High starting performance separately excited DC motor

Yasser G. Dessouky Hamdy A. Ashour Amany M. El-Zonkoly

Department of Electrical and Control EngineeringArab Academy for Science and TechnologyMiami, Alexandria, P.O. Box: 1029, EGYPT

Abstract__Electric motors have a variety of speed-torquecharacteristics during steady state and transientoperations. For a given drive applications, motors areoften selected to match the characteristics of the requiredoperation, determined by the mechanical loadcharacteristics and the available power supply. Due toadvances in power electronics, such restrictions no longerexist where the characteristics of most motors can now bealtered to match the desired performance when externalpower converters are used and advanced control strategiesare employed. Series DC motor has a high starting torque.Reversing its speed direction is normally done using arelay arrangement, which requires an off time interval andresults in waste of energy during braking. Also DC seriesmotor doesn’t run above base speed because there is noseparate control on the field current unless a field diverteris used which causes losses. Separately excited DC motorcan operate above the base speed in the field-weakeningregion by reducing the field current independently. Also itsspeed direction can be reversed by reversing the armaturevoltage. In this paper a DC drive control system issuggested to run the DC motor so as to obtain theperformance of the series DC motor below base speed andthe performance of the separately excited DC motor abovebase speed and to change speed direction in a regenerativebraking mode at any motor speed. The model of the DCmotor including the saturation is reviewed. The controlstrategy is explained and the control circuit is proposed. Asteady state and transient analysis of the motor isperformed below and above base speed.

Index terms__ series DC motor- separately excited DCmotor- DC drives

LIST OF SYMBOLS

Rf, Ra : field and armature resistance, ΩLf, La : field and armature inductance, HE, e : steady state and instantaneous induced emf, VVf, Va : steady state field and armature voltage, VIf, Ia : steady state field and armature current, Avf, va : instantaneous field and armature voltage, Vif, ia : instantaneous field and armature current, Aif

*, ia* : field and armature command control signal

J, B : inertia constant, Nm.s2/rad, friction constant, Nm.s/radTL, Te : load and electromechanical torque, Nmr, r*: actual and command control signal speed, rad/sec

I. INTRODUCTION

DC motors have variable torque/speed characteristicsand are used extensively in variable speed drives. DC

motors can provide a high starting torque and it is alsopossible to obtain speed control over a wide range. Themethods of speed control are normally simpler and lessexpensive than those of AC drives [1]. DC motors stillplay a significant role in modern industrial drives. Bothseries and separately excited DC motors are normallyused in variable speed drives, but series motors aretraditionally employed for traction applications. Due tocommutators, DC motors are not most suitable for high-speed applications and require more maintenance thando AC motors. With the recent advantages in powerconversions, control techniques, and microcomputers,the AC motor drives are becoming increasinglycompetitive with DC motor drives [2]. Although thefuture trend is toward AC drives, DC drives arecurrently used in many industries. It might be a fewdecades before the DC drives are completely replacedby AC drive. Due to their ability to supply acontinuously variable DC voltage, controlled rectifiersand DC-DC converters made a revolution in modernindustrial control equipment and variable-speed drives,with power levels ranging from fractional horsepower toseveral megawatts [3]. For the separately excited DCmotor, the flux created by the field winding is keptconstant below base speed, hence the torque developedby the motor is proportional to the armature current andthen the motor speed can be controlled by armaturevoltage. While above base speed, the speed is controlledby controlling the field current keeping the armaturevoltage, and hence the power, constant at rated voltage.Also, the speed direction can be reversed by reversingthe polarity of supply voltage across the armature inboth constant torque and power operation zones. Inseries DC motor, as the armature current, which equalsthe field current, changes with the load, the fluxproduced by the field winding also changes. Therefore,the torque developed by the motor is proportional to thesquare of the armature current as long as the motor isoperating in linear region. Since the torque developed bya series motor is also proportional to the square of theapplied voltage, its torque developed can be controlledby controlling the applied voltage. The typical speedcharacteristics of a series motor is inversely proportionalto the armature current. The series motor cannot runover base speed, as there is no individual control on thefield current. Also the direction of speed cannot bereversed by reversing supply voltage because reversingarmature voltage will reverse both field and armature

current. Series DC motor has a high starting torquetherefore it is used where the load demands a high initialtorque such as cranes, hoists, electric vehicles andelectric tractions. Researches are still being carried outto enhance the performance and operation of DC motors[4-9]. This paper proposes a drive system for the DCmotor that merges the advantages of the separatelyexcited DC motor and the series DC motor. As shown infig. (1), the armature coil is fed from a DC supply via aclass-E chopper and the field coil is fed from a DCsupply via a class A-chopper. Feed back signals fromthe speed, field current and armature current are fedbackto the control circuit, which produces the control signalsof the two chopper switches. Below base speed, withinconstant torque region, this drive will operate this motorso as to get a performance exactly like the series DCmotor having the advantage of high starting torque ofthe series DC motor where the speed is controlled usingarmature voltage. While above the base speed, the motorwill operate as a conventional separately excited DCmotor allowing field weakening operation in theconstant power region where the speed is controlled bycontrolling the field current. Moreover, the controlcircuit can change the direction of rotating at any speedthrough regenerative braking mode. The basic idea isexplained and control strategy is illustrated. The drive issimulated using MATLAB SIMULINK and theoreticalresults are obtained to show the transient and steadystate performance of the machine.

II. DYNAMIC MODEL OF DC MOTOR

Equations describing the characteristics of separatelyexcited motor can be concluded as follows [1]:

aaaa

a veiRdt

diL (1)

ffff

f viRdt

diL (2)

Lerr TTB

dt

dJ

(3)

r

ae

eiT

(4)

012

23

34

4 aiaiaiaiae

ffffr

(5)

where ao, a1, a2, a3 and a4 are factors of the curvefitting 4th order equation. Equation (5) expresses therelationship between the field current and the back emfper unit speed as can be concluded from the no loadcharacteristics of the machine including the saturation.MATLAB-SIMULINK software may be used tosimulate the dynamic model of the motor as shown infig. (2).

III. OPERATING MODES

Fig. (3) shows the schematic diagram of the closedloop control system of the proposed high startingperformance separately excited DC motor. For bothclockwise or anticlockwise speed directions, theabsolute value of the motor speed (r) is compared withthe base speed b) of the machine. Accordingly, thefield multiplexer determines the field current signal (if

*)such that below the base speed, the command signal offield current equals to the absolute value of the armaturecurrent, meaning that the machine acts exactly like aseries DC motor. Above the base speed, the commandsignal of field current is calculated so as to run themachine as a separately excited DC motor using field

Control Circuit

Vs1

is

T1

T4 D4 T2 D2

D1

D3T3

iaA1

A2

Mva

Load

speedsensor

T2 T3 T4T1 TF

TF

Vs2

DF

if

vf

F1

F2

Fig. (1) : Connection diagram of the proposed high starting separately excited DC motor

e

Te

e---

r

r

Equation (5)

2ia

1if

1/s1/s

1/s

B

1/J

Ra

1/La

Rf

1/Lf

3 TL

2Va

1Vf

(a)

DCMotor

vf

va

TL

if

ia

r

1 1

2

3

2

3

(b)Fig. (2) : DC motor model (a) Equation schematic diagram, (b)

a single block diagram

weakening mode of operation within the constant powerregion. Moreover, if the polarity of the command signalof the speed is reversed, the direction of the motor speedwill be reversed either above or below base speedregardless of operation mode of the machine. Thedecision of the command signal of the field depends onthe absolute value of both the speed and the armaturecurrent. Therefore, controlling the field current iscompletely decoupled with speed direction and vise-versa.

IV. STEADY STATE ANALYSIS

The motor performance characteristics at steady statecan be concluded below and above base speed. Equation(1) to (5) can be reduced to express the motor model atsteady state as follows:

aaa VEIR (6)

fff VIR (7)

r

ae

EIT

(8)

012

23

34

4 aIaIaIaIaE

ffffr

(9)

Equations (6) to (9) can be arranged depending onthe motor speed to develop the motor performancecharacteristics curves at the rated armature voltage asshown in fig. (4). It is seen that the machine has seriescharacteristic before base speed and separately excitedcharacteristics in the field-weakening region duringconstant power region. See Appendix for motorparameters.

V. TRANSIENT RESPONSE

The model depicted in fig. (3) has been simulated toconclude the transient response of the proposed set.Figures (5), (6) and (7) show the instantaneouswaveforms of step speed command below base speed,above base speed and reversing speed direction,respectively. In fig. (5), the machine runs at rated loadtorque where the speed command is step changed tobase speed at 0.1 second, then reduced to 50% of basespeed at 1.5 second, then reduced to zero speed at 3second. It can be seen that the machine has a highstarting torque like a series DC motor, while currentlimiters have been adjusted to limit the armature andfield current at twice the rated current. In fig. (6), thecommand speed is step changed from 80% base speed to120% base speed at 2 second. At steady state, thearmature current is controlled to equal the rated currentwhile the field current is determined to run the machinein the constant power region and therefore the torque isdecreased (field weakening). In fig. (7), the speeddirection is step reversed with in regenerative bakingmode. With respect to fig. (7-c), it could be seen that thearmature voltage in immediately reversed during thetime between to to t1 to force the armature current tochange direction, then during the time between t1 to t2,the armature voltage is positive while the armaturecurrent is negative and the power is pumped back to thesupply through the two diodes D1 and D2.

VI. DISCUSSION

The DC machine used in this drive is essentially series

if

vf

vaMr

+Vcc

-Vcc

r

+- +-

+-

b

PID

PID

PID

PWM

PWM

u u

ia*

if1*

if2*

ia

if*

CONTROL CIRCUIT

Multiplexer

CurrentLimiter

CurrentLimiter

Fig. (3) : Closed loop control circuit of high starting separately excited DC motor

1 1 5 0 1 2 0 0 1 2 5 0 1 3 0 0 1 3 5 0 1 4 0 08

9

1 0

1 1

1 2

1 3

1 4

S p e e d , r p m

Torq

ue, N

m

B e l o w b a s e s p e e d A b o v e b a s e s p e e d

S e p a r a t l e y e x c i t e d D C m o t o r

S e r i e sD C m o t o r

(a)

1 1 5 0 1 2 0 0 1 2 5 0 1 3 0 0 1 3 5 0 1 4 0 05 . 5

6

6 . 5

7

7 . 5

8

8 . 5

S p e e d , r p m

Arma

ture C

urren

t, A

B e l o w b a s e s p e e d A b o v e b a s e s p e e d

S e p a r a t l e y e x c i t e d D C m o t o r

S e r i e sD C m o t o r

(b)

1 1 5 0 1 2 0 0 1 2 5 0 1 3 0 0 1 3 5 0 1 4 0 04 . 5

5

5 . 5

6

6 . 5

7

7 . 5

8

8 . 5

9

S p e e d , r p m

Field

Curre

nt, A

B e l o w b a s e s p e e d A b o v e b a s e s p e e d

S e p a r a t l e y e x c i t e d D C m o t o r

S e r i e sD C m o t o r

(c)Fig.(4) : Steady state characteristics of high starting separatelyexcited DC motor at rated voltage, speed vesus (a) load torque,

(b) armature current and (c) field current

0 0 .5 1 1 .5 2 2 .5 3 3 .5 4 4 .5-2 0 0

0

2 0 0

4 0 0

6 0 0

8 0 0

1 0 0 0

1 2 0 0

1 4 0 0

T im e , S e c

Spee

d, rp

m

N rN r *

(a)

0 0 .5 1 1 .5 2 2 .5 3 3 .5 4 4 .5-2 0

-1 5

-1 0

-5

0

5

1 0

1 5

2 0

T im e , S e c

Torq

ue, N

.m

T LT e

(b)

0 0 .5 1 1 .5 2 2 .5 3 3 .5 4 4 .5-1 5

-1 0

-5

0

5

1 0

1 5

T im e , S e c

Curre

nts,

A

IaIf

(c)Fig. (5) : Transient response for step speed command: (a)

speeds, (b) torques and (c) currents below base speed

1 . 5 2 2 . 5 3 3 . 51 0 0 0

1 1 0 0

1 2 0 0

1 3 0 0

1 4 0 0

1 5 0 0

1 6 0 0

T i m e , s e c

Spee

d, rpm

r a t e dN r *N r

(a)

1 . 5 2 2 . 5 3 3 . 56

8

1 0

1 2

1 4

1 6

T i m e , s e c

Torqu

e, N.m

r a t e dT eT L

(b)

1 . 5 2 2 . 5 3 3 . 53

4

5

6

7

8

9

1 0

1 1

1 2

T i m e , s e c

Curre

nts, A

r a t e dI aI f

(c)Fig. (6) : Transient Response for step speed command from

80% to 120% base speed: (a) speed, (b) torque and (c) currents

1 . 2 1 . 4 1 . 6 1 . 8 2 2 . 2 2 . 4 2 . 6- 1 5 0 0

- 1 0 0 0

- 5 0 0

0

5 0 0

1 0 0 0

1 5 0 0

T i m e , S e c

Spee

d, rpm

N r *N r

(a)

1 . 2 1 . 4 1 . 6 1 . 8 2 2 . 2 2 . 4 2 . 6- 2 0

- 1 5

- 1 0

- 5

0

5

1 0

T i m e , S e c

Torqu

e, N.m

T LT e

(b)

1 . 2 1 . 4 1 . 6 1 . 8 2 2 . 2 2 . 4 2 . 6

- 2 0 0

- 1 0 0

0

1 0 0

2 0 0

T i m e , S e c

Avea

ge ar

matur

e volt

age, V

A d fg f gl k l k

A d fg f gl k l k

A d fg f gl k l k

A d fg f gl k l k

t o t 1

t 2

(c)

1 . 2 1 . 4 1 . 6 1 . 8 2 2 . 2 2 . 4 2 . 6- 1 5

- 1 0

- 5

0

5

1 0

1 5

T i m e , S e c

Curre

nts, A

I aI f

(d)

Fig. (7) : Transient response for speed reverse command: (a)speeds, (b) torques , (c) average armature voltage and (d)

currents at base speed

type such that the field winding is principally designedto carry the armature current. In the series DC motor,since the series field winding carries the rated armaturecurrent, it has few turns of heavy wire. The fieldwinding is composed of a small number of turns with alarge cross section wire. This type is designed to carrylarge currents and is connected in series with thearmature winding. The field resistance is normallysmall. Therefore, the voltage level of the field circuitshould be small and the chopping frequency is lowenough to allow continuous mode of current in the fieldcircuit. In industry, for this 6 A field current machine, a24 VDC voltage can be obtained from a single-phase220V supply, 220/24, 100 VA transformer anduncontrolled bridge rectifier. The control circuitoperates such that the voltage across the armature ispositive and the motor is running in the clockwisedirection by chopping T1 and T2 together. Conversely,the voltage across the armature is negative and themotor is running in the anti-clockwise direction bychopping T3 and T4 together. In both directions, theaverage value of the armature voltage is controlled bycontrolling the duty ratio of switching T1 to T4, whilethe field current is controlled through a single switch Tf.It should be noted that, the instantaneous waveform of

the field and armature currents may not be typicalduring transient period due to difference in timeconstant of the two circuits but at steady state they areequal as can be seen from fig. (5-c) and fig. (6-c).

VII. CONCLUSION

In this paper, a drive system for the DC motor, whichmerges the advantages of both the separately excited DCmotor and the series DC motor was introduced. Thecontrol system proposed is designed so as to run the DCmotor as a series DC motor below base speed then run itas a separately excited DC motor above base speed. Theproposed drive system was also used to change thespeed direction in a regenerative braking mode at anymotor speed. The steady state analysis of the motorbelow and above the base speed was performed. Thetransient analysis was also performed with the aid ofMATLAB-SIMULINK model. Such system can cover awide range of speed applications with differenttorque/speed characteristics such as traction, cranes andelectric cars.

VIII. REFERENCES

[1] B.S.Guru and H.R.Hiziroglu, Electric MachineryandTransformers. Oxford University Press, New York,1988.

[2] M.H.Rashid, Power Electronics: Circuits, Drives andApplications. Prentice Hall, New Jersey, 1988.

[3] N.Mohan, T.M.Undeland and W.P.Robbins, PowerElectronics: Converters, Applications and Design.Wiley Press, New York, 1989.

[4] P.B.Schmidt and R.D.Lorenz, "Design principles andimplementation of acceleration feedback to improveperformance of DC drives", IEEE Transactions onIndustry Applications, Volume 28, Issue 3, pp. 594 –599, May-June 1992.

[5] G.Jang and M.G.Kim, "A bipolar-starting and unipolar-running method to drive a hard disk drive spindle motorat high speed with large starting torque", IEEETransactions on Magnetics, Volume 41, Issue 2, pp.750 – 755,Feb. 2005.

[6] I.Avitan and V.Skormin, "Mathematical modeling andcomputer simulation of a separately excited DC motorwith independent armature/field control", IEEETransactions on Industrial Electronics, Volume 37,Issue 6, pp. 483 – 489, Dec. 1990.

[7] J.Chiasson, "Nonlinear differential-geometric techniquesfor control of a series DC motor", IEEE Transactions onControl Systems Technology, Volume 2, Issue 1, pp. 35– 42, March 1994.

[8] Zuo Zong Liu, Fang Lin Luo and M.H.Rashid, "Speednonlinear control of DC motor drive with fieldweakening", IEEE Transactions on IndustryApplications, Volume 39, Issue 2, pp. 417 – 423,March-April 2003.

[9] S.Mehta and J.Chiasson, "Nonlinear control of a series DCmotor: theory and experiment", IEEE Transactions onIndustrial Electronics, Volume 45, Issue 1, pp. 134 –141, Feb. 1998.

IX. APPENDIX

The parameters of the 220 V, 1.2 kW DC series machineis Ra = 3 ; Rf = 1.8 ;

La = 0.2 ; Lf = 0.2 ;J = 0.05 ; B = 0.001 ;

0 2 4 6 8 10 12 140

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

Feild current, A

No

load

EM

F pe

r uni

t spe

ed, V

/rpm

Best FitExperimental

(a)

b = 1280

Fig. (8) shows the no load characteristics and thepredicted relationship between the field current andmotor speed for both constant torque and constantpower region.

500 1000 1500 2000 25001

2

3

4

5

6

7

Speed, rpm

Feild

cur

rent

, A

Base Speed

(b)Fig. (8) : (a) No load characteristics (b) Relationship between

field current and speed