Electric Drives - Brushless DC / AC and Reluctance Motors
(Description and Applications)
Brushless motors such as permanent magnet and switched reluctance motors depend on electronic drive systems which produce rotating magnetic fields to pull the rotors around. The advent of new magnetic materials such as alloys of Neodymium with high levels of magnetic saturation and high coercivity, able to set up and maintain high magnetic fields, have enabled a range of innovative brushless motor designs by eliminating one set of the traditional motor's windings, either the stator or the rotor. The implementation of many of these brushless designs however has only been made possible by the availability of inexpensive high power switching semiconductors which have enabled radical new solutions to the commutation problem and much simpler mechanical designs.
Permanent Magnet Motors
By using permanent magnets, rotor windings and mechanical commutation can be eliminated simplifying manufacture, reducing costs and improving reliability. At the same time efficiency is improved by the elimination of the need for excitation of the rotor windings and by avoiding the frictional losses associated with the commutator.
Brushless versions of both DC and AC motors are available.
Brushless DC (BLDC) Motors
The speed and torque characteristics of brushless DC motors are very similar to a shunt wound "brushed" (field energised) DC motor with constant excitation. As with brushed motors the rotating magnets passing the stator poles create a back EMF in the stator windings. When the motor is fed with a three phase stepped waveform with positive and negative going pulses of 120 degrees duration, the back EMF or flux wave will be trapezoidal in shape. (See diagram below)
- Synchronous Operation
Brushless DC motors are not strictly DC motors. They use a pulsed DC fed to the stator field windings to create a rotating magnetic field and they operate at synchronous speed. Although they don't use mechanical commutators they do however need electronic commutation to provide the rotating field which adds somewhat to their complexity.
- Rotating Field and Speed Control
In the diagram below, pole pair A is first fed with a DC pulse which magnetises pole A1 as a south pole and A2 as a north pole drawing the magnet into its initial position. As the magnet passes the first magnetised pole pair, in this case poles A1 and A2, the current to pole pair A is switched off and the next pole pair B is fed with a similar DC pulse causing pole B1 to be magnetised as a south pole and B2 to be a north pole. The magnet will then rotate clockwise to align itself with pole pair B. By pulsing the stator pole pairs in sequence the magnet will continue to rotate clockwise to keep itself aligned with the energised pole pair. In practice the poles are fed with a polyphase stepped waveform to create the smooth rotating field.
A six step inverter is used to generate the three phase supply and the electronic commutation between the three pairs of stator coils needed to provide the rotating field.
Only two out of three pole pairs are energised at any one time.This also means that only two of the six inverter switches are conducting at any one time. See the Motor Control diagram below.
The speed of rotation is controlled by the pulse frequency and the torque by the pulse current. In practice the system needs some fairly complex electronics to provide the electronic commutation.
- Position Sensing and Speed Control
The inverter current pulses are triggered in a closed loop system by a signal which represents the instantaneous angular position of the rotor. The frequency of the power supply is thus controlled by the motor speed.
Rotor position can be determined by a Hall Effect device (or devices), embedded in the stator, which provide an electrical signal representing the magnetic field strength. The amplitude of this signal changes as the magnetic rotor poles pass over the sensor. Other sensing methods are possible including shaft encoders and also sensing the zero crossing points of currents generated in the unenergised phase windings. This latter method is known as "sensorless" position monitoring.
The diagram below shows the system for controlling the voltage and speed with the associated current and voltage waveforms superimposed on the circuits.
Note that though the magnetising current pulses are in the form of a stepped square wave, the back EMF is in the form of a trapezoidal wave due to the transition periods as the rotor magnet poles approach and diverge from the stator coils when the rotor magnet is only partially aligned with the stator magnets.
Power management is usually by means of a pulse width modulated controller (PWM) on the input supply which provides a variable DC voltage to the inverter.
- Mechanical Construction
No current is supplied to, nor induced, in the rotors which are constructed from permanent magnets or iron and which are dragged around by the rotating field. With no currents in the rotors these machines have no rotor I2R losses.
Without the mechanical commutator and rotor windings, the motors have low rotor inertia allowing much higher speeds to be achieved and with the elimination of this high current mechanical switch, the source of sparking and RFI is also eliminated.
The stator windings are, easy to manufacture and install, bobbin windings.
Since all the heat generating circuits are in the stator, heat dissipation is easier to control and higher currents and motor powers can also be achieved.
Some brushless motors are supplied with the control electronics incorporated into the motor body.
- The Magnets
Depending on motor size, the magnets can be arranged as a full-ring magnet, as spokes, or embedded in the rotor core.
The preferred magnets are manufactured from the rare earth element Neodymium in an alloy with Iron and Boron to produce the strongest permanent magnets currently available. (Most of the world's known supplies of Neodymium are found in China)
One drawback of permanent magnet machines is that the magnets are susceptible to high temperature complications and loss of magnetisation above the Curie temperature.
Permanent magnet motors are inherently more efficient than wound rotor machines since they don't have conduction losses associated with rotor currents.
- Synchronous Operation
The motor speed is directly proportional to the pulse frequency of the inverter. If the supply frequency is fixed and the motor operates in open loop mode then it will run at a fixed synchronous speed. Changing the supply frequency will change the motor speed accordingly.
- Variable Speed Operation
The brushless DC motor can be made to emulate the characteristics of its brushed cousin in which the speed is controlled by changing the applied voltage, rather than by changing the supply frequency. The supply frequency still changes but it does so as the result of the changing motor speed not the cause.
Using this configuration, increasing the voltage of the pulsed DC supply from the inverter will increase the current through the stator windings thus increasing the force on the rotor poles causing the motor to speed up just as in a brushed DC motor. Although the motor runs at variable speed, it is still a synchronous application since the feedback loop triggers the inverter pulses in synchronism with the motor rotation thus forcing the supply frequency to follow the motor speed. This also means that the motor will be self starting.
High efficiency and power density.
No field windings needed to produce the flux as in induction and brushed motors (this is called the "excitation penalty") and hence no conduction losses.
More torque per Amp due to lower losses.
Compact, light weight designs. The magnets are generally smaller than the windings needed to provide the equivalent field.
Lower costs due to the elimination of the field windings.
Speeds up to 100,000 RPM possible whereas the speed in brushed motors is limited by centrifugal forces on the rotor windings and the commutator.
Torque is proportional to speed as in a brushed DC motor.
Trapezoidal wave form.
No commutator, hence low maintenance and long life.
The abrupt current transitions give rise to similarly abrupt torque transitions as well as magnetostriction in the magnetic materials resulting in cogging as well as acoustic noise which may be objectionable in some applications.
Permanent magnet motors are ideal for applications up to about 5 kW. Above 5kW, the magnets needed for higher power applications become progessively more expensive reducing the economic advantage of the design. The magnets in brushless motors are also vulnerable to demagnetisation by the high fields and high temperatures used in high power applications. Inverter switching losses also become significant at higher power levels. Brushed and induction motors do not suffer from these problems.
Permanent magent motors are thus suitable for traction applications from low power wheel chairs and golf buggies for some higher power automotive uses.
Brushless DC motors are preferred over brushed motors for powering electric bikes because they don't have the friction associated with the commutator brushes in the brushed version.
Brushless AC Motors
Also known as Permanent Magnet AC (PMAC) Motors, brushless AC motors have many similarities to brushless DC motors. They do not however have salient stator poles like the DC version. Instead, the stator windings are distributed around the motor casing and the magnets are shaped to induce a sinusoidal back EMF voltage waveform in each motor phase as the rotor spins, rather than the trapezoidal back EMF waveform as found in BLDC motors. This sinusoidal back EMF waveform shape enables PMAC motors to develop nearly constant output torque when excited with a 3-phase sinusoidal current waveform.
Unlike the BLDC motors, all three sets of stator windings are allways energised to produce the rotating field in much the same way as in an induction motor. Similarly, the three phases of the inverter are also in constant use.
Because of the smoother current and torque waveforms, PMAC motors do not suffer from cogging or acoustic noise to the same extent as BLDC motors.
The reluctance motor uses the simplest of all electric machine rotors and is one of the oldest motor technologies known, dating from Robert Davidson's 1838 invention, but only recently being adopted. It does not use permanent or electromagnets in the rotor which is simply constructed from magnetic material such as soft iron.
In recent years several variations of the reluctance motor have been developed. Variable and switched reluctance motors operate on essentially the same principles but are optimised for different applications. They are both synchronous motors, similar to the permanent magnet brushless DC motor except that the rotor is constructed from iron rather than from permanent magnets. The so called "Synchronous Reluctance Motor" has a different construction and functions slightly differently.
- Variable Reluctance Motor (VRM)
The variable reluctance motor is an evolution of the stepping motor and is generally designed for use in low power, open loop position and speed control systems where efficiency is not of prime importance.
- Switched Reluctance Motor (SRM)
The switched reluctance motor was designed for use in high power, high efficiency, variable speed drives able to deliver a wide range of torque. For this they need closed loop position control.
- Synchronous Reluctance Motor
The Synchronous Reluctance Motor is similar to a synchronous AC machine and is described in the section on AC motors. The rotor has salient poles but the stator has smooth, distributed poles whereas both the switched and variable machines have salient poles for both the rotor and the stator.
- Switched and Variable Reluctance Motors
Because of their similarities, the principles of switched and variable reluctance motors are described together here. They are both synchronous motors similar to the brushless permanent magnet motors noted above except that the rotors are made from laminated "soft" magnetic materials, shaped to form salient poles.
- Operating Principle
When a piece of magnetic material is free to move in a magnetic field, it will will align itself with the magnetic field to minimise the reluctance of the magnetic circuit. To put it another way the piece will orient itself towards the magnetic pole creating the field. (This also has the effect of maximising the inductance of the field coil). The torque on the rotor created in this way is called the reluctance torque.
When the spaces or notches between the rotor poles are opposite the stator poles the magnetic circuit of the motor has a high magnetic reluctance, but when the rotor poles are aligned with the stator poles the magnetic circuit has a low magnetic reluctance. When a stator pole pair is energised the nearest rotor pole pair will be pulled into alignment with the energised stator poles to minimize the reluctance path through the machine. As with brushless permanent magnet motors, rotary motion is made possible by energising the stator poles sequentially causing the rotor to step to the next energised pole.
A polyphase inverter energises appropriate pole pairs based on shaft position. The excitation of the stator poles must be timed precisely to correspond with the rotor position so that it occurs just as the rotor pole is approaching. The reluctance motor thus requires position feedback to control the motor phase commutation. This feedback control can be provided by using position sensors such as encoders or Hall effect sensors to feedback the rotor angle to trigger the commutator at the appropriate point .
Sensorless position control is also possible at the expense of more complex electronics and software.
Motor torque and efficiency are optimised by synchronising the controller switching phase with the rotor position so that the torque angle is held at its maximum of 90 degrees.
Complex control electronics have been simplified by the availability of low cost DSPs
Practical motor designs are doubly salient, (both stator and rotor have salient poles) with multiple stator and rotor poles. The rotor however usually has fewer poles than the stator to enable self starting and bidirectional control.
Because the rotor is not a permanent magnet but is constructed from iron, no back EMF is generated, allowing the motor to reach much higher speeds than with similar permanent magnet motors.
The motor does not require sinusoidal exciting waveforms for efficient operation, so it can maintain higher torque and efficiency over broader speed ranges than is possible with other advanced variable-speed systems.
Because of the double saliency, the design suffers from torque ripple, structural resonances and acoustic noise and various methods such as multiple poles and pole shaping are needed to smooth out these variations.
The switched reluctance machine can also be driven as a generator.
No I2R loss in the rotor.
Inert rotor. No permanent magnet.
Compact size and low weight.
Efficiencies greater than 90% possible.
Inexpensive and easy to manufacture.
Lowest construction complexity of any motor. Many stamped metal elements.
High reliability (no brush wear). Rugged construction.
High start-up torque and high speed operation possible.
As with BLDC motors, reluctance motors suffer from excessive noise and cogging.
Since reluctance motors do not have permanent magnets to create the magnetic field in the air gap between the rotor and the stator, they need a very small air gap to concentrate whatever magnetic field there is. This requires tight tolerances and increased manufacturing costs.
Available with ratings up to thousands of Amps and hundreds of kiloVolts.
The automotive industry now makes extensive use of variable reluctance motors for applications such as traction drives, power steering systems, pumps and windscreen wipers.
3 or 4 phase motors used for scooters and fans.
High speed pumps and compressors.
See also Integrated Starter Generator.
- Stepping Motors
The stepper motor which includes some of the features of the modern switched reluctance motor was invented and patented in the
1920’s in Aberdeen by C.L. Walker
A stepping motor is a special case of a variable reluctance motor or a permanent magnet brushless DC motor. Instead of being fed with a constant, repetitive stream of pulses the motor can be stepped one pulse at a time enabling the motor to make very precise angular rotations.The motor is reversible, positive going pulses causing a rotation in one direction while negative going pulses drive the motor in the opposite direction.
If the motor is coupled with a leadscrew it can be used to make precise linear displacements.
The pulses may be generated by a Voltage Controlled Oscillator (VCO), but the design is particularly suited to digital and microprocessor controllers.
All of these factors make the stepping motor ideal for industrial robotics, machine tools and process controllers.
The stepping angle due to each pulse is given by:
Step angle = 360°
(rotor teeth) X (stator phases)
Position control is possible simply by counting the pulses and complex closed loop feedback systems are not necessary for the basic operation. More precise control (smaller angles) can be achieved by stacking and offsetting several rotors and stators along a single rotor shaft.
For very long movements it may be desirable to control the speed during the operation, accelerating up to a maximum slew speed then decelerating as the target is approached. For such applications a closed loop speed control may be added.
Stepper motors are categorized as permanent-magnet (PM), variable reluctance (VR) or hybrid (a combination of PM and VR).
Precise position control.
Simple open loop position control.
Amenable to simple computer control.
Used in computer plotters and printers.
Numerically controlled machine tools.
Electric Machine Fundamentals