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Unit 3
Magnetic Line Voltage Starters
| Question | Answer |
|---|---|
| HINT: What is a magnetic line voltage starter? | An electromagnetic switch with an running overload protection |
| How many poles are required on motor starters for the following motors: a. 240-volt, single phase induction motor b. 440-volt, three phase induction motor | a. 2 b. 3 |
| If a motor starter is installed according to directions but will not start, what is a common cause for the failure to start? | No overload heaters |
| HINT: What causes the AC hum or chatter in AC electromagnetic devices? | because were using AC voltage which has 0 voltage. when it reaches 0 there is no pull on the armature causing it to drop due to gravity and be pulled back in due to the solenoid principle. |
| HINT: What is the phase relationship between the flux in the main pole of a magnet and the flux in the shaded portion of the pole? | 90 degrees apart |
| in what AC devices is the principle of the shaded pole used? | Magnetic starters on contactor section Relays |
| What type of protective enclosure is most commonly used and what is its NEMA number? | NEMA 1 General purpose |
| The magnetic starter is held closed a. mechanically b. by 15% undervoltage c. by 15% overvoltage d. electrically magnetically | d. electrically magnetically |
| when a motor starter coil is de-energized, a. the contacts stay closed b. it is held closed mechanically c. gravity and spring tension open contacts d. it must cool for a restart | c. gravity and spring tension open contacts |
| Ac AC magnet may hum excessively due to a. improper alignment b. foreign matter between contact surfaces c. loose lamination d. all of these | d. all of these |
| AC magnets are made of laminated iron a. for better induction b. to reduce heating effect c. for AC and DC use d. to prevent chattering | b. to reduce heating effect |
| The purpose of overload protection on a motor is to protect the a. motor from sustained overcurrents b. wire from high currents c. motor from sustained overvoltage d. motor from short circuits | a. motor from sustained overcurrents |
| The number of magnetic starter poles refers to a. the number of power, motor, or load contacts b. the number of control contacts c. the number of north and south poles d. all of these | a. the number of power, motor, or load contacts |
| Motors may burn out because of a. overload b. high ambient temperatures c. poor ventilation d. all of the above | d. all of the above |
| The purpose of a shading coil on an AC electromagnetic pole tip is to a. prevent overheating of the coil b. limit the tripping current c. limit the closing current d. prevent chattering | d. prevent chattering |
| HINT: What advantage is there in using combination starters? | we have both disconnecting switch and starting protection |
| what safety feature does the combination starter provide that individual motor starting assemblies do not? | starting protection: fuses and circuit breakers |
| HINT: list the probable causes if the armature does not release after the magnetic starter is de-energized. | mechanical binding; the air gap in magnet is destroyed; gummy substance on magnet faces; weak tip pressure; contact tip welding |
| how is the size of the overload heaters selected for a particular installation? | Get the nameplate Full Load Amps (FLA) of the motor and look on the manufactures cover |
| the current drawn by a motor is a. low on starting b. an accurate measurement of motor load c. an inaccurate measurement of motor load d. non of these | b. an accurate measurement of motor load |
| thermal overload relays react to a. high ambient temperatures and excessive heating due to overload currents b. heavy mechanical loads c. from the motor's FLA and a manufacturer's selection chart d. by ambient temperature | a. high ambient temperatures and excessive heating due to overload currents |
| when the reset button does not reestablish the control circuit after an overload, the probable cause is a. the overload heater is too small b. the overload trip has not cooled sufficiently c. the overload heater is burned out | b. the overload trip has not cooled sufficiently |
| if an operator pusses a start button on a 3-phase induction motor and the motor starts to hum, but not run, the probable trouble is a. one fuse is blown and the motor is single phasing b. the overload trip needs resetting c. the auxiliary contact is sh | a. one fuse is blown and the motor is single phasing |
| a combination starter provides a. disconnecting means b. overload protection c. short-circuit protection d. all of these | d. all of these |
| what does IEC stand for? | international electrotechnical commission |
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aganotisi