Electricity and Magnetism

The Selection of Power Rating for a Drive Motor The primary considerations in the selection of the power rating for a drive motor are economy, efficiency, and reliability. The use of an overrated motor would entail a waste of power, unwarranted capital outlays, and an increase in the required floor area. An under­rated motor would […]
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The Basic Forms of Duty for Drive Motors The term "duty" refers to a statement of loads including no-load, rest and de-energized periods, to which the machine or apparatus is subjected, including their duration and sequence in time. With respect to motor drive, three basic forms of duty may be Singled out. They are continuous […]
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The Equation of Motion for a Motor Drive In designing a motor drive, one has to know the mechanical ar­rangement, horsepower and torque requirements, and special conditions of operation of the driven machine. All of these- factors will determine, in one way or another, the static and dynamic loads applied to the shaft of the […]
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Motor Drives General A motor drive refers to an electromechanical system consisting of a drive motor (or motors), a driven machine, a connection from the drive motor to the driven machine, and associated switchgear and controls. This topic will be mainly concerned with matters bearing on the selection of a drive motor for a particular […]
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Fractional-Horsepower Synchronous Motors The property of synchronous motors to run at constant speed despite variations in the load torque is achieved by connecting a d.c. source to the rotor winding via sliding contacts. Instead of a d.c. exci­ted electromagnet, fractional-hp syn­chronous motor use a permanent mag­net fabricated from a magnetically hard material and mounted on […]
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Starting of a Synchronous Motor The resultant torque that a synchronous motor develops due to the interaction of the stator magnetic field with the excited rotor at standstill is zero very nearly. Therefore, for starting, the rotor of a synchronous motor has to be brought up to a nearly synchronous speed in one way or […]
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Control of the Active and Reactive Power of a Synchronous Motor The active power Pm = P = 3VIac.t = ɷrotTload of a synchronous motor connected to a large power system (V = const) can be varied by varying the load torque applied to its shaft (T1oad = VAr). As the load torque is increased, […]
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The V-Curves of a Synchronous Motor The magnitude of excitation current affects both the margin of stability and the reactive current of a synchronous motor. Let us analyse this relation by referring to the phasor/vector diagram of a phase of a synchronous motor connected to a large power system (V = const), as shown in […]
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The Electromagnetic Torque and the Load-Angle Characteristic of a Synchronous Motor Similarly to Eq. (15.9) defining the electric power developed by a three-phase synchronous generator, that taken by a three-phase synchronous motor is given by P = 3VI cos φ = 3E0I cos φ0 = 3E0I cos (φ – θ) (15.17) where θ > 0. […]
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The Equation of Electric State, Equivalent Circuit and Phasor Diagram for a Phase of a Synchronous Motor in contrast to a synchronous generator, the axis of rotor poles in a synchronous motor lags behind that of the poles produced by the rotating magnetic field of the stator (see Fig 3 b). The electromagnetic torque which […]
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