In a three-phase squirrel-cage induction motor, the rotating magnetic field is produced by: a. two electromagnets placed at right angle from each other and two sine wave currents phase shifted of 90° from one another.
b. three electromagnets placed at 90° from each other and three sine wave currents phase shifted of 120° from one another.
c. three electromagnets placed at 120° from each other and three sine wave currents phase shifted of 120° from one another.
d. three electromagnets placed at 120° from each other and three sine wave currents phase shifted of 90° from one another.

Air is to be cooled in the evaporator section of a refrigerator by passing it over a bank of 0.8-cm-outer-diameter and 0.4-m-long tubes inside which the refrigerant is evaporating at -20°c. air approaches the tube bank in the normal direction at 0°c and 1 atm with a mean velocity of 4 m/s. the tubes are arranged in-line with longitudinal and transverse pitches of sl- st 1.5 cm. there are 30 rows in the flow direction with 15 tubes in each row. determine (a) the refrigeration capacity of this system and (b) pressure drop across the tube bank. evaluate the air properties at an assumed mean temperature of -5°c and 1 atm. is this a good assumption?

Which of the following components of a pid controlled accumulates the error over time and responds to system error after the error has been accumulated? a)- proportional b)- derivative c)- integral d)- on/off.

Vibration monitoring this technique uses the noise or vibration created by mechanical equipment and in seme cases by plant systems to detemine their actual condtion. a)- true b)- false