Electrical Engine Lecture 3 PDF

Al-Furat Al-Awsat Technical University Power Mechanics Techniques Engineering Second Class Electrical Engine Dr. Ali Kareem Abbas Lec. 3 Outline Characteristics of DC Motors Applications of DC Motor Starting of DC Motors...

Al-Furat Al-Awsat Technical University Power Mechanics Techniques Engineering Second Class Electrical Engine Dr. Ali Kareem Abbas Lec. 3 Outline Characteristics of DC Motors Applications of DC Motor Starting of DC Motors Characteristics of the DC Motor There are three principal types of dc motors: shunt motors, series motors and compound motors. The performance of a dc motor can be judged from its characteristic curves known as motor characteristics; following are the three important characteristics of a dc motor: Torque and Armature current characteristic (Ta/Ia): It is known as electrical characteristic of the motor. Speed and armature current characteristic (N/Ia): It is very important characteristic as it is often the deciding factor in the selection of the motor for a particular application. Speed and torque characteristic (N/Ta): It is also known as mechanical characteristic. 1. Characteristics of Series Motor 1. Ta/Ia Characteristic: We have seen that 𝑇𝑎 𝛼 Φ𝐼𝑎. In this case, as field windings also carry the armature current, Φ α Ia up to the point of magnetic saturation. Hence, before saturation, 𝑇𝑎 𝛼 Φ𝐼𝑎 and Ta α Ia2 At light loads, Ia and hence Φ is small. But as Ia increases, Ta increases as the square of the current. Hence, Ta/Ia curve is a parabola. After saturation, Φ is almost independent of Ia hence Ta α Ia only. So the characteristic becomes a straight line. The shaft torque Tsh is less than armature torque due to stray losses. It is shown dotted in the figure. So we conclude that (prior to magnetic saturation) on heavy loads, a series motor exerts a torque proportional to the square of armature current. Hence, in cases where huge starting torque is required for accelerating heavy masses quickly as in hoists and electric trains etc., series motors are used. 1. Characteristics of Series Motor 2. N/Ia Characteristics: Variations of speed can be deduced from the formula: 𝑁 𝛼 𝐸𝑏/Φ Change in Eb, for various load currents is small and hence may be neglected for the time being. With increased Ia, Φ also increases. Hence, speed varies inversely as armature current. When load is heavy, Ia is large. Hence, speed is low (this decreases Eb and allows more armature current to flow). But when load current and hence Ia falls to a small value, speed becomes dangerously high. Hence, a series motor should never be started without some mechanical (not belt-driven) load on it otherwise it may develop excessive speed and get damaged due to heavy centrifugal forces so produced. It should be noted that series motor is a variable speed motor. 3. N/Ta or mechanical characteristic: It is found from above that when speed is high, torque is low and vice-versa. 1. Characteristics of Series Motor  2. Characteristics of Shunt Motors 1. Ta/Ia Characteristic: Assuming Φ to be practically constant (though at heavy loads, Φ decreases somewhat due to increased armature reaction) we find that Ta α Ia. Hence, the electrical characteristic, is practically a straight line through the origin. Shaft torque is shown dotted. Since a heavy starting load will need a heavy starting current, shunt motor should never be started on (heavy) load. 2. N/Ia Characteristic: If Φ is assumed constant, then N α Eb. As Eb is also practically constant, speed is, for most purposes, constant. But strictly speaking, both Eb and Φ decrease with increasing load. However, Eb decreases slightly more than Φ so that on the whole, there is some decrease in speed. The drop varies from 5 to 15% of full-load speed, being dependent on saturation, armature reaction and brush position. Hence, the actual speed curve is slightly droping as shown by the dotted line in the figure. But, for all practical purposes, shunt motor is taken as a constant-speed motor. 3. N/Ta Characteristic: can be deduced from (1) and (2) above. 2. Characteristics of Shunt Motors 3. Characteristics of Compound Motors: These motors have both series and shunt windings. If series excitation helps the shunt excitation i.e. series flux is in the same direction; then the motor is said to be cumulatively compounded. If on the other hand, series field opposes the shunt field, then the motor is said to be differentially compounded. The characteristics of such motors lie in between those of shunt and series motors. (a) Cumulative-compound Motors: Such machines are used where series characteristics are required and where, in addition, the load is likely to be removed totally such as in some types of coal cutting machines or for driving heavy machine tools which have to take sudden cuts quite often. Due to shunt windings, speed will not become excessively high but due to series windings, it will be able to take heavy loads. Compound-wound motors have greatest application with loads that require high starting torques or pulsating loads (because such motors smooth out the energy demand required of a pulsating load). They are used to drive electric shovels, metal-stamping machines, reciprocating pumps, hoists and compressors etc. 3. Characteristics of Compound Motors: (b) Differential-compound Motors: Since series field opposes the shunt field, the flux is decreased as load is applied to the motor. This results in the motor speed remaining almost constant or even increasing with increase in load (because, N α Eb/Φ). Due to this reason, there is a decrease in the rate at which the motor torque increases with load. Such motors are not in common use. But because they can be designed to give an accurately constant speed under all conditions, they find limited application for experimental and research work. One of the biggest drawback of such a motor is that due to weakening of flux with increases in load, there is a tendency towards speed instability and motor running away unless designed properly. 3. Characteristics of Compound Motors:  3. Characteristics of Compound Motors: Characteristics of cumulatively compound DC motor Characteristics of differentially compound DC motor  Applications of DC Motor Shunt DC Motors Series DC Motors Cumulative Compound DC motors the fairly constant speed the high starting torque the high starting torque of and medium starting and variable speed of cumulative compound DC torque of shunt DC series DC motors: motors: motors: 1) Centrifugal and 1) Conveyors 1) Shears reciprocating pumps 2) Hoists, Elevators 2) Heavy Planers 2) Lathe machines 3) Cranes 3) Rolling mills 3) Blowers and Fans 4) Electric Locomotives 4) Elevators 4) Drilling machines 5) Milling machines 6) Machine tools Starting of DC Motors  Necessity of Starter for a DC Motor  The current drawn by the armature is given by 𝑉𝑖𝑛 − 𝐸𝑏 𝐼𝑎 = 𝑅𝑎 𝑉𝑖𝑛  At starting, as N=0 so Eb = 0 thus 𝐼𝑎 = 𝑅𝑎 Armature resistance will be very low. Therefore, the current drawn by the motor will be very high. In order to limit this high current, a starting resistance or starter is connected in series with the armature. The starting resistance will be excluded from the circuit after the motor attains its rated speed. From there on back emf limits the current drawn by the motor. Starting of DC Motors 2 3 4 5 6 1 Three-point Shunt Motor Starter 7 8 When the connected dc motor is to be started, the lever is turned gradually to the right. When the lever touches point BS, the field winding gets directly connected across the supply, and the armature winding gets connected with resistances R1 to R7 in series. During starting, full resistance is added in series with the armature winding. Then, as the lever is moved further, the resistance is gradually is cut out from the armature circuit. Starting of DC Motors Now, as the lever reaches to position 8, all the resistance is cut out from the armature circuit and armature gets directly connected across the supply. The electromagnet 'SI' (no voltage release coil) NVRC holds the lever at this position. This electromagnet releases the lever when there is no (or low) supply voltage. It can be seen that, when the arm is moved from the position BS to the last position, the starter resistance gets added in series with the field winding. But, as the value of starter resistance is very small as compared to the shunt resistance, the decrease in shunt field current may be negligible. When the motor is overloaded beyond a predefined value, 'overcurrent release electromagnet' or over load release coil (OLRC) gets activated, which short-circuits electromagnet NVRC and, hence, releases the lever and the motor is turned off.