Tuesday, May 20, 2014

Universal Motors Operational Principle and Application in Human life

Universal motors are used in following applications:-
1.      Power tools
2.      Food-processors
3.      Mixers
4.      Vacuum cleaners
5.      Sewing machines
6.      Washing Machines
Universal Motors Operating Principle:-
Universal motor winding diagram is as shown below:-
Universal Motors winding; washing Machine Motor winding

Universal Motor winding; Washing Machine Winding

From above we can see that it is Series wound DC motor having armature and field winding in series actually It is a commutated series-wound motor. In That Motor stator winding i.e. field winding is connected either in series or in parallel with rotor winding through commutator.
In case of DC series motors torque developed direction is determined by both filed polarity and the direction of current through the armature. Now let a dc series motor be connected across a single-phase ac supply. Since the same current flows through the field winding and the armature, it follows that ac reversals from positive to negative, or from negative to positive, will simultaneously affect both the field flux polarity and the current direction through the armature. This means that the direction of the developed torque will remain positive, and rotation will continue in the same direction. Thus a series motor can run both on dc and ac.
Motor that can be used with a single-phase ac source as well as a dc source of supply voltages are called universal motor.

Speed-Torque Characteristics of Universal Motor:-

The torque – speed characteristics of an universal motor is shown below and it is very similar to that of a series wound dc motor .

As in case of DC series motors It has high starting torque at low speed and low starting torque at high speed. 
In small series DC 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 predetermined 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 . 

All DC Series can’t be used at AC Supply:-
But this can’t be concluded from above that all DC series motors can be used at AC supply There are few drawbacks for operating a DC series motor on AC are as below:-
1.      Efficiency will be low due to Hysteresis and eddy current losses.
2.      Motor power factor will become low due to the large reactance of the field and the armature winding.
3.      The sparking at the brushes will be excessive.

In order to operate DC series Motor on both AC and DC Drawbacks mentioned above can be overcome by adopting following changes:-
1.      Unlike DC Series motors the rotor of universal motor is usually made of Laminated Steel sheets in order to minimize eddy current losses
2.      There motors field winding is provided with small number of turns. The field winding pole area increased so as to reduce flux density. This reduces the iron loss and the reactive voltage drop.
3.      The number of armature conductors is increased in order to get the required torque with the low flux.
4.       In order to reduce the effect of armature reaction, thereby improving commutation and reducing armature reactance, a compensating winding is used.
These motors are available even in fraction horse power sizes and are very lower in cost and used almost in every power tools.

These motors are used in small and medium size appliances. Their construction which is very in-efficient made them a poor choice for using longer duration and longer period of time.
Induction motors have longer life and better performance then universal motors. Universal motors are cheaper then Induction motors.
Universal motor is used in Washing machines as these motors have electronic speed control Universal motor used in washing machine have to agitate the drum both forwards and in reverse direction by switching the field winding with respect to the armature. Universal motor can also be used to run up the high speeds required for the spin cycle.

 The drawbacks are:
1.      Noise
2.      Brush-wear
3.      Sparking

Maintenance of Universal Motors:-
Most universal motors are permanently lubricated and sealed by the manufacturer and require no further attention. 
But Some universal motors, however have covered lubrication ports, usually marked Oil  at the ends of the motor shaft.  This type of motor should be oiled every six months.  Do not over-lubricate.

Universal motor characteristics are a lot like that of a dc series motor.
• No-load speed can be very high But will not leads to burning of motor as in case of dc series motors.
• Speed decreases as load is applied.
• High starting torque.
• Speed can be controlled by placing resistor in series.
• Usually fractional horsepower sizes.