Electric Motors Some Basic Notes

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Here are the basic question which will be useful for the ordnance factory board exam preparation. I found it much useful. share with everyone. .. mworld http://mechanicalengineersworld4u.blogspot.com/
  ElectricMotors   Objective type questions and answers   1.The synchronous speed of a motor with 6 poles and operating at 50 Hz frequency is ___.a) 1500 b) 1000 c) 3000 d) 7502.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 applicable4. Which types of following motors are most efficient?a) TEFC b) SPDP c) Both d) None5. What determines the thermal loading on the motor?a) Duty/Load cycle b) Temperature of the windingc) Age of the motor d) Ambient conditions6.Unbalance in voltages at motor terminals is caused by ____.  a) Supplying single phase loads disproportionately b) Use of different sizes of cablesc) Both (a) & (b) d)Noneoftheabove7. With decrease in speed of the motor, the required capacitive kVAr:a) Increases b) Decreases c) Does not change d) None of the above8. Application of DC motors is generally restricted to a few load speed applications because of  _____.a) cost of the motor is high b) problems with mechanical commutationc) Maintenance problems d) None of the above9.The speed of an AC motor depends on ____ .a) Frequency b) No. of polesc) both (a) and (b) d) None of the above10.The speed of the motor can be varied by ___.a) Changing supply frequency b) Changing no. of polesc) Using multi speed windings d) All the above11. Which of the following are ill suited for energy efficient motors application?a) Pumps b) Fans c) Punch Presses d) All the above12.Reduction in supply voltage by 10% will change the torque of the motor by ___.a) 38% b) 19% c) 9.5% d) no change 13. Five percent increase in supply frequency will change the synchronous speed by _____.a) -5% b) +5% c) -10% d) +10% 14. The inexpensive way to improving energy efficiency of a motor which operates consistently at below 40% of rated capacity is by _____.a) Operating in Star mode b) Replacing with correct sized motorc) Operating in delta mode d) None 15.  Which of the following needs to be measured after rewinding of motor?a) No load current b) winding resistancec) air gap d)all the above 16. Output power requirements of constant torque loads vary with _____.a) Speed b) voltage c) Current d) power factor 17.  Which of the following is an AC motor?a) Slip ring motor b) Synchronous motorc) Squirrel cage Induction d) all of the aboveStray losses in an induction motor generally are ____ .  18. a) proportional to the square of the stator current b) proportional to the square of the rotor currentc) proportional to the rotor currentd) inversely proportional to the square of rotor current 19.  Which of the following information available on name plate of a motor?a) HP rating b) RPM c) Frame model d) all the above 20. Machine tools are a typical example of a) Constant power load b) Constant torque loadc) Variable torque load d) a & b Short type questions and answers 1.Name the two important parameters that attribute to efficiency of electricity use by ACinduction Motors?The important parameters that attribute to efficiency of electricity use by AC induction Motorsare1. Efficiency of the motor2. Power Factor2.How do you define percentage unbalance in voltage?Percentage unbalance in voltage is defined as[(V  max – V  avg )/V  avg ] x 100, where V  max and V  avg is the largest and the average of the three phase voltages respectively. 3.  Why is it beneficial to operate motors in star mode for under loaded motors?For motors which consistently operate at loads below 50 % of rated capacity, an inexpensiveand effective measure might be to operate in star mode. A change from the standard deltaoperation to star operation involves re-configuring the wiring of the three phases of powerinput 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 performancecharacteristics as a function of load remain unchanged. Thus, full-load operation in star modegives 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, typicalpower factors of standard motors can provide the basis for conservative estimates of capacitorratings 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 pumps Variable torque loads: Centrifugal pumps and fans. 6.  What is an energy efficient motor? An energy efficient motor produces the same shaft output power (HP), but uses less inputpower (kW) than a standard-efficiency motor. 7.  What is synchronous speed and how to determine the % slip of a motor?The speed of a motor is the number of revolutions in a given time frame, typically revolutionsper minute (RPM). The speed of an AC motor depends on the frequency of the input power andthe number of poles for which the motor is wound. The synchronous speed in RPM is given by the following equation, where the frequency is in hertz or cycles per second:a) PolesofNo.Frequency 120 (RPM) Speed sSynchronou×= b) Slip (%) = [(Synchronous Speed–Full Load Speed)/Synchronous Speed] x 100 8. Listdown some of the important parameters that influence the motor selection?The following parameters influence the motor selection:(a) Torque requirement/load characteristics(b) Ambient operating conditions(c) Anticipated switching frequency (d) Reliability (e) Inventory (f) Price(g) efficiency  9.  What are the types of losses in any motor?The losses in any motor are shown below:Fixed Loses – core loss, friction & windage loss. Variable Losses - Copper & Stray Losses10. Write the relation to determine the energy savings by motor replacement with energy efficientone.kW savings = kW output x [1/η old – 1/η new  ]11. What steps should an energy manager take to minimize voltage unbalance?1. Balancing any single phase loads equally among all three phases2. Segregating any single phase loads which disturb the load balance and feed them from aseparate line/transformer
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