Electric Machine DC Generator (3) By Dr. Shorouk Ossama

References Stephen J. Chapman, “Electric Machinery Fundamentals”, 5th Edition, 2012.

Compound -Wound Generator A dc generator with two methods of connection are used, both having a mixture of shunt and series windings, designed to combine the advantages of each. The Equivalent Circuit:

The Characteristics : In this type the magnetic flux produced by the series and shunt fields. The degree of compounding obtained depending on the number of turns of wire on the series winding.

Applications Compound-wound generators are used in electric arc welding.

Example 5: A 10 kw, 250 v, DC shunt generator has an armature resistance of 0.05 ohm and field resistance of 60 ohm. With the generator operating at rated voltage, determine the induced voltage at: (a) full load and (b) half-load.

(a) At full-load VT = Ea – Ia Ra Ia = IL + Lf = 40 + 4. 17 = 44 (a) At full-load VT = Ea – Ia Ra Ia = IL + Lf = 40 + 4.17 = 44.17 Amp The generated emf is: Ea = VT + Ia Ra = 250 + (44.17 x 0.05) = 252.2 v

(a) At half-load Ia = IL + Lf = 200 + 4. 17 = 24 (a) At half-load Ia = IL + Lf = 200 + 4.17 = 24.17 Amp The generated emf is: Ea = VT + Ia Ra = 250 + (24.17 x 0.05) = 251.2 v

DC Machine Losses A generator is a machine for converting mechanical energy into electrical energy. When such conversions take place, certain losses occur which are dissipated in the form of heat.

The principle losses of machines are: Copper Loss, due to I2R heat losses in the armature and field windings. Iron (or core) loss, due to hysteresis and eddy current losses in the armature.

3. Friction and windage losses, due to bearing and brush contact friction and losses due to air resistance against moving parts (called windage). At constant speed, these losses are assumed to be constant.

4. Brush contact loss between the brushes and commutator 4. Brush contact loss between the brushes and commutator. This loss is approximately proportional to the load current. The total losses of a machine can be quite significant and operating efficiencies of between 80% and 90% are common.

Efficiency of a DC generator The efficiency of an electrical machine is the ratio of the output power to the input power and is usually expressed as a percentage. The greek letter, Ƞ (eta) is used to signify efficiency and since the units are, power/power, then efficiency has no units.

input power (mechanical) – total losses Output power (electrical) = input power (mechanical) – total losses Input power = power developed on the armature + rotational losses Pin = Pdel + Prot The maximum efficiency occurs when the variable loss = the constant loss

Example 7: A 10 kw, 125 v compound generator has rotational losses amounting to 580 w. The shunt field resistance is 62.5 ohm, the armature resistance is 0.12 ohm and the series field resistance is 0.022 ohm. Calculate the full-load efficiency.

Ia = IL + If = 80 + 2 = 82 Amp, which is the current through the series winding. The I2R losses can now be determined: Armature: (82)2 x 0.12 = 807 W Series field: (82)2 x 0.022 = 148 W Shunt field: (2)2 x 62.5 = 250 W Rotational loss = 580 W Total loss = 807 + 148 + 250 + 580 = 1785 W

10000+ 1785

DC Machine Voltage Regulation The regulation of a generator refers to the VOLTAGE CHANGE that takes place when the load changes. It is usually expressed as the change in voltage from a no-load condition to a full-load condition

Where: VnL = is the no-load terminal voltage VFL = is the full-load terminal voltage of the generator

Example 8: Calculate the percent of regulation of a generator with a no-load voltage of 462 volts and a full-load voltage of 440 volts. Given that: - No-load voltage 462 V - Full-load voltage 440 V The Percent of regulation =

Voltage control is either Manual or Automatic Voltage control is either Manual or Automatic. In most cases the process involves changing the resistance of the field circuit. By changing the field circuit resistance, the field current is controlled. Controlling the field current permits control of the output voltage.

Manual Voltage Control: The field rheostat is connected in series with the shunt field circuit. Automatic voltage control: device "senses" changes in output voltage and causes a change in field resistance to keep output voltage constant.

VOLTAGE REGULATION is an internal action occurring within the generator whenever the load changes. VOLTAGE CONTROL is an imposed action, usually through an external adjustment, for the purpose of increasing or decreasing terminal voltage.

Thanks