Universal Motors

Generally, the electric motors are opearted either in DC Power or AC Power. But for some specific applications , it is desirable to introduce a motor that operates on either ac or dc supply. The word ‘Universal’ signifies that something which is compaible with versatile inputs. We have build small series motors upto ½ KW rating which operates on single phase ac supply as well as on dc . Such motors are called universal motors . An universal motor is a specifically designed series wound motor, that operates at approximately the same speed and output on either ac or dc voltage. In case of universal motor, the speed of rotation is slightly lesser when opearting in AC. Becuase, the reactance voltage drop is present on ac but not on dc. So, the motor speed is somewhat lower for same load in ac operation than dc . This takes place especially at high loads . Most universal motors are designed to operate at speeds exceeding 3500 rpm

Construction Of Universal Motor

Basically , an universal motor is similar to a dc series motor in construction. However it is constructed with few series field turns, laminated armature and field circuits, low reluctance magnetic path, increased armature conductors and commutator segments. The frames of universal motor are usually made up of alumunium , rolled steel and cast iron. The commutation on ac is much poorer than on dc , due to current induced in the short circuited armature coils .If we use wide brushes then the short circuited current is excessive and motor starting torque is reduced . Brushes used are high resistance carbon ones so as to aid commutation . Compensating windings which are used in most large rating motors to improve commutation are not used in universal motors . In this motor armature current is quite small which cannot cause any commutation problems .

Working principle of universal motor

Now let us discuss the operation of this kind of motor in brief . In series circuit, same amount of current flows throgh all components. Similarly in a series wound motor, the same current flows through field windings and armature winding both. In an universal motor, both windings connected in series with each other. When the motor is supplied from ac or dc supply, magnetic fields are developed around the armature winding and field winding. They  reacts on each other to produce an unidirectional torque forcefully. In some other words, the interaction in between series magnetic field and armature field causes to develop a torque and this torque leads to rotate the shaft. However a series motor which is specifically designed for dc operation suffers from following drawbacks when it is used on single phase ac supply :-

• Its efficiency is low due to hysteresis and eddy current losses .
• The p.f is low due to large reactance of the field and armature windings .
• The sparking at the brushes is excessive.

Characteristics Of Universal Motor

The torque – speed ( τ – N) characteristics of an universal motor is quite similar to that of a series wound dc motor . It has high starting torque at low speed and low starting torque at high speed. In small series motors losses are large at no-load to restrict the speed to a definite value. A centrifugal switch is placed on the motor shaft. The tension of the springs of the switch is adjusted so that the switch opens at a pre determined speed . An external resistor R is placed in series with armature to reduce the speed. When speed drops due to increase of load ,the switch contacts close thereby shorting the resistor R thus raising the speed . Universal motors are high speed , small size motors as compared to other motors of same output . Here the full load power factor is high (0.9).

Speed control of universal motor

Speed control of universal motor is best obtained by solid-state devices. Since the speed of these motors is not limited by the supply frequency and may be as high as 20,000 rpm , they are most suitable for applications requiring high speeds . The factors that determine the speed for any dc motor are the same as those for ac series or universal motors i.e flux and generated voltage . Generated voltage change is rarely employed in speed control method. Instead line voltage is varied .This has been accomplished by means of tapped resistor , rheostat in series with the line.

Another method is by using a tapped field , thereby reducing the flux and hence raising the speed . This can be achieved by any one of the methods that follow :

• By using field poles wound in various sections with wires of different size and bringing out the taps from each section .

• By using tapped nichrome wires coils wound over a single field pole. In this method torque decreases with increase in speed .

Applications Of Universal Motor

Universal motors finds its applications in various devices . These are :

• The very small power output rating universal motors, which usually does not exceed 5 to10 watts are employed in equipments such as sewing machines , fans , portable hand tools, hair dryers , motion picture projectors and electric shavers.

• The higher rating (5-500 W) universal motor are used in vacuum cleaners, food mixers, blenders, cameras and calculating machines .

• This type of machine is used in table fans , polishers , portable drills and other kitchen appliances .a

Corona Effects

The presence of corona can reduce the reliability of a system by degrading insulation.
While corona is a low energy process, over long periods of time, it can substantially
degrade insulators, causing a system to fail due to dielectric breakdown. The effects of
corona are cumulative and permanent, and failure can occur without warning. Corona
causes:

• Light
• Ultraviolet radiation
• Sound (hissing, or cracking as caused by explosive gas expansions)
• Ozone
• Nitric and various other acids
• Salts, sometimes seen as white powder deposits
• Other chemicals, depending on the insulator material
• Mechanical erosion of surfaces by ion bombardment
• Heat (although generally very little, and primarily in the insulator)
• Carbon deposits, thereby creating a path for severe arcing
• It also causes degradation of insulators.

crona is accomplained by blue light & hizzing sound which leads to losses in it

Prevention of crona

Corona can be avoided by minimizing the voltage stress and electric field gradient. This is
accomplished by using utilizing good high voltage design practices, i.e., maximizing the
distance between conductors that have large voltage differentials, using conductors with Page 4
large radii, and avoiding parts that have sharp points or sharp edges. Corona inception
voltage can sometimes be increased by using a surface treatment, such as a semiconductor
layer, high voltage putty or corona dope.

Also, use a good, homogeneous insulator. Void free solids, such as properly prepared
silicone and epoxy potting materials work well. If you are limited to using air as your
insulator, then you are left with geometry as the critical parameter.

Finally, ensure that steps are taken to reduce or eliminate unwanted voltage transients,
which can cause corona to start.

Crona

 A corona discharge is an electrical discharge brought on by the ionization of a fluid surrounding a conductor that is electrically energized. The discharge will occur when the strength (potential gradient) of the electric field around the conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects. It is often seen as a bluish (or other color) glow in the air 

For corona effect to occur effectively, two factors here are of prime importance as mentioned below:-

1) Alternating potential difference must be supplied across the line.
2) The spacing of the conductors, must be large enough compared to the line diameter.

Corona Effect in Transmission Line

When an alternating current is made to flow across two conductors of the transmission line whose spacing is large compared to their diameters, then air surrounding the conductors (composed of ions) is subjected to di-electric stress. At low values of supply end voltage, nothing really occurs as the stress is too less to ionize the air outside. But when the potential difference is made to increase beyond some threshold value of around 30 kV known as the critical disruptive voltage, then the field strength increases and then the air surrounding it experiences stress high enough to be dissociated into ions making the atmosphere conducting. This results in electric discharge around the conductors due to the flow of these ions, giving rise to a faint luminescent glow, along with the hissing sound accompanied by the liberation of ozone, which is readily identified due to its characteristic odor. This phenomena of electrical discharge occurring in transmission line for high values of voltage is known as the corona effect in power system. If the voltage across the lines is still increased the glow becomes more and more intense along with hissing noise, inducing very high power loss into the system which must be accounted for

Protection relays

A relay is an electrically operated switch.A relay is automatic device which senses an abnormal condition of electrical circuit and closes its contacts. These contacts in turns close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped for disconnecting the faulty portion of the electrical circuit from rest of the healthy circuit.

Pickup level of actuating signal: The value of actuating quantity (voltage or current) which is on threshold above which the relay initiates to be operated.
If the value of actuating quantity is increased, the electro magnetic effect of the relay coil is increased and above a certain level of actuating quantity the moving mechanism of the relay just starts to move.

Terms related to relay:-

Reset level: The value of electric current or voltage below which a relay opens its contacts and comes in original position.

Operating time of relay -Just after exceeding pickup level of actuating quantity the moving mechanism (for example rotating disc) of relay starts moving and it ultimately close the relay contacts at the end of its journey. The time which elapses between the instant when actuating quantity exceeds the pickup value to the instant when the relay contacts close.

Reset time of relay – The time which elapses between the instant when the actuating quantity becomes less than the reset value to the instant when the relay contacts returns to its normal position.

Reach of relay – A distance relay operates whenever the distance seen by the relay is less than the pre-specified impedance. The actuating impedance in the relay is the function of distance in a distance protection relay. This impedance or corresponding distance is called reach of the relay.

SL	LINES TO BE PROTECTED	RELAYS TO BE USED
1	400 KV Transmission Line	Main-I: Non switched or Numerical Distance Scheme Main-II: Non switched or Numerical Distance Scheme
2	220 KV Transmission Line	Main-I : Non switched distance scheme (Fed from Bus PTs) Main-II: Switched distance scheme (Fed from line CVTs) With a changeover facility from bus PT to line CVT and vice-versa.
3	132 KV Transmission Line	Main Protection : Switched distance scheme (fed from bus PT). Backup Protection: 3 Nos. directional IDMT O/L Relays and 1 No. Directional IDMT E/L relay.
4	33 KV lines	Non-directional IDMT 3 O/L and 1 E/L relays.
5	11 KV lines	Non-directional IDMT 2 O/L and 1 E/L relays.

Input/outputs pins,ports circuits of 8051 microcontroller

One of the main feature of micro-controller 8051 is the built in input & output ports that connects the micro-controller to the outside world. To make it commercially viable it should incorporate a many functions as were technically & economically feasible. The micro-controller has 40 pins & it’s design success depends upon the flexibility in the use of pins.

It has 24 such pins which can be used to serve one of two entirely different function yielding a total configuration of 64 pins. The function that a pin performs depends on what is physically connected to it & what software commands are used to program the pins. Both these factors are under the control of 8051 programmer.

Given this flexibility, 8051 can simply be applied as a single component I/O only or can be expanded to include memory, parallel ports & serial data communication.

Each port has D-type output latch for each pin. SFR’s (Special Function Registers)  for each port is made up of these 8 output latches, which can be addressed at the SFR address for that port.

For example:- 8 latches for port 0 are addressed at location 80 H

There are 2 different paths that read the latch & pin data which is done by using two different buffers. The upper buffer is enable when latch data is to be read & lower buffer is enable when pin data is to read. The status of each latch can be read from  latch buffer while an input buffer is connected to each pin so that pin status can be read independently.

Port 1 pins do not have alternate functions where as all other ports i.e.  0,2,3 are  programmable ports. The programmable ports have different alternate functions. The configuration between the output latch & the port pins determines the function nature of a particular port pin.