Universal Motor - An Overview

Universal Motors are motors which run on both AC as well as DC power.  These motors find wide application in household electric appliances such as mixers, blenders.

The small size, high power and speed of universal motors make them ideal in these applications.  The Universal Motor is the motor of choice for washing machines as they can be used to agitate the drum by easily reversing direction. 

A universal motor is a dc series motor which usually has some modifications which enable it to run efficiently on AC as well.  Running the motor on AC leads to eddy currents being developed in the core of the motor.  Hence, the core and poles of universal motors are made of laminated sheets.  The windings of the motors are made thicker to reduce the reactance.   

Like all DC machines, universal motors suffer from the effects of commutation such as sparking.  Hence, universal motors are used mainly below 1000W.  The high speed of the universal motor enables it to deliver a higher power even while having a small size.  Universal motors are usually kept coupled to the loads such as gears or blowers as they tend to run at very high speeds on no load conditions. 

Speed Control is usually done by Electronic means.  

Interpoles

Interpoles are additional poles which are placed between the main

poles in a dc machines. These poles are designed to counteract the

effects of armature reaction in the dc machines.

Interpoles are placed at the geometric mean axis. They are wound byconductors which are connected in series to the main armature winding.

The polarity of the interpoles is the same of the next approaching pole.

The chief function of the interpoles is to compensate the reactanceemf and to minimize sparking across the brushes.

Armature Reaction in DC Machines

Armature Reaction refers to a phenomenon in DC machines where the magnetic field of the poles is distorted by the magnetic field produced by the armature current. 

In a generator operating on no load, the armature current is zero and the only field inside the machine is that produced by the poles.  When this generator is connected to a load, current starts to flow in the armature windings.  This current causes a magnetic field around the armature conductors. 

This magnetic field interacts with the magnetic field caused by the generator poles.  The effect of this magnetic field is both demagnetising and cross-magnetising as reduces the net effect of the main field on the armature conductors while distorting the magnetic field at the same time. 

Effects of the armature reaction. 

The principal effect of armature reaction is related to commutation.  The brushes on a dc machine are placed in the neutral plane.  The neutral plane refers to the plane where the armature brushes move in parallel to the magnetic field of the poles.  At this point the emf of the armature conductor is zero.  This facilitates easy commutation across the commutator segments. 

Due to armature reaction, the natural direction of the magnetic field of the poles is distorted.  Thus the neutral plane is also altered.  Therefore, the armature conductors are not at zero potential when they come in contact with the brushes.  This leads to sparking across the brushes and loss of power.

Overcoming Armature reaction.

There are two methods of addressing the effects caused by armature reaction.  One method is through the use of compensating winding in the field poles. Another method is the use of interpoles between the main poles to prevent distortion of the main field.

Infrared Emitting Diodes

IREDs or InfraRed Emitting Diodes are a type of LEDs which emit light in the infrared range of the electromagnetic spectrum. IREDs are widely used in process control, optical switching circuits and Logic circuits.

Like LEDs, Infrared Emitting Diodes are PN junctions which emit light when connected in a forward bias.  Under forward bias conditions, the carriers are given energy to cross the depletion layer.  On crossing the depletion layer, the carriers, both holes and electrons recombine.  Energy is released in the process of recombination in the form of a photon.

One common application where Infrared Emitting Diodes are used is the TV remote

Infrared Emitting Diodes are made of materials such as Gallium Arsenide (GaAs) and Gallium Aluminium Arsenide (GaAlAs). 

IREDs emit radiation at wavelengths such as 880nm and 940 nm (nanometres).  Thus, they are ideal for switching on phototransistors which are sensitive to electromagnetic radiation at such wavelengths.

Gravity Powered Lights

A British Company, deciwatt.org has developed a simple, low cost light which can be used by remote communities isolated from the main electricity grid.  This simple low cost light source can be an alternative to the kerosene lamp which is polluting and expensive as it consumes kerosene which consumes a significant part of the family's income. Kerosene has also been linked to accidents.

The Gravity lamp is powered by a dynamo which is drive by a weight (a bag of sand) which is raised to a certain level.  As the weight gradually descends, it drives the dynamo which powers the LED.

This gravity powered light can last for half an hour by when the weight comes to its lowest position.  After that, it has to be lifted to its higher position which takes about 3 seconds.  It then starts again.  The power from this dynamo can be used to power LEDs for illumination, to charge batteries or to power a radio.

This lamp would ideal for households in remote communities.

The company says that with increased research and production, the efficiency and cost would improve further..

Visit www.deciwatt.org

Magnetic Pick-up sensors

Magnetic pickup sensors are used to measure the speed of rotating objects such as shafts, flywheels, etc.  These sensors generate a sinusoidal waveform with a variable frequency.  The frequency of the signal depends on the speed of the rotating object. 

The magnetic pickup sensor works by the changing magnetic field when a metallic gear passes through it. The sensor consists of a permanent magnet and a coil mounted on the same axis.  Under static condition, the flux from the magnet flows from the north pole to the south pole.  The flux passes through the coil.  However, no voltage is produced as there is no relative motion between the magnet and the coil as they are fixed.  When the tooth of a gear to be measured passes through the sensor.

The magnetic flux lines are distorted as the tooth comes in front of the sensor. And when the tooth passes, the flux lines return to their original position.  They change again when the next tooth comes in front of the sensor.  This change in the flux induces a voltage in the coil placed in the sensor.  The frequency of this emf is dependent on the speed of the gear teeth. 

Thus by measuring the frequency, the speed of the rotating object is measured.   Magnetic pickups can