Category: Electrical Machines
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Common Causes of Failures of Power Transformers
As with most of electrical equipment, the key causes of failure of power transformers are either due to electrical failure or mechanical failure. These failures don’t occur that often as compared with other failures in other electrical equipment like switchgear. Common failures of power transformers and their causes are discussed in the following sections. Typical…
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Testing Power and Distribution Transformers
The typical practice for testing power and distribution transformers is to carry out a comprehensive set of tests at the manufacturer’s works. The number and nature of the tests depend on whether the transformer is the first of a new design or not, additionally a few relatively simple tests are done after installation at site…
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Fuses & Circuit Breakers vs. Overload Heaters for Industrial Motor Protection
A key component of any motor control circuit is some kind of device to detect a condition of excessive overload and interrupt the power to the motor before thermal damage occurs to it. An overload heater is a simple and common overload protective device that consists of resistive elements connected in series with the three…
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Permanent Magnet DC Motors – Operation & Applications
Permanent magnet dc motors employ a permanent magnet instead of stationary field winding (as with the case of some other types of dc motors) for producing the required magnetic flux. These stationary magnets are normally fixed to the outer shell of the motor as illustrated in Figure 1.0. Features of a Permanent Magnet DC Motor…
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How to Perform a Brake Test on a DC Motor
Testing is usually performed on DC machines to establish efficiency and power losses. A brake test is a direct technique that involves applying a brake to a water-cooled pulley mounted on the motor shaft as demonstrated in Figure 1.0. This brake test method is typically used on small motors. The brake band is fixed with…
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How solid-state soft starting of Induction motors is done
Solid-state soft starting is the most commonly used technique for starting induction motors. It provides a smooth build-up of current and torque; the maximum current and acceleration time are easily adjusted. The only problem with this technique over conventional starters is that the mains currents during run-up aren’t sinusoidal, which can lead to interference with…
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Basic Features of Autotransformer Starter
A 3-phase autotransformer is typically employed where star or delta starting provides insufficient starting torque. Each phase of an autotransformer consists of a single winding on a laminated core. This incoming supply is connected across the ends of the coils, and one or more tapping points (or a sliding contact) provide a reduced voltage output…
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Types of AC Motors
Classification of AC Motors The AC motors works by rotating the stator field, they make use of the natural alternating nature of the AC wave to turn the field coils on and off sequentially. Generally, AC motors can be classified into two groups: single-phase and polyphase, with each group being further subdivided into induction and…
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Direct-on-line (DOL) Starter
Let’s consider an illustration of a typical three-phase direct-on-line (DOL) starter below: When the start button is pressed, the 415V contactor coil is energized, the main and auxiliary contacts close, and the motor starts. The auxiliary contact in parallel with the start button holds the coil on. Overload or overcurrent protection is provided by either…
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How cycloconverter is used as a direct frequency converter
The cycloconverter provides direct ac to ac transformation with control over both the output voltage magnitude and output frequency. It employs phase-angle-controlled bridges to synthesize voltages. The most common form is the three-phase to three-phase converter illustrated below: Each output needs two three-phase thyristor bridges. One bridge is used to synthesize positive voltages at the…