Today we got low insulation of 440V on the main switchboard megohmmeter when starting the emergency fire pump. Checking the power part of the circuit showed an insulation resistance relative to the electric motor housing of 0 MOhm. Has the motor burned out or is there a chance to restore normal operation of the mechanism?
Checking the power part of the circuit (main contactor and autotransformer) with a portable megohmmeter showed 0 MOhm. Have to climb into the pump shaft and measure the insulation resistance of the electric motor at the terminal box with disconnecting of the power cable.
This procedure is standard when looking for low motor insulation. It is necessary to find out whether this is a problem with the electric motor or the power part, for example, a cable.
A test without cable showed that the electric motor gives infinity, so the problem is only in the power part of the circuit (cable, contactors and autotransformer).
Visually, there is increased humidity in the starter panel. Perhaps the panel was flooded with sea water, because... it is located in the steering gear compartment opposite the aft exit. In such a situation, you can try to dry out the electrical circuit inside the panel.
For this, a regular incandescent light bulb is suitable, which we install and connect inside the switchboard next to the autotransformer.
Also, several bags of silica gel were placed in the panel so that the moisture that evaporates is absorbed by the silica gel and does not settle on the walls inside.
During drying, the circuit insulation rose significantly to almost 50 MΩ.
On the walls of the shield and on the cable, salt formations are visible, which have settled after the moisture evaporates. Remove the salt using contact cleaner and continue drying. It is advisable to replace the silica gel with a new or dried one over time.
In this way, the insulation of the power part of the electric motor was restored and the problem of low insulation, which could lead to burnout of the circuit, was eliminated.
An autotransformer is a type of electrical transformer that has a single winding which acts as both the primary and secondary winding, instead of having separate primary and secondary windings. This design allows the autotransformer to be more compact, efficient, and cost-effective for certain applications compared to conventional transformers. Here’s an overview of its key characteristics and uses:
Key Characteristics
- Single Winding: Unlike traditional transformers with separate primary and secondary windings, an autotransformer uses a single winding that shares part of its turns between the input (primary) and output (secondary) circuits.
- Voltage Adjustment: The voltage transformation ratio can be adjusted by tapping at different points along the winding, allowing for variable output voltage.
- Size and Efficiency: Due to the single winding design, autotransformers are generally smaller and more efficient than conventional transformers for the same power rating.
- Cost: The reduced amount of copper and core material used in autotransformers makes them cheaper to manufacture and purchase.
Applications
- Voltage Regulation: Autotransformers are often used in applications where slight adjustments in voltage are needed, such as in power distribution systems to stabilize voltage levels.
- Motor Starting: In industrial settings, autotransformers are used to start induction motors with a reduced voltage, which minimizes the inrush current and mechanical stress on the motor.
- Laboratory Equipment: They are used in variable autotransformers (variacs) to provide adjustable AC voltage for testing and research purposes.
- Power Distribution: They can be used to step down high voltage in power distribution networks, especially where the voltage difference is not too large.
Advantages and Disadvantages
Advantages:
- Higher Efficiency: Less copper loss and core loss due to the single winding design.
- Compact Size: Smaller and lighter than conventional transformers.
- Cost-Effective: Lower material and manufacturing costs.
Disadvantages:
- No Isolation: There is no electrical isolation between the primary and secondary circuits, which can be a safety concern in certain applications.
- Limited Voltage Range: Less suitable for applications requiring a wide range of voltage transformation.
- Harmonic Distortion: More susceptible to harmonic distortion compared to conventional transformers.
Autotransformers are a practical solution for many electrical applications requiring efficient voltage transformation with minimal cost and size, provided that the lack of electrical isolation is not a critical issue.
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