Electric Motor

3242 WordsApr 12, 201113 Pages
Question bank for Energy Managers & Energy Auditors Chapter 3.2: Electric Motors Part–I: Objective type questions and answers 1. The synchronous speed of a motor with 6 poles and operating at 50 Hz frequency is ___. a) 1500 2. b) 1000 c) 3000 d) 750 The efficiency figures for energy efficient motors (in comparison with standard efficiency motor) can be generally higher by____%. a) 1% b) 3-7% c) 10% and above d) 8-10% 3. The power consumption, in case of centrifugal loads (like pump, fan, blower etc.), proportional to ____. a) speed b) square of speed c) cube of speed d) not applicable 4. Which types of following motors are most efficient? a) TEFC b) SPDP c) Both d) None 5. What determines the thermal loading on the…show more content…
A change from the standard delta operation to star operation involves re-configuring the wiring of the three phases of power input at the terminal box. Operating in the star mode leads to a voltage reduction by a factor of ‘ 3 ’. Motor output falls to one-third of the value in the delta mode, but performance characteristics as a function of load remain unchanged. Thus, full-load operation in star mode gives higher efficiency and power factor than partial load operation in the delta mode. However, motor operation in the star mode is possible only for applications where the torque-to-speed requirement is lower at reduced load. 4. What is the thumb rule for installing capacitors to motor terminal? The size of capacitor required for a particular motor depends upon the no-load reactive kVA (kVAR) drawn by the motor, which can be determined only from no-load testing of the motor. In general, the capacitor is then selected to not exceed 90 % of the no-load kVAR of the motor. (Higher capacities could result in over-voltages and motor burn-outs). Alternatively, typical power factors of standard motors can provide the basis for conservative estimates of capacitor ratings to use for different size motors. 5. Write some applications of constant torque and variable torque loads. Constant Torque Loads: Conveyors, rotary kilns, constant displacement
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