torque speed characteristics of dc shunt motor experiment

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Lab 11 Shunt DC Motors

Electrical engineering laboratory (enee-3518), university of new orleans, recommended for you, students also viewed.

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Enee 3518 lab report, date: ............... section: .......... group: ............, name: .................................., experiment 11, the dc shunt motor.

• To study the torque vs. speed characteristics of a shunt wound dc motor.
• To calculate the efficiency of the shunt wound dc motor.

The speed of any dc motor depends mainly upon its armature voltage and the strength of the magnetic field. In a shunt motor, the field winding, as well as the armature winding, is connected in parallel (shunt) directly to the dc supply lines. If the dc line voltage is constant, then the armature voltage and the field strength will be constant. It is, therefore, apparent that the shunt motor should run at a reasonably constant speed. The speed does tend to drop with an increasing load on the motor. This drop in speed is mainly due to the resistance of the armature winding. Shunt motors with low armature winding resistance run at nearly constant speeds. Just like most energy conversion devices, the de shunt motor is not 100% efficient In other words, all of the electric power which is supplied to the motor is not converted into mechanical power. The power difference between the input and output is dissipated in the form of heat, and constitutes what arc known as the "losses" of the machine. These losses increase with load, with the result that the motor gets hot as it delivers mechanical power. In this Laboratory Experiment you will investigate the efficiency of a dc shunt motor.

INSTRUMENTS AND COMPONENTS

Power Supply Module 120Vac, 0-120Vdc) EMS 8821 DC Metering Module (200V, 5A) EMS 8412 DC Motor/Generator Module EMS 8211 Electrodynamometer Module EMS 8911 Hand Tachometer EMS 8920 Connection Leads EMS 8941 Timing Belt EMS 8942

Caution: High voltages are present in this Laboratory Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement!

 1. Using your EMS Power Supply, DC Motor/ Generator, DC Metering and Electrodynamometer Modules, connect the circuit shown in Fig. 24-1.

DO NOT APPLY POWER AT THIS TIME!

Notice that the motor is wired for shunt field operation and is connected to the variable dc output of the power supply (terminals 7 and N). The electrodynamometer is connected to the fixed 120V ac output of the power supply (terminals 1 and N).

 b) Measure the line current and motor speed. Record these values in Table 24-1.  c) Repeat for each of the torque values listed in the Table, while maintaining a constant 120Vdc input.  d) Return the voltage to zero and turn off the power supply.

 7. a) Plot the recorded motor speed values from Table 24- 1 on the graph of Fig 24-2.  b) Draw a smooth curve through your plotted points.  c) The completed graph represents the speed vs torque characteristics of a typical de shunt- wound motor. Similar graphs for series wound and com pound wound de motors will be constructed in the following two Laboratory Experiments. The speed VS torque characteristics for each ty pe of motor will then be compared and evaluated.

 8. C a l c u l a t e t h e s p e e d v s t o r q u e r e g u l a t i o n ( f u l l l oa d = 9 l b f. i n ) u si n g t h e e q u a t i o n:

% regulation = 100 (full load speed)

(no load speed) - (full load speed) 

Speed regulation = ________________ %

 9. Set the dynamometer control kno b at its full cw position (to provide the maximum starting load for the shunt-wound motor).

 10) Turn on the power supply and gradually increase the dc voltage until the motor is drawing 3 amperes of line current. The motor should turn very slowly or not at all.  b) Measure and record the dc voltage and the torque developed.

E = _______________ V, torque = _______________ lbf

 c) Return the voltage to zero and turn off the power supply.

 11. a) The line current in Procedure 10 is limited only by the equivalent dc resistance of the shunt-wound motor.  b) Calculate the value of the starting current if the full line voltage (120Vdc) were applied to the shunt-wound dc motor.

Starting current =_____________ A

TEST YOUR KNOWLEDGE

• Calculate the hp develope d by the shun t -wound dc motor when the torque i s 9lbf. Use the equation:

r/( min)(lbf)(.1 59 ) hp

______________________________________________________________________

_____________________________________ hp = _________________.

• K n o w i n g t h a t 1 h p i s e q u i v a l e n t t o 746 wa tts, what is the equivalent "watts

output" of the motor in Question 1? __________________________________________

_____________________________________watts output = ______________ W.

• What is the power inp ut (in watts) o f the motor in Question 1? Pin = I×E

_________________________________ watts input = ___________________ W.

• Multiple Choice

Course : Electrical Engineering Laboratory (ENEE-3518)

University : university of new orleans.

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Characteristics of DC motors - Shunt, Series & Compound

Dc motors are widely used in various applications like variable-speed drives, constant-speed drives, and many more. For selecting a type dc motor for a particular application it is very important to study the characteristics of various dc motors. Dc motor characteristics give the performance under various load conditions for recommending their field of applications.

Some of the important characteristics of dc motor are, Speed-armature current characteristics (N vs I a ), Torque-armature current characteristics (T vs I a ), and Speed-torque characteristics (N vs T).

Characteristics of dc shunt motor :, speed-armature current characteristics :.

The equation for back emf induced in a dc motor is given as E b = PɸNZ / 60A. From this, the relation between speed and back emf is expressed as N ∝ E b /Φ. The expression for speed N of a dc motor is given as,

In a dc shunt motor, the field is connected across the armature terminals. Hence at constant supply voltage V, the field current remains constant producing constant Φ. But in practice due to the armature reaction effect, the distribution of air-gap flux gets distorted.

Thus reducing the resultant flux. Suppose if the load on the motor increases, the armature current I a increases, thereby increasing the drop I a R a . This causes a small drop in speed because at constant Φ there will be very little change in the difference ( V - I a R a ) since the armature resistance R a of a dc motor is kept very small.

The curve between speed and armature current with slightly dropping from no-load to full-load is shown in figure (a) below.

Torque-Armature Current Characteristics :

For a dc motor, the expression for torque is given as T = K Φ I a i.e., torque is directly proportional to flux and armature current. At constant field current with constant Φ, T ∝ I a .

Now if the load increases, the load current increases ( i.e., armature current increases ), thereby increasing the armature torque. Therefore the curve will be a straight line from the origin as shown in figure (b).

But in practice, the output torque from the motor is less than armature torque due to frictional losses at the bearings. So that in order to generate high torque, the armature current should be very high which can damage the motor. Hence the dc shunt motor is well-suited for constant speed applications like conveyors, line shafts, etc.

Speed-Torque Characteristics :

From T vs I a and N vs I a characteristics, the N vs T can be drawn as shown in figure (c). These characteristics are similar to speed and armature current characteristics, the speed remains almost constant for various load conditions below the full-load value.

Characteristics of DC Series Motor :

In a dc series motor, both field and armature windings are connected in series with the supply. Hence the supply voltage has to overcome the drop in the series winding also. Therefore, the expression for speed N of a dc series motor is given as,

Due to the series field connection, the field current is proportional to the armature current and hence the flux, Φ ∝ I a . Generally, R a and R se are kept small ( with small I a R a and I se R se drops ), thus for constant V i.e., V ≅ E b , if the load increases the speed decreases. The relation between N and I a will be in the shape of a rectangular hyperbola as shown in figure (a).

It is seen that, if the load is high, I a will be high, and speed N will be low. But at no-load condition, I a will be low and speed N will be very high. That is why a dc series motor never allowed to start without any load connected to it or with low-load conditions.

For a dc motor, the expression for torque is given as T = K Φ I a . In a series motor, Φ ∝ I a , thus T ∝ I a 2 . Here torque is proportional to the square of the armature current, thus curve T vs I a will be parabolic in nature as shown in figure (b). Also the curve, the parabolic will be up to a certain limit of I a , then after since I a = I se , the field winding gets saturated and produces constant flux and hence constant T (curve will be a straight line).

Therefore, from T vs I a characteristics, a series motor when started with a load it develops high starting torque. Hence dc series motors are well suited for applications like electric trains, hoists, trolleys, etc.

Similar to the dc shunt motor, the N vs T relation of the dc series motor is similar to N vs I a (a parabolic curve) as shown in figure (c). At no-load, I a will be low, and hence torque, thus speed will be very high. As the load increases, torque increases, and speed decreases.

Characteristics of DC Compound Motor :

We know that the compound motors are made with a combination of shunt and series field windings with the armature winding. Hence the characteristics of dc compound motors will be combined characteristics of shunt and series motors. But the characteristics depend upon how the two field windings are connected.

In a cumulative compound motor, the flux produced by the series field winding assists the flux produced by the shunt field winding. In differential compound motor, the series field flux opposes the shunt field flux ( reduction in resultant flux ).

The speed and armature current characteristics of dc cumulative and differential compound wound motors are shown in figure (a) below. Since resultant flux is more in cumulative compound motor with respect to load, the curve will be slightly more dropping compared to shunt motor.

If the load on the cumulative compound motor increases the torque developed also increases. But the torque decreases with an increase in load in the case of a differential compound motor as shown in figure (b).

When there is a sudden application of heavy loads, the speed decreases, the cumulative compound motors can develop large torque similar to series motors. At no-load, they run at definite speed due to shunt field flux, unlike series motor attain dangerous speeds.

The speed remains almost constant in differential compound motors with increases in load. Hence shunt motors are more preferred compared to differential compound motors and are rarely used.

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EE 448 Lab Experiment No. 4 Introduction to DC Motors

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Speed Torque Characteristics of DC Motor (Shunt & Series)

Speed-torque characteristics of dc shunt motor.

The speed-torque characteristics of DC shunt motor are also known as mechanical characteristics. They can be obtained from torque-current and speed-current characteristics of DC shunt motor.

The expression for back e.m.f in a DC motor is given by,

\[{{E}_{b}}={{k}_{a}}\phi N\text{     }\left[ {{k}_{a}}=\frac{ZP}{60A} \right]…(1)\]

Also, we have,

\[{{E}_{b}}=V-{{I}_{a}}{{R}_{a}}…(2)\]

On equating equations (1) and (2), we get,

\[{{k}_{a}}\phi N=V-{{I}_{a}}{{R}_{a}}\]

\[N=\frac{1}{{{k}_{a}}\phi }\left[ V-{{I}_{a}}{{R}_{a}} \right]…(3)\]

The expression for torque of a DC motor is given by,

\[T={{k}_{a}}\phi {{I}_{a}}\]

\[{{I}_{a}}=\frac{T}{{{k}_{a}}\phi }\]

Substituting the above value of I a in equation (3), we get,

\[N=\frac{1}{{{k}_{a}}\phi }\left[ V-\left( \frac{T}{{{k}_{a}}\phi } \right){{R}_{a}} \right]\]

\[N=\frac{V}{{{k}_{a}}\phi }-\frac{T{{R}_{a}}}{{{({{k}_{a}}\phi )}^{2}}}…(4)\]

As the torque in a DC motor increases, flux (ϕ) decreases. This is because when torque is increased, armature current increases resulting in reduction of air gap flux ϕ i.e., due to armature reaction and saturation. Therefore, for increased torque there is an increase in the value \(\frac{T}{{{\phi }^{2}}}\) of equation (4) causing a drop in the speed of the motor. However, when the effect of armature reaction is neglected, the value of flux (ϕ) remains constant due to which for increase in torque there will not be much drop in speed of the DC shunt motor. The speed-torque characteristics of a DC shunt motor are drawn as shown below. Dotted line in above figure represents the ideal speed- torque characteristics of a DC shunt motor. But, due to the reduction in flux caused by the armature reaction, the curve is obtained as shown by the thick line.

Speed-Torque Characteristics of DC Series Motor

The speed-torque characteristics of DC series motor can be obtained or derived from the torque-current and speed-current characteristics of DC series motor. Therefore, armature torque and speed has inverse relationship as shown in figure (3).

We know that,

\[N\propto \frac{{{E}_{b}}}{\phi }\propto \frac{{{E}_{b}}}{{{I}_{a}}}\]

Since on no-load the speed is dangerously high, the series motors are never started on no-load. When the motor is connected across supply mains without load, the current drawn is small and hence ϕ is small and the speed tends to increase \(\left[ N\propto \frac{{{E}_{b}}}{\phi } \right]\). With increase in speed, E b increases \(\left[ {{E}_{b}}=\frac{\phi ZN}{60}\times \frac{P}{A} \right]\) thus the field current decreases \(\left[ {{I}_{a}}=\frac{V-{{E}_{b}}}{{{R}_{a}}} \right]\) which in tum leads to decrease in flux and hence speed increases gradually. This process continues until the armature gets damaged. Hence, series motors are not suitable for the services where the load may be entirely removed.

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