DC motor
A DC motor is a mechanically commutated electric motor powered from direct current (DC). The stator is stationary in space by definition and therefore its current. The current in the rotor is switched by the commeuator to also be stationary in space. This is how the relative angle between the stator and rotor magnetic flux is maintained near 90 degrees, which generates the maximum torque.
DC motors have a rotating armature winding (winding in which a voltage is induced) but non-rotating armature magnetic field and a static field winding (winding that produce the main magnetic flux) or permanent magnet. Different connections of the field and armature winding provide different inherent speed/torque regulation characteristics. The speed of a DC motor can be controlled by changing the voltage applied to the armature or by changing the field current. The introduction of variable resistance in the armature circuit or field circuit allowed speed control. Modern DC motors are often controlled by power electronics systems called DC drives.
The introduction of DC motors to run machinery eliminated the need for local steam or internal combustion engines, and line shaft drive systems. DC motors can operate directly from rechargeable batteries, providing the motive power for the first electric vehicles. Today DC motors are still found in applications as small as toys and disk drives, or in large sizes to operate steel rolling mills and paper machines.
Brush
A brushed DC electric motor generating torque from DC power supply by using internal mechanical commutation, space stationary permanent magnets form the stator field. Torque is produced by the principle of Lorentz force, which states that any current-carrying conductor placed within an external magnetic field experiences a force known as Lorentz force. The actual (Lorentz) force ( and also torque since torque is F x l where l is rotor radius) is a function for rotor angle and so the green arrow/vector actually changes length/magnitude with angle known as torque ripple) Since this is a single phase two pole motor the commutator consists of a split ring, so that the current reverses each half turn ( 180 degrees).
The brushed DC electric motor generates torque directly from DC power supplied to the motor by using internal commutation, stationary magnets (permanent or electromagnets), and rotating electrical magnets.
Like all electric motors or generators, torque is produced by the principle of Lorentz force, which states that any current-carrying conductor placed within an external magnetic field experiences a torque or force known as Lorentz force. Advantages of a brushed DC motor include low initial cost, high reliability, and simple control of motor speed. Disadvantages are high maintenance and low life-span for high intensity uses. Maintenance involves regularly replacing the brushes and springs which carry the electric current, as well as cleaning or replacing the commutator. These components are necessary for transferring electrical power from outside the motor to the spinning wire windings of the rotor inside the motor. Brushes are made of conductors.
Brushless
Typical brushless DC motors use a rotating permanent magnet in the rotor, and stationary electrical current/coil magnets on the motor housing for the rotor, but the symmetrical opposite is also possible. A motor controller converts DC to AC. This design is simpler than that of brushed motors because it eliminates the complication of transferring power from outside the motor to the spinning rotor. Advantages of brushless motors include long life span, little or no maintenance, and high efficiency. Disadvantages include high initial cost, and more complicated motor speed controllers. Some such brushless motors are sometimes referred to as "synchronous motors" although they have no external power supply to be synchronized with, as would be the case with normal AC synchronous motors.
Uncommutated
Other types of DC motors require no commutation.
- Homopolar motor – A homopolar motor has a magnetic field along the axis of rotation and an electric current that at some point is not parallel to the magnetic field. The name homopolar refers to the absence of polarity change.
Homopolar motors necessarily have a single-turn coil, which limits them to very low voltages. This has restricted the practical application of this type of motor.
- Ball bearing motor – A ball bearing motor is an unusual electric motor that consists of two ball bearing-type bearings, with the inner races mounted on a common conductive shaft, and the outer races connected to a high current, low voltage power supply. An alternative construction fits the outer races inside a metal tube, while the inner races are mounted on a shaft with a non-conductive section (e.g. two sleeves on an insulating rod). This method has the advantage that the tube will act as a flywheel. The direction of rotation is determined by the initial spin which is usually required to get it going.
Connection types
There are three types of electrical connections between the stator and rotor possible for DC electric motors: series, shunt/parallel and compound ( various blends of series and shunt/parallel) and each has unique speed/torque characteristics appropriate for diffent loading torque profiles/signatures.[1]
Series connection
A series DC motor connects the armature and field windings in series with a common D.C. power source. The motor speed varies as a non-linear function of load torque and armature current; current is common to both the stator and rotor yielding I^2 (current) squared behavior[citation needed]. A series motor has very high starting torque and is commonly used for starting high inertia loads, such as trains, elevators or hoists.[2] This speed/torque characteristic is useful in applications such as dragline excavators, where the digging tool moves rapidly when unloaded but slowly when carrying a heavy load.
With no mechanical load on the series motor, the current is low, the counter-EMF produced by the field winding is weak, and so the armature must turn faster to produce sufficient counter-EMF to balance the supply voltage. The motor can be damaged by over speed. This is called a runaway condition.
Series motors called "universal motors" can be used on alternating current. Since the armature voltage and the field direction reverse at (substantially) the same time, torque continues to be produced in the same direction. Since the speed is not related to the line frequency, universal motors can develop higher-than-synchronous speeds, making them lighter than induction motors of the same rated mechanical output. This is a valuable characteristic for hand-held power tools. Universal motors for commercial power frequency are usually small, not more than about 1 kW output. However, much larger universal motors were used for electric locomotives, fed by special low-frequency traction power networks to avoid problems with commutation under heavy and varying loads.
Shunt connection
A shunt DC motor connects the armature and field windings in parallel or shunt with a common D.C. power source. This type of motor has good speed regulation even as the load varies, but does not have the starting torque of a series DC motor.[3] It is typically used for industrial, adjustable speed applications, such as machine tools, winding/unwinding machines and tensioners.
Compound connection
A compound DC motor connects the armature and fields windings in a shunt and a series combination to give it characteristics of both a shunt and a series DC motor.[4] This motor is used when both a high starting torque and good speed regulation is needed. The motor can be connected in two arrangements: cumulatively or differentially. Cumulative compound motors connect the series field to aid the shunt field, which provides higher starting torque but less speed regulation. Differential compound DC motors have good speed regulation and are typically operated at constant speed.
QUESTIONS & ANSWERS
What is the difference between a DC motor and ac motor?
Answer:
D.C motor works on direct current. and by changing its polarity we can change the direction of motor shaft. Where in A.C motor this all not happening it and it works on alternative current
Difference between ac motor and dc motor?
Answer:
Direct current or DC electric motors work for situations where speed needs to be controlled. DC motors have a stable and continuous current. DC motors were the first and earliest motors used. They were found, however, to not be as good at producing power over long lengths. Electric companies found using DC motors to generate electric did not work because the power was lost as the electric was transmitted. Brush DC motors use rings that conduct the current and form the magnetic drive that powers the rotor. Brushless DC motors use a switch to produce the magnetic drive that powers the rotor. Direct current motors are often found in appliances around the home.
Alternating current or AC electric motors are used differently based on what type of AC motor it is. Single phase AC motors are known as general purpose motors. They work well in many different situations. These AC motors work great for systems that are hard to start because they need a lot of power up front. Three phase, also called polyphase, AC motors are usually found in industrial settings. These motors also have high starting power built transmit lower levels of overall power. AC power gets its name from the fact that it alternates in power. The amount of power given off by an AC motor is determined by the amount of power needed to operate the system.
Difference between DC motor and AC generator?
Answer:
Let me tell you what does
each word means:
Motor: is a
device which converts electrical energy(Voltage/Current) into
mechanical energy (eg: rotating shaft)
Generator:
is a device which converts mechanical energy (eg: rotating shaft)
into electrical energy(Voltage/Current).
DC:
electricity in which voltage has constant polarity and current
flows in same direction.
AC:
electricity in which voltage switches polarity (+ve to -ve &
vice versa) and current flows backforth (Changes direction
periodically)
An AC generator can act as
an AC motor (ie what is called "motoring", and is undesireable in
the power industry).
The most obvious difference
between what you're asking is their purpose as the prevous answerer
points out. Second is one uses AC vs. DC., which effects the
physical makeup of the device. There are many more, but I will stop
with three - a DC motor's speed is variable / not constant, while
AC generators are typically spun at "synchronous" speed, and kept
there.
What is the difference between dc motor and dc shunt motor?
]
Answer:
D.C. motors are categorised according to the way in which the field winding is connected in relation to the armature winding. The term 'shunt' is an archaic word for 'parallel', so a d.c. 'shunt motor' is one in which the field winding is connected in parallel with its armature winding. Another category is the d.c. 'series motor', in which the field winding is connected in series with the armature winding. Yet another category is the d.c. 'compound motor', in which there are two field windings; one in parallel, and the other in series, with the armature winding. Each of these connections gives a d.c. motor a different operating characteristic, making them suitable for different types of load.
What is difference between stab shunt dc motor and shunt dc motor?
Answer:
A Stab-Shunt wound motor
is a type of compound motor. Is has a field in series with the
armature and also a field in parallel (shunt).
You can tell the difference
between the leads as the field in series with the armature has to
be able to handle the amperage load of the armature itself. It will
have leads identical in size to the armature.
The key issue to watch for
is wiring the shunt and the series field in opposition to each
other. This will create a differentially wound motor (suicide
motor). The fields will (in effect) offset each other as
speed/current increases. The faster it rotates, the more the series
field will cancel the shunt field. This is DC motors 101, less
field = faster rotation. This can cause a runaway motor.
What is difference between shunt series dc motor?
Answer:
field winding is connected in series with armature
DC shunts motor application?
Answer:
Centrifugal pumps
What are the applications of a DC shunt motor?
Answer:
Dc shunt motor is a constant flux and constant speed motor . So that it is used in centrifugal pumps ,machine tools ,blowers and fans etc.
What is the application of dc shunt generator?
Answer:
DC shunt motor is used where constant speed is required.
Applications of a dc shunt generator?
Answer:
1.for lightening
purposes
2.for battery
charging
3.for
electroplating
What are the characteristics of dc shunt generator?
Answer:
The open-circuit characteristics and the load characteristics
Why you perform open circuit characteristics of a dc shunt generator?
Answer:
to find the relation between emf generated and Field current .....
What will happen when shunt field is kept open when generator is running in a dc shunt generator?
Answer:
n=emf/flux according to
this formula field open means flux
will be zero. Flux zero
means speed will be infinity, but we
know that no machine can't
reach infinite speed so that
theoretically ok but
practically starting speed will
increase then it will
reduced zero.
What will happen if the field of a DC shunt motor is opened under running condition?
Answer:
Typical applications; "the motor will have a limited amount of torque, lesser in value than what is needed for normal operation. Thus, is real world workings the unit will overload, stop, not work, trip OL.
What will happen when field winding of dc shunt motor is disconnected?
Answer:
when the field winding of dc shunt motor is disconnected, the armature takes full line current and it runs very high speed
What will happen when armature winding of a dc shunt motor is disconnected?
Answer:
In D.C Shunt motor, the field winding is connected parallel with the armature winding. If you remove the supply to armature winding, the motor will stop after some time.
What are field windings and armature windings in a dc motor and generator?
Answer:
Field winding is for producing magnetic field (flux) both in DC Motor and DC Generator. EMF is induced in armature winding of DC Generator which finally gives power supply/load current to electrical load circuit (lamp, fan etc.). Power supply is fed to Armature winding in a DC Motor to produce torque for running the motor.
What Three types of windings are used on dc generator armature?
Answer:
Shunt, series, and compound windings.
Which type of dc armature winding requires equalizer rings?
Answer:
Certain "Lap" wound armatures.
Armature reaction in a DC generator?
Answer:
ARMATURE REACTION in DC Machine
All current-carrying conductors produce magnetic fields. The magnetic field produced by current in the armature of a dc generator affects the flux pattern and distorts the main field. This distortion causes a shift in the neutral plane, which affects commutation. This change in the neutral plane and the reaction of the magnetic field is called armature reaction.
In a dc generator brushes are placed in which side to avoid armature reaction?
Answer:
along the M.N.A axis
along the M.N.A axis
In a DC generator if the brushes are given a small amount of forward shift the effect of armature is?
Answer:
is fully diamagnetism
Why do armature resistance is low in dc generators?
Answer:
Because it is made of pure wire with nothing beyond its length and interaction with its neigh boring wires to create resistance
Why the value of armature resistance is low?
Answer:
because it depends inversely to speed
How do you measure armature resistance practically?
Answer:
by using measuring meter
Why armature resistance is maximun in motors?
Answer:
avoid high stating currents
Why external resistances are added to armature circuit in a dc motor?
Answer:
extra resistance is added in order to decrease starting current and improve starting torque
Why is the armature rheostat of dc motor kept at maximum resistance position?
Answer:
to limit the high current
Why motor rheostat kept minimum position generator rheostat maximum position?
Answer:
the back emf increases so that high currents doesn't pass through the field windings
Why rheostat position kept in minimum position in shunt motor load test experiment?
Answer:
it is kept at minimum position to produce more torque which is required for starting a motor
What is Swinburne’s test of dc shunt motor?
Answer:
Swinburne’s test on dc shunt machine is to predetermine the efficiency of the dc machine , but it is not accurate it is just like estimation of efficiency of dc shunt machine when it is run as a motor and a generator............................
Why is Swinburne's test done on dc machines?
Answer:
because we cannot apply to series motor
Why Swinburne’s test cannot be performed on series machine?
Answer:
This is a no load test and so it cannot be performed on series machine
How can Swinburne's test be adopted for series motor?
Answer:
the method cannot be used in the case of a d.c series motor
What is Swinburne's test?
Answer:
For a d.c shunt motor
change of speed from no load to full load is quite small.
Therefore, mechanical loss can be assumed to remain same from no
load to full load. Also if field current is held constant during
loading, the core loss too can be assumed to remain
same.
In this test, the motor is
run at rated speed under no load condition at rated voltage. The
current drawn from the supply IL0 and the field current If are
recorded. Since the motor is operating under no load condition, net
mechanical output power is zero. Hence the gross power developed by
the armature must supply the core loss and friction & wind age
losses of the motor.
The biggest advantage of
Swinburne's test is that the shunt machine is to be run as motor
under no load condition requiring little power to be drawn from the
supply; based on the no load reading, efficiency can be predicted
for any load current. However, this test is not sufficient if we
want to know more about its performance (effect of armature
reaction, temperature rise, commutation etc.) when it is actually
loaded. Obviously the solution is to load the machine by connecting
mechanical load directly on the shaft for motor or by connecting
loading rheostat across the terminals for generator operation. This
although sounds simple but difficult to implement in the laboratory
for high rating machines (say above 20 kW), Thus the laboratory
must have proper supply to deliver such a large power corresponding
to the rating of the machine. Secondly, one should have loads to
absorb this power.
What are the advantages of Swinburne's test?
Answer:
we can predetermine the efficiency of a dc machines working as a motor or generator
What are the advantages and disadvantages of Swinburne's method?
Answer:
Everything has its
advantages and disadvantages both,
so accept these logics and
their aspects in your life.
What are the advantages and disadvantages of Registration Method?
Answer:
Advantage is that people will be see you as you become the member and anybody can access you anywhere in the world and the disadvantage is that people can hack your info
Explain advantages and disadvantages of software registration?
Answer:
The advantage is that that the company will be able to send you updates or new add-ons and more. The downside is that you have to submit you personal information to the software company.
BRUSHLESS MOTORS
Brushless Motors
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.
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.
- Characteristics
No I2R loss in
the rotor.
Inert rotor. No permanent
magnet.
Compact size and low
weight.
Low cost.
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 efficiency.
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.
- Applications
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.
Household appliances.
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).
- Characteristics
Precise position control.
Simple open loop position control.
Amenable to simple computer control.
- Applications
Used in computer plotters and printers.
Industrial controls.
Numerically controlled machine tools.
Robotic equipment.
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 theresult 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.
- Characteristics
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.
- 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.
Reluctance 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.