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Posted on 01 November 2019

Getting the Most From Your Brushless DC Motor

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It is possible to run a sensor-less 3 phase motor on a PIC16 microcontroller

With a wide variety of motors and drive solutions available for Brushless Direct Current Motors (BLDC) are you sure you have considered all the options and selected the best combination for your application. This article discusses 3 phase permanent magnet brushless motors, makes qualitative comparisons for the different types and reviews the various control solutions available from Microchip. The aim is to provide the non-expert with insight into the terminology, technology and applicable electronic control solutions.

By Martin Hill, Microchip Technology

 

Motor types and differences

BLDCs were designed to replace the electro-mechanical commutator sub-system in a conventional Brushed DC Motor (BDC). The benefits of the BLDC over the BDC are many-fold with higher reliability, lower maintenance, audible noise, electromagnetic emissions and more power per unit volume due to more thermal efficiency being the salient ones. However the BLDC has not been able to obsolete the BDC due to the associated cost of the alternative electronic commutation sub-system and the fact that BLDCs are still produced at very low cost and in their millions in the Far East. The ease of connection to either the AC or DC supply in the case of the Universal brushed motor is also a benefit.

There is a re-surgent, associated type of permanent magnet brushless motor which is the Permanent Magnet Synchronous Motor (PMSM) which shares the same fundamental construction of the BLDC. However, the PMSM came from a different design and market requirement where higher efficiency than an Alternating Current Induction Motor (ACIM) was required, particularly at low speed. Also, the PMSM has inherently lower audible noise and smoother torque delivery (less torque ripple) than a traditional BLDC. Despite the construction similarities, the mathematical treatment for the PMSM is very different compared with the BLDC.

There are other similarities between the BLDC and PMSM i.e. they are synchronous machines whereby the frequency of the applied electrical drive signals are directly related to the motor mechanical speed. The electrical frequency may be the same or higher than the mechanical speed depending on the number of magnetic poles used in the design. The direct speed relationship is given by:

Mechanical _ Speed = Electrical _ Frequency / Number _ of _ pole _ pairs

In order to produce continuous torque the applied drive signals also have to be synchronized to the rotor position. Detection of rotor position is commonly achieved using sensors such as Halls or more precise devices such as Quadrature Encoders and Resolvers. These provide additional cost overhead and so sensorless control methods have been developed for certain types of applications usually where the system parameters are known not to vary dramatically or are confined in some way. To view where the BLDC and PMSM fit in the motor spectrum we can consider the traditional Motor Classification Tree Diagram – figure 1.

Motor Classification Tree

So fundamentally the BLDC and PMSM belong to the alternating current (AC) and Synchronous groups under motor classification. The way these motors produce torque are the same i.e. A rotating magnetic field in the stator is set up by the commutation sub-system and this interacts with the rotor magnets to produce torque and speed.

Although these motors have existed for years they are receiving more interest of late due to the world governments adopting more energy and environmentally conscious policies. In addition consumers want more reliability and less maintenance and so the car manufacturers for example have responded by converting traditional automotive BDC applications to BLDC equivalents. More-over automotive OEMs want to avoid EMC issues, reduce audible noise and increase efficiency/fuel consumption as well. The PMSM has come back to the fore due to its inherent higher motor efficiency and ability to provide smoother torque delivery for those applications requiring it. The higher efficiency comes from the fact that the PMSM back emf and applied electrical drive signals are sinusoidal and so less parasitic energy is present to allow the motor structure to be excited in an uncontrolled way and in effect wasting energy.

Motor fundamentals

So what are the constructional differences between the BLDC motor and PMSM? To answer this question you have to consider the nature of the motor’s back emf characteristics. In the case of the BLDC this is trapezoidal and for the PMSM it is sinusoidal. Therefore, the more informative way of describing the two types of motor might be Trapezoidal Back Emf PMSM and Sinusoidal Back Emf PMSM but there is a lot of terminology out there due to history. With this difference in mind we can review the mechanism by which these wave shapes are produced from first principles by considering the flux linkage from the rotor to the stator. Figure 2 shows the flux linkage and corresponding induced back emf signal.

Flux linkage and back emf for BLDC and PMSM

The induced back emf in each stator coil is basically down to Faraday’s Law. As the rotor magnets rotate pass the 3 phase coils more flux is linked with each coil and the integral of the flux linkage is a ramp and sinusoid in the BLDC and PMSM case respectively. Then, according to the law the back emf is proportional to the rate of change of flux with respect to time or rotor position as shown in the figure (2). Hence the magnitude of back emf is proportional to speed of the motor.

The area under the curve of the back emf signal will dictate how much torque the motor will produce. Typically there is more area under the curve for a comparatively sized BLDC with respect to a PMSM and so inherently the BLDC can produce more torque.

If we consider the basic electrical motor model as shown in figure 3 this depicts a stator phase winding connected to a power supply and this would be via the electronic drive system in the practical case. Considering the basic model the battery is supplying current through the winding resistance R, causing heat dissipation. The same current flows through the winding inductance L causing energy storage in its magnetic field. Then there is the back emf signal e and the current is flowing through this generator and so due to the conservation of energy principle this is converted to mechanical energy. With other losses ignored, the instantaneous electrical power given by the product of current x voltage = torque x speed.

Basic Electric Model

The physical construction which creates the different shapes of back emf is largely influenced by the stator design and winding layout. In addition the number of stator slots per pole per phase (Ns/pp/pp/pp) is key. If the Ns/pp/pp is an integer the waveshape is likely to be trapezoidal. Conversely, if the Ns/pp/pp is fractional the waveshape is likely to be sinusoidal. Rotor magnets can also be shaped so as to create a sinusoidal flux linkage with the stator coils.

Control possibilities

From the perspective of the electronic drive the applied motor current should be the same shape as and synchronized to the motor back emf signal for normal operation. There are exceptions where more speed maybe desired using field weakening or starting the motor from rest in an open loop manner without rotor position sensors. In BLDC motors this is not so much of a problem because the motor currents are approximately rectangular shape and the commutation process occurs at discrete points that can be detected by Hall sensors. These pick up the rotor position of separate magnets mounted on the end of the rotor assembly. This form of control is commonly referred to as six step or block commutation because the process involves six discrete steps or blocks. In the case of the PMSM the problem is not trivial because matching motor phase current to back emf relies on rotor position detection to a much higher resolution and continuously due to the continuous variation of back emf. Quadrature Encoders (Q.E.) and Resolvers are commonly used to provide the position measurement for PMSM commutation but Hall sensors again can provide a low cost alternative depending on the application requirements. The Q.E. and Resolver devices also provide the feedback in position servo systems using either type of motor.

It is possible to run each of these motor types without position sensors for commutation purposes. There are various techniques to do this and back emf sensing or estimation can be applied to the BLDC and PMSM respectively. For the BLDC motor the commutation process involves switching on only one pair of transistor switches in a 3 phase (6 transistors) power bridge at any instant in time. As such this means that for a Y (star) connected motor, current passes through two phase windings at any instant in time. This is convenient because the non driven phase can be monitored for back emf. In effect this signal indicates the position of the rotor when the voltage is equal to half the applied supply, this is known as the zero crossing point. The system can also be used with D (delta) connected motors. For the PMSM, back emf sensing cannot be done in the aforementioned way because the commutation process (Space vector PWM) involves driving current through each of the 3 phases continuously but it can be estimated using a model of the motor in software and then comparing the actual measured motor current with the estimated version. When the two are equal the calculated back emf signal is used to estimate the rotor position on a continuous basis.

When the position of the rotor is known the control system can provide the correct commutation sequence for the type of motor. In the case of the BLDC it should be apparent that the rotor position detection is discrete whereas the PMSM is continuous. For the PMSM that means the commutation process and applied phase current is continuous and this leads to very low torque ripple when compared with the BLDC. In addition a very effective way to realise optimal torque control in high performance PMSM systems is to employ the Field Oriented Control method. Through mathematical transformations it allows separate control of torque and flux and the associated control variables effectively become d.c quantities which extends the bandwidth of the control system. Having the ability to control the magnetic flux coupling between rotor and stator also permits field weakening control and the possibility for extended speed range operation.

Microchip Solutions

For the wide range of control possibilities for BLDC and PMSM Microchip has provided solutions in the form of Application Notes and hardware development boards and associated power modules. These can form the basis of a customers design and be enhanced as necessary during development and in-line with marketing requirements. In addition, in-house tools are continually updated to provide the engineer with the most efficient route to project completion.

The following table (Figure 4) summarises the applications, provides some qualitative comparisons and also shows the relevant motor control Microchip Application Notes for those sensor and sensor-less based solutions.

3 Phase BLDC-PMSM Motor-Drive Comparison

For the most cost sensitive BLDC motor applications it is possible to run a sensor-less 3 phase motor on a PIC16 microcontroller which has two internal comparators and a PWM module with pin steering ability. The complete low cost reference design is available now and would be particularly suited to fan and pump applications although others are possible.

PIC18 and dsPIC® devices feature dedicated motor control peripherals which enable the off loading of CPU tasks. This frees up more CPU time for those applications performing more than the motor control task. PIC18 and dsPIC devices are very capable of controlling high performance BLDC based systems. For the highest performance PMSM and BLDC systems the dsPIC is an ideal fit with enough MIPs and dedicated motor control peripherals to suit these types of applications. In addition a variety of interfaces including CAN are available on the motor control type parts.

Summary

In summary there are many motor applications and control possibilities using both BLDC and PMSMs. PMSMs offer certain advantages over the former and are receiving attention from designers now. However, at present the BLDC motor prevails due to its good performance/ cost trade off. In support of this Microchip has a wide range of motor control solutions available to cover all application segments and dedicated technical support set up to address the motor control market. There are also Regional Training Centres (RTCs) where you will find motor control training available. Contact your local Microchip representative to find out more and how we can help you with your applications.

 

 

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