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Solid-state control of DCmotor

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Content

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Part 1: Four quadrant operation of DC motor• Part 2: Phase controlled of DC motor• Part 3: Chopper controlled of DC motor

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Part 1

Four quadrant operation ofDC motor

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Principle of DC Motor Speed Control

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The principle of speed control for dc motor (separately-excited) isderived from the basic emf equation of the motor.

• Two types of speed control are available: armature voltage and fieldcurrent control.

• These methods are combined to yield a wide range of speed control.

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Field Control

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In field control, the applied armature voltage V is maintainedconstant. Then the speed is represented as

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Field Control

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By weakening the field flux, the speed can be increased.• Normally, it is not possible to strengthen the field flux beyond its

rated value, an account of saturation of the steel laminations.• Hence, the field control for speed variation is not suitable below the

rated speed.• At rated speed, the field current by design is at rated value.

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Armature Control

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In this mode, the field current is maintained constant. Then, thespeed is represented as

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Armature Control

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Armature control is limited in speed by the limited magnitude of theavailable dc supply voltage and armature winding insulation.• If the supply dc voltage is varied from zero to its rated value, then the

speed can be controlled from zero to its rated speed as well.• Therefore, armature control is ideal for speeds lower than the rated

speed. And the field control is suitable above for speeds greater thanrated speed.

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Field and armature control

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For speed lower than that of the rated speed, the applied armaturevoltage is varied while keeping the field current constant (at its ratedvalue).

• For speed above the rated speed, the field current is decreased whilekeeping the armature voltage constant (at its rated value).

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Field and armature control

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Four quadrant operation

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Consider that the machine is operating at a speed of ω m anddesired to bring the speed to zero.• There are two ways to achieve it:

• (1) Cut off the armature supply to machine and let the motor come to zerospeed.

• (2) The machine can be made to work as a dc generator, thereby the storedkinetic energy can be effectively transferred to the source. This save energyand brings the machine rapidly to zero speed.

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Four quadrant operation

• To make the dc machine operating in the motoring mode go to thegenerating mode, all that needs to be done is to reverse the armaturecurrent flow in the dc machine.

• The power flows from the machine armature to the dc source. Thismode of operation is termed “ regenerative braking ”.

• The braking is accomplished by regeneration. This implies that anegative torque is generated in the machine.

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Four quadrant operation

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Four quadrant operation

• Hence, a mirror reflection of torque-speed curve of required on the IVquadrant for regeneration. The first and fourth quadrants are for onedirection of rotation, say forward.

• For reverse direction of rotation, the III quadrant signifies the reversemotoring and the II quadrant; the reverse regeneration mode.

• Since

• The speed axis becomes the armature voltage axis and the torque axisis equivalent to the armature current axis.

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Converter requirement

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Part 2

Phase controlled of DCmotor

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Two quadrant operation

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Phase controlled converter

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Phase controlled converter

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One quadrant operation

• Now the reversal of output voltage (V) is not possible now.• Hence, the converter operates only in the first quadrant, delivering a

positive voltage and current.

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Four quadrant operation

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Part 3

Chopper controlled of DCmotor

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Review

• The basic schematic of chopper is shown as follows:

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Review

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Full bridge DC-DC converter

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First-Quadrant Operation (FM)

• First quadrant operation with positive voltage and current in the load(ia increasing) (T1, T2 on).

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First-Quadrant Operation (FM)

• First quadrant operation with zero voltage across the load (iadecreasing) (T2, D4 on).

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Second-Quadrant Operation (RR)

• Second quadrant operation with negative load voltage and positivecurrent (D3, D4 on).

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Third-Quadrant Operation (RM)

• Third quadrant operation with negative voltage and current in theload (ia increasing) (T3, T4 on).

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Third-Quadrant Operation (RM)

• Third quadrant operation with zero voltage across the load (iadecreasing) (T3, D1 on).

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Fourth-Quadrant Operation (FR)

• Fourth quadrant operation with positive load voltage and negativecurrent (D1, D2 on).