Unit 4 AC & Motor Theory Exam questions and answers 2024

For a given number of poles, which of the following determines the speed of an AC motor? Frequency The terminals of a motor can be identified using a battery and a(n) _____. voltmeter The stationary magnet of a DC motor is represented by the ______. field poles Any exposed live parts must be guarded if they are operating at ______. 50V or more between terminals The synchronous speed at which a four-pole, 60Hz induction motor operates is _____. 1,800rpm In a split-phase motor, how much greater is the starting current when compared to the full-load current? 6 to 8 times Opening the secondary circuit transformer under load will cause a dangerous level of ______. induced high voltage A delta-connected motor has _______. 9 leads The power factor of an induction motor at no-load conditions can be as low as: 15% Which of the following motors is used when required starting torque is high and speed control is desirable? Compound DC injection braking is used on ______. the stationary windings of AC motors In an inductive AC circuit, when the applied voltage is at its maximum negative value, the current is? zero What is the current flow in a purely resistive AC circuit if the resistance is 15ohms and the voltage is 150V? 10A If the phase angles of two sine waves are both 0 degrees, then the waves ______. are in phase In a circuit that has both capacitance and resistance, the current will lead the voltage _______. by some angle between 0 degrees and 90 degrees If the time for one cycle is 0.1 second, the frequency is _____. 10 Hz What is the true power in a 2A series RL circuit with a 100-ohm resistor? 400W According to NEMA definitions, which of the following is a type of totally enclosed motor? Water-cooled A polyphase synchronous motor has three stator windings that are _____. 120 degrees apart Which motor should be used for applications that require frequent starts without overheating the motor? Wound rotor induction motor A device that uses electromagnetic induction to transfer electrical energy from one circuit to another is a(n) _______. transformer What is the step-down turn ratio of a transformer with 200 turns in the primary and 100 turns in the secondary? 2:1 The NEC requires motor disconnecting means be _____. within sight of the controller location According to the power triangle, the apparent power is ______. greater than the true power The capacitor-start motor is the single-phase motor type that has the ______. highest starting torque In a DC series motor, how much of its full-load torque is developed at starting? 500% If the area of the plates of a capacitor is increased, the amount of capacitance ______. increases The value indicated in rpm on the nameplate is the ______. rated full-load speed A typical constant-torque load would be a(n) _____. conveyor Reactive power is always ______. 90° out of phase with true power Several motors are permitted to be supplied by a single branch circuit as long as each motor is no larger than _____. 1 hp The primary method of varying the speed on a DC shunt motor is to adjust the voltage and current on the field winding or the ______. armature Which of the following is most likely to use dynamic braking? Forklift Transformers in which the primary and secondary voltages are 180 degrees out of phase are known as _______. unlike-wound transformers Which of the following motors provides constant speed from no-load conditions to full-load conditions? Synchronous If a transformer has 200 turns in the primary, 100 turns in the secondary, and 150VAC applied to the primary, the voltage across the secondary is ______. 75VAC