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When the motor is switched off, the motor contactor relay not only disconnects the AC current supply to the motor windings, but also causes a braking relay contactor to close. This initiates a sequence beginning with a time delay of about 300ms that allows current still flowing in the motor windings to diminish to a safe level.
AC electric motors can be run in fixed-speed operation determined by the number of stator pole pairs in the motor and the frequency of the alternating current supply. AC motors can be made for "pole changing" operation, reconnecting the stator winding to vary the number of poles so that two, sometimes three, speeds are obtained.
Small C-frame shaded-pole squirrel-cage motor. With the poles shown, the rotor will rotate in the clockwise direction. Shading coils (copper bars) within the magnetic circuit of the field coil. The shaded-pole motor is the original type of AC single-phase electric motor, dating back to at least as early as 1890. [1]
When an electric motor, AC or DC, is first energized, the rotor is not moving, and a current equivalent to the stalled current will flow, reducing as the motor picks up speed and develops a back EMF to oppose the supply. AC induction motors behave as transformers with a shorted secondary until the rotor begins to move, while brushed motors ...
The AC power supply and frequency converter pioneer Miro Zorič and his first AC power electronics have also enabled this to be possible with AC supplies. [ citation needed ] The supply frequency must also be matched (this mainly applies to locomotives where an AC supply is rectified for DC motors).
Most three-phase AC field coils are used to generate a rotating field as part of an electric motor. Single-phase AC motors may follow either of these patterns: Small motors are usually universal motors, like the brushed DC motor with a commutator, but run from AC. Larger AC motors are generally induction motors, whether these are three-phase or ...
The competitive power rating span of standardized CCVs ranges from few megawatts up to many tens of megawatts. CCVs are used for driving mine hoists, rolling mill main motors, [4] ball mills for ore processing, cement kilns, ship propulsion systems, [5] slip power recovery wound-rotor induction motors (i.e., Scherbius drives) and aircraft 400 Hz power generation. [6]
During flux braking the motor is under DTC control which guarantees that braking can be made according to the specified speed ramp. This is very different from the DC injection braking typically used in drives. In the DC injection method DC current is injected to the motor so that control of the motor flux is lost during braking.