VIEW ON THE CONTROL ASPECTS OF UNIVERSAL MOTOR SUBRAMANIAN.R Edited By Sarath S Nair www.technologyfuturae.com www.technologyfutuare.com

SYNOPSIS Basics of universal motor Characteristics Control aspects Equivalent circuit Analysis Conclusion References www.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 watts www.technologyfutuare.com

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 www.technologyfutuare.com

UNIVERSAL MOTOR FROM A BLENDER Fig.1 Fig.2 www.technologyfutuare.com

CONSTRUCTION Permanent magnets in dc motor is replaced by electromagnets manufactured in two types Concentrated-pole, non-compensated Distributed-field compensated www.technologyfutuare.com

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 www.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. www.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 www.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 www.technologyfutuare.com

ADVANTAGES Simple structure Robust Small size & compact Portable Good power density characteristics Rotates more than 3000 to 25000 rpm www.technologyfutuare.com

APPLICATIONS 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. www.technologyfutuare.com

CHARACTERISTICS www.technologyfutuare.com

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 www.technologyfutuare.com

CONTROL ASPECTS In domestic appliances, Electric motors the ability to control its speed Two control methods Phase angle control Chopper control www.technologyfutuare.com

PHASE ANGLE CONTROL www.technologyfutuare.com

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. www.technologyfutuare.com

Contd… Advantages Disadvantages simplicity cost effectiveness full speed control Disadvantages high brush noise short lifetime high current ripple www.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 www.technologyfutuare.com

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 DC chopper speed control DC chopper speed and current control www.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 changes www.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 changes www.technologyfutuare.com

EQUIVALENT CIRCUIT The equivalent circuit of a proposed universal motor with speed and current control is shown in Figure www.technologyfutuare.com

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. www.technologyfutuare.com

Contd… Lp =La+Lf V(t) = [R + Lp – k1w]i(t) Where R=Ra+Rf The mechanical equation can be written as a serial excitation motor the motor torque is proportional to the square of the motor current www.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. www.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 www.technologyfutuare.com

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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. www.technologyfutuare.com