Tag Archives: BLDC Motors

Controlling BLDC Motors in Trapezoidal Form

One of the easiest motor control methods for brushless DC motors is the trapezoidal, six-step, or 120° block commutation control. Optimum torque generation requires applying square-wave currents to motor phases in alignment with the trapezoidal back-EMF profile of BLDC motor. MOSFETs of the inverter drive can exhibit only six combinations of on/off states. Therefore, this method has another name—the six-step—resulting in six possible orientations of the stator field within the plane of rotation of the magnetic field of the rotor.

Depending on the desired direction of rotation of the motor, the six possible inverter states must follow a specific sequence. This is necessary so that the orientation of the stator and rotor magnetic fields produces the maximum torque. There are two ways of sensing the rotor position for determining proper commutation timing—sensing through Hall-effect sensors on the motor, or a sensorless way of back-EMF sensing of the rotating motor phases.

Of the two, using sensors requires no voltage or current feedback signals for proper operation. Rather, the position feedback from the Hall sensors is adequate to determine the proper sequence for energizing the motor phases. Hall sensors in strategic positions in the motor can sense rotor position as a result of the rotating magnetic field of the permanent magnets in the rotor. Trapezoidal control using sensors is easier to implement, as it allows for proper commutation even during startups—the information about the rotor position is available even at zero speed.

For trapezoidal control without sensors, the proper motor commutation sequence depends on the back-EMF that the motor’s rotation generates. Such trapezoidal control requires energizing only two motor phases at a time. As the non-energized phases have no current flowing through them, it is possible to sense the back-EMF they are producing during the non-energized times. Typically, such back-EMF positional feedback in BLDC motors is trapezoidal and is either linearly increasing or decreasing. Therefore, most positional feedback techniques using back-EMF use a zero-crossing detection for determining the moment when it crosses a reference point. This can be either half the DC bus voltage or the neutral motor voltage.

Sensorless control has a major drawback. As the magnitude of the back-EMF is proportional to the rotational speed, the rotor must be rotating at a minimum speed to generate a back-EMF of adequate magnitude for sensing the rotor position properly. Therefore, it is necessary to use a startup mechanism for kick-starting the motor until it reaches an adequate rotational speed.

Although it is easier to implement a trapezoidal control with sensors, the Hall sensors add an increased cost. Additionally, signals from Hall sensors may be noisy and may require hardware or software filtering. The motor also requires more wiring, which in some environments, may be a challenge. On the other hand, sensorless control is more complex. It is necessary to tune it to meet specific loads or operating conditions and may face difficulties in starting up under heavy loads. That makes sensorless control well-suited for applications with a well-known load profile that increases with speed, such as for a fan.

Intelligent Phase Control for BLDC Motors

Many applications use BLDC or Brushless DC motors for powering several types of high-speed equipment. These include industrial machines, data center cooling fans for servers and home vacuum cleaners. One of the challenges designers face is to ensure the motors operate effectively and reliably. Now, Toshiba is making it easy for designers to do this with its intelligent phase control motor controller.

While other manufacturers also offer intelligent phase control devices, they usually meet a specific design need. Toshiba’s TC78B016FTG has a driver rated for 40 VDC and 3 A maximum. The fully integrated motor control driver requires a power supply ranging from 6 to 36 VDC, and provides a sine wave output drive. ON resistance of the driver is only 0.24 ohms, representing the total of low and high sides. This typically reduces the self-heating of the device during operation and allows driving 1 to 1.5 A loads without a heat sink.

TC78B016FTG uses a simple speed control mechanism using pulse width modulation. It has several built-in protections, and these include protection from over-current, thermal runaway, and motor lock. Toshiba offers the TC78B016FTG in a 5 x 5 mm VQFN32 package.

Other controllers from Toshiba include the TC78B941FNG and TC78B042FTG. These intelligent phase controllers allow users to tailor the power requirement of an application by selecting a proper MOSFET and its gate driver for the design. Toshiba offers these devices in SSOP30 and VQFN32 packages respectively. Both measure 5 x 5 mm.

Another controller from Toshiba is the TC78B027FTG, which incorporates a gate driver, for which the user can select the proper MOSFETs according to the application. This controller also has a one-Hall drive system for the user to drive a less expensive one-sensor BLDC motor. Toshiba offers the device in a VQFN24 device measuring 4 x 4 mm.

Conventional drive technology adjusts the phase or lead angle of the voltage and current it feeds to the motor for achieving high-level efficiency. However, high-speed rotation prevents the magnetic drive from reaching maximum power, as phase lag delays the voltage applied to the coil from rising until the current has increased to a maximum.

Intelligent phase controllers avoid the above situation by advancing the rotor by a certain angle from the calculated position. This is the new lead angle that depends on the BLDC motor’s characteristics, its rotational speed, and load conditions.

Designers try to achieve optimal efficiency over rotational speeds ranging from almost zero rpm at motor startup to several thousand rpm at high speeds. As this requires several characterizations for adjusting the phase, they achieve optimal efficiency only for a limited range of speeds. Intelligent phase controllers allow BLDC motors to rotate at high speeds with uniform accuracy and efficiency.

Compared to earlier technologies, the approach taken by Toshiba is different. Rather than adjust the phase difference between the voltage and current to the motor at different points in its operating range, Toshiba automatically and continually adjusts the phases of voltage and current the controller feeds to the motor. Intelligent phase controllers from Toshiba thereby achieve the highest possible efficiency for the entire operating range of the motor.