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.
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2. Circuit Diagram of a Compound Motor
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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.
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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
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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!
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6. Reversing the Armature Current
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7. Using NEMA standard terminal markings
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8. Series Motor Series field
Heavy windings
Must conduct the armature current
Potentially dangerous problem if the shaft load is removed!
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9. Field winding is in series with the armature
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10. More Details When shaft load is removed, TD>Tload
Motor speed increases
cemf increases
armature current decreases
series field flux decreases
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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
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12. Reversing the Armature Current
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13. Using NMEA standard Terminal Markings
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14. Using NEMA standard terminal markings
Reversing the series field
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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
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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.
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17. Developed Torque and Speed
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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)
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19. Solve Problems with Proportions
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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 = Ω RIP = Ω Rs = Ω Rshunt = 99.5 Ω
Turns/pole series - ½ shunt
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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.
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23. Solution for Armature Current
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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.
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25. net mmf = 0.70 T
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27. Ff = 2.3 X 1000 = 2300 A-t/pole
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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.
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30. Approximate Equations for Torque and Speed
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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.
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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.
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34. From Example 11.1, the value of resistance was determined to be 28.8 Ω
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35. Calculate the Percent Error
This lower value of resistance would cause a slightly higher field current, and therefore, a speed slightly lower than r/min.
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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.
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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%
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39. Relatively Constant Speed
Linear Torque
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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
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41. Lower Speed at Higher Torque
Higher Torque above base speed than Shunt motor
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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!
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43. Wide Speed Range
High Starting Torque
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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.
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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.
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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.
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47. Compound Motor Example
Normal Operation
Dynamic Braking
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48. Normal Operation
Closed
Open
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49. Dynamic - Braking
Open
Closed
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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
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51. Plugging The electrical reversal of a motor before it stops
Reverse the voltage applied to the armature
Current in the series and shunt fields is not reversed
Insert resistance in series with the armature to limit the current
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52. Normal Operation
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53. Plugging
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54. Jogging Very brief application of power to a motor
Fraction of a revolution
Used for positioning the load
Place resistance in series with the armature to limit the current
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55. Example 11.7
A 240-V, compensated shunt motor driving a 910 lb-ft torque load is running at 1150 r/min. The efficiency of the motor at this load is 94.0%. The combined armature, compensating winding, and interpole resistance is Ω, and the resistance of the shunt field is 52.6Ω. Determine the resistance of a dynamic-braking resistor that will be capable of developing 500 lb-ft of braking torque at a speed of 1000 r/min. Assume windage and friction at 1000r/min are essentially the same as at 1150 r/min.
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56. Circuit for Dynamic Braking
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