view on the control aspects of universal motor subramanian.r edited by sarath s nair 1
TRANSCRIPT
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VIEW ON THE CONTROL ASPECTS OF UNIVERSAL MOTOR
SUBRAMANIAN.REdited By
Sarath S Nair
www.technologyfuturae.com1www.technologyfutuare.com
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SYNOPSISBasics 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 watts3www.technologyfutuare.com
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PARTSStator: 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 winding4www.technologyfutuare.com
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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 commutator7www.technologyfutuare.com
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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.8www.technologyfutuare.com
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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 9www.technologyfutuare.com
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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 10www.technologyfutuare.com
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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.12www.technologyfutuare.com
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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.17www.technologyfutuare.com
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Contd…
Advantages
1. simplicity
2. cost effectiveness
3. full speed control
Disadvantages
4. high brush noise
5. short lifetime
6. high current ripple 18www.technologyfutuare.com
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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 control20www.technologyfutuare.com
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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 changes21www.technologyfutuare.com
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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 changes22www.technologyfutuare.com
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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.24www.technologyfutuare.com
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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
current25www.technologyfutuare.com
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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.26www.technologyfutuare.com
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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. 29www.technologyfutuare.com