Download In a typical DC motor, there are permanent magnets on the outside

Building Energy Systems, 2009 Fall. Electric Motor Supplemental Notes
In a typical DC motor, there are permanent magnets on the outside and a spinning
armature on the inside. The permanent magnets are stationary, so they are called
the stator. The armature rotates, so it is ca lled the rotor.
The armature contains an electromagnet . When you run electricity into this
electromagnet, it creates a magnetic field in the armature that attracts and repels the
magnets in the stator. So the armature spins through 180 degrees. To keep it spinning, you
have to change the poles of the electromagnet . The brushes handle this change in polarity.
They make contact with two spinning electrodes attached to the armature and flip the
magnetic polarity of the electromagnet as it spins.
•
The brushes eventually wear out.
•
Because the brushes are making/breaking connections,
you get sparking and electrica l noise.
•
The brushes limit the maximum speed of the motor.
•
Having the electromagnet in t he center of the motor
makes it harder to coo l.
•
The use of brushes puts a li mit on how many poles the
armature can have.
How Brushless Motors Work
A brushless motor looks a lot like a brushed motor that has been turned inside out . The
magnets are attached to the armature instead of the can and the windings are attached to
the case instead of the armature. In operation your magnets spin and
not your windings.
As shown in the figures, the rotor has permanent magnets mounted to
it and is supported by a pair of ball bearings . These are the only moving
parts in the motor and are also the only parts to wear out. Compare this
to a brushed motor with its brush and commutator interface wear areas.
The stator or armature is made up of a laminated stack and wound with
copper windings. Magnetic sensors detect the angular position of the
rotor with respect to the stator. These sensors tell the controller (ESC) wh ich loop needs to
have the current on and which ones need the current off. This allows the motor to produce a
continuous torque in one direction.
What are the Advantages Brushless Motors
•
•
•
•
•
•
Less radio interference since there is no brush arching
Less resistance in the windings since there is no brush to commutator interface . Additionally
you can have thicker wires (lower resistance) than a brushed motor of equ ivalent size . Both of
these result in overall lower resistance and higher efficiency and torque
Lower friction loss since there is no spring/brush pressure and drag on the commutator.
They are efficient over a much wider range of power. Efficiencies range from 75% up to nearly
90% depending on style/brand.
Better cooling since the heat generating wind ings are aga inst the outer case and not buried
inside.
No springs to change, no brushes to rep lace, no commutator to cut.
,
./
Pa.9JA 5ij{~
t /+
Building Energy Systems, 2009 Fall. Electric Motor Supplemental Notes
•
•
•
•
The motor can be easily synchronized to an internal or external clock, leading to precise speed
control.
Brushless motors have no chance of sparking, unlike brushed motors, making them better
suited to environments with volatile chemicals and fuels. Also, sparking generates ozone which
can accumulate in poorly ventilated buildings risking harm to occupants' health.
Brushless motors are usually used in small equipment such as computers and are generally
used to get rid of unwanted heat .
They are also very quiet motors which is an advantage if being used in equipment that is
affected by vibrations.
Modern DC brush less motors range in power from a fraction of a watt to many kilowatts. Larger
brush less motors up to about 100 kW rating are used in electric vehicles . Brushless DC motors are
commonly used where precise speed control is necessary, as in computer disk drives or in video
cassette recorders, the spindles within CD, CD-ROM (etc.) drives, and mechanisms within office
products such as fans , laser printers and photocopiers . They also find significant use in highperformance electric model aircraft.
Induction motor
An induction motor (1M) is a type of alternating current motor where power is supplied to the
rotating device by means of electromagnetic induction . It is also called asynchronous motor.
Three-phase induction motors aka squirrel-cage AC motor
An electric motor converts electrical power to mechanical power in its rotor(rotating part). There are
several ways to supply power to the rotor. In aDC motor this power is supplied to
the armature directly from a DC source, while in an induction motor this power is induced in the
rotating device. An induction motor is sometimes called a rotating transformer because the
stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part)
is the secondary side . The primary side's currents evokes a magnetic field which interacts with the
secondary sides mmf to produce a resultant torque ,henceforth serving the purpose of producing
mechanical energy. Induction motors are widely used, especially polyphase induction motors, which
are frequently used in industrial drives.
The most common rotor is a squirrel-cage rotor . It is made up of bars of either solid copper (most
common) or aluminum that span the length of the rotor, and are connected through a ring at each
end. The rotor bars in squirrel-cage induction motors are not straight, but have some skew to reduce
noise and harmonics.
Induction motors are now the preferred choice for industrial motors due to their rugged construction,
absence of brushes (which are required in most DC motors) and - thanks to modern power
electronics - the ability to control the speed of the motor.
The induction motor with a wrapped rotor was invented by Nikola Tesla in 1882 in France but the
initial patent was issued in 1888 after Tesla had moved to the United States . In his scientific work,
Tesla laid the foundations for understanding the way the motor operates. The induction motor with a
Building Energy Systems, 2009 Fall. Electric Motor Supplemental Notes
cage was invented by Mikhail Dolivo-Dobrovolsky about a year later
in Europe . Technological development in the field has improved to where a
100 .!:ill (74 .6 kW ) motor from 1976 takes the same volume as a 7.5 hp (5.5
kW) motor did in 1897 . Currently, the most common induction motor is the
cage rotor motor. A 3-phase power supply provides a rotating magnetic field
in an induction motor
The basic difference between an induction motor and a synchronous
current is supplied onto the rotor. This then creates a magnetic field
interaction, links to the rotating magnetic field in the stator which in
called synchronous because at stead y state the speed of the rotor is
rotating magnetic field in the stato r:..
~. ; ~ '
~
AC motor is that in the latter a
which, through magnetic
turn causes the rotor to turn. It is
the same as the speed of the
By way of contrast, the induction motor does not have any direct su pp ly onto the rotor ; instead, a
seconda ry current is induced in the rotor. To achieve this, stator windings are arranged around the
rotor so that when energized with a polyphase supply they create a rotating magnetic field pattern
which sweeps past the rotor. This changing magnetic field pattern induces current in the rotor
conductors. These currents interact with the rotating magnetic field created by the stator and in effect
cause a rotational motion on the rotor.
However, for these currents to be induced, the speed of th e/p hysical rotor Jand theCspeed of t he
~tatlng magnetic fiel d)in the stator must be different. or else the magnetic field will not be moving
relative to the rotor conductors and no currents will be induced . If by some chance this happens, the
rotor typically slows slightly until a current is re-induced and then the rotor continues as before . This
Qlfte reo'-.e between the speed of the rotor and speed of the rotating magnetiC field in the stator is
called slJ.E: It is unit less and is the ratio between the relative speed of the magnetic field as seen by
the rotor (the slip speed) to the speed of the rotating stator field . Due to this an ~ uction motor is
sometimes referred to as an asynchronous m ~
The relationship between the supply frequency, f, the number of poles, p, and the synchronous speed
(speed of rotating field),n s is given by :
f = pn6
120
From this relationship:
S n hronous peed n& =
•
•
•
120f
p
[rev jmin]
n = Revolutions per minute (rpm)
f = AC power frequency (hertz)
p = Number of poles (an even number), For example a 6 pole motor would have 3 pole pairs
The rotor speed is:
Rot.or p ed nr = n (1 - ,,)
where s is the slip.
Slip is calculated using:
A synchronous motor always runs at synchronous speed with
0% slip .
Speed control
The synchronous rotational speed of the rotor (i.e . the theoretica l unloaded speed with no slip) is
controlled by the number of pole pairs (number of windings in the stator) and by the frequency of the
supply voltage. Before the development of cheap power electroniCS, it was difficult to vary the
frequency to the motor and therefore the uses for the induction motor were limited .
.
/-=Page- ~ ':&
Building Energy Systems, 2009 Fall. Electric Motor Supplemental Notes
The general term for a power electronic device that controls the speed of a motor as well as other
parameters is inverter. A typical unit will take the mains AC supply, rectify and smooth it into a "link"
DC voltage, and, then convert it into the desired AC waveform . In general, a DC-to-AC converter is
called an inverterL which is probably where the motor-control inverter gets its name.
Because the induction motor has no brushes and is easy to control, many older DC motors are being
replaced with induction motors and accompanying inverters in industrial applications .
Starting of induction motors
The simplest way to start a three- phase induction motor is to connect its terminals to the line. This
method is often called "direct on line" and abbreviated DOL.
In an induction motor, the magnitude of the induced emf in the rotor circuit is proportional to the
stator field and the slip speed (the difference between synchronous and rotor speeds) of the motor,
and the rotor current depends on this emf. When the motor is started, the rotor speed is zero. The
synchronous speed is constant, based on the frequency of the supplied AC voltage . So the slip speed
is equal to the synchronous speed, the slip ratio is 1, and the induced emf in the rotor is large. As a
result, a very high current flows through the rotor. This is similar to a transformer with the secondary
coil short circuited, which causes the primary coil to draw a high current from the mains.
When an induction motor starts DOL, a very high current is drawn by the stator, in the order of 5 to 9
times the full load current. This high current can , in some motors, damage the windings; in addition,
because it causes heavy line voltage drop, other appliances connected to the same line may be
affected by the voltage fluctuation. To avoid such effects, several other strategies are employed for
starting motors.