Three-Phase Motor Starters and Control Circuits

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1. 1

   What is the primary function of a contactor in a three-phase motor starter?

   A Electrically switch power to the motor B Limit motor current during every start C Convert three-phase power into direct current
2. 2

   What fault is a motor overload relay primarily designed to detect?

   A A high-energy short circuit between phases B Sustained motor current above an acceptable level C A momentary drop in control voltage
3. 3

   In the illustrated three-wire starter circuit, which component keeps the contactor coil energized after the START button is released?

   

   A The normally closed overload contact B The control transformer secondary C The normally open auxiliary contact wired across START
4. 4

   Which type of device is represented by the highlighted symbol at the beginning of this starter control rung?

   

   A Normally closed stop pushbutton B Normally open start pushbutton C Normally open contactor auxiliary contact
5. 5

   Why is a control transformer commonly installed in a motor starter circuit?

   A To change the motor's direction of rotation B To provide a lower control voltage from the line supply C To correct the motor's power factor under all loads
6. 6

   Which highlighted control contact opens when the overload relay detects excessive motor current?

   

   A The normally open START contact B The normally open seal-in contact C The normally closed overload contact
7. 7

   What normally happens in a three-wire starter circuit after control power is lost and then restored?

   A The motor remains stopped until START is pressed again B The motor automatically restarts at full speed C The overload relay automatically reverses the motor
8. 8

   Which feature is characteristic of two-wire motor control?

   A It always requires separate momentary START and STOP buttons B It uses a maintained contact that can command an automatic restart C It requires two contactors for every motor
9. 9

   The diagram shows forward and reverse connections for the same three-phase motor. What electrical change reverses the motor's rotation?

   

   A Connecting all three phases in parallel B Removing one phase from the motor C Interchanging any two phase conductors
10. 10

    What is the purpose of the normally closed F and R auxiliary contacts shown in series with the opposite contactor coils?

    

    A To prevent the forward and reverse contactors from energizing together B To increase coil voltage during acceleration C To bypass overload protection during reversal
11. 11

    How does a mechanical interlock improve a reversing starter?

    A It automatically adjusts the overload trip current B It physically prevents both contactors from closing simultaneously C It reduces the motor's rated operating speed
12. 12

    In the illustrated control circuit, why does the contactor release when the JOG pushbutton is released?

    

    A JOG immediately trips the overload relay B JOG disconnects one of the motor phases C The jogging path energizes the coil without maintaining the seal-in circuit
13. 13

    What should be the primary current reference when setting an adjustable motor overload relay?

    A The motor nameplate current, adjusted as required by applicable instructions and rules B The contactor's maximum short-circuit rating C The control transformer's primary current
14. 14

    What commonly occurs when a running three-phase motor loses one supply phase?

    A The motor automatically converts to single-phase operation at normal current B Current rises in the remaining phases and the motor may overheat C Motor speed doubles while torque remains unchanged
15. 15

    Which IEC utilization category is commonly specified for starting and stopping standard squirrel-cage induction motors?

    A AC-1 B DC-1 C AC-3
16. 16

    In the illustrated rung, several control contacts are connected in series ahead of the coil. What logic does this arrangement implement?

    

    A Every series condition must be closed for the coil to energize B Any one series condition can energize the coil C The coil remains energized regardless of contact state
17. 17

    Under common IEC terminal numbering, which designation normally identifies a normally open auxiliary contact?

    A A1-A2 B 13-14 C 21-22
18. 18

    With power isolated, a meter displays infinite resistance across disconnected contactor coil terminals A1 and A2. What does this indicate?

    

    A The coil has normal continuity B The main contacts are welded closed C The coil winding is open