DC Machines PDF

Electrical Machines (110405323) DC Machines By Prof. Mohammad Salah Mechatronics Engineering Department The Hashemite University Electrical Machines (110405323) Dr. Mohammad Salah...

Electrical Machines (110405323) DC Machines By Prof. Mohammad Salah Mechatronics Engineering Department The Hashemite University Electrical Machines (110405323) Dr. Mohammad Salah 2 DC Machines – Definition ► DC machines are generators that convert mechanical energy to DC electric energy and motors that convert DC electric energy to mechanical energy. ► Most DC machines are like AC machines in that they have AC voltages and currents within them. DC machines have a DC output only because a mechanism exists that converts the internal AC voltages to DC voltages at their terminals. ► Since this mechanism is called a commutator, DC machinery is also known as commutating machinery. Electrical Machines (110405323) Dr. Mohammad Salah 3 DC Machines – Simple Rotating Loop ► The simplest possible rotating dc machine consists of a single loop of wire rotating about a fixed axis. ► The rotating part of this machine is called the rotor, and the stationary part is called the stator. ► The magnetic field for the machine is supplied by the magnetic north and south poles on the stator. ► Notice that the loop of rotor wire lies in a slot carved in a ferromagnetic core. ► The magnetic field is perpendicular to the rotor surface everywhere under the pole faces. ► The magnetic flux is uniformly distributed (constant) everywhere under the pole faces. Electrical Machines (110405323) Dr. Mohammad Salah 4 DC Machines – Simple Rotating Loop Electrical Machines (110405323) Dr. Mohammad Salah 5 DC Machines – Simple Rotating Loop Let’s suppose that a battery is connected to the machine and see how the torque is developed inside it. Electrical Machines (110405323) Dr. Mohammad Salah 6 DC Machines – Simple Rotating Loop Electrical Machines (110405323) Dr. Mohammad Salah 7 DC Machines – Simple Rotating Loop Electrical Machines (110405323) Dr. Mohammad Salah 8 DC Machines – Simple Rotating Loop ► The torque produced in the machine is the product of the flux in the machine and the current in the machine, times some quantity representing the mechanical construction of the machine (the percentage of the rotor covered by pole faces). ► In general, the torque in any real machine will depend on the same three factors: 1. The flux in the machine 2. The current in the machine 3. A constant representing the construction of the machine Electrical Machines (110405323) Dr. Mohammad Salah 9 DC Machines – Operation DC Motor, How it works Electrical Machines (110405323) Dr. Mohammad Salah 10 DC Machines – Power Flow and Losses ► In all DC machines, there is always some loss associated with the operation. ► The losses that occur in DC machines can be divided into five basic categories: 1. Electrical or copper losses (I2R losses) 2. Brush losses 3. Core losses 4. Mechanical losses 5. Stray load losses Electrical Machines (110405323) Dr. Mohammad Salah 11 DC Machines – Power Flow and Losses ► Electrical or copper losses are the losses that occur in the armature and field windings of the machine. ► The copper losses for the armature and field windings are given by Electrical Machines (110405323) Dr. Mohammad Salah 12 DC Machines – Power Flow and Losses ► Brush losses is the power lost across the contact potential at the brushes of the machine. It is give n by the equation ► The reason that the brush losses are calculated in this manner is that the voltage drop across a set of brushes is approximately constant over a large range of armature currents. Unless otherwise specified. the brush voltage drop is usually assumed to be about 2 V. Electrical Machines (110405323) Dr. Mohammad Salah 13 DC Machines – Power Flow and Losses ► Core losses are the hysteresis losses and eddy current losses occurring in the metal of the motor. ► These losses vary as the square of the flux density (B2) and for the rotor, as the 1.5th power of the speed of rotation (n1.5). ► Mechanical losses are associated with mechanical effects and there are two basic types: 1. Friction losses are losses caused by the friction of the bearings in the machine. 2. Windage losses are caused by the friction between the moving parts of the machine and the air inside the motor's casing. ► These losses vary as the cube of the speed (n3). Electrical Machines (110405323) Dr. Mohammad Salah 14 DC Machines – Power Flow and Losses ► Stray losses (or miscellaneous losses) are losses that cannot be placed in one of the previous categories. No matter how carefully losses are accounted for, some always escape inclusion in one of the above categories. ► All such losses are lumped into stray losses. For most machines, stray losses are taken by convention to be 1% of full load input power. Electrical Machines (110405323) Dr. Mohammad Salah 15 DC Machines – Power Flow and Losses Electrical Machines (110405323) Dr. Mohammad Salah 16 DC Machines – Power Flow and Losses The efficiency of a DC machine is defined by the equation 𝑷𝑷𝒐𝒐𝒐𝒐𝒐𝒐 𝜼𝜼 = × 𝟏𝟏𝟏𝟏𝟏𝟏𝟏 𝑷𝑷𝒊𝒊𝒊𝒊 𝑷𝑷𝒊𝒊𝒊𝒊 − 𝑷𝑷𝑷𝑷𝑷𝑷𝑷𝑷𝑷𝑷 𝜼𝜼 = × 𝟏𝟏𝟏𝟏𝟏𝟏𝟏 𝑷𝑷𝒊𝒊𝒊𝒊 𝑷𝑷𝒐𝒐𝒐𝒐𝒐𝒐 𝜼𝜼 = × 𝟏𝟏𝟏𝟏𝟏𝟏% 𝑷𝑷𝒐𝒐𝒐𝒐𝒐𝒐 + 𝑷𝑷𝑷𝑷𝑷𝑷𝑷𝑷𝑷𝑷 Electrical Machines (110405323) Dr. Mohammad Salah 17 DC Machines – Power Flow and Losses Example 9.8 A 50 hp, 250 V, 1200 rpm shunt dc motor has a rated armature current of 170 A and a rated field current of 5 A. When its rotor is blocked, an armature voltage of 10.2 V (exclusive of brushes) produces 170 A of current flow and a field voltage of 250 V produces a field current flow of 5 A. The brush voltage drop is assumed to be 2 V. At no load with the terminal voltage equal to 240 V. the armature current is equal to 13.2 A. the field current is 4.8 A. and the motor's speed is 1150 rpm. (a) How much power is output from this motor at rated conditions? (b) What is the motor's efficiency? Electrical Machines (110405323) Dr. Mohammad Salah 18 DC Machines – Power Flow and Losses Electrical Machines (110405323) Dr. Mohammad Salah 19 DC Machines – Power Flow and Losses Electrical Machines (110405323) Dr. Mohammad Salah 20 DC Machines – Types DC Machines Motors Generators Permanent Magnet Separately Excited Shunt Series Compound These various types of DC machines differ in their output (voltage-current) characteristics if they are generators or (speed-torque) characteristics if they are motors, and therefore in the applications to which they are suited. Electrical Machines (110405323) Dr. Mohammad Salah 21 DC Machines – Types ► In a separately excited machine, the field flux is derived from a separate power source. ► In a shunt machine, the field flux is derived by connecting the field circuit directly across the terminals of the armature. ► In a series machine, the field flux is produced by connecting the field circuit in series with the armature. ► In a cumulatively compounded machine, both a shunt and a series field are present, and their effects are additive. ► In a differentially compounded machine, both a shunt and a series field are present, but their effects are subtractive. Electrical Machines (110405323) Dr. Mohammad Salah 22 DC Machines – Types Types of DC Motors - Classification of DC Motors Electrical Machines (110405323) Dr. Mohammad Salah 23 DC Machines – Motors ► Today, induction motors with solid-state drive packages are the preferred choice over DC motors for most speed control applications. However, there are still some applications where dc motors are preferred. ► DC motors are often compared by their speed regulations. ► The speed regulation (SR) of a motor is defined by 𝝎𝝎𝒏𝒏𝒏𝒏 − 𝝎𝝎𝒇𝒇𝒇𝒇 𝑺𝑺𝑺𝑺 = × 𝟏𝟏𝟏𝟏𝟏𝟏𝟏 𝝎𝝎𝒇𝒇𝒇𝒇 𝒏𝒏𝒏𝒏𝒏𝒏 − 𝒏𝒏𝒇𝒇𝒇𝒇 𝑺𝑺𝑹𝑹 = × 𝟏𝟏𝟏𝟏𝟏𝟏𝟏 𝒏𝒏𝒇𝒇𝒇𝒇 Electrical Machines (110405323) Dr. Mohammad Salah 24 DC Machines – Motors ► It is a rough measure of the shape of a motor's speed– torque characteristic. A positive speed regulation means that a motor's speed drops with increasing load, and a negative speed regulation means a motor's speed increases with increasing load. ► The magnitude of the speed regulation tells approximately how steep the slope of the torque- speed curve is. ► It is a rough measure of the shape of a motor's torque- speed characteristic. A positive speed regulation means that a motor's speed drops with increasing load, and a negative speed regulation means a motor's speed increases with increasing load. Electrical Machines (110405323) Dr. Mohammad Salah 25 DC Machines – Motors ► The magnitude of the speed regulation tells approximately how steep the slope of the torque- speed curve is. ► It should be noted that in order to get the maximum possible power per pound of weight out of a machine, most motors and generators are designed to operate near the saturation point on the magnetization curve (at the knee of the curve). ► This implies that a fairly large increase in field current is often necessary to get a small increase in 𝑬𝑬 A when operation is near full load. Electrical Machines (110405323) Dr. Mohammad Salah 26 DC Machines – Motors Separately Excited and Shunt DC Motors Electrical Machines (110405323) Dr. Mohammad Salah 27 DC Machines – Motors ► The output characteristic of a separately excited and shunt DC motors can be derived from the induced voltage and torque equations of the motor plus Kirchhoff's voltage law as VT = EA + IARA VT = Kϕω + IARA 𝝉𝝉𝒊𝒊𝒊𝒊𝒊𝒊 VT = Kϕω + RA where 𝝉𝝉𝒊𝒊𝒊𝒊𝒊𝒊= IAKϕ Kϕ RA 𝑽𝑽𝑻𝑻 ⇒ ω = Kϕ − 𝟐𝟐 𝝉𝝉𝒊𝒊𝒊𝒊𝒊𝒊 Kϕ Electrical Machines (110405323) Dr. Mohammad Salah 28 DC Machines – Motors It is important to realize that, in order for the speed of the motor to vary linearly with torque, the other terms in this expression must be constant as the load changes. Electrical Machines (110405323) Dr. Mohammad Salah 29 DC Machines – Motors Example 9.1 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 0.06Ω. Its field circuit has a total resistance Radj+RF of 50Ω, which produces a no-load speed of 1200 r/min. There are 1200 turns per pole on the shunt field winding (see Figure 9-7). (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 to torque-speed characteristic of this motor. Electrical Machines (110405323) Dr. Mohammad Salah 30 DC Machines – Motors Electrical Machines (110405323) Dr. Mohammad Salah 31 DC Machines – Motors Electrical Machines (110405323) Dr. Mohammad Salah 32 DC Machines – Motors Electrical Machines (110405323) Dr. Mohammad Salah 33 DC Machines – Motors Electrical Machines (110405323) Dr. Mohammad Salah 34 DC Machines – Motors Electrical Machines (110405323) Dr. Mohammad Salah 35 DC Machines – Motors Electrical Machines (110405323) Dr. Mohammad Salah 36 DC Machines – Motors There are two common ways in which the speed of a separately excited and shunt DC motors can be controlled: 1. Adjusting the field resistance RF (and thus the field flux) 2. Adjusting the terminal voltage applied to the armature. 3. Inserting a resistor in series with the armature circuit (the less common method of speed control). Electrical Machines (110405323) Dr. Mohammad Salah 37 DC Machines – Motors Changing the Field Resistor Control Method 1. Increasing RF causes IF (= VF /RF ↑) to decrease. 2. Decreasing IF decreases ϕ. 3. Decreasing ϕ lowers EA (= Kϕ↓ω). 4. Decreasing EA increases IA (= (VT −EA↓)/RA) 5. Increasing IA increases τind (= Kϕ↓IA⇑), with the change in IA dominant over the change in flux. 6. Increasing τind makes τind > τload , and the speed ω increases. 7. Increasing ω increases EA (= Kϕω↑) again. 8. Increasing EA decreases IA 9. Decreasing IA decreases τind until τind = τload at a higher speed ω Electrical Machines (110405323) Dr. Mohammad Salah 38 DC Machines – Motors Electrical Machines (110405323) Dr. Mohammad Salah 39 DC Machines – Motors Changing the Armature Voltage Control Method 1. An increasing VT increases IA (= (VT ↑ −EA)/RA) 2. Increasing IA increases τind (= KϕIA↑) 3. Increasing τind makes τind > τload and the speed ω increases 4. Increasing ω increases EA (= Kϕω↑) 5. Increasing EA decreases IA (= (VT −EA↑)/RA) 6. Decreasing IA decreases τind until τind = τload at a higher speed ω Electrical Machines (110405323) Dr. Mohammad Salah 40 DC Machines – Motors Electrical Machines (110405323) Dr. Mohammad Salah 41 DC Machines – Motors Inserting a Resistor in Series with the Armature Circuit 1. An increasing RA decreases IA (= (VT − EA)/RA↑) 2. Decreasing IA decreases τind (= KϕIA↓) 3. Decreasing τind makes τind < τload and the speed ω decreases 4. Decreasing ω decreases EA (= Kϕω↓) 5. Decreasing EA increases IA (= (VT −EA↓)/RA) 6. Increasing IA increases τind until τind = τload at a lower speed ω Electrical Machines (110405323) Dr. Mohammad Salah 42 DC Machines – Motors Electrical Machines (110405323) Dr. Mohammad Salah 43 DC Machines – Motors Example (A) A 230V dc shunt motor drives a load at 900RPM and drawing a current of 30A. The resistance of armature circuit is 0.4Ω. The torque of the load is proportional to the speed. Calculate the resistance to be connected in series with the armature to reduce the speed to 600RPM. Ignore armature reaction. Electrical Machines (110405323) Dr. Mohammad Salah 44 DC Machines – Motors Electrical Machines (110405323) Dr. Mohammad Salah 45 DC Machines – Motors Example (B) A 250V dc shunt motor has an armature current of 20A when running at 1000RPM against full load torque. The armature resistance is 0.5Ω. What resistance must be inserted in series with the armature to reduce the speed to 500RPM at the same load torque, and what will be the speed if the load torque is halved with this inserted resistance? Electrical Machines (110405323) Dr. Mohammad Salah 46 DC Machines – Motors Electrical Machines (110405323) Dr. Mohammad Salah 47 DC Motors Nameplate/Tag Electrical Machines (110405323) Dr. Mohammad Salah 48 DC Motors Nameplate/Tag