When a DC motor is adapted to be supplied from a single phase AC power source, i
ID: 2072614 • Letter: W
Question
When a DC motor is adapted to be supplied from a single phase AC power source, it is called a Universal Motor. Describe the operation and construction of a universal motor. Explain how the torque-speed characteristic differs when fed with AC as opposed to DC and discuss how speed is controlled. This is not covered in the lecture notes, so you will have to do your own research. 2 or 3 pages should be enough to provide a full answer. Presentation, referencing and writing quality will be taken into consideration.Explanation / Answer
i) Construction :- Basically a universal motor is similar to a dc series motor in construction. How ever it is constructed with few series field turns , laminated armature and field circuits, low reluctance magnetic path , ncreased armature conductors and commutator segments and using low flux densities.
This is done to minimize the adverse effects caused by high field reactance , eddy curent and hysteresis losses.
The brushed employed are made of high resistance carbon.
Universal motors may be either distributed field or concentrated field type .
The motor of the concentrated fied type is usually a salient pole machine with two poled and a winding of relatively few turns to give opposite magnetic polarity .
Distributed field universal motors are of two types :- a) single field compensated motor
b) two pole compensated motor
The frames of universal motor are usually made of aluminium , cast iron and rolled steel.
OPERATION :-
In a series wound motor the same current flows through field windings aand armature being connected in series with each other ,when motor is connected to either dc or ac supply . The magnetic filed developed by the series field winding and armature currents react with each other and evelop uni directional torque .
CHARACTERISTICS OF TORQUE SPEED:-
The torque speed characteristics of a universal motor is quite similar to series wound dc motor i.e high starting torque and high no load speed.
In a smal series motor the losses are usually large enough at no load to restrict the speed to a definite value . Where there is a danger of high no load speed a governor may be employed to keep speed with in specified limits .The governor consists of a centrifugla switch mounted on the motor shaft .
The capacitor C is placed across govenrnor to reduce sparking .
UNIVERSAL MOTORS BEING IGH SPED MOTORS ARE smaller size and liger in weight compared to others motors of same rating .
Full load power factor is high ,but it is poor at starting and overloads.
Related Questions
drjack9650@gmail.com
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.