1. If the input voltage is 100V and output voltage is 200V, which side of the transformer winding will be primary?(a) 100V side (c) winding with loss turns(b) 200V side (d) winding with more turns2. Which of the following statement about humming of transformer is true?(a) The frequency of humming tone is 100Hz(b) The frequency of humming tone is 50 Hz(c) The humming is caused by a vibration of the low voltage winding through which the high current flows(d) The humming is caused by the private force of attraction between the lamination of the core3. In a 10 KVA, 230/1000V 1-? transformer, the no load current will be around(a) 3 A(c) 10 A(b) 0.5 A (d) 0.9 A
A transformer can be used as step-up or step-down, but normally use is as step-down so b. for the first question. Second question, the humming is caused by d. and will be at the frequency of the supply. Third question would be b. or d. since the only current drawn on no-load is the magnetising current for the core.
Each turn produces about the same voltage, and the primary is the input side. 1. The input voltage is lower than the output voltage, so the input winding (primary) must have the lower number of turns. 2. If the line frequency is 50 Hz, then a and d, since the vibration force is proportional to the magnitude of the current, but not the direction, so both half cycles produce the same force and there are two cycles of force each voltage cycle. The main vibration forces involve the magnetic attraction between the laminations and their magneto-striction (change in dimension as magnetic flux magnitude varies) 3. The load full load primary current will be roughly the power divided by the voltage or 10,000/23043 A. The no load current will be a few percent of that current, but I do not know the typical fraction for a transformer of that size. My seat-of-the-pants guess is a, about 7%. -- Regards, John Popelish
