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**BASIC ELECTRICAL TECHNOLOGY DET 211/3**

Chapter 7 Direct Current (DC) Motor & Direct Current (DC) Generator

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DC Motor There are four major types of DC motor in general use: Separately excited DC motor Shunt DC Motor Series DC Motor Compounded DC Motor

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Speed Regulations DC motors are often compared by their speed regulations. Speed Regulations (SR) of the a motor is defined by

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**The Equivalent Circuit of a DC Motor**

RA Armature circuit represented by voltage source, EA and a resistor RA. The brush voltage drop represented by battery, Vbrush opposing the direction of current flow in the machine. The field coils, which produce the magnetic flux are represented by inductor LF and resistor RF. The separate resistor, Radj represents an external variable resistor used to control the amount of current in the field circuit.

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**The Equivalent Circuit of a DC Motor**

The brush voltage drop is often only a very tiny fraction of the generated voltage in a machine so the voltage drop may be left out or approximately included in the value of RA. The internal resistor in the field coils is sometimes lump together with the variable resistor, and the total is called RF.

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**The Equivalent Circuit of a DC Motor**

The internal generated voltage in DC motor is The induce torque developed by DC motor is These two equations, the KVL equation of the armature circuit and the machine’s magnetization curve are all the tools necessary to analyze the behavior and performance of the dc motor.

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**Separately Excited and Shunt DC motors**

Separately excited DC motor Shunt DC motor

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Example A 50hp, 250V, 1200rpm DC shunt motor with compensating windings has an armature resistance of 0.06Ω. Its field circuit has a total resistance of 50Ω, which produces a no load speed of 1200rpm. There are 1200 turns per pole on the shunt field winding. Find: the speed of this motor when its input current is 100A the speed of this motor when its input current is 200A the speed of this motor when its input current is 300A. the induced torque of this motor for above conditions.

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Solution The relationship between the speeds and internal generated voltages of the motor at two different load conditions is (1) The flux is constant and no armature reaction (2)

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Solution At no load, IA = 0A 1)

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Solution EA at this load will be The resulting speed of this motor is

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Answer 2) IA = 195A, EA = 238.3V, n2 = 1144rpm 3) IA = 295A, EA = 232.3V, n2 = 1115rpm

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Solution 4) At IL = 100A At IL = 200A At IL = 300A

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Series DC Motor

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Compounded DC Motor A compounded DC motor is a motor with both a shunt and a series field. Current flowing into a dot produces a positive magnetomotive force. If current flows into the dots on both field coils, the resulting magnetomotive forces add to produces a larger total magnetomotive force. It is called cumulative compounding.

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**Equivalent circuit of compounded DC motor**

Long shunt connection The net magnetomotive force Effective shunt field current Short shunt connection

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**Applications of DC motor types**

Separately-excited dc motor applications: Golf cars (buggy) Forklift Aerial lift equipment

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**DC Generator There are five major types of DC generators:**

Separately excited generator. In a separately excited generator, the field flux is derived from a separately power source independent of the generator itself. Shunt generator. In a shunt generator, the field flux is derived by connecting the field circuit directly across the terminals of the generator. Series generator. In a series generator, the field flux is produced by connecting the field circuit in series with the armature of the generator. Cumulatively compounded generator. In a cumulatively compounded generator, both a shunt and a series field are present, and their effects are additive. Differentially compounded generator. In a differentially compounded generator, both a shunt and a series field are present, but their effects are subtractive.

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DC Generator DC generators are compared by their voltages, power ratings, efficiencies, and voltage regulations. Voltage regulation, VR is defined by

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**Separately Excited Generator**

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Example If no load voltage of a separately-excited dc generator is 135V at 850 r/min, what will be the voltage if the speed is increased to 1000 r/min? Assume constant field excitation

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**Constant field excitation means; if1 = if2 or constant flux; 1 = 2**

Solution Constant field excitation means; if1 = if2 or constant flux; 1 = 2

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Shunt DC Generator

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Series DC Generator

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**The Cumulatively Compounded DC Generator**

Total Magnetomotive force Cumulatively compounded dc generator with a long shunt connection Cumulatively compounded dc generator with a short shunt connection

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**Differentially Compounded DC Generator**

With a long shunt connection Equivalent shunt field current,

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Example A short-shunt compound generator delivers 50A at 500V to a resistive load. The armature, series field and shunt field resistance are 0.16, 0.08 and 200, respectively. Calculate the armature current if the rotational losses are 520W, determine the efficiency of the generator

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**Solution Armature Copper Loss: Series Field Copper Loss:**

Shunt Field Copper Loss: Friction + Stray + windage + etc: So,Total Losses =

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Continued… Efficiency, η =

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**The equivalent circuit of the separately-excited dc motor**

Assignment 6 QUESTION 1 The equivalent circuit of the separately-excited dc motor Figure above shows fixed field voltage VF of 240V and armature voltage VA that can be varied from 120 V to 240 V. What is the no-load speed of this separately-excited dc motor when Radj = 175 and a) VA = 120V, b) VA = 180V, c) VA = 240V ?

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**The equivalent circuit of the shunt dc motor**

Assignment 6 QUESTION 2 The equivalent circuit of the shunt dc motor a) If the resistor Radj is adjusted to 175, what is the rotational speed of the motor at no-load conditions? b) Assuming no armature reaction, what is the speed of the motor at full load? What is the speed regulation of the motor?

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**The magnetization curve for the dc motor of Question 1 and Question 2**

The magnetization curve for the dc motor of Question 1 and Question 2. This curve was made at a constant speed of 1200 r/min.

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