DEBREMARKOS UNIVERSITY

INISTITUTION OG TECHNOLOGY
DEPARTMENT OF ECE
ELECTRICAL ENGINEERING LAB V GROLUP ASSIGNMENT

GROUP 2
NO NAME ID
1, AYICHELUM MILLION 1404946
2, ADERAJEW KUMIE 1404776
3, ZELALEM TENA 1404551
4, NITSUH MEKICHA 1404580
5, ALEMU MULU 1404824

SUBMITTED TO GIRMA.K

SUBMISSION DATE MAR 6 2025

 DC motor speed and position control using PWM.

 Synchronous motor speed control

 Driving stepper motor

I.INTRODUCTION

Any rotary electrical motor that converts direct current electrical energy into mechanical energy is
referred to as a DC motor. The most common varieties rely on magnetic fields to produce forces.
Almost all DC motors contain an internal mechanism, either electromechanical or electronic, that
changes the direction of current in a section of the motor on a regular basis.Because they could be
supplied by existing directcurrent lighting power distribution networks, DC motors were the first
type of motor to become widely employed. The speed of a DC motor can be varied across a large
range by varying the supply voltage or adjusting the current intensity in the field windings. Tools,
toys, and appliances all employ small DC motors. The universal motor can run on direct
current.However, it is a little brushed motor that is utilized in portable power equipment and
appliances. Larger DC motors are being employed in electric vehicle propulsion, elevator and hoist
drives, and steel rolling mill drives. With the introduction of power electronics, it is now possible to
replace DC motors with AC motors in a variety of applications.

II,THE PRINCIPLE OF WORKING &CONSTRUCTION OF DC MOTOR

A DC motor is a type of electric motor that converts electrical energy into mechanical energy. The
DC motor's primary working concept is that whenever a current carrying conductor enters the
magnetic field, it is subjected to a mechanical force. The magnitude of Fleming's left-hand rule
determines the force's
A DC motor is a device that converts direct current electrical energy into mechanical energy (find
out more about DC motors).

A DC motor is constructed with:

• A Stator

• A Rotor
• A Yoke
• Poles
• Field windings
• Armature windings
• Commutator
• Brushes
The field windings and supplies are housed in the stator, which is the static element of the DC
machine. The revolving portion of a DC machine that causes mechanical rotations is called a rotor.
The complete construction of a DC MOTOR is made up of all of these pieces combined together.
III.PULSE WIDTH MODULATION

PWM is a technique that is used to reduce the total harmonic distortion (THD) in a load current. It
uses a pulse wave in rectangular/square form that results in a variable average waveform value f(t),
after its pulse width has been modulated. T specifies the modulation time interval. As a result, the
waveform average value is bar{y}=\frac{1}{T}\int_{0}^{T}f\left ( t \right )dt

• Sinusoidal Pulse Width Modulation:

The switches in a simple source voltage inverter can be switched on and off as required. The switch
is switched on and off once during each loop. As a consequence, the waveform is square. When the

switch is turned on repeatedly, however, a harmonic profile with a better waveform is

obtained.The sinusoidal PWM waveform is created by comparing the desired modulated waveform
to a high frequency triangular waveform. The resultant output voltage of the DC bus is either
negative or positive, depending on whether the signal voltage is smaller or
larger than that of the carrier waveform.
• Sinusoidal Pulse Width Modulation:
The switches in a simple source voltage inverter can be switched on and off as required. The switch
is switched on and off once during each loop. As a consequence, the waveform is square. When the
switch is turned on repeatedly, however, a harmonic profile with a better waveform is
obtained.The sinusoidal PWM waveform is created by comparing the desired modulated waveform
to a high frequency triangular waveform. The resultant output voltage of the DC bus is either
negative or positive, depending on whether the signal voltage is smaller or larger than that of the
carrier waveform.Am denotes the sinusoidal amplitude, while Ac denotes the carrier triangle. The
modulating index m is equal to Am/Ac for sinusoidal PWM.

• Modified Sinusoidal Waveform PWM:

For power control and power factor optimization, a modified sinusoidal PWM waveform is used.
The key idea is to change the PWM converter to transfer current delayed on the grid to the voltage

grid. As

A Am denotes the sinusoidal amplitude, while Ac denotes the carrier triangle. The modulating
index m is equal to Am/Ac for sinusoidal PWM.
• Modified Sinusoidal Waveform PWM:
For power control and power factor optimization, a modified sinusoidal PWM waveform is used.
The key idea is to change the PWM converter to transfer current delayed on the grid to the voltage
grid. As a result, there is an increase in power production as well as a reduction in power factor.
• Multiple PWM:
The multiple PWM has a number of outputs with different values, but the time period over which
they are produced is the same for all of them. PWM enabled inverters can operate at high voltage
• Multiple PWM:
The multiple PWM has a number of outputs with different values, but the time period over which
they are produced is the same for all of them. PWMenabled inverters can operate at high voltage
output

PWM (pulse width modulation) or PDM (pulse duration modulation) is a technique for decreasing
the average power produced by an electrical signal by splitting it up into discrete parts. By rapidly
flipping the switch between supply and load on and off, the average value of voltage (and current)
provided to the load can be regulated. The longer the switch is on relative to the time it is off, the
better. The higher the total power delivered to the load, the better. It is one of the principal
methods of decreasing the output of solar panels to that which can be used by a battery, along with
maximum power point tracking (MPPT). 1st PWM is especially well suited for running inertial
loads like motors, which are less influenced by discrete switching due to their inertia.Depending on
the load and application, the rate (or frequency) at which the power supply must switch can vary
substantially. An electric stove, for example, must switch several times per minute; a lamp dimmer
must switch 120 Hz; a motor drive must switch between a few kilohertz (kHz) and tens of kHz; and
audio amplifiers and computer power supplies must switch far into the tens or hundreds of kHz. The key
benefit of PWM is that it has a very low power loss in the switching devices. There is nearly no current
when a switch is turned off, and there is almost no voltage drop across the switch when it is turned on
and power is transferred to the load. Because power loss is the product of voltage and current, it is near
to zero in both circumstances. PWM also work well with digital controls, which are more common these
days.Because of its on/off nature, the duty cycle can be readily set. PWM has also been utilized in
communication systems, where the duty cycle is employed to transmit data over a communications
channel. Many modern micro controllers (MCUs) include PWM controllers that are exposed to external
pins as peripheral devices that are controlled by software via internal programming Interfaces. These are
extensively used in robotics and other applications to control direct current (DC) motors.
• Principle:
Pulse-width modulation employs a rectangular pulse wave with a modulated pulse width, resulting in a
variation in the waveform's average value. The average value of a pulse waveform f(t) with time period
T, low value y min, high value y max, and duty cycle D There are three types of pulse-width modulation
(PWM):

1. The pulse centre can be fixed at the time window's

centre, and the pulse's two edges can be changed to compress or expand the width.
2. The lead edge of the window can be kept at the lead
edge and the tail edge modified.
3. The lead edge can be modulated while the tail edge
is fixed.there are three general methods for controlling the speed of D.C. motors.
• Armature resistance control, also known as Rheostat control, is a way of varying the resistance
in the armature circuit.

• Field flux fluctuation Field flux control is the name of this technique.
• Changes in the applied voltage Armature voltage control is another name for this method.

V.DC SPEED MOTOR CONTROL USING PWM METHOD

This method is also known as armature voltage control. To reduce excessive heat dissipation in
linear power amplifiers, pulse width modulation is used in DC motor control. Large heat sinks and,
 in some cases, forced cooling are used to solve the heat dissipation problem. Because of their
substantially higher power conversion efficiency, PWM amplifiers considerably decrease this
difficulty. Furthermore, the PWM driver's input signal can be obtained straight from any digital
system, eliminating the need for any D/A converters. There are some drawbacks to using a
PWM power amplifier. The required signal is converted to the time duration (or duty cycle) of a
pulse rather than a voltage amplitude. This is clearly not a linear process. However, the PWM can
be approximated as linear using a few assumptions that are usually applicable in motor control (i.e.,
a pure gain). The average voltage is equal to the integral of the voltage waveform in the linear
model of the PWM amplifier.Consequently, Veq * T = VS * Ton The supply voltage (+12 volts) is
denoted by VS.Ton = Time between pulses Veq is the motor's average or equivalent voltage.
T = Time between switches (1/f) The dc motor speed is regulated by a power electronic
device, which uses PWM to control the dc motor speed. The needed input speed will determine the
speed of the speed pulse train. This circuit can be used to run dc motors at the desired speed with
little losses and at a low cost. The circuit has a quick response time. As a result, excellent reliability
is possible. The circuit was successfully tested for a variety of speed inputs. The approach already
uses a traction system and has a bright future ahead of it. Synchronous motor is a type of AC
motor that runs at a constant speed. And before discussing the speed control methods of a
Synchronous motor, let us see how to find the speed of a synchronous motor.

Using this simple formula: Ns = (120xf) / p , we can calculate the synchronous speed of any synchronous
motor.

Let’s look at this formula in more detail.

Table of Contents

What is Ns=(120xf)/p all about?

Why do we require speed control of synchronous motors?

What parameters can we vary for speed control of Synchronous motors?

Why do we require speed control of synchronous motor?

What parameters can we vary for speed control of synchronous motor?

Separate control or open loop mode

What is Ns=(120xf)/p all about?

Ns here stands for synchronous motor speed and as evident by the formula, it depends on two factors:

Frequency (f)

We know that an AC supply system has a particular frequency. The frequency indicates the number of
times the rotating magnetic field rotates inside the armature. In the US, this frequency is 60Hz, while in
India / UK, this frequency is 50 Hz. So, the (f) in the formula represents the frequency of the AC supply
connected to the synchronous motor.

Number of stator poles (p)

The stator of a Synchronous motor

Every AC motor consists of a Stator (the stationary part of the motor). The inner periphery of the stator
contains slots for the windings. So, according to the winding distribution inside these slots, we can
create the desired number of Stator poles. The (p) in the formula represents the number of stator poles.

So, from the formula given above, it is clear that we can achieve the speed control of synchronous
motor by either of the two methods:

1. Changing the number of Stator poles

2. Changing the frequency of AC supply

 Why do we require speed control of synchronous motor?

We know that a Synchronous motor is a constant-speed motor. Its speed does not alter despite the load
variations. So, why do we need speed control of synchronous motor?

It is because some industries prefer Synchronous motors over induction motors for speed control.
Synchronous motors are more efficient and have fewer losses than induction motors. So, using them
improves reliability.

 What parameters can we vary for speed control of synchronous motor?

After designing an AC motor, it is difficult to change the speed by changing the stator poles. To make it
happen, for every speed variation that we require, we have to open the motor and rearrange the
windings to modify the number of Stator poles. This method doesn’t look practical, agree? So, the only
option that remains is to vary the frequency of incoming AC supply to control the motor speed.

To vary the frequency, we use a combination of rectifiers and inverters and use them in either of the
following modes:

Separate control or Open loop mode

Self-control or Closed-loop mode

Let us first look at how to use them in an Open-loop mode.

for speed control of the synchronous motor.

Using the rectifier and inverter combination, we can vary the frequency of AC supply.

There are two methods to vary the frequency of AC supply: Separate control (Open-loop control) and
Self-control (Closed-loop control).

The Separate control method does not use feedback, and the chances of speed error are more.

The Self-control method uses a feedback sensor to monitor the rotor speed. The chances of error get
reduced in this method.

The closed-loop method is preferred over the open-loop method for precise speed control of the
synchronous motor.

for speed control of the synchronous motor.

Using the rectifier and inverter combination, we can vary the frequency of AC supply.

There are two methods to vary the frequency of AC supply: Separate control (Open-loop control) and
Self-control (Closed-loop control).

The Separate control method does not use feedback, and the chances of speed error are more.

The Self-control method uses a feedback sensor to monitor the rotor speed. The chances of error get
reduced in this method.
The closed-loop method is preferred over the open-loop method for precise speed control of the
synchronous motor.

 Working

A rectifier unit first converts the Alternating current to pulsating Direct current. The inductor and
capacitor in the circuit act as a filter and refine the DC waveform.

We can easily set the new frequency for the AC waveform using the frequency circuit. The inverter uses
the Pulse Width Modulation (PWM) technique to convert the DC to AC. The AC obtained now has a
different frequency as compared to the original AC supply.

Hence in this way, we can vary the frequency to control the speed of the synchronous motor.

 Drawbacks

As said earlier, this is an open-loop system, i.e., once we set the frequency in the circuit, the system
does not know about the output condition. If somehow the speed of the motors changes, the system
can’t take any actions to regulate the speed of the motors. Thus, to reduce the chances of speed error,
we require a better speed control method.

 Applications

This method finds applications where slight changes in motor speed do not vary the load connected to
the motor.

The open-loop control is a suitable way to control parallel-connected synchronous motors. By setting
the frequency in the circuit, the speed of all the motors changes simultaneously.

Self-control or closed-loop mode

Block diagram of Self-control or Closed-loop control method

The closed-loop method uses a feedback system to monitor the speed of the motor. The following figure
shows the block diagram of the closed-loop speed control system.

 Working
The working procedure of this system is similar to the above-discussed method. Here also, we can easily
vary the frequency of the output AC wave using the frequency circuit.

But here, a sensor continuously monitors the rotor speed. An error detector compares the preset speed
and the actual speed of the rotor and sends the difference value to the rectifier circuit. The rectifier
circuit suitably adjusts the firing angle to control the magnitude of the output DC wave. Thus, this
method reduces the chances of speed error and is more reliable than the open-loop speed control
method.

 Applications

We use the closed-loop speed control method where accurate speed control is required.

This method is suitable for large gearless drives, viz. mine hoists, drives for mills, etc.

 Conclusion

Let us quickly conclude the main points of the speed control of the synchronous motor discussed in our
article.

We can achieve the speed control of synchronous motor either by changing the number of Stator poles
or changing the frequency of AC supply.

Varying the frequency is a feasible way for speed control of the synchronous motor.

Using the rectifier and inverter combination, we can vary the frequency of AC supply.

There are two methods to vary the frequency of AC supply: Separate control (Open-loop control) and
Self-control (Closed-loop control).

The Separate control method does not use feedback, and the chances of speed error are more.

The Self-control method uses a feedback sensor to monitor the rotor speed. The chances of error get
reduced in this method

3.What is a stepper motor?

A stepper motor is a type of brushless synchronous DC motor that, unlike many other standard types of
electric motors, doesn’t just rotate continuously for an arbitrary number of spins until the DC voltage
passing to it is shut off.Instead, stepper motors are a type of digital input-output device for precision
starting and stopping. They’re constructed so that the current passing through it hits a series of coils
arranged in phases, which can be powered on and off in quick sequence. This allows the motor to turn
through a fraction of a rotation at a time - and these individual predetermined phases as what we refer
to as ‘steps’.A stepper motor is designed to break up a single full rotation into a number of much smaller
(and essentially equal) part rotations. For practical purposes, these can be used to instruct the stepper
motor to move through set degrees or angles of rotation. The end result is that a stepper motor can be
used to transfer minutely accurate movements to mechanical parts that require a high degree of
precision.Stepper motors are typically digitally controlled, and function as key components in an open-
loop motion-control positioning system. They’re most commonly used in holding or positioning
applications where their ability to assert much more clearly defined rotational positions, speeds and
torques make them ideally suited to tasks demanding extremely rigorous movement control.

Browse All Stepper Motors How do stepper motors work?

In a normal brushed DC motor, voltage is applied to terminals which in turn causes a wire coil to rotate
at speed inside a fixed magnet housing (the ‘stator’).In this setup, the spinning wire coil (the ‘rotor’)
effectively becomes an electromagnet, and turns rapidly at the centre of the motor based on the
familiar principle of magnetic attraction and repulsion. A combination of brushes (electrical contacts)
and a rotary electrical switch known as a commutator allows the direction of the current running to the
wire coil to be alternated quickly. This creates continuous unidirectional spinning of the rotor coil for as
long as the assembly is being fed with sufficient voltage.A potential downside of this type of motor is
that it spins continuously and for an arbitrary number of rotations until power is cut off. This makes it
very hard to control the exact stopping point of the motor, rendering it unsuitable for applications
requiring greater precision control. Manually controlling the on/off flow of power to the motor can’t
give you the required start-stop precision for performing minutely accurate movements.In a stepper
motor, the setup is quite different. Instead of a wire coil rotor spinning inside a fixed housing of
magnets, stepper motors are built with a fixed wire housing (the stator in this case) arranged around a
series of ‘toothed’ electromagnets spinning at the centre. The stepper motor converts a pulsing
electrical current, controlled by a stepper motor driver, into precise one-step movements of this gear-
like toothed component around a central shaft.
Each of these stepper motor pulses moves the rotor through one precise and fixed increment of a full
turn. As the current switches between the wire coils arranged in sequence around the outside of the
motor, the rotary part can complete full or partial turns as required, or it can be made to stop very
abruptly at any of the steps around its rotation.Ultimately, the real strength of a stepper motor versus
normal DC brushed motors is that they can quickly locate themselves to a known and repeatable
position or interval, and then hold that position for as long as required. This makes them extremely
useful in high-accuracy applications such as robotics and printing. Learn Engineering have created the
below video that demonstrates how a stepper motor works:

Types of Stepper Motors

There are a variety of stepper motors available, but most of them can be separated into two groups:

•Permanent-magnet (PM) stepper motor — This kind of motor creates rotation by using the forces
between a permanent magnet and an electromagnet created by electrical current. An
interesting characteristic of this motor is that even when it is not powered, the motor exhibits
somemagnetic resistance to turning.
Variable-reluctance (VR) stepper motor — Unlike the PM stepper motor, the VR stepper motor does not
have a permanent-magnet and creates rotation entirely with electromagnetic forces. This motor does
not exhibit magnetic resistance to turning when the motor is not powered.

Dedicated integrated circuits have dramatically simplified stepper motor driving. To apply these ICs
designers need little specific knowledge of motor driving techniques, but an under-standing of the basics
will helpin finding the best solution. This note explains the basics of stepper motor driving and describes
the drive techniques used today

From a circuit designer’s point of view stepper motors can be divided into two basic types : unipolar and
bipolar.A stepper motor moves one step when the direction
of current flow in the field coil(s) changes, reversing the magnetic field of the stator poles. The
difference between un ipolar and bipolar motors lies in the may that this reversal is achieved (figure 1) :
Figure 1a : BIPOLAR - with One Field Coil and Two Charge over Switches That are Switched in the
Opposite Direction.
Figure 1b : UNIPOLAR - with Two Separate Field Coils and are Charge over Switch. The advantage of the
bipolar circuit is that there is only one winding, with a good bulk factor (low winding resistance). The
main disapuantages are thetwo changeover switches because in this case more semiconductors are
needed.The uni polar circuit needs only one changeover switch. Its enormous disadvantage is, however,
that a double bi filar winding is required. This means that at a specific bulk factor the wire is thinner and
the resistance is much higher. We will discuss later the problems involved.Unipolar motors are still
popular today because the drive circuit appears to be simpler when implemented with discrete devices.
However with the integrated circuits available today bipolar motors can be driver with no more
components than the unipolar motors. Figure 2 compares integrated unipolar and bipolar devices.

BIPOLAR PRODUCES MORE TORQUE

The torque of the stepper motor is proportional to the magnetic field intensity of the stator windings. It
may be increased only by adding more windings or by increasing the current.A natural limit against any
current increase is the danger of saturating the iron core. Though this is of minimal importance. Much
more important is the maximum temperature rise of the motor, due to the power loss in the stator
windings. This shows one advantage of the bipolar circuit, which, compared to unipolar systems, has
only half of the copper resistance because of the double cross section of the wire. The winding current
may be increased by the factor √2 and this produces a direct proportional affect on the torque. At their
power loss limit bipolar motors thus deliver about 40 % more torque (fig. 3) than unipolar motors built
on the same frame. If a higher torque is not required, one may either reduce the motor size or the
power loss. Figure 3 : Bipolar Motors Driver DelivThe advantage of the bipolar circuit is that there is
only one winding, with a good bulk factor (low winding resistance). The main disapuantages are the
two changeover switches because in this case more semiconductors are needed. unipolar circuit needs
only one changeover switch. Its enormous disadvantage is, however, that a double bifilar winding is
required. This means that at a specific bulk factor the wire is thinner and the resistance is much higher.
We will discuss later the problems involved. Unipolar motors are still popular today because the drive
circuit appears to be simpler when implemented with discrete devices. However with the integrated
circuits available today bipolar motors can be driver with no more components than the unipolar
motors. Figure 2 compares integrated unipolar and bipolar devices.

BIPOLAR PRODUCES MORE TORQUE

The torque of the stepper motor is proportional to the magnetic field intensity of the stator windings. It
may be increased only by adding more windings or by increasing the current. A natural limit against any
current increase is the danger of saturating the iron core. Though this is of minimal importance. Much
more important is the maximum temperature rise of the motor, due to the power loss in the stator
windings. This shows one advantage of the bipolar circuit, which, compared to unipolar systems, has
only half of the copper resistance because of the double cross section of the wire. The
windingcurrentmay be increased by the factor √2 and this produces a direct proportional affect on the
torque. At their power loss limit bipolar
motors thus deliver about 40 % more torque (fig. 3) than unipolar motors built on the same frame.
If a higher torque is not required, one may either reduce the motor size or the power loss.
Figure 3 : Bipolar Motors Driver Deliv The advantage of the bipolar circuit is that there is
only one winding, with a good bulk factor (low winding resistance). The main disapuantages are the
two changeover switches because in this case more semiconductors are needed. The unipolar circuit
needs only one changeover switch. Its enormous disadvantage is, however, that a double bifilar winding
is required. This means that at a specific bulk factor the wire is thinner and the resistance is much
higher. We will discuss later the problems involved.Unipolar motors are still popular today because the
drive circuit appears to be simpler when implemented with discrete devices. However with the
integrated circuits available today bipolar motors can be driver with no more components than the
unipolar motors. Figure 2 compares integrated unipolar and bipolar devices.

BIPOLAR PRODUCES MORE TORQUE

The torque of the stepper motor is proportional to the magnetic field intensity of the stator windings. It
may be increased only by adding more windings or by increasing the current. A natural limit against any
current increase is the danger of saturating the iron core. Though this is of minimal importance. Much
more important is the maximum temperature rise of the motor, due to the power loss in the stator
windings. This shows one advantage of the bipolar circuit, which, compared to unipolar systems, has
only half of the copper resistance because of the double cross section of the
wire. The winding current may be increased by the factor √2 and this produces a direct proportional
affect on the torque. At their power loss limit bipolar motors thus deliver about 40 % more torque (fig.
3) than unipolar motors built on the same frame. If a higher torque is not required, one may either
reduce the motor size or the power loss.
Figure 3 : Bipolar Motors Driver DelivThe advantage of the bipolar circuit is that there is only one
winding, with a good bulk factor (low winding resistance). The main disapuantages are the
two changeover switches because in this case more semiconductors are needed. The unipolar circuit
needs only one changeover switch. Its enormous disadvantage is, however, that a double bifilar winding
is required. This means that at a specific bulk factor the wire is thinner and the resistance is much
higher. We will discuss later the problems involved. Unipolar motors are still popular today because the
drive circuit appears to be simpler when implemented with discrete devices. However with the
integrated circuits available today bipolar motors can be
driver with no more components than the unipolar motors. Figure 2 compares integrated unipolar and
bipolar devices.
BIPOLAR PRODUCES MORE TORQUE
The torque of the stepper motor is proportional to the magnetic field intensity of the stator windings. It
may be increased only by adding more windings or by increasing the current. A natural limit against any
current increase is the danger of saturating the iron core. Though this is of minimal importance. Much
more important is the maximum temperature rise of the motor, due to the
power loss in the stator windings. This shows one advantage of the bipolar circuit, which, compared to
unipolar systems, has only half of the copper resistance because of the double cross section of the wire.
The winding current may be increased by the factor √2 and this produces a direct proportional affect on
the torque. At their power loss limit bipolar motors thus deliver about 40 % more torque (fig. 3) than
unipolar motors built on the same frame. If a higher torque is not required, one may either reduce the
motor size or the power loss.

Figure 3 : Bipolar Motors Driver Deliv

The advantage of the bipolar circuit is that there is only one winding, with a good bulk factor (low
winding resistance). The main disapuantages are the two changeover switches because in this case more
semiconductors are needed.
The unipolar circuit needs only one changeoverswitch. Its enormous disadvantage is, however, that a
double bifilar winding is required. This means that at a specific bulk factor the wire is thinner and
theresistance is much higher. We will discuss later the problems involved.Unipolar motors are still
popular today because the drive circuit appears to be simpler when implemented with discrete devices.
However with the integrated circuits available today bipolar motors can be
driver with no more components than the unipolar motors. Figure 2 compares integrated unipolar and
bipolar devices.
BIPOLAR PRODUCES MORE TORQUE

The torque of the stepper motor is proportional to the magnetic field intensity of the stator windings. It
may be increased only by adding mor ewindings or by increasing the current.
A natural limit against any current increase is the danger of saturating the iron core. Though this is of
minimal importance. Much more important is the maximum temperature rise of the motor, due to the
power loss in the stator windings. This shows one advantage of the bipolar circuit, which, compared to
unipolar systems, has only half of the copper resistance because of the double cross section of the wire.
The winding current may be increased by the factor √2 and this produces a direct proportional affect on
the torque. At their power loss limit bipolar motors thus deliver about 40 % more torque (fig. 3)than
unipolar motors built on the same frame.
If a higher torque is not required, one may either reduce the motor size

MORE TORQUE AT A HIGHER NUMBER OF REVOLUTIONS

Higher torque at faster speeds are possible if a current generator as shown in Fig. 4b is used. In this
application the supply voltage is chosen as high possible to increase the current’s rate of change. The
current generator itself limits only the phase current and becomes active only the moment in which
thecoil current has reached its set nominal value. Up to this value the current generator is in saturation
and the supply voltage is applied directly to the winding.
Fig. 6, shows that the rate of the current increase is now much higher than in Figure 5. Consequently at
higher step rates the desired current can be maintained in the winding for a longer time. The torque
decrease starts only at much higher speeds.
Fig. 7 shows the relation between torque and speed in the normal graphic scheme, typical for the
stepper motor. It is obvious that the power increases in the
upper torque range where it is normally needed, asthe load to be driven draws most energy from the
motor in this range.
EFFICIENCY - THE DECISIVE FACTOR
The current generator combined with the high supply voltage guarantees that the rate of change of the
current in the coil is sufficiently high.At the static condition or at low numbers of revolutions, however,
this means that the power loss in the current generator dramatically increases, although
the motor does not deliver any more energy in this range ; the efficiency factor is extremely bad

ADVANTAGES AND DISADVANTAGES OF THE HALF-STEP

An essential advantage of a stepper motor operating at half-step conditions is its position resolution
increased by the factor 2. From a 3.6 degree motor
you achieve 1.8 degrees, which means 200 steps per revolution.This is not always the only reason. Often
you are forced to operate at half-step conditions in order to avoid that operations are disturbed by the
motor resonance. These may be so strong that the motor has no more torque in certain step frequency
ranges and looses completely its position (fig. 10). This is due to the fact that the rotor of the motor,
andthe changing magnetic field of the stator forms a springmass-system that may be stimulated to
vibrate. In practice, the load might deaden this system, but only if there is sufficient frictional force.In
most cases half-step operation helps, as the course covered by the rotor is only half as long and
thesystem is less stimulated.
The fact that the half-step operation is not the dominating or general solution, depends on certain
disadvantages :
- the half-step system needs twice as many clock-pulses as the full-step system ; the
clock-frequency is twice as high as with the full-step.in the half-step position the motor has only about
half of the torque of the full-step.

TORQUE LOSS COMPENSATION IN THE HALF-STEP OPERATION

It’s clear that, especially in limit situations, the torque loss in half-step is a disadvantage. If one has to
choose the next larger motor or one with a double resolution operating in full-step because of some
insufficient torque percentages, it will greatly influence the costs of the whole system.In this case, there
is an alternative solution that does not increase the coats for the bipolar chopping stabilized current
drive circuit.The torque loss in the half-step position may be compensated for by increasing the winding
current by the factor √2 in the phase winding that remains active. This is also permissible if, according to
the motor data sheet, the current limit has been reached, because this limit refers always to the
contemporary supply with current in both windings in the full-step position. The factor √2 increase in
current doubles the stray power of the active phase. The total dissipated power is like that of the full-
step because the non-active phase does not dissipate power.The resulting torque in the half-step
position amounts to about 90 % of that of the full-step, that means dynamically more than 95 % torque
compared to the pure full-step ; a neglectable factor.The only thing to avoid is stopping the motor at
limit current conditions in a half-step position because it would be like a winding thermal phase
overload concentrated in one.The best switch-technique for the half-step phase current increase will be
explained in detail later on Fig. 11 shows the phase current of a stepping motor in half-step control with
an without phase current increase and the pertinent curves of stap frequency
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