals.
And then once you’ve read those, check out AN857  Brushless DC Motor Control Made Easy. This links the theory with a practical implementation of the hardware and firmware, supported with examples.
A competing document is NXP’s AN10661  Brushless DC Motor Control Using The LPC2141. For the visual learners, this one has more colourful pictures than Microchip’s, but it has less depth.
RoboTeq ( www.roboteq.com) have some very cool BLDC motor controllers. They are highly configurable, offer a huge number of features, and can drive very powerful motors (+100A).
If your a fan of animations (who isn’t, visual learning rocks!), then check out Nanotech’s Stepper/BLDC animations ( http://en.nanotec.com/support/tutorials/steppermotorandbldcmotorsanimation/). A flash program which allows you visually watch the motors operation as you step through commutation cycles. Also let’s you change the type of motor!
If you want to delve into the arena of Sensor Field Orientated control, the Atmel’s AVR32723  Sensor Field Orientated Control For Brushless DC Motors with AT32UC3B0256 should be the first port of call. “ Brushless Permanent Magnet Servomotors” is a good article explaining the differences between BLDC motors and PMSMs, as well as equations for FOC.
Authors
This work is licensed under a Creative Commons Attribution 4.0 International License .
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Tags
 BLDC
 motors
 trapezoidal
 sinusoidal
 FOC
 PWM
 Clark transformation
 Park transformation
 control loops
 deadtime
 feedback
 closedloop
 maths
 enclosures
 equations
 encoders