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VIEW ON THE CONTROL
ASPECTS OF UNIVERSAL
MOTOR
SUBRAMANIAN.R
Edited By
Sarath S Nair
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SYNOPSIS
 Basics
of universal motor
 Characteristics
 Control
aspects
 Equivalent circuit
 Analysis
 Conclusion
 References
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UNIVERSAL MOTOR
 Can
run on either dc or ac power
 A series
wound dc motor
 A double-excited motor
 High
starting torque and a variable speed
characteristics
 Runs
at dangerously high speed on no load
 Input
range is few watts to 2000 watts
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PARTS
 Stator: field
windings to generate the radial
magnetic flux

Rotor: armature winding supplied with
current through the carbon brushes to generate
magnetic flux

Brushes: mechanical linking of power supply
and rotor windings
 Commutator
& a compensating winding
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UNIVERSAL MOTOR FROM A
BLENDER
Fig.1
Fig.2
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CONSTRUCTION
 Permanent magnets
in dc motor is replaced
by electromagnets
 manufactured in
two types
1.
Concentrated-pole, non-compensated
2.
Distributed-field compensated
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Contd…
Concentrated-pole, non-compensated

low rating
 Stator:
Two salient poles & like a two pole dc
series motor except that whole of its magnetic
path is laminated
 Rotor:
wound type and a small dc motor with
laminated core having straight slots having
armature winding is connected to the
commutator
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Contd…
Distributed-field compensated

high rating
 Stator:
Similar to that of a split phase motor
 Rotor:
Wound type similar to that of a small d.c.
motor
 Compensating
winding is used to reduce the
reactance voltage present in the armature when
motor runs on ac supply
 The
reactance of the field winding can be kept low
by limiting the number of turns.
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OPERATION

With input DC power, the stator coils as permanent magnets
like a DC motor & continues to rotate in the same direction

the current through the rotor also reverses the current through
the electromagnets

every pole of the motor changes from north to south or from
south to north
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Contd…

With input AC power, it reverses the poles and thus the
direction of rotation is obtained.

experiences no torque during current reversal, but the
average torque is sufficient to turn the motor as if it were
connected to a DC power supply.
 different
rotational speeds can be achieved by varying
the amount of current reaching the electromagnets.

The starting torque of a universal motor is determined by
the current that flows through the armature
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ADVANTAGES
 Simple
structure
 Robust
 Small
size & compact
 Portable
 Good
power density characteristics
 Rotates
more than 3000 to 25000 rpm
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DISADVANTAGES

Sparks occurring between brushes & commutator due to their
imperfect electrical contact leads to loud noise while operating

As a result life of brushes is reduced

High maintenance requirements

High peak to peak current gains poor motor efficiency

High brush temperature results in limited motor lifetime
APPLICATIONS

Speed control and high torque are required

Typical applications are in the field of power tools and home
appliances like mixers, washing machine.
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CHARACTERISTICS
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Contd…
 Torque
is inversely proportional to speed & directly
proportional to the current
 More
current flows at high speed, torque increases
as magnetic flux density also increase
 low
speed tends to cause magnetic saturation,
deteriorates the characteristic of the motor
 Rotation
speed is determined by the no. of
windings
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CONTROL ASPECTS
 In
domestic appliances, Electric motors the ability
to control its speed

Two control methods
1.
Phase angle control
2.
Chopper control
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PHASE ANGLE CONTROL
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Contd…
 Phase
angle control is a simple and cost-effective
solution

The speed of a universal motor is controlled by
using a triac.
 The
gate of the triac can be controlled directly by
the output ports of a microcontroller
 The
whole circuitry is usually connected directly to
the mains.
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Contd…
Advantages
1.
simplicity
2.
cost effectiveness
3.
full speed control
Disadvantages
1.
high brush noise
2.
short lifetime
3.
high current ripple
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CHOPPER CONTROL
 PWM
control
 The AC
voltage from the mains is rectified and then
switched at a high frequency by a Power MOSFET
or an IGBT
 lower
current ripple which reduces the harmonics
 reduces
the copper and iron losses
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Contd…
Advantages
1. Higher motor efficiency
2. Lower acoustic noise
3. Smooth current behavior
Disadvantages
1. More components needed
2. Filtering of chopper frequency required
 Two
chopper control method is used
1.
DC chopper speed control
2.
DC chopper speed and current control
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1. DC CHOPPER SPEED CONTROL

The speed regulation is poor at low and medium speeds

The average error is large

The speed regulation improves at high speeds

The speed dynamic response is slow

Control system takes a long time to get back to a steady state
when load changes
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2. DC CHOPPER SPEED AND
CURRENT CONTROL
 The
current control loop is designed to be much
faster than the speed control
a
fast dynamic response to load changes
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EQUIVALENT CIRCUIT
 The
equivalent circuit of a proposed universal
motor with speed and current control is shown in
Figure
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Contd…
 The
model of the proposed universal motor control can
be described mathematically as:

e the motor back emf

k1 is the motor constant dependent on the motor
characteristics.
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Contd…

V(t) = [R + Lp – k1w]i(t)
Where

R=Ra+Rf
 Lp =La+Lf

The mechanical equation can be written as

a serial excitation motor

the motor torque is proportional to the square of the motor
current
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MODEL OF UNIVERSAL MOTOR

Dynamics of universal motor is governed by the equations:

Term ωφ in (1) represents the Counter electromotive force

Term iφ in (2) is the electromagnetic Torque produced by the motor.

The flux φ is a function of the current I, a non-linear system of
equations.

The relation between Φ and I is
Φ(i ) =k(| I |)I
(3)
Where the coefficient k is the mutual inductance Between the field and
armature coils.
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CONCLUSION
 The
basic outline of the universal motor & the dc
chopper speed & current control are discussed here
 Equivalent
circuit & model of the universal motor
are also explained briefly
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REFERENCES
[1] Huangsheng Xu, Kevin King, and Yashvant Jani Renesas Technology America, Inc. 450 Holger
Way, San Jose, CA 95134 “ High Performance DC Chopper Speed and Current Control of Universal
Motors Using a Microcontroller” 0197-2618/07/$25.00 © 2007 IEEE
[2] Hac1 Bodur, A. Faruk Bakan, M. Hadi Sarul Yildiz Technical University, Dept. of Electrical
Engineering Istanbul 80750, TURKEY “Universal Motor Speed Control with Current Controlled
PWM AC Chopper by using a Microcontroller 0-7803-5812-0/00/$10.00~2000IE
[3] L. C. PACKER Associate. A. I. E. E. “Universal Type Motors” Westinghouse Electric & Mfg.
Co., Springfieldl Mass.- Presented at the Regional Meeti,ng of the A. I. E. E., Swarmp- scott, Mass.,
May 7-9, 1925
[4] Rex Miller and Mark Richard Miller “Small Electric Motors” 2nd Edition Wiley-dreamtech India
Pvt. Ltd
[5] Charles I. Hubert “Electrical Machines” 2nd Edition Pearson Education
[6] B.L.Theraja, A.K. Theraja “ A Textbook of Electrical Technology” Volume II AC &DC Machines
S.Chand & Company Ltd.
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