Tag Archives: Brushless DC Motors

Stepper Servo Motors

Although many designers prefer to relegate stepper motors to the realm of low-cost low-performance technology, a new technique is bringing the step motors a fresh lease of life. This new drive technique is the stepper servo, and it uses the generic stepper motor, yet extracts significantly more performance out of it. The technique requires adding an encoder and operating the motor effectively as a commuted two-phase brushless DC motor.

While the inclusion of an encoder makes the stepper servo idea non-suitable for low-cost applications, designers are increasingly considering the technique an alternate approach to applications requiring a brushless DC motor.

This is because the cost of a stepper servo motor is considerably less than a comparable brushless DC motor, while the former actually outperforms brushless DC motors in areas of torque output and acceleration. Therefore, designers are considering the stepper servo motor as a candidate for high-speed applications such as coil winding, point-to-point moves, textile equipment, high-speed electronic cams, and more.

Stepper motors are easy to use, making them popular. They maintain their position without external aids such as encoders. Neither do they require a servo control loop when designers use them for positioning, as other DC motors do. Their brushless operation, high torque output, and low cost are their biggest advantages. However, their limited speed range, noisy operation, and vibrations are their main disadvantages.

Being a multi-phase device, stepper motors require the excitation of multiple coils and driving control waveforms for their operation. The usual configuration for stepper motors will have 1.8 mechanical degrees for a full step of 90 electrical degrees—making it 200 full steps for every mechanical rotation. Other stepper motors may have 7.2- or 0.9-degree configurations in place of the customary 1.8.

A stepper servo motor has an encoder attached to the shaft. For a typical 1.8-degree stepper motor, the resolution of the encoder must be of the order of 2000 counts per mechanical rotation. The encoder verifies the final position of the rotor through a traditional step motor control scheme.

The stepper servo motor operates more like a brushless DC motor, with the actual encoder position commuting the phase angle, instead of the commanded position. The phase angle and amplitude of the driving waveform need to vary continuously depending on the output from a position PID loop. This allows the motor to servo to the commanded position.

The presence of the encoder frees the stepper servo from losing steps—the encoder determines the location. The motor operation is now more efficient, causing much lower heat generation. Traditional stepper motors require driving at large currents adequate for handling worst-case motions.

Traditional stepper motors always have problems achieving positional accuracy. With the encoder driving the stepper servo motor to its location, these vagaries of position do not arise. The encoder frees the stepper servo motor from the restrictions of the 1.8 degrees per step of the regular stepper motor. Simply increase the resolution of the encoder to get better positional accuracy.

The addition of the encoder also produces a smooth acceleration to the desired position without the customary bouncing and noise.

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.

What is a brushless DC motor?

Most electrical appliances have an electric motor that rotates to displace an object from its initial position. Various motors are available in the market such as servomotors, induction motors, stepper motors, DC motors (both brushless and brushed), etc. The choice of a motor depends on the requirements of an application. Most new designs favor brushless DC motors, also referred to as BLDC motors.

The working principle of brushless DC motors is similar to that of brushed DC motors, but their construction is very close to that of AC motors. Like all motors, a brushless DC motor too has a stator and a rotor as its major parts.

The stator of a brushless DC motor, similar to the stator of an induction AC motor, is made up of laminated CRGO steel sheets stacked up to carry the windings. The stator windings follow one of two patterns, star and delta. Motors with stators wound in star pattern produce high torque at low RPM compared to motors whose stators are wound in a delta pattern. For motors required to run at very high speeds, the stator core has no slots, as this lowers the winding inductance.

Lack of slots in the lamination stack means the stator has no teeth, which reduces the cogging torque. Teeth in the stator align with the permanent magnets in the rotor, holding the rotor in a stationary position. When starting to move, additional torque, known as the cogging torque is required to make the rotor break free. However, slotless cores are more expensive as a larger air gap is necessary and that means more winding to compensate.

A typical brushless DC motor has its rotor made out of permanent magnets. The number of poles in the rotor depends on the requirements of the application, as more number of poles gives better torque. However, this reduces the maximum possible speed. Torque produced in a brushless DC motor also depends on the flux density of the material of the permanent magnet; higher flux density material produces higher torque.

Brushless DC motors are popular due to several advantages they offer over other types of motors. Compared to brushed type of motors, a BLDC motor produces higher torque because it has no brushes where power may be lost. Lack of brushes also means higher operating life and lower maintenance. Compared to AC motors, the rotor construction is simpler as it has no windings.

The cost to performance ratio of brushless DC motors is the lowest among all the types of motors available. One reason for this is the stator of a BLDC motor is on its outer periphery, which makes it dissipate a larger amount of heat. Additionally, commutation of brushless DC motors is simpler through electronic switches. That makes it easier to control the speed of BLDC motors.

Whether you are looking at single-speed, adjustable speed, position control or low-noise applications, brushless DC motors are the clear winners over all other types. As they are easier to control, maintaining speed of brushless DC motors is simpler with variations in load. A brushless DC motor generates very low amounts of EMI and audible noise.