How to succeed with a modular approach to motor-control design

READ THIS TO FIND OUT ABOUT: Motor-control developments, such as BLDC, PMSM, inverter drives and AC induction Considerations for effective motor-controller design Generic controller boards that support development across motor types

The push to improve energy efficiency in applications that make extensive use of electric motors, such as domestic appliances and light industrial equipment, will result in the rapid adoption of inverter drives in the coming years. At the same time, the choices facing engineers when selecting the optimal motor type are becoming more complex. Parmjeet Dahele, Senior Systems Engineer, European System Design Centre, Future Electronics (EMEA) explains.

The relative costs of Brushless DC (BLDC) and Permanent Magnet Synchronous Motors (PMSM) are falling quickly compared to AC Induction (ACI) motors, bringing functional advantages such as higher reliability and more versatile controls within the price range of many more applications. Technology supporting sensorless drives for ACI motors has reached a level where it can now be used as a low-cost alternative to servo drives in applications requiring accurate load positioning.

So, with a greater variety of motor types to support, a generic controller design that can be easily optimised for an individual motor type would be very attractive to designers. To prove the effectiveness of the concept, Future Electronics’ European System Design Centre (SDC) has produced a generic controller board that supports sensored or sensorless control for both BLDC and ACI motors.

 

 

Comparing motor-controller topologies and their requirements

The basic topology of a motor driver, comprising a controller, a PWM generator, a three-phase half-bridge, sensing or feedback network and isolation, is applicable to the majority of three-phase motor types serving domestic-appliance and light industrial applications. These include ACI, brushless and brushed DC motors, and permanent-magnet motors.

Figure 1 illustrates the functional blocks of a generic controller suitable for ACI motors and BLDC motors from 100W up to 1kW (assuming single-phase mains operation in territories excluding the USA). The main difference at the hardware level between an ACI-motor controller and a BLDC-motor controller concern the sensing and feedback circuitry.

Assuming operation from a single-phase 240V AC mains supply, the threephase bridge will most likely use 600V-rated IGBTs operating from the approximately 400V DC bus produced from the rectified AC input. An abundant choice of microcontrollers and digital signal controllers are available, optimised for motor-control applications. Basic requirements include:

• Sufficient processing power to execute the relevant algorithm
• 6 PWMs
• Fast ADCs for current measurement in sensorless designs
• Capabilities to decode quadrature encoders or Hall-effect sensors

While an 8-bit CPU may be sufficient to perform a simple BLDC control algorithm, a 16-bit device is a more suitable candidate for a generic design and this is the approach Future uses on its boards.

 

Generic motor controller

At the most basic level, Future Electronics’ generic motor controller consists of core control blocks combined with separate sensored and sensorless feedback modules, to create a controller board that can be quickly and easily tailored for a variety of motor types and ratings.

The board design also allows for extra functions such as emergency braking, networked communications, protection mechanisms including temperature sensing, and mandated features such as power-factor correction to be designed and implemented on a modular basis.

 

Trade-off considerations

Some trade-offs are immediately apparent when considering the choice of central controller. A sufficiently powerful controller for accurate vector control of an ACI motor will be more powerful, and therefore more expensive, than is required for a simple BLDC controller, for example. Conversely, the price differential can be quite small, and outweighed by the savings in design time and effort involved in supporting multiple motor types across a variety of applications.

600V IGBT modules (containing gate driver and 3-phase half bridge) will be relatively expensive components on the board, but it is possible to specify a pin-compatible family of devices with a wide range of ratings that can be chosen to match the requirements of the system.

Modules of this type are available from several vendors: examples include International Rectifier’s IRAMxxx family and Fairchild Smart Power Modules (SPM®s). The use of an IGBT module provides reduced footprint, lower component count, and reduced layout complexity. IGBT modules also tend to integrate fault-sensing circuitry, making it easy to add features such as IGBT over-temperature, DC bus under-voltage and over-current protection. The designer also needs to consider the most cost-effective heatsink solution: this might mean fitting the maximum rated heatsink to all variants, or sizing the heatsink according to the rating of the power module.

 

A hardware platform for motor control

As described, a 16-bit microcontroller is sufficient to host the majority of AC and DC motor-control applications in the 100W to 2.5kW range. In this case we chose a 16-bit MCU architecture with embedded DSP functionality to accelerate the execution of motor-control algorithms. This might be necessary to achieve the desired range of speed and torque control, for example in highspeed applications, or to perform complex waveform calculations for ACI motor control. Several suitable devices are available from suppliers, including Microchip, Freescale Semiconductor and NXP.

The motor-control MCU provides complementary PWM outputs to drive each of the high-side and low-side inputs to the integrated gate driver and IGBT modules directly without requiring external circuitry to generate complementary signals via optocouplers. Gate-driving circuitry is implemented internally by the IGBT module. Programmable dead-time insertion is another key feature of motor-control MCUs, and essential to prevent potentially damaging shoot-through.

As far as feedback and sensing arrangements are concerned, sensorless control is possible for both DC and AC motor types, but speed and position calculations for each type are based on current or voltage signals detected at different locations. The generic controller must support sensing circuitry appropriate to both motor types. For example, BLDC rotor speed and position are typically calculated from the back EMF measured at the motor windings. Conversely, sensorless control of ACI motors depends on current measurements that can be taken directly from either the motor-phase lead, the IGBT low-side emitter connection or the DC bus rail current draw.

 

 

For sensorless ACI motor control, the motor-phase lead signal provides the most direct reading of the rotor position and speed. But typically this is a relatively small differential signal that floats on a common-mode voltage above 600V. This can be overcome by using International Rectifier’s IR21771/IR22771 High Voltage IC (HVIC) family, which also provides a fast over-current signal for IGBT protection. Figure 2 shows a complete driver solution including modular sensing circuitry for ACI and BLDC motors.

For a generic design supporting sensorless control for both DC and AC motor types, an ADC of 12-bit resolution and speed in the range of 10ksample/s to 100ksample/s would be suitable. Figure 3 shows a generic representation of the MCU interfaces required to support drive, sense, synchronisation, protection and user-interface signals.

 

 

Sensorless control becomes more difficult when either more precise speed control is required or larger motors are required. For example, the magnitude of the back EMF of a BLDC motor operating at low speeds is not sufficient to enable accurate zero-crossing detection. As an alternative, devices such as Halleffect sensors or quadrature encoders may be considered. Timers/counters implemented in typical motor-control MCUs are able to convert the Hall-effect or encoder outputs into speed and position information to drive the motorcontrol algorithm. Alternatively, some MCUs provide an integrated interface module dedicated to the support of feedback devices.

The provision of isolation components for common-mode rejection, as well as to protect the user, is another key design decision when creating a generic controller. Isolation of digital signals, such as the PWM signals between the MCU and IGBT module for common-mode rejection purposes, can be implemented using relatively low-cost opto-isolators. On the other hand, isolation of analogue signals presented to the ADC in a generic controller design is much more difficult and expensive, requiring high-performance isolators to achieve effective coupling and to maintain signal integrity. For this reason, in a generic design aiming to implement sensorless control, isolation for safety purposes may need to be located between the user interfaces and the controller, while additional isolation for common-mode rejection stays between the MCU and IGBT.

 

Other considerations

A generic controller board may also need to include provision for features such as an emergency brake or communications over a network such as CAN or RS-485. To implement braking, only one additional high-rated switch element

is required, as well as one additional PWM output, which can often be found on an MCU with the required communications peripherals. Ethernet, too, is becoming more widely used in a range of networking applications, including industrial systems. High-speed opto-isolators are required to protect the system-level communications against common-mode effects in the motor controller, and these are likely to be relatively expensive items in the bill of materials. Suitable isolators are available from a number of suppliers including Avago Technologies, Fairchild Semiconductor and Clare.

From a regulatory standpoint, the need for active power-factor correction will depend on the application area and geographical deployment. To achieve the lowest-cost solution, power-factor correction circuitry should be implemented as an optional module. A wide range of active power-factor correction ICs are available, if necessary.

Finally, an effective generic motor controller board should give due consideration to over-temperature protection, particularly for the switching elements and the motor. Commercially available IGBT modules usually integrate temperature-sensing circuitry, and additional temperature sensing for the motor itself is also highly advisable.

 

Conclusion

The hardware platform approach to electronic design has brought advantages in a number of applications, including audio amplifiers, set-top boxes and personal computers. Extending this philosophy to the design of motor controllers offers extra freedom to quickly customise motor implementations that meet system cost, size and performance targets. Even so, many component selection and integration challenges remain. A proof-of-concept performed by Future Electronics’ SDC has successfully overcome many of the challenges: this board will be available for use by customers as a working reference design for a generic motor-controller board.

 

 

Future-Blox PowerDrive boards are available to members of the Future Board Club. To apply for the development board, and membership of the Board Club, go to www.my-boardclub.com This offer is free and subject to qualification.