- 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 dial shows liquid level between its minimum and maximum marks. Depending on the specified layout, it may also include intermediate marks such as 1/4, 1/2 and 3/4. The dial markings and float travel are selected for the conservator geometry.
Some dials include a reference filling-level mark for a stated oil or ambient temperature, often 30°C or 35°C. Alarm contacts can be set to operate near low or high level. Older gauges may use a mercury switch, while current products commonly use adjustable mechanical switch contacts. The switch type is therefore not an inherent feature of every MOG.
A ball, drum or other float is attached to a float arm inside the conservator. Depending on the model, the arm drives the internal magnet through a lever or gear mechanism. A magnetic coupling transfers movement across the sealed tank wall to the external pointer. This arrangement lets the dial and electrical contacts remain outside the oil space and avoids a rotating shaft seal through the conservator wall.
Working Principle of Magnetic Oil Gauge or MOG
Liquid-filled transformers can use several oil-preservation designs. A conservator-equipped transformer uses an expansion vessel above the main tank to accommodate changes in liquid volume. As the insulating liquid warms and expands, its level in the conservator rises.
When the liquid cools and contracts, its level in the conservator falls. The expected reading therefore changes with oil temperature. The level must remain within the operating range specified for the transformer at the observed temperature.
Conservator-equipped power transformers commonly use a magnetic oil level indicator or magnetic oil gauge. In a conventional conservator, the float follows the oil surface. Its up-and-down movement changes the float-arm angle and rotates the lever or gear mechanism. The exact float shape and drive arrangement depend on the gauge and conservator design.
The internal mechanism turns a driving magnet. Magnetic coupling moves the external pointer without a shaft passing through the tank wall. As the oil level changes, the pointer moves across the MOG dial and provides a local indication. Some models also provide switch contacts or an electronic output for remote monitoring.
When the pointer reaches a configured low-level point, an optional contact can operate the transformer’s alarm or control circuit. Treat that alarm as a condition to investigate, not an automatic instruction to add oil. Check the oil temperature, gauge operation and transformer for leakage. Any oil addition must use compatible processed liquid and the manufacturer’s de-energised maintenance procedure.
In an air-cell conservator, the oil surrounds a flexible air cell. The MOG float normally follows the underside of that cell rather than being rigidly attached to it. As oil volume changes, the air cell inflates or deflates and its lower surface moves with the oil level. The float arm follows that movement and drives the pointer. Installation geometry and float-arm length must match the conservator and air-cell design.
