Direct – Current Motor Characteristics and Applications

1. Direct – Current Motor Characteristics and Applications • Straight Shunt Motor • Essentially a constant speed motor • Compound or Stabilized – Shunt Motors • Has both shunt and series field windings • Series field generates mmf in the same direction as the shunt field mmf. ECE 441

2. Circuit Diagram of a Compound Motor ECE 441

3. Differential Connection of Fields • Both the series and shunt fields must provide fluxes that are additive. • If the series field is reversed with respect to the shunt field, the net flux decreases, and the speed increases. • The time constant of the series field is such that the current increases faster than the shunt field current. ECE 441

4. Differential Connection of Fields • If the series field is reversed, • The motor will start in the wrong direction • Depending upon the load and the structure of the series field, the motor could • slow down and stop, tripping the breaker • slow down, stop, reverse direction, and accelerate • slow down, stop, reverse direction, slow down, stop, reverse direction, etc. until a breaker trips ECE 441

5. Reversing the Direction of Compound Motors • Reverse either the armature current or reverse both the series and shunt fields. • If only one field is reversed, a “differential” connection results! • The field mmfs will be reduced, resulting in excessive speed! ECE 441

6. Reversing the Armature Current ECE 441

7. Using NEMA standard terminal markings ECE 441

8. Series Motor • Series field • Heavy windings • Must conduct the armature current • Potentially dangerous problem if the shaft load is removed! ECE 441

9. Field winding is in series with the armature ECE 441

10. More Details • When shaft load is removed, TD>Tload • Motor speed increases • cemf increases • armature current decreases • series field flux decreases ECE 441

11. Reversing the Direction of a Series Motor • Reverse the current in the armature-interpole-compensating branch • Reverse the current in the series field windings ECE 441

12. Reversing the Armature Current ECE 441

13. Using NMEA standard Terminal Markings ECE 441

14. Using NEMA standard terminal markings Reversing the series field ECE 441

15. Effect of Magnetic Saturation on DC Motor Performance • Pole flux is not directly proportional to the applied mmf due to magnetic saturation • Net mmf is made up of the following components, as applicable • Fnet = Ff + Fs - Fd • Fnet = net mmf (A-t/pole) • Ff = shunt field mmf (NfIf)(A-t/pole) • Fs = series field mmf (NsIa)(A-t/pole) • Fd = equivalent demagnetizing mmf due to armature reaction (A-t)/pole ECE 441

16. Effect of Magnetic Saturation on DC Motor Performance • Note that Fd is not exactly proportional to the armature current, but is assumed to be. • If a compensating winding is used, Fd = 0. ECE 441

17. Developed Torque and Speed ECE 441

18. Defining Parameters • Racir = resistance of armature circuit (Ω) • Ra = resistance of armature windings (Ω ) • RIP = resistance of interpole windings (Ω) • RCW = resistance of compensating windings (Ω) • Rs = resistance of series field winding (Ω) • Bp = air-gap flux density (T) • Φp = pole flux (Wb) ECE 441

19. Solve Problems with Proportions ECE 441

20. Example 11.1 • A 240-V, 40-hp, 1150 r/min stabilized-shunt motor, operating at rated conditions, has an efficiency at rated load of 90.2%. The motor parameters are • Ra = 0.0680 Ω RIP = 0.0198 Ω Rs = 0.00911 Ω Rshunt = 99.5 Ω • Turns/pole series - ½ shunt - 1231 ECE 441

21. Example 11.1 (continued) • The circuit diagram and magnetization curve are shown on the next slide. Determine (a) the armature current when operating at rated conditions; (b) the resistance and power rating of an external resistance required in series with the shunt field in order to operate at 125% rated speed. Assume the shaft load is adjusted to a value that limits armature current to 115% of rated current. ECE 441

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23. Solution for Armature Current ECE 441

24. Solution for External Resistance • The series field of a compound motor is designed to be approximately equal and opposite to the equivalent demagnetizing mmf of armature reaction. Therefore, the net flux is due to the shunt field alone. ECE 441

25. net mmf = 0.70 T ECE 441

26. ECE 441

27. Ff = 2.3 X 1000 = 2300 A-t/pole ECE 441

28. ECE 441

29. Linear Approximations • If the magnetization curve is not available • rough approximation obtained by assuming magnetization effects are negligible • Do not use approximations if the motor is operating under heavy overload or locked rotor conditions. • If the net mmf is to be reduced below its rated value, approximation using the linear assumption is OK. ECE 441

30. Approximate Equations for Torque and Speed ECE 441

31. For the Series Motor If the range of operation is in the unsaturated region, and armature reaction effects are either negligible or compensated for, The developed torque is proportional to the square of the armature current. ECE 441

32. Example 11.2 • Example 11.1 is re-solved using the linear approximation, and the solution is compared to the results obtained in Example 11.1. ECE 441

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34. From Example 11.1, the value of resistance was determined to be 28.8 Ω ECE 441

35. Calculate the Percent Error This lower value of resistance would cause a slightly higher field current, and therefore, a speed slightly lower than 1437.5 r/min. ECE 441

36. Comparison of Steady – State Operating Characteristics of DC Motors • The steady-state operating characteristics of typical shunt, compound, and series motors of the same torque and speed ratings are shown on the next slide. ECE 441

37. ECE 441

38. Comparisons (continued) • Shunt Motor • relatively constant speed from no-load to full-load • does not have high starting torque • essentially constant flux • torque varies linearly with armature current • speed regulation around 5% ECE 441

39. Relatively Constant Speed Linear Torque ECE 441

40. Comparisons (continued) • Compound Motor • Higher torque, lower speed than shunt motor • speed regulation between 15 and 25% • used with loads requiring high starting torques or have pulsating loads • smoothes out the energy required by the pulsating load, lowering the demand on the electrical supply ECE 441

41. Lower Speed at Higher Torque Higher Torque above base speed than Shunt motor ECE 441

42. Comparisons (continued) • Series Motor • high starting torque • wide speed range • REMOVING THE LOAD CAUSES IT TO RUN AWAY! • CONNECT LOAD BY GEARS OR SOLID COUPLING – NO BELT DRIVES! ECE 441

43. Wide Speed Range High Starting Torque ECE 441

44. Dynamic Braking, Plugging, and Jogging • Dynamic Braking is the deceleration of the motor by converting the energy stored in the moving masses into electrical energy and dissipating it as heat via resistors. Also called resistive braking. ECE 441

45. Dynamic Braking (continued) • Disconnect the armature from the electrical supply lines and connect across a suitable resistor while maintaining the field at full strength. • The motor behaves as a generator, feeding current to the resistor, dissipating heat. ECE 441

46. Dynamic Braking (continued) • Choose the resistance for current between 150 and 300% of rated current. • The armature current is in a direction to oppose the armature motion, producing a negative, or, counter-torque, slowing down the load. ECE 441

47. Compound Motor Example Normal Operation Dynamic Braking ECE 441

48. Normal Operation Closed Open ECE 441

49. Dynamic - Braking Open Closed ECE 441

50. Regenerative Braking • Convert energy of overhauling loads into electrical energy and pumps it back into the electrical system. • The overhauling load drives a DC motor faster than normal, causing the cemf to become greater than the supply voltage and results in generator action. • Trains, elevators, hybrid automobiles ECE 441