view on the control aspects of universal motor subramanian.r edited by sarath s nair 1

VIEW ON THE CONTROL ASPECTS OF UNIVERSAL MOTOR

SUBRAMANIAN.REdited By

Sarath S Nair

www.technologyfuturae.com1www.technologyfutuare.com

SYNOPSISBasics of universal motor

Characteristics

Control aspects

Equivalent circuit

Analysis

Conclusion

References

2www.technologyfutuare.com

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

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

UNIVERSAL MOTOR FROM A BLENDER

Fig.1 Fig.2


CONSTRUCTION

Permanent magnets in dc motor is replaced

by electromagnets

manufactured in two types

1. Concentrated-pole, non-compensated

2. Distributed-field compensated


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

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

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

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

ADVANTAGES

Simple structure

Robust

Small size & compact

Portable

Good power density characteristics

Rotates more than 3000 to 25000 rpm


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

CHARACTERISTICS


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


CONTROL ASPECTS

In domestic appliances, Electric motors the ability

to control its speed

Two control methods

1. Phase angle control

2. Chopper control


PHASE ANGLE CONTROL


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

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

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


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

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

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

EQUIVALENT CIRCUIT

The equivalent circuit of a proposed universal

motor with speed and current control is shown in

Figure


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

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

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

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

view on the control aspects of universal motor subramanian.r edited by sarath s nair 1

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