Types of Motors - 4

Coreless DC motors

Coreless dc motors are small motors which do not have a core.  The armature winding is self-supporting wound in the form of a basket.

These motors can have extremely high acceleration due to reduced weight of the rotor.  The rotor armatures also have low inductance.  This helps in extending the life of the brushes and the commutator.

These rotors have low inertia and low noise and vibration and thus can be easily controlled.

The downside to these motors is that the rotor gets heated quickly as it does not have the core which can serve as a heat sink.  These motors can be used only for low power application such as the motor in magnetic tape devices, optical rotary encoders, etc..


Motor Selection Guide - Online

www.SpecAmotor.com is a website that helps engineers quickly select a motor based on their requirement.  You just have to only key in the required speed and torque and specify the type of motor you are looking for, induction, dc, synchronous, etc.

The site launched in 2008, now claims to have a database of over 10000 configurations.  The site is brand independent and includes specifications from all leading brand. 

You can also test the characteristics of motors which fit your search online.


Flame Proof Motors

Flame Proof motors are used in hazardous locations usually in chemical, petroleum and gas industries, where the environment may contain gases or inflammable vapours. Electric equipment are notorious for being the source of sparks during switching. This could be catastrophic in areas where inflammable gases are present.

Hazardous areas classified into zones depending on the possibility of the presence of gases and combustible dust-air mixtures.

The following are the classification for hazardous areas

Zone 0, An Explosive atmosphere is continuously

Zone 1, An Explosive atmosphere is present for 1000 hours in a year.

Zone 2, An Explosive atmosphere is present for 10 hours in a year.

Flameproof motors are specially designed so that any explosion which occurs within the motor is contained within the motor frame itself. This prevents any secondary explosion in the surrounding area. The flame-proof motor is specially enclosed within a protective shield which covers the entire motor including the bearings.

Flame proof motors are heavier that motors of similar rating due to the increased weight of the reinforced design of the end shields so as to withstand the impact of an explosion.

Flame proof motors have special arrangements for cooling. Since conventional air circulation is not possible with these motors, special arrangements such as using the internal air to carry heat to the surface of the motor where the heat is transferred to the external air by means of special tubes have been devised.

Another method of cooling is the use of a coolant such as water which circulate in the stator and rotor of the motor. This water is brought to the near the inside of surface of the motor where it is cooled by means of the atmospheric air.

The cooling system ensures the isolation of the internal cooling medium from the external atmospheric air.


Totally Enclosed Fan Cooled (TEFC) Motors

The Totally Enclosed Fan Cooled Motor is the most widely used type of motor.  It consists of frame which totally encloses the motor.  Thus there is no airflow between the inside of the motor and the outside.  However, the frame is not airtight.

The term "Totally Enclosed" means that the motor is dust proof.  It is however not submersible.

Cooling occurs by means of a fan which blows air over the frame.  The air which passes over the frame removes the heat created by the motor.  The enclosure has fins to maximize the surface area to enhance cooling.


Open Drip Proof (ODP) Motors


Open Drip Proof Motors(ODP)  are motors which are used in applications where there may be dripping water.  These motors are covered by a metal enclosure which ensures that any water which drips will not flow into the motor.

Drops of water which fall at an angle of 15% will be caught by this enclosure.  Open drip proof motors usually have a fan for cooling the motor.  These motors are relatively inexpensive and used widely in the industry.

Open Drip Proof motors are used in clean, dry environments.  Sometimes, the air from the blower is made to pass through a filter before entering the motor


Can Single Phase motors be reversed by changing the polarity?

Single phase motors have two windings, the main winding and the auxilliary winding. The auxiliary winding is used to start the motor and may be disconnected once the motor picks up sufficient speed.

Reversing a single phase motors cannot be done by reversing the polarity of the supply to the entire motor. To reverse the single phase motor, the polarity of the supply to only one of the windings needs to be changed.

This can be done by reconfiguring special links which may be provided in the terminal box of the motor.