Machines ranging from simple devices such as drills to complex equipment such as industrial robots all make use of brushless DC motors that convert electrical energy into rotational motion. Brushless DC motors, also referred to as BLDC motors, offer many advantages over their brushed counterparts. BLDC motors provide higher efficiency and require lower maintenance, which is why they have replaced brushed motors in many applications.
Both types of motors operate based on a similar principle: rotational motion is generated through the attraction and repulsion of magnetic poles produced by permanent magnets and electromagnets. However, their control methods differ significantly. BLDCs require a complex controller to convert DC power to three-phase voltage, whereas brushed motors can be controlled by regulating the DC voltage.
Brushed DC motors
- High maintenance due to worn brushes
- Low efficiency
- Easy control (DC voltage)
Brushless DC motors (BLDC)
- Low maintenance
- High efficiency
- Complex control (DC to AC)
Traditional Brushed DC Motors
In a brushed DC motor, the DC current passing through the coil windings of the rotor generates an electromagnet with the poles as shown in the animation below. These rotor magnetic poles interact with the poles of the fixed permanent magnet, called the stator, to make the rotor spin. The motor keeps spinning through a process called commutation:
- After every half turn, the rotor poles need to be flipped by switching the polarity of the current in the coil windings.
- This switching of current polarity is called commutation.
- Commutation occurs mechanically: Every half turn, electrical contacts called brushes complete a circuit to the other half of the commutator on the rotor as it spins.
- Due to this physical contact, brushes wear out over time, preventing the motor from working.
Brushless DC Motors
BLDC motors overcome the shortcomings of brushed motors by replacing mechanical commutation with electronic commutation. To better understand this concept, it is useful to take a closer look at the BLDC motor structure.
A BLDC motor is a kind of flipped version of a brushed motor. The permanent magnets are installed in the rotor, whereas the coil windings become the stator.
Motors can have different magnet arrangements, including different numbers of stator windings and rotor pole pairs, as shown in the following animation.
A similarly structured motor is the permanent magnet synchronous machine, or PMSM. The following section explains the differences between BLDC and PMSM motors and the control algorithms used to operate them.
BLDC motors and PMSMs are similarly structured. Both have permanent magnets in the rotor and are defined as synchronous motors. In a synchronous motor, the rotor is synchronized with the stator magnetic field; that is, the rotor turns at the same speed as the stator magnetic field.
A key difference between BLDC motors and PMSMs is the shape of their back electromotive force (EMF). When an electrical motor rotates, it also acts as a generator, inducing a voltage in the stator that opposes the voltage driving the motor. This back EMF is an important motor characteristic because its shape influences the control method used to achieve optimal performance.
Due to their design, BLDC motors have a trapezoidal back-EMF shape and are commonly controlled using trapezoidal commutation.
BLDC
- Trapezoidal back EMF
- Controlled by trapezoidal commutation
PMSMs have a sinusoidal back-EMF shape and typically use field-oriented control.
PMSM
- Sinusoidal back EMF
- Typically use field-oriented control
Sometimes the terms PMSM and BLDC are used interchangeably in the motor control community, which may cause confusion about their back-EMF profiles. For this discussion, BLDC motors will strictly be machines with a trapezoidal back EMF.
An easy way to observe the back-EMF shape of a motor modeled in Simscape Electrical™ is to simulate it.
The image above shows a simulation of a single pole-pair BLDC motor with open-circuit terminals, meaning no current flows through the coils. When the rotor is driven, the motor acts like a generator. Measuring the voltage at phase A over time shows the shape of the resulting back EMF. The voltage scope shows that the BLDC motor produces a trapezoidal back-EMF waveform, with regions where the voltage remains constant.
The following tutorial demonstrates how to model and simulate a BLDC motor with Simscape Electrical using a setup like the one shown above.
You can model a BLDC motor using Simscape Electrical and investigate its back-EMF profile.
To better understand BLDC motor behavior when an external voltage is applied, consider the configuration shown earlier, in which the rotor has a single pole pair and the stator consists of three coils spaced 120 degrees apart. The coils, referred to as phases A, B, and C, are energized by passing a current through them. The north pole of the rotor is shown in red; the south pole is shown in blue.
Initially, none of the coils are energized and the rotor is stationary. Applying voltage across two phases, A and C in this example, generates a combined magnetic field along the dashed line. The rotor then turns to align itself the stator magnetic field, as shown in the animation.
There are six possible switching states, as shown below. By commutating two phases at a time, the stator magnetic field rotates, causing the rotor to turn to the corresponding position. The animation shows the rotor angle relative to the horizontal axis, with six possible rotor alignments spaced 60 degrees apart.
By commutating the appropriate phases every 60 degrees, the motor continues to rotate, as shown in the following animation. This technique is known as six-step commutation, or trapezoidal control.
Additional pole pairs can be added to these motors, requiring more frequent commutation. To properly commutate the motor at the right times with the correct phases, the controller needs to know the rotor position, which is typically measured using Hall sensors.
The arrows represent the relative magnetic forces, with the arrow thickness indicating field strength. The two identical poles repel each other, making the rotor turn counterclockwise. At the same time, the opposite poles attract each other, adding torque in the same direction.
Once the rotor completes 60 degrees of rotation, the next commutation occurs.
Superimposing the stator magnetic field discussed earlier on the animation shows how commutation keeps the rotor from aligning with the stator magnetic field. Instead, the rotor continuously follows the rotating field.
There are two reasons for switching phases in BLDC motors in this sequence. First, if the rotor and stator magnetic fields align, the motor produces zero torque, which is unfavorable. Second, maximum torque occurs when the fields are 90 degrees apart. The goal, therefore, is to keep the angle between the fields as close to 90 degrees as possible.
However, with six-step commutation, the angle cannot remain at 90 degrees continuously. Instead, it varies between 60 and 120 degrees, as shown in the following animation. The variation results from the discrete switching used in trapezoidal control. More advanced techniques, such as field-oriented control commonly used with PMSMs, as discussed before, can more precisely maintain a 90-degree angle between the stator and rotor magnetic fields.
To control the phases during six-step commutation, a three-phase inverter directs DC power to the motor phases, switching between positive (red) and negative (blue) current. To supply positive current to a phase, the corresponding high-side switch is turned on. To supply negative current, the low-side switch is turned on.
Following the switching pattern sequence described above keeps the motor rotating while maintaining the angle between the rotor and stator magnetic fields between 60 and 120 degrees. Motor speed can be varied by adjusting the applied voltage. Alternatively, pulse-width modulation (PWM) can be used to vary motor speed without changing the DC supply voltage.
