DC Machines Fundamentals DET 205 Chapter 2 DC Machines Fundamentals

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.

4. Cumulatively compounded generator 4. Cumulatively compounded generator. In a cumulatively compounded generator, both a shunt and a series field are present and their effects are additive. 5. Differentially compounded generator. In a differentially compounded generator, both a shunt and a series field are present, but their effects are subtractive. DC generators are compared by their voltages, power ratings, efficiencies and voltage regulations. Voltage regulation, VR is defined by

Separately Excited Generator

Example 2.1 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.

Solution 2.1 Constant field excitation means; if1 = if2 or constant flux; 1 = 2

Shunt DC Generator

Series DC Generator

Total Magnetomotive force 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

Differentially Compounded DC Generator With a long shunt connection Equivalent shunt field current,

Example 2.2 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

Solution 2.2 Armature Copper Loss Series Field Copper Loss Shunt Field Copper Loss Friction + Stray + windage + etc: Total Losses =

Efficiency, η =

The equivalent circuit of the separately-excited dc motor Assignment 2.1 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

The equivalent circuit of the shunt dc motor QUESTION 2 Assignment 2.2 The equivalent circuit of the shunt dc motor If the resistor Radj is adjusted to 175, what is the rotational speed of the motor at no-load conditions? Assuming no armature reaction, what is the speed of the motor at full load? What is the speed regulation of the motor?

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.

Power flow and losses in DC machines DC generators take in mechanical power and produce electric power while DC motors take in electric power and produce mechanical power Efficiency

The losses that occur in DC machine can be divided into 5 categories Copper losses (I2R) Brush losses Core losses Mechanical losses Stray load losses Ia = armature current If = field current Ra = armature resistance Rf = field resistance VBD = brush voltage drop - Usually assumed to be 2V

Power Losses Core losses – Hysteresis losses and Eddy current losses Mechanical losses – The losses that associated with mechanical effects. Two basic types of mechanical losses: Friction & Windage. Friction losses caused by the friction of the bearings in the machine. Windage are caused by the friction between the moving parts of the machine and the air inside the motor casing’s Stray losses (Miscellaneous losses) – Cannot placed in one of the previous categories.

The Power Flow Diagram Pout = VTIL For generator

The Power Flow Diagram For motor

Equivalent circuit of DC generator Vf = field voltage Rfw = rheostat resistance If = field current Rf = Rfc + Rfw = field circuit resistance Ra = armature resistance

Ea = KФω where Ф = flux generated by field current, If VT = terminal voltage Ia = armature current

Equivalent circuit of DC motor Vf = field voltage Rfw = rheostat resistance If = field current Rf = Rfc + Rfw = field circuit resistance Ra = armature resistance