A three-phase Y-connected synchronous motor with a line-to-line voltage of \(440\) V and a synchronous speed of \(900\) rpm operates with a power of \(9\) kW and a lagging power factor of \(0.8\). The synchronous reactance per phase is \(10\) ohms. Determine the torque angle in electrical degrees.
A\(36.33^\circ\)
B\(33.51^\circ\)
C\(35.51^\circ\)
D\(38.46^\circ\)
#2
A \(10\)-hp, \(230\)-V, three-phase Y-connected synchronous motor has a reactance of \(3\) ohms per phase and a negligible resistance. To what voltage must the motor be excited in order to deliver full load at unity power factor and \(90\%\) efficiency?
A\(262\) V
B\(254\) V
C\(248\) V
D\(242\) V
#3
A \(1{,}000\) kVA, \(11\) kV, \(3\)-phase Y-connected synchronous motor has a resistance of \(3\) ohms and a reactance of \(25\) ohms per phase. Determine the induced emf per phase when fully loaded at \(0.8\) power factor lagging.
A\(5521\) V
B\(5362\) V
C\(5475\) V
D\(5582\) V
#4
A \(500\)-hp, \(2200\) V, \(3\)-phase star-connected synchronous motor has a synchronous impedance of \(0.3 + j3.0\ \Omega\) per phase. Determine the induced emf per phase if the motor works on full load with an efficiency of \(94\%\) and a power factor of \(0.8\) leading.
A\(1360\) V
B\(1354\) V
C\(1402\) V
D\(1522\) V
#5
A three-phase, star connected synchronous motor takes \(50\) kW at \(660\) V, the power factor being \(0.8\) lagging. This machine has a synchronous reactance of \(1\) ohm per phase with a negligible resistance. If the emf is increased by \(30\%\), the power taken remaining the same, find the new leading pf current.
A\(94.1\) A
B\(85.2\) A
C\(90.3\) A
D\(88.6\) A
#6
A \(6.6\) kV, Y-connected synchronous motor has a synchronous reactance per phase of \(10\) ohms and a negligible resistance. For a certain load, the motor draws \(990\) kW at normal voltage. At this load, the induced emf per phase is \(5160\) V. Determine operating pf.
A\(0.478\) leading
B\(0.502\) leading
C\(0.462\) leading
D\(0.518\) leading
#7
A \(75\)-hp, \(600\)-volt, \(1000\)-rpm, three-phase Y-connected synchronous motor has an armature resistance of \(0.05\) ohm per phase and a leakage reactance of \(0.45\) ohm per phase. The efficiency at rated load, \(80\%\) pf leading is \(92\%\). Determine mechanical power developed within the armature.
A\(64\) kW
B\(62\) kW
C\(60\) kW
D\(67\) kW
#8
A \(230\) V, \(60\)-Hz, \(3\)-phase, Y-connected synchronous motor draws a current of \(20\) A and delivers \(8\) bhp. Armature resistance per phase is \(0.5\ \Omega\). If the iron and friction losses amount to \(300\) W, determine the operating power factor of the motor.
A\(0.809\)
B\(0.807\)
C\(0.866\)
D\(0.862\)
#9
A \(2200\) V, \(200\) hp, \(3\)-phase mesh connected synchronous motor is working on full load at an efficiency of \(0.88\) and \(0.8\) power factor leading. The armature reactance per phase is \(5\) ohms. Determine the induced emf per phase. Neglect resistance.
A\(2377\) V
B\(2345\) V
C\(2380\) V
D\(2307\) V
#10
A \(15\)-hp, \(230\) V single-phase synchronous motor is taking a current of \(50\) A at a power factor of \(0.85\) leading. Effective armature resistance is \(0.2\) ohm. If this motor drives a mechanical load of \(12\) hp, determine its rotational losses.
A\(323\) W
B\(340\) W
C\(376\) W
D\(314\) W
#11
A \(440\)-V, single-phase synchronous motor is drawing a line current of \(50\) A at \(0.9\) pf lagging. This machine has an armature resistance and reactance of \(0.8\) and \(10\) ohms respectively. If stray power and excitation losses amount to \(500\) W and \(800\) W respectively, determine bhp output of the motor.
A\(23.2\)
B\(20.4\)
C\(25.8\)
D\(18.5\)
#12📜 EE Board April 1993
A single-phase synchronous motor gives an output of \(9.46\) kW at \(0.9\) pf lagging with \(600\) V. If stray power loss is \(600\) W and field losses is \(900\) W, while effective armature resistance is \(0.6\) ohm, find the efficiency of the synchronous motor.
A\(82.35\) %
B\(81.24\) %
C\(85.25\) %
D\(84.64\) %
#13
A \(230\)-V, \(60\)-Hz, single-phase synchronous motor is taking a current of \(20\) A at \(0.866\) lagging pf. Effective armature resistance is \(0.3\) ohm. Iron and friction losses amount to \(450\) W. Determine the efficiency of the motor at this load. Neglect losses in the excitation.
A\(86.15\) %
B\(85.30\) %
C\(88.52\) %
D\(83.20\) %
#14
A three-phase star-connected synchronous motor draws \(70\) A at \(0.8\) pf leading when connected from a \(6.6\) kV, \(60\)-Hz, \(3\)-phase source. At this load the machine operates with an efficiency of \(90\%\). If the synchronous impedance of the machine is \(2 + j15\) ohms, determine stray power losses.
A\(40.5\) kW
B\(28.6\) kW
C\(32.8\) kW
D\(34.6\) kW
#15
A \(20\) hp, \(440\) V, \(60\)-Hz, \(3\)-phase star-connected synchronous motor has an armature resistance per phase of \(0.4\) ohm. It is giving its full load output at a power factor of \(0.9\) leading. If the stray power losses amount to \(500\) watts, find the armature current.
A\(23.44\) A
B\(25.28\) A
C\(22.53\) A
D\(20.67\) A
#16
A \(500\)-V, \(60\)-Hz, single-phase synchronous motor develops \(20\) hp and is operating at a pf of \(0.8\) lagging. Effective resistance of armature is \(1\) ohm. Iron and friction losses amount to \(500\) W. Determine power drawn by the motor from the source.
A\(18{,}220\) W
B\(21{,}546\) W
C\(17{,}288\) W
D\(20{,}065\) W
#17📜 EE Board April 1991
A three-phase synchronous motor is measured by two A and B wattmeters. It reads \(28.6\) kW and \(46.5\) kW, respectively. The input power of the motor is under excited and assumed constant. What are the readings of A and B wattmeters if the power factor is \(85\%\) and the motor is over-excited?
A\(48.6\) kW, \(26.5\) kW
B\(42.5\) kW, \(32.6\) kW
C\(52.6\) kW, \(22.5\) kW
D\(51.0\) kW, \(24.1\) kW
#18
The input power to a \(600\)-V synchronous motor is measured by means of a two-wattmeter method. One wattmeter reads \(64\) kW and the other reads \(36.5\) kW. The motor is known to be operating at leading pf. Determine the line current drawn.
A\(104\) A
B\(110\) A
C\(107\) A
D\(112\) A
#19
A group of induction motors rated \(2200\)-V, \(3\)-phase, \(60\) Hz delivers a total of \(600\) hp and is operating at an average power factor of \(0.8\) lagging. If half of this total load is replaced by a group of synchronous motors operating at an average pf of \(0.8\) leading, determine the percentage reduction in the line current? Assume all motors operate at an average efficiency of \(92\%\).
A\(23\) %
B\(25\) %
C\(20\) %
D\(21\) %
#20
A three-phase transmission line having a capacity of \(10{,}000\) kVA serves a balanced load that draws \(5{,}000\) kW at a lagging power factor such that the transmission line is loaded to its capacity. An additional load of induction motors operating at an average pf of \(0.80\) is connected to the line. In order to make use of the line to its full capacity in active power, a synchronous condenser having losses of \(300\) kW will be connected to the line. What is the kVA rating of the synchronous motor required?
