Electric motors Essay Example | Topics and Well Written Essays - 1250 words

Electric motors - Essay Example (i) DC Motor: This motor uses a DC current. An armature is placed between magnetic poles, known as field poles. Field poles remain stationary and provide the static magnetic field which runs from North Pole to South Pole. The armature is mounted on bearing and there is winding of conduction wire on it. When a DC current is made to pass through the windings on the armature, it becomes an electromagnet and the interaction between the pole magnet and the armature electromagnet forces the armature to turn until North and South poles of the two magnets align. Once this happens direction of the current in the armature windings is reversed to switch the North and South poles using a Commutator and thus the armature keeps on rotating [2]. (ii) AC Motor: It consists of a Stator and a Rotor as shown in Fig. 1 [3]. As implies by the name Stator is the stationary component. It consists of number of magnetic poles arranged in a cylindrical symmetry around the rotating component or the Rotor. As a n AC current passes through the Rotor, it experiences a torque which rotates it and this rotation is coupled to external load through a suitable drive mechanism. Fig. 1: Schematic Diagram Showing Parts of an AC Motor [3] (iii) Servo Motor: It is primarily a DC motor, but is much more refined one. It consists of a sensor to sense position of the shaft and used this as feedback to control speed of the DC motor [4]. Q1. (b) (i) Applications of Motors DC motor: It is much easier to control speed in case of DC motors and therefore, these motors are preferred for applications where speed control is very important. Computer disc drive is one very important application of DC motor. AC motor: It is inexpensive; however, the speed control is not good. One typical application of an AC motor is to drive fans where speed control is not so critical. Servo Motor: It is feedback controlled system and hence it is used for applications which need precise position and speed control. One such applicati on is drive for CNC stage, where control of position and speed is very critical. Q1. (b) (ii) Speed Control DC Motor: In a DC motor the back electromagnetic force and torque is given by the following equations. Back electromagnetic force: E = K?N Torque: T = K?Ia Where: E = electromagnetic force developed at armature terminal (volt) ? = field flux which is directly proportional to field current N = speed in RPM (revolutions per minute) T = electromagnetic torque Ia = armature current K = an equation constan Therefore, speed of DC motors can be controlled by controlling either armature voltage or field current. AC Motor: Control of speed is difficult for AC motors. Speed of a synchronous AC motor is given by the following equation [5]. Ns = 120 f / P Where: f = frequency of the supply frequency P= number of poles It can however, be controlled by using variable frequency drive, because speed of an AC motor can be controlled by controlling frequency of AC supply only. Servo Motor: It i s a DC motor only so speed can be controlled in the same manner as in case of a DC motor. It uses a feedback system to control its speed. Q1. (b) (iii) Two Limitations DC Motor: Power is limited due to difficulty in commutation at high size. This limits application of DC motors for high power applications. Besides, there can be sparks from the commutator especially at higher powers. Therefore, a DC motor cannot be used in hazardous and fire prone areas. AC Motor: Speed control is more difficult. Cannot be