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Three Phase Analysis and Power Calculation

Three-phase systems differ from single-phase systems in that they use a set of three voltages instead of one. Three-phase systems are used for the generation and transmission of bulk electric power. To get single-phase ac from a three-phase system, we simply tap off one of its phases. Three-phase systems may be balanced or unbalanced. If a system is balanced, it can be analyzed by considering just one of its phases. Since most systems operate close to balance, many practical problems can be dealt with by assuming balance. Three-phase systems posses economic and operating advantages over single-phase systems.

Popularity: 18%

Tags: Engineering, phase_ac, three_phase_system

Magnetic Levitation

Magnetic levitation, maglev, or magnetic suspension is a method by which an object is suspended above another object with no support other than magnetic fields. The electromagnetic force is used to counteract the effects of the gravitational force.

Magnetic levitation transport, or maglev, is a form of transportation that suspends, guides and propels vehicles via electromagnetic force. This method can be faster than wheeled mass transit systems, potentially reaching velocities comparable to turboprop and jet aircraft .

There are two primary types of maglev technology:

Electromagnetic suspension (EMS) uses the attractive magnetic force of a magnet beneath a rail to lift the train up.

Electrodynamic suspension (EDS) uses a repulsive force between two magnetic fields to push the train away from the rail.

Popularity: 4%

Tags: Engineering, magnetic-levitation

Operation of three-phase induction motor on a single supply

If one line of a three-phase line is accidentally opened while the three-phase motor is running the machine will continue to run as single-phase motor.

The current drawn from the remaining two lines will almost double, and the motor will begin to overheat.

The torque-speed curve is seriously affected when a three-phase motor operates on single phase.

The pull-out torque (maximum torque) decreases to about 40% of its original value, and the motor develops no starting torque at all.

Popularity: 4%

Tags: Engineering, induction-motor, operation, power-supply, three-phase

Permanent-split capacitor motor

In this motor, the capacitor in series with the auxiliary winding is not switched out after starting.

It remains in the circuit; therefore the centrifugal switch is not needed.

Since the capacitor and the auxiliary winding are in the circuit continuously, they should be properly designed.

The value of the capacitor is chosen such that the phase angle difference between the currents in the main and auxiliary windings are exactly 90o.

Such a motor will have a perfectly uniform rotating magnetic field at some specific load.

Popularity: 4%

Tags: capacitor-motor, Engineering, permanent-split

What is Single Phase Motors

Single-phase induction motors are the most familiar of all electric motors because they are used in home appliances, businesses, and small industries. Single-phase induction motors have usually two poles or four poles and rated at 2 hp or less.

In a single-phase motor we have only a single field winding excited with alternating current; therefore, it does not have a revolving field like three-phase motors. Thus, it does not self-starting. Several methods have been devised to initiate rotation of the squirrel-cage rotor and the particular method employed to start the motor will designate the specific type.

Popularity: 4%

Tags: Engineering, motors, single-phase-motors

Braking method: Plugging

The terminal voltage Vt of the source connected to the armature is suddenly reversed keeping the field current unchanged.

The net voltage acting on the armature circuit becomes (Ea + Vt) and it will produce enormous reverse current, perhaps 50 times greater than the full-load armature current.

We must limit the reverse current by introducing a resistor in series with the reversing circuit.

When the machine reaches zero speed we must immediately open the armature circuit, otherwise it will begin to run in reverse direction.

Popularity: 4%

Tags: braking, Engineering, motors, plugging