CHAPTER 2: DC DRIVES (Part 3) PLT 303 – ELECTRICAL DRIVES

SPEED CONTROL OF SHUNT @ SEPARATELY EXCITED MOTORS For a given torque, the motor speed is a function of: Resistance in armature circuit – when a resistance is inserted in the armature circuit, the speed drop, ∆ω increases & the motor speed decreases. Terminal voltage (armature voltage) – reducing the armature voltage, Vt of the motor reduces the motor speed. Field flux (or field voltage) – reducing the field voltage reduces the flux, φ & the motor speed increases. PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADJUSTING ARMATURE VOLTAGE This method involves changing the armature voltage (applied to the armature circuit) without changing the field voltage. This method is highly efficient, stable and simple to implement. Basic configuration of controlling speed by adjusting the armature voltage is shown in figure. PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADJUSTING ARMATURE VOLTAGE Assuming the field voltage & load torque are kept constant. By adjusting the armature voltage, Vt it will not affect the speed drop, ∆ω but it will affect the no-load speed, ω0. Hence, by adjusting Vt, the speed-torque characteristic is shifted in parallel. PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADJUSTING ARMATURE VOLTAGE If the armature voltage keeps decreasing, the motor will operates at point 4. Point 4 is a holding operation where the motor speed is: PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADJUSTING ARMATURE VOLTAGE PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADJUSTING FIELD VOLTAGE A basic configurations of controlling speed by adjusting the field voltage are shown below. OR PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADJUSTING FIELD VOLTAGE Field voltage reduce, field current and flux also reduced Characteristics show that because of the change in speed drop, lines not parallel PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADJUSTING FIELD VOLTAGE Considerations when adjusting the field voltage: Vf must not exceed the absolute maximum rating. Large reductions in field may result in excessive speed. If the load is constant, large reductions in field may result in excessive Ia. PLT 303 – ELECTRICAL DRIVES

EXAMPLE 2 A 150 V, DC shunt motor drives a constant torque load at a speed of 1200 rpm. The armature & field resistances are 2 Ω & 150 Ω, respectively. The armature current before inserting a resistance in the field circuit is 10 A. Assume that a resistance is added in the field circuit to reduce the field current by 20%. Calculate the armature current, motor speed, value of the added resistance & extra field losses. PLT 303 – ELECTRICAL DRIVES

EXERCISE 2 A 200 V, dc shunt motor drives a constant torque load at a speed of 1300 rpm. The armature and field resistances are 3 Ω and 200 Ω, respectively. The motor draws a line current of 15 A. Assume that a resistance is added in the field circuit to reduce the field current by 25 %. Calculate the armature current, motor speed and value of the resistance added. Determine the extra field losses. PLT 303 – ELECTRICAL DRIVES

SPEED CONTROL OF SERIES MOTOR Equation of speed-torque characteristics: Three methods that can be used to control the motor speed: Adding a resistance in the armature circuit Adjusting the armature voltage Adjusting the field current PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADDING RESISTANCE TO ARMATURE CIRCUIT For a given load torque, armature current of the motor is constant. The change in armature current is only proportional to the change in the load torque. PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADDING RESISTANCE TO ARMATURE CIRCUIT By assuming φ ≈ CIa : Assume that a resistance Radd is inserted in series with the armature circuit: If assume the load torque is unchanged, the armature current is constant. If the supply voltage is unchanged, the motor speed is reduced when resistance is added to armature circuit. (due to increase in speed drop Δω) PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADDING RESISTANCE TO ARMATURE CIRCUIT Shapes of the characteristics are similar when a resistance is added but speed drop increases when the value of added resistance increases. PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADJUSTING ARMATURE VOLTAGE Similar effect as the insertion of an armature resistance. This type of control is suitable for speed reduction below rated speed. PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADJUSTING ARMATURE VOLTAGE PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADJUSTING FIELD CURRENT Two simple methods to control field current: Add shunt resistance to the series field circuit. Use solid-state switching device across the field windings to regulate the field current. 1 2 PLT 303 – ELECTRICAL DRIVES

CONTROLLING SPEED BY ADJUSTING FIELD CURRENT PLT 303 – ELECTRICAL DRIVES