MTS 231 Actuating System

Dr. Basharat Ullah

Week 13

Department of Mechatronics
College of Electrical and Mechanical
Engineering
Compounded DC Motor
❑ A compounded DC motor is a motor with both a shunt and a series field.
❑ Current flowing into a dotted end of a
coil (shunt or series) produces a
positive mmf.
❑ If current flows into the dotted ends of Long-shunt
connection
both coils, the resulting mmfs add to
produce a larger total mmf –
cumulative compounding.
❑ If current flows into the dotted end of
one coil and out of the dotted end of
another coil, the resulting mmfs Short-shunt
subtract – differential compounding. connection
Compounded DC Motor
❑ The Kirchhoff’s voltage law equation for a compounded DC motor is
𝑉𝑇 = 𝐸𝐴 + 𝐼𝐴 𝑅𝐴 + 𝑅𝑆
❑ The currents in a compounded DC motor are 𝐼𝐴 = 𝐼𝐿 − 𝐼𝐹
𝑉𝑇
𝐼𝐹 =
𝑅𝐹
Cumulatively compounded

❑ The mmf of a compounded DC motor: 𝔉𝑛𝑒𝑡 = 𝔉𝐹 ± 𝔉𝑆𝐸 − 𝔉𝐴𝑅

Differentially compounded

❑ The effective shunt field current in a compounded DC motor:

∗
𝑁𝑆𝐸 𝔉𝐴𝑅
𝐼𝐹 = 𝐼𝐹 + 𝐼𝐴 −
𝑁𝐹 𝑁𝐹
Number of turns
Cumulatively Compounded Motors: Torque-Speed Characteristic

❑ In a cumulatively compounded motor, there is a constant component of flux and

a component proportional to the armature current (and thus to the load).
❑ These motors have a higher starting torque than shunt motors (whose flux is
constant) but lower than series motors (whose flux is proportional to the
armature current).
❑ The series field has a small effect at light loads – the motor behaves as a shunt
motor. The series flux becomes quite large at large loads – the motor acts like a
series motor.
Differentially Compounded Motors: Torque-Speed Characteristic

❑ Since the shunt mmf and series mmf subtract from each other in a differentially
compounded motor, increasing load increases the armature current 𝐼𝐴 and
decreases the flux.
❑ When flux decreases, the motor speed increases further increasing the load. This
results in an instability (much worse than one of a shunt motor) making
differentially compounded motors unusable for any applications.
❑ In addition to that, these motors are not easy to
start. The motor typically remains still or turns
very slowly consuming enormously high
armature current.
❑ Stability problems and huge starting armature
current lead to these motors being never used
intentionally.
Example 8.6
❑ A 100 hp, 250 V compounded DC motor with compensating windings has an
internal resistance, including the series winding of 0.04 . There are 1000 turns
per pole on the shunt field and 3 turns per pole on the series windings. The
magnetization curve is shown below. The field resistor has been adjusted for the
motor speed of 1200 rpm. The mechanical, core, and stray losses may be
neglected.
❑ Find the no-load shunt field current.
❑ If the motor is cumulatively compounded, find
its speed when 𝐼𝐴 = 200 𝐴.
❑ If the motor is differentially compounded, find
its speed when 𝐼𝐴 = 200 𝐴.
The Permanent Magnet DC Motor
❑ A permanent magnet DC (PMDC) motor is a motor whose poles are made out of
permanent magnets.
❑ Advantages
o Since no external field circuit is needed, there are no field circuit copper losses;
o Since no field windings are needed, these motors can be considerable smaller.
❑ Disadvantages
o Since permanent magnets produces weaker flux densities then externally supported
shunt fields, such motors have lower induced torque.
o There is always a risk of demagnetization from extensive heating or from armature
reaction effects (via armature mmf).
❑Note: Topic 8.8 and 8.9 are not part of the syllabus.
Example 8.8
❑ A 50-hp, 250-V, 1200 r/min shunt dc motor has a rated armature current of 170A
and a rated field current of 5 A. When its rotor is blocked, an armature voltage of
10.2V (exclusive of brushes) produces 170A of current flow, and a field voltage of
250V produces a field current flow of 5A. The brush voltage drop is assumed to
be 2V. At no load with the terminal voltage equal to 240V, the armature current is
equal to 13.2A, the field current is 4.8A, and the motor’s speed is 1150 r/min.
(a) How much power is output from this motor at rated conditions?
(b) What is the motor’s efficiency?
Questions?
Lecture 26
AC Machines
❑ Chapter-3: AC Machinery Fundamentals (5th edition)
Page: 152 - 190
❑ Chapter-4: Synchronous Generator (5th Edition)
Page: 191 – 270
❑ Chapter-5: Synchronous Motors (5th Edition)
Pages: 271-306
❑ Chapter-6: Induction Motors (5th Edition)
Pages: 307-402
AC Motor
❑ AC motor - no need rectification, so don't need split rings.
AC Generator
❑ This process can be described in terms of Faraday's law when you see that the
rotation of the coil continually changes the magnetic flux through the coil and
therefore generates a voltage.
AC Machines

AC Machines

Synchronous Induction

❑ Alternating current (ac) is the primary source of electrical energy

❑ Synchronous machines are motors and generators whose magnetic field current
is supplied by a separate dc power source.
❑ Induction machines are motors and generators whose field current is supplied by
magnetic induction (transformer action) into their field windings.
AC Machines
❑ The induced voltage,

❑ If the loop is rotating at a constant angular velocity ω, then angle θ of the loop
will increase linearly with time. So,

❑ The final form will become,

Synchronous Machines
❑ Synchronous machines are AC machines that have a field circuit supplied by an
external DC source.
❑ In a synchronous generator, a DC current is applied to the rotor winding
producing a rotor magnetic field. The rotor is then turned by external means
producing a rotating magnetic field, which induces a 3-phase voltage within the
stator winding.
❑ In a synchronous motor, a 3-phase set of stator currents produces a rotating
magnetic field causing the rotor magnetic field to align with it. The rotor
magnetic field is produced by a DC current applied to the rotor winding.
❑ Field windings are the windings producing the main magnetic field (rotor
windings for synchronous machines); armature windings are the windings where
the main voltage is induced (stator windings for synchronous machines).
Construction of Synchronous Machines
❑ The rotor of a synchronous machine is a large electromagnet. The magnetic poles
can be either salient (sticking out of rotor surface) or non-salient construction.

Non-salient-pole rotor: usually two-pole rotors Salient-pole rotor: four and more poles

❑ Rotors are made laminated to reduce eddy current losses

Construction of Synchronous Machines

Salient pole without

field windings – observe
laminations
A synchronous rotor with 8
salient poles Salient pole with field
windings
Construction of Synchronous Machines
❑ Two common approaches are used to supply a DC current to the field circuits on
the rotating rotor:
❑ Supply the DC power from an external DC source
to the rotor by means of slip rings and brushes;
❑ Supply the DC power from a special DC power
source mounted directly on the shaft of the
machine.

❑ Slip rings are metal rings completely encircling the shaft of a machine but
insulated from it. One end of a DC rotor winding is connected to each of the two
slip rings on the machine’s shaft. Graphite-like carbon brushes connected to DC
terminals ride on each slip ring supplying DC voltage to field windings regardless
the position or speed of the rotor.
Construction of Synchronous Machines

Brush

Slip rings
Construction of Synchronous Machines
❑ Slip rings and brushes have certain disadvantages: increased friction and wear
(therefore, needed maintenance), brush voltage drop can introduce significant
power losses. Still this approach is used in most small synchronous machines.
❑ On large generators and motors, brushless exciters are used.
❑ A brushless exciter is a small AC generator whose field circuits are mounted on
the stator and armature circuits are mounted on the rotor shaft.
❑ The exciter generator’s 3-phase output is rectified to DC by a 3-phase rectifier
(mounted on the shaft) and fed into the main DC field circuit. It is possible to
adjust the field current on the main machine by controlling the small DC field
current of the exciter generator (located on the stator).
❑ Since no mechanical contact occurs between the rotor and the stator, exciters of
this type require much less maintenance.
Construction of Synchronous Machines
 Construction of Synchronous Machines
❑ To make the excitation of a
generator completely independent
of any external power source, a
small pilot exciter is often added to
the circuit.
❑ The pilot exciter is an AC generator
with a permanent magnet
mounted on the rotor shaft and a
3-phase winding on the stator
producing the power for the field
circuit of the exciter.
 Construction of Synchronous Machines
❑ A rotor of large synchronous
machine with a brushless
exciter mounted on the same
shaft.

❑ Many synchronous generators

having brushless exciters also
include slip rings and brushes
to provide emergency source
of the field DC current.
Construction of Synchronous Machines

A large synchronous
machine with the exciter
and salient poles.
Questions?