DC Machines: Speed Regulation, Equivalent Circuits, and Speed Control, Slides of Electrical Engineering

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Electrical Machines

DC Machines-

Separately Excited

& Shunt

Nuha Nadhiroh

• Speed Regulation
• Equivalent Circuit
• Magnetization Curve
• Speed Control
• Excercises

Overview

Equivalent Circuit of DC Motors Simplified Equivalent Circuit

Equivalent Circuit of Separately Excited DC Motors Sometimes, Radj and Rf are lumped together and called Rf

• A separately excited dc motor is a motor whose field circuit is supplied from a separate constant-voltage power supply.

Terminal Characteristic A terminal characteristic of a machine is a plot of the machine's output quantities versus each other. For a motor, the output quantities are shaft torque and speed, so the terminal characteristic of a motor is a plot of its output torque versus speed.

Excercise A 50 - hp, 250 - V, 1200 r/min dc shunt motor with compensating windings has an armature resistance (including the brushes, compensating windings, and interpoles) of 0. 06 Ω. Its field circuit has a total resistance Radj + RF of 50 Ω, which produces a no-load speed of 1200 r/min. There are 1200 turns per pole on the shunt field winding (see Figure). a) Find the speed of this motor when its input current is 100 A. b) Find the speed of this motor when its input current is 200 A. c) Find the speed of this motor when its input current is 300 A. d) Plot the torque-speed characteristic of this motor.

Answer for d) To plot the output characteristic of this motor, it is necessary to find the torque corresponding to each value of speed. At no load, the induced torque Tind is clearly zero. The induced torque for any other load can be found from the fact that power converted in a dc motor is Therefore, the induced torque when IL = 100 A is Find the rest, and plot torque-speed characteristic for this motor

Speed Control Changing The Field Resistance if there is a 1% decrease in flux, 49% increase in armature current 𝑰𝑨 = 𝟐𝟓𝟎 − 𝟐𝟒𝟓 𝟎. 𝟐𝟓 𝑰𝑨 = 𝟐𝟎𝑨

Excercise a 1000hp. 250-V, 1200 r/min shunt dc motor with an armature resistance of 0.03 Ω and a field resistance of 41.67 Ω. The motor has compensating windings. so armature reaction can be ignored. Mechanical and core losses may be assumed to be negligible for the purposes of this problem. The motor is assumed to be driving a load with a line current of 126 A and an initial speed of 1103 r/min. To simplify the problem, assume that the amount of armature current drawn by the motor remains constant a) If the machine's magnetization curve is shown in next Figure. what is the motor’s speed if the field resistance is raised to 50 Ω

Answer

4. The ratio of the internal generated voltage at one speed to the internal generated voltage at another speed is given by the ratio of Equation (&--41) at the two speeds

Any Question