Tuesday, 11 November 2014

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Generally, the electric motors are operated 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 compatible with versatile inputs. We have build small series motors up to ½ KW rating which operates on single phase ac supply as well as on dc . Such motors are called universal motors . A 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 operating in AC. Because, 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 . We will explain discuss the construction of this type of motor

The universal motor is basically a series DC motor which is specially designed to operate on AC as well as on DC. A standard DC series motor has very poor characteristics when operated on AC, mainly due to two reasons:

a) The high reactance of both the armature and field windings limits AC current to a much lower value than DC current (for the same line voltage).

b) If solid steel is used for the stator frame, AC flux will produce large eddy currents in the frame with consequent heating. 
To insure satisfactory operation of the universal motor from an AC power source, some modifications are necessary. The reactance of the series field and armature windings must be reduced as much as practicable. The reactance of the series field winding can be somewhat reduced by using fewer turns of heavier wire. However, it would not be practical to eliminate the reactance voltage drop due to the series field since that would also eliminate the magnetic field. The reactance voltage drop due to the armature winding can be practically eliminated by use of a compensating winding. The compensating winding is connected in series with the armature winding (conductive compensation) and arranged such that the ampere-turns of the compensating winding oppose and neutralize the ampere-turns of the armature. To realize this compensation, the compensating winding is displaced by 90 electrical degrees from the field winding. Since the motor used in this experiment us a 4-pole motor, the mechanical displacement is 450. The compensating winding also improves commutation considerably. This is a great adventure since the field of a universal motor commutation considerably. is weakened by lowering the reactance of the series field winding. If the compensating winding is short circuited (inductive compensation), the alternating currents in the armature are induced by transformer action into the shorted compensating winding, thus, effectively cancelling the reactive armature currents.
To reduce losses due to hysteresis and eddy currents, the field structure is laminated. Few universal motors operate at the same speed on AC as on DC. Whether it runs faster on AC or DC is a matter of design. The reactance of the armature winding can be lowered by placing a compensating winding on the stator so that the fluxes oppose or "cancel" each other. This same compensating winding can be connected in series with the armature winding. In this case, the motor is said to be conductively compensated. Under these conditions, the universal motor will have similar operating characteristics whether on AC or DC power

The compensating winding may be simply shorted upon itself, so that it behaves like a short circuited secondary of a transformer (the armature winding acting as the primary). The induced AC current in the compensating winding again opposes or "bucks" the armature current and the motor is said to be inductively compensated. The reactance of the field winding can be kept low by limiting the number of turns
The starting torque of a universal motor is determined by the current that flows through the armature and field windings. Due to the inductive reactance of these windings the AC starting current will always be less than the DC starting current (with equal supply voltages). Consequently, the starting torque on AC power will be lower than the starting torque on DC power.

The compensating winding has the important role of reducing the overall reactance of the motor. However, it also has an equally important part in opposing armature reaction, thereby improving commutation. An uncompensated universal #motor will lose most of its power. Sparking at the brushes will also be markedly worse

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

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