- Magnetic Oil Gauge Definition: A Magnetic Oil Gauge (MOG) is defined as a device that indicates the oil level in a transformer’s conservator tank.
- Main Parts: The MOG includes a float, bevel gear arrangement, and indicating dial, essential for its operation.
- Working Principle: The MOG operates by moving a float, which adjusts a bevel gear and pointer to show oil levels on a dial.
- Alarm Feature: The MOG has a mercury switch that sounds an alarm when the oil level is too low, ensuring timely maintenance.
- Air Cell Conservator: In air cell conservators, the float arm changes with the air cell’s size due to oil expansion and contraction, maintaining oil levels.
A Magnetic Oil level Guage (MOG) is a device used to indicate the position of transformer insulating oil level in conservator of a transformer. This is a mechanical device. A mog in a transformer consists of three main parts:
- One float
- Bevel gear arrangement
- An indicating dial
Construction of Magnetic Oil Gauge or MOG
The construction of a magnetic oil gauge (MOG) starts with its dial. The dial has a scale ranging from empty to full, with intermediate divisions like 1/4, 1/1, and 3/4.
The dial can also show the prescribed oil level at 30°C or 35°C ambient temperature. A mercury switch and bevel gear are attached to the pointer. As the pointer rotates, the mercury switch’s alignment changes with it.
A ball-type or drum-type float is attached to a long float arm, with a bevel gear on one end. The other end is magnetically coupled to the pointer and mercury switch. The bevel gear is inside the conservator tank, while the dial, pointer, and mercury switch are outside.
Working Principle of Magnetic Oil Gauge or MOG
All oil-immersed distribution and electrical transformers are provided with expansion vessel which is known as conservator of transformer. This vessel takes care of oil expansion due to temperature rise. When transformer insulating oil is expanded, the oil level in the conservator tank goes up.
Again when oil volume is reduced due to fall in oil temperature, the oil level in the conservator goes down. But it is essential to maintain a minimum oil level in the conservator tank of transformer even at lowest possible temperature.
All large electrical power transformers are therefore provided with a magnetic oil level indicator or magnetic oil gauge. In conventional conservator tank, a light weight hollow ball or drum floats on the transformer insulating oil. The float arm is attached with bevel gear as we already explained during the discussion on the construction of magnetic oil gauge. Naturally, the position of the float goes up and down depending upon the oil level in the conservator and consequently the alignment of float arm changes. Consequently, the bevel gear rotates.
The bevel gear’s movement is transmitted to the pointer outside the conservator via magnetic coupling. The pointer is also connected to a mercury switch. Thus, as the oil level changes, the pointer moves on the MOG dial to indicate the oil level in the conservator tank.
As the alignment of mercury switch changes along with the pointer, this switch closes and actuates an audible alarm when pointer reaches near empty position on the dial of magnetic oil gauge. This event alerts us for topping up oil in electrical power transformer.
In air cell conservator, the float arm is attached with air cell and alignment of float arm changes with the changing size of air cell due to the expansion and contraction of oil.
