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Motor Types- Special Types of Motor

Motor Types- Special Types of Motor

1. Iron less or core-less rotor motor

Nothing in the principle of any of the motors described above requires that the iron (steel) portions of the rotor actually rotate. If the soft magnetic material of the rotor is made in the form of a cylinder, then (except for the effect of hysteresis) torque is exerted only on the windings of the electromagnets. Taking advantage of this fact is the core less or iron less DC motor, a specialized form of a PM DC motor. Optimized for rapid acceleration, these motors have a rotor that is constructed without any iron core. The rotor can take the form of a winding-filled cylinder, or a self-supporting structure comprising only the magnet wire and the bonding material. The rotor can fit inside the stator magnets; a magnetically soft stationary cylinder inside the rotor provides a return path for the stator magnetic flux.
A second arrangement has the rotor winding basket surrounding the stator magnets. In that design, the rotor fits inside a magnetically soft cylinder that can serve as the housing for the motor, and likewise provides a return path for the flux.
Because the rotor is much lighter in weight (mass) than a conventional rotor formed from copper windings on steel laminations, the rotor can accelerate much more rapidly, often achieving a mechanical time constant under one ms. 

This is especially true if the windings use aluminium rather than the heavier copper. But because there is no metal mass in the rotor to act as a heat sink, even small core-less motors must often be cooled by forced air. Overheating might be an issue for core-less DC motor designs.

2.Axial rotor motor:

A rather unusual motor design, the printed armature or pancake motor has the windings shaped as a disc running between arrays of high-flux magnets. The magnets are arranged in a circle facing the rotor with space in between to form an axial air gap. This design is commonly known as the pancake motor because of its extremely flat profile, although the technology has had many brand names since its inception, such as Servo Disc.

The printed armature (originally formed on a printed circuit board) in a printed armature motor is made from punched copper sheets that are laminated together using advanced composites to form a thin rigid disc. The printed armature has a unique construction in the brushed motor world in that it does not have a separate ring commutator. The brushes run directly on the armature surface making the whole design very compact.

3.Reluctance motors: A synchronous‐induction motor. The rotor has salient poles and a cage so that it starts like an induction motor, and runs like a synchronous motor.Uses soft magnet rotor that is magnetized by the application of current in the stator coils.The magnet is spun by a 3 phase AC motor with no rotor voltage.The gear teeth are designed to increase the magnetic flux driving the rotor while providing asymmetry that gives it the

rotational acceleration

4.Hysteresis motor: hysteresis produces the torque, can be very tiny, used as the driver for electric clock.

5.Stepper motor: a special type of synchronous motors. Rotates a number of degrees with each electric pulse.

6. Brush-less DC motor: Brush less DC motors were developed from conventional brushed DC motors with the availability of solid state power semiconductors. So, why do we discuss brushless DC motors in a chapter on AC motors? Brushless DC motors are similar to AC synchronous motors. The major difference is that synchronous motors develop a sinusoidal back EMF, as compared to a rectangular, or trapezoidal, back EMF for brushless DC motors. Both have stator created rotating magnetic fields producing torque in a magnetic rotor. 
Synchronous motors are usually large multi-kilowatt size, often with electromagnet rotors. True synchronous motors are considered to be single speed, a submultiple of the powerline frequency. Brushless DC motors tend to be small– a few watts to tens of watts, with permanent magnet rotors. The speed of a brushless DC motor is not fixed unless driven by a phased locked loop slaved to a reference frequency. The style of construction is either cylindrical or pancake.

7.Universal motor: A universal motor is a series wound electric motor that is designed to run on either AC or DC current. Advantages of a universal motor over an AC or DC motor are higher start up torque, smaller size, and the ability to operate at very high rpms. An example of this is a Dremel tool, which run around 30,000 rpm. The disadvantage is a shorter lifespan, so these motors are generally used in applications where the motor is only used intermittently.

For more related information please refer:
Motor Types-Introduction
Motor Types- DC Motor
Motor Types- AC Motor
Motor Types- Special Types of Motor
Servo Motors
Stepper Motor

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