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