Fundamentals of Motor Control

Motor is widely used in many electronic applications, so they are everywhere. In our home, we have many examples, such as fans, hair dryers, heaters, kitchen appliances and so on. If we consider the car again, we will soon find that there are different types of motors in the car: heating system, radiator cooling fan, electric window, electric rear view mirror, seat control device, etc.

Based on the "For dummies" series of e-books [1] published by Qorvo, this paper introduces the basic concepts that each designer, manufacturer or student must master when facing the application of motor control.

Brushed and Brushless Motor

There are different types of motors in the market, and the designer must select them according to the technical and economic requirements of the specific application. The main categories of motor are as follows: brush, brushless (also divided into BLDC and PMSM), induction, step.

Figure 1 shows the four types of motors and summarizes the main advantages and disadvantages of each.

Figure 1: Four main types of motors (:[1])

Figure 1: Four main types of motors (:[1])

For various applications, two closely related brushless motors, BLDC and PMSM, are becoming more and more popular. These motors do not require a brush or commutator to make them more efficient than brushed motors and significantly extend their life.

The brush/commutator interface in a brushed motor generates commutation, which is the process of switching the current in the phase to generate a rotating magnetic field, thereby causing motion. This interaction causes friction and arcs, which are both undesirable.

BLDC motors and PMSMs use electronically generated rotating magnetic fields to eliminate brushes and commutators. The voltage and current supplied to the phase are modulated using an external circuit designed to accomplish this task.

Although more complex, BLDC motor and PMSM have significant advantages over traditional brush motors. Their electronic commutation technology is more reliable, smaller, lighter, quieter and 20% to 30% more energy efficient than brushed motors operating at the same speed.

In brushed motors, the windings are on the rotator (rotation), while in brushless motors, they are on the stator (stationary). Brushes are not required because of the arrangement of permanent magnets and windings in the motor. An electronic controller is required to control the current flowing to the BLDC motor or PMSM stator coil.

Compared with AC induction motors, BLDC and PMSM motors can achieve speed control, are more suitable for variable speed applications, and have excellent speed and torque characteristics.

Motor Controller

Since BLDC motors and PMSMs both use electronic commutation, do they require dedicated circuits to provide accurate coils? Power-on timing ensures accurate speed control, torque control and efficiency optimization.

Today, the circuit (also known as the motor controller) is integrated into a high-performance microcontroller that drives high-power MOSFETs externally (and in some cases internally). There are two main benefits of an integrated solution:

It simplifies the design of motor drive circuit and transfers complexity to the microcontroller

It reduces the number of external components (BOMs), which reduces costs.

Important functions implemented in the motor controller are as follows:

Adjust motor speed, torque or power output

Start-up phase control (soft start)

Prevent circuit failure and overload

Acceleration and deceleration curves

Motor Drive Type

Both BLDC motor and PMSM can be considered synchronous motors. The commutation applied to the stator phase produces a rotating magnetic field, while the poles on the rotor try to keep up with the synchronization. This causes the motor to rotate, and if commutation is applied, the motor will continue to rotate.

When a current-carrying conductor is placed in a magnetic field or when a conductor cuts off the magnetic field, an electromotive force (EMF) is induced or generated in the conductor. Closed paths are paths that allow current to flow through. Because the induced EMF in the motor is opposite to that of the generator, the EMF generated by motion in any motor is called back EMF.

The geometry of the stator windings of BLDC and PMSM motors is different, which results in different back-EMF (BEMF) responses.

More precisely, BLDC BEMF responses are trapezoidal, whereas PMSM has sinusoidal BEMF (see Figure 2). This is because in PMSM, the coil is wound in a sinusoidal curve, resulting in a sinusoidal BEMF characteristic (very similar to three sinusoidal waves with phase separation of 120 degrees).

Figure 2: BEMF of BLDC and PMSM motors (:[1])

Figure 2: BEMF of BLDC and PMSM motors (:[1])

Because BEMF waveforms are different, each type of motor needs different control.

Trapezoid and FOC control

In order to obtain the sinusoidal waveform needed to control PMSM motor, FOC (Field Orientation Control) algorithm is used. FOC is a frequency conversion control of the stator in a three-phase motor with two orthogonal components. One is the magnetic flux generated by the stator, and the other is the torque defined by the motor speed determined by the position of the rotor.

In sinusoidal commutation, all three wires are continuously powered on by a sinusoidal current 120 degrees apart from each other. This creates a north-south magnetic field that rotates in the motor cage. For proper operation, the FOC algorithm needs to know the position and speed of the motor.

BLDC motor control is simpler, simpler and cheaper than PMSM motor control. However, the latter achieves lower noise and harmonics in the current waveform. Generally, BLDC motors use six-step trapezoidal algorithm for better performance, while PMSM motors use sinusoidal commutation algorithm for better performance.

Sensor and Sensorless Motor

BLDC and PMSM motors with sensors use a Hall sensor (one for each phase) embedded in the motor's stator. This allows the controller to know the position of the rotor to determine which sector needs power and when.

Motor with sensors is more expensive, requires more wiring and increases the complexity of production. For these reasons, sensorless motors have become popular in many applications. Sensorless motors require an algorithm to use the motor as a sensor and rely on BEMF information to operate. In the traditional six-step trapezoidal commutation algorithm for controlling BLDC motors, only two phases are connected at any given time. The other phase is floating and provides a window to learn about the motor BEMF. By sampling the BEMF, the position of the rotor can be inferred without using a hardware-based sensor.

The main disadvantage of the sensorless algorithm is that BEMF (proportional to motor speed) is zero during startup. Without BEMF, the position of the rotor cannot be determined. However, a new algorithm for inferring the position of the rotor by injecting high frequency signals into three phases can overcome this problem.

Motor Controller

Today's integrated motor controllers, the on-chip system (SoC) shown in Figure 3, contain all the analog and digital components needed to control the operation of the motor on a single chip.

As shown in Figure 3, the microcontroller core has analog front-end, power driver, power management, pulse width modulation (PWM) generator, and sequence-driven data collection. The power manager also handles system functions, including internal reference generation, timers, sleep mode management, and power and temperature monitoring.