Chapter 7 DC Machines Copyright © 2014 The McGraw-Hill Companies, Inc. Permission required for presentation or display 6/1/2016DC Machines1.

Apresentação em tema: "Chapter 7 DC Machines Copyright © 2014 The McGraw-Hill Companies, Inc. Permission required for presentation or display 6/1/2016DC Machines1."— Transcrição da apresentação:

1 Chapter 7 DC Machines Copyright © 2014 The McGraw-Hill Companies, Inc. Permission required for presentation or display 6/1/2016DC Machines1

2 Introduction The DC machine has a field on the stator, which sets up the main air-gap flux, and an armature on the rotor, which handles the electrical power The armature is connected to stationary terminals by brushes riding on the rotating commutator –The commutator-brush combination acts as a mechanical converter from DC at the terminals to time-varying armature currents that set up a stationary MMF –The magnetic axis of the armature is 90 electrical degrees from the magnetic axis of the field 6/1/2016DC Machines2

3 Schematic representation of a two-pole dc machine the direct axis is aligned with the field axis the armature axis is perpendicular 6/1/2016DC Machines3

4 Electromagnetic torque T mech is due to interaction of d- axis flux per pole  d and space fundamental armature mmf F a1 6/1/2016DC Machines4 The development of this equation uses several results from Chapter 4, and evaluated the angle between the d axis and armature axis as 90 electrical degrees The torque is shown to be proportional to the flux  d times the armature current i a

5 The generated voltage is sum of the rectified voltages of the coils in series between the brushes 6/1/2016DC Machines5 This voltage is also called the speed voltage, and with all quantities in SI units

6 6/1/2016DC Machines6 Four possible circuit connections: (a) separate excitation, (b) series, (c) shunt, (d) compound. A shunt field must be rated for armature voltage and a series field must be rated for armature current

7 6/1/2016DC Machines7 Generator volt-ampere characteristics

8 6/1/2016DC Machines8 Motor speed-torque characteristics

9 Commutator action 6/1/2016DC Machines9

10 Electric-circuit analysis 6/1/2016DC Machines10

11 Effect of armature MMF Armature MMF increases flux under half of field pole but decreases it under the other half This hinders commuatation and decreases net flux (because of saturation in the iron 6/1/2016DC Machines11

12 Steady-state generator performance Self-exicted shunt generator will have voltage build-up due to residual magnetism, which causes a small field current, causing voltage build-up The voltage will build up along with the field current until steady-state operating point is reached at the intersection of the magnetization curve and the field resistance line 6/1/2016DC Machines12

13 6/1/2016DC Machines13 Field resistance line V = R f I f Magnetization curve

14 DC motor analysis Example 7.9 and practice problem 7.6 illustrate the use of the equivalent circuit to analyze the shunt motor Example 7.10 and practice problem 7.7 show the analysis of a DC permanent- magnet motor Example 7.11 is discussed on the next few slides, again for the permanent-magnet motor 6/1/2016DC Machines14

15 Example 7.11 Given R a = 1.03 , V t = 50 V, I a = 1.25 A, n= 2100 r/min. Find K m, the no-load rotational loss and the power output when operating from a 48-V source at 1700 r/min 6/1/2016DC Machines15 Equivalent circuit for permanent- magnet DC motor.  d = constant, so K m = K a  d.

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18 Interpole and compensating windings Interpole windings: –wound on small poles between the main field poles –used for improving commutation by nulling out the armature flux in the region where coils are undergoing commutation Compensating windings: –Placed in the main pole faces to compensate for demagetizing effect of armature flux These windings are connected in series with the armature 6/1/2016DC Machines18

19 6/1/2016DC Machines19 Schematic section of DC machine showing interpoles and compensating windings

20 Series universal motors A small series-field commutator motor can be designed with laminated stator and rotor iron to be used in either AC or DC DC operation is as an ordinary series motor, and AC operation is similar since the time variations in the armature current coincide with those of the flux (since the armature and field are in series) With AC operation, the field and armature both have reactance voltage drops, affecting performance 6/1/2016DC Machines20

21 6/1/2016DC Machines21 With AC operation, saturation at the peaks will reduce the rms flux, further affecting performance

22 Summary The conventional DC machine with a commutator and brushes is useful as a motor or a generator The primary disadvantages are complexity of construction and cost Many AC motors with electronic drives have been replacing DC motors, but the DC motor is still important in certain applications The DC permanent-magnet motor finds many low-power applications 6/1/2016DC Machines22