Power Electronics

Motor Characteristics Drive Bridge, Controller Development

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The advanced capabilities permitted by integrated electronics continue to improve the precision, efficiency and reliability of motor control. One aspect of this continued evolution is the emergence of circuit architectures and device features that address the motor's physical and electrical characteristics. This not only has the benefit of improving the mechanical and electrical performance of the motor, but can also benefit its driver or controller ICs in such areas as controllability and thermal management. Additionally, it has led to motion-control modules optimized for specific applications.

Some characteristics of motors can be highly undesirable. The current spike generated at the end of a brushless dc (BLDC) motor's commutation cycle is one example. Zetex's ZXBM1016 uses a technique called tail-end current control to gradually reduce the stator current to zero during the final portion of each commutation cycle, eliminating the current spike.

This technique also reduces drive current without sacrificing motor speed, thereby allowing further optimization of the drive circuitry, according to Neil Chadderton, director of technology for Zetex (www.zetex.com). However, the original motivation for developing the technique was to reduce acoustical noise generated by the motor. The key enabler is the ability to monitor the rotor's position precisely.

Precision monitoring is also used in three-phase motor drives, where back electromotive force (EMF) from the idle winding can be used to monitor rotor position. Microchip (www.microchip.com) is preparing to release another sensorless method for rotor-position sensing in which the three phase-separated pulse width modulation (PWM) signals are sine-wave modulated and respectively applied to each of the motor's three-phase windings.

The current in each winding is monitored using shunt resistors and compared to an ideal value from an electrical model based on the motor's characteristics. This comparison is used to extrapolate back EMF, which in turn is used to calculate rotor position. The advantage is smoother operation and more precise controllability. Multiple features, in particular the on-chip DSP in the dsPIC30/33F families, will enable this functionality. The parts also include other features such as direct-control signals that operate independently of the processor cycles for improved safety and break-before-make bridge driver outputs from the PWMs.

Another vendor developing microcontroller-based control techniques is Renesas Technology (www.renesas.com). Ritesh Tyagi, senior marketing manager of the LSI business unit for Renesas Technology America, stated that Renesas has developed algorithms for driving one-phase, three-phase, ac-induction and BLDC motors. Successful implementations result when devices with the proper features are chosen based on a careful top-down analysis of the application.

For example, the R8/Tiny products are well suited for brushed dc and single-phase ac induction motors. An M16C/28, with its dedicated three-phase motor controller timer, would probably be sufficient for the vector control of BLDC motors. However, Renesas also has developed a new control algorithm optimized for compressor applications using sinusoidal modulation and current sensing from a shunt resistor on each phase that gives comparable performance to vector control at lower cost.

While digital processing is certainly an evolving practice in the controllers that supply drive signals to discrete or bridge drivers, interesting features also can be found in the drivers themselves. An example shared by Sam Robinson of Apex Microtechnology (www.apexmicrotech.com) is the internally generated 100-kHz ramp of the PWM controller in the SA56, which enables it to directly drive a loud speaker with sufficient audio quality for public address applications. Furthermore, both the SA56 for brushed motors and the SA305 for BLDC motors, use complimentary MOSFETs and require no boost caps or charge pumps for driving the high-side devices.

Complementing these motor-driving techniques, another driver series, Allegro MicroSystem's (www.allegromicro.com) EasyStepper microstepping drivers with translator (A3967, A3977, A3979, A3980, A3982, A3983, A3984) perform motor PWM current control through carefully controlled switching of its internal DMOS driver devices in a patented process called mixed decay, which avoids the sine-wave distortions caused by slow decay and the ripple current caused by fast decay.

Fig. 1 shows the motor-phase current for the three decay modes. According to Tom Rowan, strategic marketing manager for Allegro, the use of synchronous rectification output-switching techniques also reduces dissipated power in the motor drive by up to 30%.

Another part that includes a stepper motor translator is STMicroelectronics' L6208, the last member of a product family that also includes the 05, 06 and 07. All the products in this line feature a high-side nondissipative overcurrent detection mechanism, eliminating the need for a current-sensing resistor. Peak output current is limited to 5.6 A (or 2.8 A for the low-power versions) by default.

The L6206 provides a user-adjustable overcurrent limit through a programming resistor. Another device from ST (www.st.com) is the L6235, which integrates three Hall sensor interfaces, their logic decoders and three half-bridge drivers for BLDC motor-drive applications.

While highly integrated devices can find use in a broad range of applications, modules can be optimized as complete solutions for specific applications. According to Vajapeyam Sukumar, a technologist at Fairchild Semiconductor (www.fairchildsemi.com), modules will be changing the form factors of motor-drive electronics. In some cases, the complete control and driver electronics can be contained on a single “doughnut board” and fitted over the motor shaft.

The patented Smart Power Modules (SPMs) for motion control from Fairchild incorporate six transistors, six diodes and four control ICs, including wave-shaping control ICs. This allows the modules to be designed for minimal electromagnetic interference. It also allows the package to be thermally optimized for the application as a low-cost ceramic or high-performance direct bonding on copper (DBC). The Motion- and SRM-SPMs drive ac and switched-reluctance motors, respectively.

Another module is the iMotion platform from International Rectifier (www.irf.com). It is designed specifically for driving interior permanent magnet (IPM) motors in air-conditioning applications, providing separate outputs for the fan and compressor motors. The iMotion provides torque control, reducing acoustical noise in the compressor caused by torque ripple. It executes an algorithm within its motor control engine to develop additional torque by harnessing the reluctance torque of the IPM motor. Reluctance torque arises from the dependency of the inductance on rotor position.

Fig. 2 illustrates how reluctance torque increases the overall torque in the IPM motor as the phase of the drive current is advanced. The technology and techniques used in the iMotion to accomplish this can also be extended to other specialized applications.

New types of electric motors have emerged to fill in the parameter space of torque, power, efficiency, reliability and cost. Through the use of specialized drivers and controllers, designers have a wider range of options when selecting motors for their applications.

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