Improving the performance efficiency of your motor-control based design

This “Product How-To” article focuses how to use a certain product in an embedded system and is written by a company representative.

Brushless DC (BLDC) and Permanent Magnet Synchronous(PMSM) motors are technologies that have seen an increase in demand,because of high efficiency and increasingly cheaper production cost.These types of motors are expected to gain market importance in the lowpower range 750W to 5kW particularly.

There is pressure therefore on designers to cut the cost of motorinstallation, including the control systems, and this is where moreefficient low-cost microcontrollers can help. Understanding therequirements however can help illustrate what features control circuitdesigners need to focus on to get the best performance for the smallestinvestment in silicon.

Furthermore modular software blocks and hardware reference designsoffer fast motor control solutions. The core motor control softwareroutines are proven and remain the same, independent of the motor size,so applications including white goods through to major industrialinstallations can be driven by the same core device.

Understanding motors
A motor has two primary parts; the non-moving part is called thestator and the moving part, typically inside the stator, is called therotor. Depending on the motor type, the stator and rotor can consist ofcoil windings or permanent magnets.

In order to enable a motor to rotate, two magnetic fluxes arerequired, one from the stator and the other from the rotor. Bycontrolling the current applied, a rotating magnetic field can begenerated. The motor rotates because of the interaction of the rotatingmagnetic fields, as the magnetic field from the rotor attempts to alignwith that of the stator.

Brushed DC motors depend on a mechanical system to transfer current.The brushed motors have a wound rotor attached to the centre with apermanent magnet stator bonded to a steel ring surrounding the rotor. Acommutator provides a means for connecting a stationary power source tothe rotating coils, typically via conductive brushes that ride onsmooth conductive plates. As the brushes come into contact with thecommutator, current passes through to the rotor coil. The uneven torquethat results from a single coil rotor can be smoothed by addingadditional coils and commutator segments.

AC induction motors, on the other hand, do not depend upon themechanical system to control current, but instead pass current throughthe stator which is connected to an electrical supply directly or via asolid-state circuit.

The motor stator has a number of coil windings, that when driven byan alternating current, operates as a set of electromagnets to generatethe required flux. It typically has a squirrel-cage rotor, consistingof a ring at either end of the rotor, with bars connecting the ringsrunning the length of the rotor. In a 3-phase motor, the stator coilsenergise and de-energise sequentially, creating a rotating magneticfield. This induces current to flow in the bars of the squirrel-cagerotor, which in turn creates another magnetic field.

Brushless DC motor specifics
The term 'BLDC' is a marketing driven label to promote the idea thata BLDC motor with the appropriate drive is a suitable drop-inreplacement for a brushed DC motor and its drive. Technically, a BLDCmotor is still an AC motor; that is, alternating current inputs arerequired in order for the motor to move.

A BLDC motor is a rotating electric machine where the stator is aclassic 3-phase wound stator, like that of an induction motor. Ratherthan inducing the rotor, the BLDC rotor has surface-mounted permanentmagnets which provide the steady-state magnetic field.

When the wound stator is energised by a 3-phase alternating current,it creates a rotating magnetic field that causes the rotor to rotatesynchronously with it. The BLDC motor, with trapezoidally distributedwindings, is driven by trapezoidal currents coupled with the givenrotor position.

Similar to the BLDC motor, the rotor of a PMSM consists of permanentmagnets. The stator of a PMSM has its 3-phase windings distributedsinusoidally, as opposed to the trapezoidal distribution found in aBLDC motor. It operates in the same way as a BLDC motor, when the woundstator is energised by a 3-phase alternating current, it creates arotating magnetic field that causes the rotor to rotate synchronouslywith it.

The PMSM, with sinusoidally distributed windings, is driven bysinusoidal currents coupled with the given rotor position. A motor willdraw only as much power and consume only as much energy as it takes tosatisfy the load, thus to save energy, the options are to reduce theload, reduce the operating time, or increase efficiency.

The efficiency of a motor is a measure of how well it convertselectrical energy into useful work. The difference between the outputmechanical power and the input electrical power is due to fivedifferent kinds of losses occurring in the motor including::

Electrical losses -expressed as I2R are consequently mostsignificant and increase rapidly with the motor load. These appear asheat generated by electric resistance to current flowing in the statorwindings and in the rotor conductors;

Magnetic losses -occur in the steel laminations of thestator and rotor due to hysteresis and eddy currents;

Mechanical losses – fromfriction in the bearings,ventilation and windage;

Stray load losses -due to leakage flux associated with airgap imperfection, and;

Brush contact losses -result from the voltage drop betweenthe brushes and the commutator, in the case of a brushed DC motor.

Higher Efficiency BLDC and PMSM
Using permanent magnets in the rotor helps keep BLDC motor and PMSMrotors small and inertias low. More significantly, the motors have lesselectrical losses than induction motors because they do not have thesecondary windings in their rotors, and the rotor magnetic losses arealso much lower.

Without current flow in the rotor, the motor also generates muchless heat. The wound stator construct of BLDC motor and PMSM furtherallow any heat that is produced to be dissipated more efficiently,compared to dissipation via the AC induction motor air gap or thebrushed DC motor shaft.

The inertia on the rotor is also less when compared to thesquirrel-cage and wound rotor constructs of the AC induction motor andbrushed DC motor respectively. This high torque to inertia ratiosallows the BLDC motor and PMSM to provide a much better accelerationrate than the other motor types.

In the low power range and in applications requiring variable speedcontrol, adopting BLDC motor and PMSM can lead to efficiencyimprovements of up to 10% to 15% when compared with AC inductionmotors, and allow the possibility of 90% operating efficiency.

At the same time, BLDC motor and PMSM are also more energy efficientthan brushed DC motors. This arises because the motors eliminate theexcitation circuit losses and does not suffer from friction due to thebrushes. The enhanced efficiency is more apparent in the low-loadregion of the motors' performance curve.

In addition, both BLDC and PMSMs, for the same mechanical workoutput, will always be smaller than an AC induction motor and usuallybe smaller than a brushed DC motor, This arises because the motors'inherent construction facilitates better thermal efficiency, thus themotor body has less heat to dissipate. From this standpoint, BLDC andPMSMs require fewer raw materials to build, and are potentially morecost effective.

Figure1: eCOG1X motor control specific peripherals

16 bit microcontrollers such as the peripheral-rich eCOG1X naturallylend themselves to embedded motor control applications. These deviceshost a number of motor control specific peripherals that aid andsimplify the motor control application software executed by themicrocontroller processor core. These peripherals are as summarised in Figure 1 above.

The six channel MCPWM digital outputs can control directly theswitching transistors of the power driver stage, which in turn appliesthe appropriate phase signals to the motor phases via the inverterbridge. Full four quadrant drive operation is possible, providingacceleration and deceleration torque with the motor running in eitherdirection.

In applications where a potentiometer is used to input the speeddemand and motor running direction, this can be monitored by one of theeCOG1X ADC channels. Another ADC channel can be used to observe thetotal stator current and check for over-current fault conditions.

Information on the rotor position and speed feedback can be obtainedby capturing changes in the motor Hall effect sensor outputs using theeCOG1X input capture timer.

In order to reduce design time, modular software blocks and hardwarereference design are also provided, an example of the latter is asshown in Figure 2 below .

Figure2: Control circuits for Brushless DC Motors and Permanent MagnetSynchronous Motors are increasingly being implemented using low cost,highly integrated microcontrollers.

As we have shown, both Brushless DC motors and Permanent MagnetSynchronous Motors are gaining widespread use in various consumer andindustrial applications, due to the high efficiency and linearspeed/torque characteristics which meet the need to cut power andimprove performance.

Modern microcontroller offerings have therefore evolved to ensurethat products can be generated quickly and cheaply while meeting theserequirements.

Ee Beng Lam is an applications engineer at Cyan

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