TRANSFORMER -2

Hello folks,

Let us now understand what is this magnetic humming in transformer in detail. It is also a well-known interview question.

Q. What is Magnetic humming in transformer? How can it be minimized?

The   process of bunching of all the laminations is called core staggering. If laminations are not properly staggered, there is possibility of thin air gap between laminations. It increases reluctance of magnetic path, thereby increasing the excitation current in transformer. So transformer makes more noise which is magnetic humming.

The basic reason behind the production of noise in transformer is Magnetostiction phenomena. It is the tendency of any magnetic material due to which slight changes in the dimensions of magnetic material take place whenever it is subjected to alternating nature of magnetic force. These dimensional changes in laminations when the core is subjected to alternating mmf is (10^-8) cm. When thickness of all laminations increase, transformer core area also increases. Core is subjected to continuous expansion and compression due to magnetostiction and hence there is mechanical stress on core. Now core starts vibrating due to mechanical stresses on transformer. This noise is in audible frequency range (20Hz-20KHz), which is called magnetic humming.

Consider the sinusoidally varying flux. In first cycle, there is increment and decrement in flux. When there is strengthening of flux (0-45) , thickness of lamination increases slightly, whereas there is no change in core dimensions from (45-90). The same process repeats for negative half cycle of sine wave. So in 1 cycle of flux, 2 times dimensional changes are taking place in core. Therefore, frequency of noise is double than supply frequency. If supply frequency = 50 Hz, then noise frequency = 100 Hz. Noise level depends on the flux density in the core.

In solid core, there is no humming sound as there is no noise produced. We cannot get noise in transformer with DC excitation because here we have steady mmf, so there is no magnetization-demagnetization (there is only magnetization).  There is no noise from transformer in electronics because they operate at higher frequencies (MHz) which is not in the audible range. (Here noise is present but not audible).

We get more noise in Power transformer (in Substations) as compared to distribution transformer (normal usage) because power transformers have higher flux densities. We cannot curtail noise problems but we can reduce noise level by proper core staggering.

Thank you for your time.