Electric Motor Control
10th Edition
ISBN: 9781305177611
Author: Herman
Publisher: Cengage
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Chapter 57, Problem 4SQ
To determine
Explain the advantage of motor-generator type of variable speed drive over the rectifier-type DC supplies.
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Answer question D only using by hand
first darw cylinder then calculate show me st
The phase currents in a delta-connected three-phase
load are as follows: between the red and yellow lines,
30 A at p.f. 0.707 leading; between the yellow and blue
lines, 20 A at unity p.f.; between the blue and red
lines, 25 A at p.f. 0.866 lagging. Calculate the line
currents and draw the complete phasor diagram.
ANS:
21.6 A in R, 49.6 A in Y, 43.5 A in B
. Two wattmeters connected to measure the input to
a balanced three-phase circuit indicate 2500 W and
500 W respectively. Find the power factor of the
circuit: (a) when both readings are positive; (b) when
the latter reading is obtained after reversing the con
nections to the current-coil of one instrument. Draw
the phasor and connection diagrams.
ANS:
0.655, 0.359
Chapter 57 Solutions
Electric Motor Control
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- Explain the advantage of connecting the low-voltage winding of distribution transformers in star. A factory has the following load with power factor of 0.9 lagging in each phase. Red phase 40 A, yellow phase 50 A and blue phase 60 A. If the supply is 400 V, three phase, four-wire, calculate the current in the neutral and the total active power. Draw a phasor diagram for phase and line quantities. Assume that, relative to the current in the red phase, the current in the yellow phase lags by 120° and that in the blue phase leads by 120°. ANS: 17.3 A, 31.2 kWarrow_forwardA three-phase, 400 V system has the following load connected in delta: between the red and yellow lines, a non-reactive resistor of 100 Ω; between the yellow and blue lines, a coil having a reactance of 60 Ω and negligible resistance; between the blue and red lines, a loss-free capacitor having a reactance of 130 Ω. Calculate: (a) the phase currents; (b) the line currents. Assume the phase sequence to be R–Y, Y–B and B–R. Also, draw the complete phasor diagram. ANS: 4.00 A, 6.67 A, 3.08 A, 6.85 A, 10.33 A, 5.8 Aarrow_forwardWith the aid of a circuit diagram, show that two wattmeters can be connected to read the total power in a three-phase, three-wire system. Two wattmeters connected to read the total power in a three-phase system supplying a balanced load read 10.5 kW and −2.5 kW respectively. Calculate the total active power. Drawing suitable phasor diagrams, explain the significance of: (a) equal wattmeter readings; (b) a zero reading on one wattmeter. ANS: 8 kWarrow_forward
- A factory has the following load with power factor of 0.9 lagging in each phase. Red phase 40 A, yellow phase 50 A and blue phase 60 A. If the supply is 400 V, three-phase, four-wire, calculate the current in the neutral and the total active power. Draw a phasor diagram for phase and line quantities. Assume that, relative to the current in the red phase, the current in the yellow phase lags by 120° and that in the blue phase leads by 120°.arrow_forwardFundimentals of Energy Systems Q8arrow_forwardTwo wattmeters are used to measure power in a three phase, three-wire network. Show by means of connection and complexor (phasor) diagrams that the sum of the wattmeter readings will measure the total active power. Two such wattmeters read 120 W and 50 W when connected to measure the active power taken by a balanced three-phase load. Find the power factor of the load. If one wattmeter tends to read in the reverse direction, explain what changes may have occurred in the circuit ANS: 0.815arrow_forward
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