Ctrl of DC Drives

8/6/2019 Ctrl of DC Drives

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www.anoopmathew.110mb.com

- Anoop Mathew

Asst. ProfessorElectronics & Commn. Engg. Dept.Vedavyasa Institute of Tech.Malappuram Dist. 673632Kerala

Speed Control of DC Motors


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Speed Control of DC MotorsMerits of DC Drives

Versatile control characteristics. High starting torque. (Traction purposes)

Control over a large speed range.

Speed control methods are simpler andcheaper as compared to AC machines.


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Tailor-made machines.

Bulky as compared to AC counterpart. Commutator sparking not suitable for

petrochemicals, mine and chemical applications.

Above 500 kW, manufacturing of machine itself istedious.

Demerits of DC Drives


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Fundamentals of Drives

Quadrants of operation


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Fundamentals of DC machines

DC machine

Separatelyexcited

Series Shunt Compound


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Fundamentals of DC machines Consists of stationary field winding and rotating

armature winding. Field magnetic field interacts with the current in the

armature conductors and this generates a torque. Armature current distribution is maintained by the

Commutators. Commutators reverse the armature current as

armature pass from one field pole to next. Due to the field, a back EMF is induced in the

armature windings and it appears at thecommutators.


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Fundamentals of DC machinesMotoring

Vb = V-I aRa

= KNeglecting I a Ra drop,

V= Vb = K

ie, 1/

Also VWhere,V terminal voltageVb back emf rotational speed fluxRa armature resistance

(V/ )

(1)


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n Drive Speed can be controlled by:

Armature Voltage control Field Flux control

Speed control options


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Speed control using Armature voltage control


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Semi-converter fedseparately-excited motor


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Waveforms (Continuous motor current)


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Waveforms (Discontinuous Motor current)High power

motorsoften havecontinuouscurrent


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Terminal Voltage Avg. teminalVoltage, V a =

)cos1( /

)(1

+

m

m

V

t d t SinV

Vdc (min) = zero when =

Vdc (max) = 2V m / when = 0Range of V dc , 0 to 2V m /


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Torque-Speed Characteristics = (Va-IaRa)/Ka --------------- (2)But Ia =T/K a

And V a =

? Substitute these values in Eqn. (2)? Find No-Load speed (At no-load, T=0)

)cos1( / +mV


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Torque-Speed Characteristics


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Waveforms (continuous current)


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Home work....

? Find No-Load speed

? Find terminal voltage


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Speed-Torque Characteristics


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Motoring & Breaking operation


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Motoring & Breakingn Rectifier supplies power to the load for

0 90 0

n Reversing the rectifier output current byreversing the rectifier output voltage .

n Rectifier output voltage can be reversed by180 o> >90 o rectifier works as a linecommuted inverter .

n Regenerative braking cannot be obtained byhalf controlled rectifier since o/p voltagecannot be reversed.


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Converter fed DC series motor


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Single phase semi converter fed

Series motor


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Class work ...n Derive equation for torque from the

series motor fundamentals andterminal voltage equations.


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Speed-Torque characteristics


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Single phase full converter drive


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Waveforms


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Home work ...n Derive equation for torque from the

series motor fundamentals andterminal voltage equations.


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Speed-torque characteristics


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Speed control using Armature voltage control

Chopper-fed DC drive


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Chopper-fed DC series Motor- Power Circuit


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Waveform- Continuous motor current


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Waveform- Discontinuous motor

current


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Chopper-fed series DC motorTorque

Ia armature current

Average motor voltageFor continuous current:

Va=Edc.

For discontinuous current:

Va= E dc . +Kres .N.(T-t p)/T duty ratio

2a

I T


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References[1] A. Maamoun, DEVELOPMENT OF CYCLOCONVERTERS,IEEE Trans. 2003.

[2] Ned Mohan et al, Power Electronics, Weily publications 2005

[3] M.D. Singh & K.B Khanjandani, Power Electronics, TMH

2006

[4] Cyril W. Lander Power Electronics, McGraw Hill

International Editions 2007

[5] Dr.S. Rama Reddy Power Electronics, Nirosha Publications

[6] Vedam Subramanyam Power Electronics, New Age

Publications 2004


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Thank You

Quest ions ???

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Ctrl of DC Drives

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