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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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direct current motor characteristics and applications
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

differential connection of fields
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

differential connection of fields1
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

reversing the direction of compound motors
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

series motor
Series Motor
  • Series field
    • Heavy windings
    • Must conduct the armature current
  • Potentially dangerous problem if the shaft load is removed!

ECE 441

more details
More Details
  • When shaft load is removed, TD>Tload
    • Motor speed increases
    • cemf increases
    • armature current decreases
    • series field flux decreases

ECE 441

reversing the direction of a series motor
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

using nema standard terminal markings1
Using NEMA standard terminal markings

Reversing the series field

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effect of magnetic saturation on dc motor performance
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

effect of magnetic saturation on dc motor performance1
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

defining parameters
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

example 11 1
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

example 11 1 continued
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

solution for external resistance
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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linear approximations
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

for the series motor
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

example 11 2
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

calculate the percent error
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

comparison of steady state operating characteristics of dc motors
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

comparisons continued
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

slide39

Relatively Constant Speed

Linear Torque

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comparisons continued1
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

slide41

Lower Speed at Higher Torque

Higher Torque above base speed than Shunt motor

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comparisons continued2
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

slide43

Wide Speed Range

High Starting Torque

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dynamic braking plugging and jogging
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

dynamic braking continued
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

dynamic braking continued1
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

compound motor example
Compound Motor Example

Normal Operation

Dynamic Braking

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normal operation
Normal Operation

Closed

Open

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dynamic braking
Dynamic - Braking

Open

Closed

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regenerative braking
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

plugging
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

ECE 441

plugging1
Plugging

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jogging
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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example 11 7
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 0.00707Ω, 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.

ECE 441