Óbuda University, Bánki Donát Faculty of Mechanical &
Safety Engineering
Institute of Mechatronics & Vehicle Engineering
Electrical Engineering - 1
5. Lecture
Electrical Machines – AC, Stepper
motors
Arranged by:
Istvá
István Nagy
Ingrid Langer
Main themes
Rotating Electrical
Machines
Generators
AC Generators
Synchronous
Asynchronous
DC Generators
shunt
series
Motors
AC Motors
Synchronous
Asynchronous
separately
DC Motors
Universal
motors
Stepper motors
shunt
series
separately
compound
Mechanical energy ⎯generator
⎯⎯
⎯→ Electrical energy
Electrical energy ⎯motor
⎯
⎯→ Mechanical energy
Topics of AC motors
Asynchronous (induction) motors
Most AC motors are induction motors. Induction motors are
favored due to their ruggedness and simplicity. In fact, 90% of
industrial motors are induction motors.
Most large ( > 1 hp or 1 kW) industrial motors are poly-phase
induction motors. By poly-phase, we mean that the stator
contains multiple distinct windings per motor pole, driven by
corresponding time shifted sine waves. In practice, this is two or
three phases. Large industrial motors are 3-phase.
Induction motor: the stator windings induce a current flow in
the rotor conductors, like a transformer, unlike a brushed DC
commutator motor.
Construction:
Asynchronous motors’ construction - Stator
The stator is wound with pairs of coils corresponding to the phases of electrical energy
available. The 2-phase induction motor stator (see figure a.)) has 2-pairs of coils, one pair for
each of the two phases of AC (figure b.) 3-phase winding). The individual coils of a pair are
connected in series and correspond to the opposite poles of an electromagnet. That is, one coil
corresponds to a N-pole, the other to a S-pole until the phase of AC changes polarity. The other
pair of coils is oriented 90o in space to the first pair. This pair of coils is connected to AC shifted
in time by 90o in the case of a 2-phase motor. The terminals of the coils are outputted, and can
be connected in ∆ or Y connection to the to the 3-phase power supply.
Asynchronous motors’ construction - Rotor
The key to the popularity of the AC induction motor is simplicity as evidenced by the simple rotor . The rotor
consists of a shaft, a steel laminated rotor, and an embedded (or separated) copper or aluminum squirrel
cage.
As compared to a DC motor armature, there is no commutator. This eliminates the brushes, arcing,
sparking, graphite dust, brush adjustment and replacement, and re-machining of the commutator.
Slip ring (with wounded rotor )
Cage (short(short-circuited rotor)
The rortor contains 33-phase winding, which
is usually connected to the Y. The terminals
of the winding are connected to the slipring
located at the end of the shaft.
The rotor is consist from the shortshort-circutted cage,
what can be an embedded squirrel cage or aa
conductive cage removed from rotor. (See figure:
figure: ⇒
)
Construction of the slip ring (wounded)
asynchronous motor
Construction of the slip ring (wounded)
asynchronous motor
Comparation of the slip ring vs. cage motors
Slip ring motors
Complicated construction
Expensive
☺ High starting torque
☺ Starting is with rheostat
Cage motors
☺ Simply construction
☺ Fool proof operation
☺ Cheap
Complicated speed control
The 90% of asynchronous motors used in industry are cage
motors.
Principle of operation
In both induction and synchronous motors, the AC power supplied to the motor's stator creates a magnetic
field that rotates in time with the AC oscillations. Whereas a synchronous motor's rotor turns at the same rate
as the stator field, an induction motor's rotor rotates at a slower speed than the stator field. The induction
motor stator's magnetic field is therefore changing or rotating relative to the rotor. This induces an opposing
current in the induction motor's rotor, in effect the motor's secondary winding, when the latter is short-circuited
or closed through an external impedance. The rotating magnetic flux induces currents in the windings of the
rotor; in a manner similar to currents induced in transformer's secondary windings. These currents in turn
create magnetic fields in the rotor that react against the stator field. Due to Lenz's Law, the direction of the
magnetic field created will be such as to oppose the change in current through the windings. The cause of
induced current in the rotor is the rotating stator magnetic field, so to oppose this the rotor will start to rotate in
the direction of the rotating stator magnetic field. The rotor accelerates until the magnitude of induced rotor
current and torque balances the applied load. Since rotation at synchronous speed would result in no induced
rotor current, an induction motor always operates slower than synchronous one. The difference between
actual and synchronous speed or slip varies from about 0.5 to 5% for standard induction motors. The
induction machine's essential character is that it is created solely by induction instead of being separately
excited as in synchronous or DC machines or being self-magnetized as in permanent magnet motors.
For these currents to be induced, the speed of the physical rotor must be lower than that of the stator's
rotating magnetic field (Ω0), or the magnetic field would not be moving relative to the rotor conductors and no
currents would be induced. As the speed of the rotor drops below synchronous speed, the rotation rate of the
magnetic field in the rotor increases, inducing more current in the windings and creating more torque. The
ratio between the rotation rate of the magnetic field as seen by the rotor (slip speed) and the rotation
rate of the stator's rotating field is called slip. Under load, the speed drops and the slip increases enough
to create sufficient torque to turn the load. For this reason, induction motors are sometimes referred to as
asynchronous motors. An induction motor can be used as an induction generator, or it can be unrolled to form
the linear induction motor which can directly generate linear motion.
Principle of operation
A three-phase power supply provides a rotating magnetic field in an
induction motor.
Operation
Slip rings
Short-cutting
ring
Brushes
Rotor
Starting rheostat
Stator
To the three coils of stator located in 120°
120° from each other, we are
connecting 3 phase power supply, where the phases are shifted
120°
120° from each other. The result is rotating magnetic field in stator,
with synchronous speed n0 (or synchronous angularangular-speed Ω0)
n0 =
60 ⋅ f
p
⎡ 1 ⎤
⎢ min ⎥
⎣
⎦
Where: f [Hz]: the frequency of supplying voltage
p:
number of polepole-pairs
See more: http://www.youtube.com/watch?v=N8LUOTQKXlk
Slip
The relative lagging of the rotor related to the synchronous speed
speed (Ω
(Ω0 – speed of the rotating magnetic field): slip
s=
n0 − n
n
= 1−
n0
n0
p
s
n = (1 − s)n0
1
n0
Reverse current
braking mode
n0
nn
[1/min]
[%]
1
3000
280028002880
4 -6
2
1500
141014101450
3 -5
3
1000
920920970
3 -8
4
750
710710730
2 -5
n
Generator mode
Motor mode
sn
[1/min]
Power ballance and, efficiency
P1 = 3U1 ⋅ I1 ⋅ cos ϕ
Pl
Input power
Airgap
Power
Pm = (1 − s)Pl
Mechanical
power
P2 = M ⋅ Ω 2
Output power
η=
• Stator copper-winding
losses Pt1=3If12R1, where R
is resistance of one phasecoil in
stator
• Magnetical (iron) losses
Piron
Stator losses
Rotor loses
Rotor copper losses
Pt 2 = 3⋅ I 2f 2 ⋅ R2 = s ⋅ Pl
(Piron2 ≈ 0)
Frictional and ventilation
losses
P2
P1
The torque of asynchronous motor
Pl = M ⋅ Ω 0
Synchronouse angular speed
Pt 2 = Pl − Pmech = M ⋅ Ω 0 − M ⋅ (1 − s )Ω 0 = M ⋅ s ⋅ Ω 0 = 3 ⋅ I 22 ⋅ R2 (= s ⋅ Pl )
Voltage on the terminals
of rotor
I2 =
U2 = s ⋅ U20
U2
R22 + X L22
The angular speed of
voltage, induced in rotor
X L 2 = ω2 ⋅ L2 = s ⋅ ω0 ⋅ L2 = s ⋅ X L 20
Reactance of 1
phase in rotor in
steady state mode
Reactance of 1
phase in rotor
Resistance of 1
phase in rotor
3 ⋅ s ⋅ U ⋅ R2
= M ⋅ s ⋅ Ω0
R22 + s 2 ⋅ X L220
2
ω2 = s ⋅ ω0
Voltage on the terminals
of rotor in steady state
2
20
3 ⋅ s ⋅ U 202 ⋅ R2
M= 2 2
R2 + s ⋅ X L220 ⋅ Ω 0
(
)
The result: The motor torque is proportional to the
square of voltage measured in rotor, in steady
state (stand) mode.
The adequate circuit of the asynchronous motor
Stator winding
Air gap
Rotor winding
R1 :Stator resistance in 1 phase
Xs1: Stator reactance in 1 phase
Xm: magnetizing reactance
X's2: Rotor reactance of 1 phase
related to the stator’s nr. of turns
in 1 phase
R'2: Rotor resistance of 1 phase
related to the stator’s nr f turns in
1 phase
Rv: Resistance of iron losses
R't: the adequate resistance of load
Torque-speed characteristic
Motor (normal) mode
Motor brake mode
Generator mode
Pullout (max. torque)
Performance criteria of asynchronous motors
The performance of an asynchronous motor can be characterized by the following major factors:
•Efficiency
•Power factor (cos ϕ)
•Starting torque
•Starting current
•Pullout (or maximum) torque
Starting of asynchronous motor
The problem:
problem: Istart=(3...9)xIn this cause voltage drops in
supplying network circuit.
Torque of the
driven device
The starting current must
be reduced.
1. Starting of slip ring (wound)
asynchronous motors
M
Starting current reduction is achieved with:
Series resistance inserted in three lines are used to limit the
starting current.
These resistances are shortshort-circuited, once the motor has
gained speed. (inefficient(inefficient- lot of thermal losses)
High rotor resistance results in high starting torque!
m
0
The characteristics after the continuous short-circuiting the
resistors
n0
Starting of asynchronous motor
2. Starting of cage motors
•
•
Direct starting:
Can be applied in case of small induction motors connected to the large
performance network.
Reduced-Voltage Starting (large motor’s starting)
starting)
With inserting some resistors (or startingstarting-coils) between the motor and
supplying network.
I.
The starting torque is also reducing
Starting of asynchronous motor
II. Starting with transformers (applicable for highhigh-voltage motors)
motors)
A transformer is inserted between the motor and supplying network.
network.
First the switches 2K and 1K are closed,
closed, the
then, close to the operational speed 2K will be opened
and 3K closed.
The network drop and the starting torque is significantly decreasing
decreasing (1/a2).
Starting of asynchronous motor
III. Y-∆ (wye-delta) starting (small voltage motors,
motors, over 3 kW)
At the start the stator windings
are connected in Y, then close to
the operational speed they are
reconnected to ∆.
The current consumption in Y is
1/3 (one(one-third) of current
consumption in ∆, but the
starting torque is decreasing by
1/3 (one(one-third).
Advantages: inexpensive.
I∆
=
IY
2K close,
close, 3K open:
open: Y (wye)
wye) connection
3K close,
close, 2K open:
open: ∆ (delta) delta
3 ⋅ I f∆
=
I fY
3
U f∆
Zf
U fY
Zf
=
M∆
U2
U2
= 2f ∆ =
= 3
2
MY
U fY
⎛ U ⎞
⎜
⎟
⎝ 3 ⎠
3 ⋅U
=3
U
3
Starting of asynchronous motor
3. Deep bar doubledouble-cage motors
motors
DeepDeep-bar – deep slots are used, where the slot depth is 2 or 3 times greater
greater than the slot
width.
DoubleDouble-cage – the inner cage is deeply embedded in iron and has lowlow-resistance bars. The
outer cage has relatively high resistance bars close to the stator.
or.
stat
Operation:
• DeepDeep-bar cagge:
cagge: The conductors are tall and tight bars.
bars. The inductivity of lower part of bar
is many times higher than inductivity of upper part. At the start,
start, when the rotor’s current
frequency is high (f2=sf0), the current is flowing in the upper (containing smaller reactance)
reactance)
part of bar ( skin effect). This effects, like increasing the resistance of rotor:
Mstart ⇑ Istart⇓.
Double cage : The resistance of the cage near to the air gap is large, consequently
consequently the
inductivity is small. The resistance of the inner cage is small → large inductivity. At the start
the current is flowing through the outer cage with small inductivity
inductivity and near to the
operational speed is continuously replacing to the conductors of inner cage.
Speed control of asynchronous motors
n = n 0 ⋅ (1 − s )
f
n0 =
p
n=
f
⋅ (1 − s )
p
The speed can be controlled by the:
the:
• slip changing
• number of poles
• frequency changing
1. Slip changing (asynchronous slip ring (wound) motors
motors)
The airair-gap power (Pl) is depending on load torque.
torque. If load torque (on figure Mt) does not changing, then Pl does not changing
too.
Pl =
Pt 2
Pt 2
= const.
s
Pt 2
changing
s is changing
changing:
changing: with resistors (∆R’) inserted into rotor circuit.
R2' + ∆R' R2'
=
s1
s
Speed control of asynchronous motors
2. Changing of numbers of poles (cage motors
motors)
LossLoss-less solution
Commonly used method is:
The Dahlander’s
Dahlander’s winding: The phase winding are divided for two parts, which can be connected
connected in series or
parallel.
Speed control of asynchronous motors
3. Frequency changing in stator
The most frequently used method, usually realized with variablevariable-frequency drivers (VFD), or different types of
inverters.
Advantages:
•LossLoss-less
•Contiuous control of speed
•It is reachable the speed over 3000 RPM.
3 phase
supply
Energy
store
Asynchronous motor
Sensors
Reference
speed
Controller
Electronically control of asynchronous motor
PCU – (power conditioning unit) includes the energy source (which may be a CD source, too) a means of producing an
AC variable-frequency source (the inverter) from the available DC source, and some means of controlling the output
voltage [the adjustable voltage inverter (AVI) or the PWM (pulse-wide modulated) inverter].
In practice, it would be desirable to keep the voltage-to-frequency (U/f) ratio fixed, as we shall see.
INVERTERS:
Basically has DC input
and AC output. Is a
backbone of the AC
drive systems.
Types:
•AC transistor inverter
•AC SCR McMurray
inverter
•AC SCR loadcommutated inverter
•AC SCR current inverter
(ASCI)
•AVI – in AVI the output
voltage and frequency
can both varied.
Block diagram for asynchronous motor
Control of AC motors - summary
Braking of asynchronous motors
1. Regenerate braking
When the loading is speeding up, over the synchronous speed, the rotor. The motor is acting as generator and the
mechanical energy changing to the electrical one.
A pullout torque can not be exceeded, because the load can not be
braked more!
more!
Motor mode
Braking
mode
Braking of asynchronous motors
2. Reverse current braking
Changing the direction of rotating flux:
flux:
• with changing two phases:
phases: slowing
• when the rotor’s rotation direction is changing, but the rotation’s
rotation’s direction of flux is still remain: load sinking
3. Dynamical braking (braking by DC field in stator)
stator)
The stator is disconnected from the network, and excited by DC,
and to the slipslip-rings of the motor are connected, so called
„braking„braking-resistors”
resistors”.
Single-phase asynchronous motors
Small-power motors where 3-phase is not available. (eg. Domestic compressors, pumps).
Construction:
Stator: contain 1-phase winding
Rotor: Copper or aluminium squirrel cage, (sometimes 3ph slip ring)
The single coil of a single phase asynchronous motor does not produce a rotating magnetic field, but a pulsating field
reaching maximum intensity at 00 and 1800 electrical. In another view is that the single coil excited by a single phase
current produces two counter rotating magnetic field phasors coinciding twice per revolution at 00 and 1800. (see Figure
below a-e).
• Thus no
n starting torque is developed.
• will develope torque once the rotor is started
•Must be started with assistant device (usually
some capacitor)
Permanent-split capacitor asynch. motor
Embedded stator cols in singlephase asynchronous motors.
One way to solve the single phase problem is to build a 2-phase motor,
deriving 2-phase power from single phase. This requires a motor with two
windings spaced apart 900 electrical, fed with two phases of current
displaced 900 in time.
Capacitor-start asynch. motor
Switchin off the
2nd phase
A larger capacitor may be used to
start a single phase induction motor
via the auxiliary winding if it is
switched out by a centrifugal switch
once the motor is up to speed.
Moreover, the auxiliary winding may
be many more turns of heavier wire
than used in a resistance split-phase
motor to mitigate excessive
temperature rise. The result is that
more starting torque is available for
heavy loads like air conditioning
compressors.
Capacitor-run asynch. motor
A variation of the capacitor-start motor is to start the motor with a relatively large capacitor
for high starting torque, but leave a smaller value capacitor in place after starting to improve
running characteristics while not drawing excessive current.
A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could
be two + to + (or - to -) series connected polarized electrolytic capacitors. Such AC rated
electrolytic capacitors have such high losses that they can only be used for intermittent duty (1
second on, 60 seconds off) like motor starting. A capacitor for motor running must not be of
electrolytic construction, but a lower loss polymer type.
Synchronous machines
Most important machines at the producing of AC electrical energy
Can operate as motor and generator equally
Usually made in 3-phase realization
Permanently can operate only at the synchronous revolution
The armature is excited with DC
n0 =
60 ⋅ f ⎡ 1 ⎤
p ⎢⎣ min ⎥⎦
Construction:
Construction:
Stator:
3-phase winding
Stator: laminated, contain 3Rotor:
Rotor: can be cylindrical or salient pole
Salient pole:
pole:
Cylindrical (non(non-salient):
salient):
more number of poles ⇒
smaller speed of rotation
less number of poles ⇒ higher speed of
rotation
A synchronous electric motor is an
AC motor in which, at steady state,
the rotation of the shaft is
synchronized with the frequency of
the supply current; the rotation
period is exactly equal to an integral
number of AC cycles. Synchronous
motors contain electromagnets on
the stator of the motor that create a
magnetic field which rotates in time
with the oscillations of the line
current. The rotor turns in step with
this field, at the same rate.
Operation of synchronous machines
The operation of a synchronous motor is due to the interaction of the magnetic fields of the stator and the rotor.
The stator winding, when excited by a poly-phase (usually 3-phase) supply, creates a rotating magnetic field
inside the motor. The rotor locks in with the rotating magnetic field and rotates along with it.
The rotor locks in with the rotating magnetic field and rotates along with it. Once the rotor locks in with the
rotating magnetic field, the motor is said to be in synchronization. A single-phase (or two-phase derived from
single phase) stator winding is possible, but in this case the direction of rotation is not defined and the machine
may start in either direction unless prevented from doing so by the starting arrangements.
Once the motor is in operation, the speed of the motor is dependent only on the supply frequency. When the
motor load is increased beyond the breakdown load, the motor falls out of synchronization and the field winding
no longer follows the rotating magnetic field. Since the motor cannot produce (synchronous) torque if it falls out
of synchronization, practical synchronous motors have a partial or complete squirrel-cage damper (amortisseur)
winding to stabilize operation and facilitate starting. Because this winding is smaller than that of an equivalent
induction motor and can overheat on long operation, and because large slip-frequency voltages are induced in the
rotor excitation winding, synchronous motor protection devices sense this condition and interrupt the power
supply (out of step protection).
http://vimeo.com/groups/37089/videos/10291411
The equivalent circuit
Ua
Us
jXa
jXs
Up
UR
Ia
Ia
jX
R
Ui
Uk
Up
Uk
X = X a + X s ; R << X
Ia =
Uk − Up
jX
Ui = Up + Ua
Ui :Induced voltage (Generator: Uemf, Motor: Ucemf)
Ua: Armature voltage
Up: pole voltage
UT: Terminal voltage
Xa: Armature reactance
Xs: leakage reactance
X: synchronous reactance
Torque of synchronous machine
Stable mode
M ⋅ ω0 = 3 ⋅ Uk ⋅ Ia ⋅ cos ϕ
Ia ⋅ X d ⋅ cos ϕ = Up ⋅ sin δ
M=
3 Uk ⋅ Up
⋅
⋅ sin δ
ω0
Xd
M: torque
δ: loading angle (between Up
and Uk
on Figure: ÚT=Uk
)
Starting of synchronous machine
Synchronous motors are not self-starting motors. This property is due to the inertia of the rotor; it cannot instantly
follow the rotation of the magnetic field of the stator. Since a synchronous motor produces no inherent average torque at
standstill, it cannot accelerate to synchronous speed without some supplemental mechanism.
Large motors operating on commercial power frequency include a "squirrel cage" induction winding which provides sufficient
torque for acceleration and which also serves to damp oscillations in motor speed in operation. Once the rotor nears the
synchronous speed, the field winding is excited, and the motor pulls into synchronization. Very large motor systems may
include a "pony" motor that accelerates the unloaded synchronous machine before load is applied. Motors that are
electronically controlled can be accelerated from zero speed by changing the frequency of the stator current.
Small synchronous motors are commonly used in line-powered electric mechanical clocks or timers that use the powerline
frequency to run the gear mechanism at the correct speed. Synchronous motors in clocks typically use an anti-reversing
mechanism to ensure starting in the correct direction.
Very small synchronous motors are able to start without assistance if the "moment of inertia of the rotor and its mechanical
load is sufficiently small (because the motor) will be accelerated from slip speed up to synchronous speed during an
accelerating half cycle of the reluctance torque".
Stepping into the
synchronous mode
Synchron
ous
Asynchronous
mode
Stepper motors
A stepper motor is a “digital” version of the electric motor. The rotor moves in discrete steps as commanded,
rather than rotating continuously like a conventional motor. When stopped but energized, a stepper (short
for stepper motor) holds its load steady with a holding torque. Wide spread acceptance of the stepper motor
within the last two decades was driven by the ascendancy of digital electronics. Modern solid state driver
electronics was a key to its success. And, microprocessors readily interface to stepper motor driver circuits.
Application wise, the predecessor of the stepper motor was the servo motor. Today this is a higher cost
solution to high performance motion control applications. The expense and complexity of a servomotor is due
to the additional system components: position sensor and error amplifier. It is still the way to position heavy
loads beyond the grasp of lower power steppers. High acceleration or unusually high accuracy still requires a
servo motor. Otherwise, the default is the stepper due to low cost, simple drive electronics, good accuracy,
good torque, moderate speed, and low cost.
A stepper motor positions the read-write heads in a floppy drive. They were once used for the same purpose
in hard-drives. However, the high speed and accuracy required of modern hard-drive head positioning
dictates the use of a linear servomotor (voice coil).
Stepper motor’s characteristic
Stepper motors are rugged and inexpensive because the rotor contains no winding slip rings, or commutator.
The rotor is a cylindrical solid, which may also have either salient poles or fine teeth. More often than not the
rotor is a permanent magnet. Determine that the rotor is a permanent magnet by un-powered hand rotation
showing detent torque, torque pulsations. Stepper motor coils are wound within a laminated stator, except
for can stack construction. There may be as few as two winding phases or as many as five. These phases are
frequently split into pairs. Thus, a 4-pole stepper motor may have two phases composed of in-line pairs of
poles spaced 900 apart.
There may also be multiple pole pairs per phase. For example a 12-pole stepper has 6-pairs of poles, three
pairs per phase. Stepper motors move one step at a time, the step angle, when the drive waveforms are
changed. The step angle is related to motor construction details: number of coils, number of poles, number
of teeth. It can be from 900 to 0.750, corresponding to 4 to 500 steps per revolution. Drive electronics may
halve the step angle by moving the rotor in half-steps.
The torque available is a function of motor speed, load inertia, load torque, and drive electronics as
illustrated on the speed vs torque curve. An energized, holding stepper has a relatively high holding torque
rating. There is less torque available for a running motor, decreasing to zero at some high speed. This speed
is frequently not attainable due to mechanical resonance of the motor load combination.
Variable reluctance stepper
A variable reluctance stepper motor relies upon magnetic flux seeking the lowest reluctance
path through a magnetic circuit. This means that an irregularly shaped soft magnetic rotor will
move to complete a magnetic circuit, minimizing the length of any high reluctance air gap. The
stator typically has three windings distributed between pole pairs , the rotor four salient poles,
yielding a 30o step angle. A de-energized stepper with no detent torque when hand rotated is
identifiable as a variable reluctance type stepper.
Permanent magnet stepper
Construction:
Construction:
The permanent magnet stepper only has two windings, yet has 24-poles in each of two phases. This style of construction is
known as can stack. A phase winding is wrapped with a mild steel shell, with fingers brought to the center. One phase, on a
transient basis, will have a north side and a south side. Each side wraps around to the center of the doughnut with twelve
interdigitated fingers for a total of 24 poles. These alternating north-south fingers will attract the permanent magnet rotor.
If the polarity of the phase were reversed, the rotor would jump 3600/24 = 150. We do not know which direction, which is not
useful. However, if we energize φ-1 followed by φ-2, the rotor will move 7.50 because the φ-2 is offset (rotated) by 7.50 from
φ-1. And, it will rotate in a reproducible direction if the phases are alternated. Application of any of the above waveforms will
rotate the permanent magnet rotor.
Permanent magnet stepper
A permanent magnet stepper motor has a cylindrical permanent magnet rotor. The stator usually has two
windings. The windings could be center tapped to allow for a unipolar driver circuit where the polarity of the
magnetic field is changed by switching a voltage from one end to the other of the winding. A bipolar drive of
alternating polarity is required to power windings without the center tap. A pure permanent magnet stepper
usually has a large step angle. Rotation of the shaft of a de-energized motor exhibits detent torque. If the
detent angle is large, say 7.50 to 900, it is likely a permanent magnet stepper rather than a hybrid stepper .
Permanent magnet stepper motors require phased alternating currents applied to the two (or more)
windings. In practice, this is almost always square waves generated from DC by solid state electronics.
Bipolar drive is square waves alternating between (+) and (-) polarities, say, +2.5 V to -2.5 V. Unipolar drive
supplies a (+) and (-) alternating magnetic flux to the coils developed from a pair of positive square waves
applied to opposite ends of a center tapped coil. The timing of the bipolar or unipolar wave is wave drive, full
step, or half step.
Wave Drive:
Conceptually, the simplest drive is wave drive. The rotation sequence left to right is positive φ-1 points rotor
north pole up, (+) φ-2 points rotor north right, negative φ-1 attracts rotor north down, (-) φ-2 points rotor
left.
Hybrid stepper
The hybrid stepper motor combines features of both the variable reluctance stepper and the permanent
magnet stepper to produce a smaller step angle. The rotor is a cylindrical permanent magnet, magnetized
along the axis with radial soft iron teeth. The stator coils are wound on alternating poles with corresponding
teeth. There are typically two winding phases distributed between pole pairs. This winding may be center
tapped for unipolar drive. The center tap is achieved by a bifilar winding, a pair of wires wound physically in
parallel, but wired in series. The north-south poles of a phase swap polarity when the phase drive current is
reversed. Bipolar drive is required for un-tapped windings.
The stator teeth on the 8-poles correspond to the 48-rotor teeth, except for missing teeth in the space
between the poles. Thus, one pole of the rotor, say the south pole, may align with the stator in 48 distinct
positions. However, the teeth of the south pole are offset from the north teeth by half a tooth. Therefore, the
rotor may align with the stator in 96 distinct positions.