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Editor's Notes

• #2: Alternating current (AC) motors use an electrical current, which reverses its direction at regular intervals. An AC motor has two basic electrical parts: a "stator" and a "rotor". The stator is in the stationary electrical component. The rotor is the rotating electrical component, which in turn rotates the motor shaft. The main advantage of DC motors over AC motors is that speed is more difficult to control for AC motors. To compensate for this, AC motors can be equipped with variable frequency drives but the improved speed control comes together with a reduced power quality. There are two types of AC motors: synchronous (see figure) and induction. The main difference between the synchronous motor and the induction motor is that the rotor of the synchronous motor travels at the same speed as the rotating magnetic field.
• #3: Alternating current (AC) motors use an electrical current, which reverses its direction at regular intervals. An AC motor has two basic electrical parts: a "stator" and a "rotor". The stator is in the stationary electrical component. The rotor is the rotating electrical component, which in turn rotates the motor shaft. The main advantage of DC motors over AC motors is that speed is more difficult to control for AC motors. To compensate for this, AC motors can be equipped with variable frequency drives but the improved speed control comes together with a reduced power quality. There are two types of AC motors: synchronous (see figure) and induction. The main difference between the synchronous motor and the induction motor is that the rotor of the synchronous motor travels at the same speed as the rotating magnetic field.
• #6: Induction motors are the most common motors used for various equipments in industry. Their popularity is due to their simple design, they are inexpensive (half or less of the cost of a DC motor) High power to weight ratio (about twice that of a DC motor) easy to maintain can be directly connected to an AC power source
• #7: Induction motors work as follows: Electricity is supplied to the stator, which generates a magnetic field. This magnetic field moves at synchronous speed around the rotor, which in turn induces a current in the rotor. The rotor current produces a second magnetic field, which tries to oppose the stator magnetic field, and this causes the rotor to rotate.
• #8: Induction motors can be classified into two main groups: single-phase and three-phase induction motors Single-phase induction motors. These only have one stator winding, operate with a single-phase power supply, have a squirrel cage rotor, and require a device to get the motor started. This is by far the most common type of motor used in household appliances, such as fans, washing machines and clothes dryers, and for applications for up to 3 to 4 horsepower.
• #17: A synchronous motor is an AC motor, which runs at constant speed fixed by frequency of the system. It requires direct current (DC) for excitation and has low starting torque, and synchronous motors are therefore suited for applications that start with a low load, such as air compressors, frequency changes and motor generators. Synchronous motors are able to improve the power factor of a system, which is why they are often used in systems that use a lot of electricity. This motor rotates at a synchronous speed, which is given by the following equation Ns = 120 f / P Where: f = frequency of the supply frequency P= number of poles
• #19: A synchronous motor is an AC motor, which runs at constant speed fixed by frequency of the system. It requires direct current (DC) for excitation and has low starting torque, and synchronous motors are therefore suited for applications that start with a low load, such as air compressors, frequency changes and motor generators. Synchronous motors are able to improve the power factor of a system, which is why they are often used in systems that use a lot of electricity. This motor rotates at a synchronous speed, which is given by the following equation Ns = 120 f / P Where: f = frequency of the supply frequency P= number of poles
• #21: Arrow in stator depicts magnetic field of windings according to the left hand rule. X is out of plane; dot is into plane.
• #22: Induction motors can be classified into two main groups: Single-phase induction motors. These only have one stator winding, operate with a single-phase power supply, have a squirrel cage rotor, and require a device to get the motor started. This is by far the most common type of motor used in household appliances, such as fans, washing machines and clothes dryers, and for applications for up to 3 to 4 horsepower. Three-phase induction motors. The rotating magnetic field is produced by the balanced three-phase supply. These motors have high power capabilities, can have squirrel cage or wound rotors (although 90% have a squirrel cage rotor), and are self-starting. It is estimated that about 70% of motors in industry are of this type, are used in, for example, pumps, compressors, conveyor belts, heavy-duty electrical networks, and grinders. They are available in 1/3 to hundreds of horsepower ratings.
• #24: An induction motor has two main electrical components as shown in the figure Rotor. Induction motors use two types of rotors: A squirrel-cage rotor consists of thick conducting bars embedded in parallel slots. These bars are short-circuited at both ends by means of short-circuiting rings. A wound rotor has a three-phase, double-layer, distributed winding. It is wound for as many poles as the stator. The three phases are wired internally and the other ends are connected to slip-rings mounted on a shaft with brushes resting on them. Stator. The stator is made up of a number of stampings with slots to carry three-phase windings. It is wound for a definite number of poles. The windings are geometrically spaced 120 degrees apart
• #25: From wikipedia: http://en.wikipedia.org/wiki/File:3phase-rmf-noadd-60f-airopt.gif WARNING Animation shows flaws when GIF is resized. The reason for this behavior is unclear, but it is common both to Mozilla and Konqueror on Linux. Any advice welcome (mtodorov3_69@yahoo.com). Note: IE 7.0 does not seem to share this deficiency. (The error seems to affect Chrome on Windows XP too) Model of 3 phase synchronous electric motor with animated vector adding of stator coil magnetic fields. Stator phases R, S and T have sine current shifted by 120 degrees between each. Magnetic field is proportional to current in linear approximation. Magnetic field vectors of the phases add up on the axis of the motor as vectors, combining into single rotating vector according to parallelogram law, which is clearly visible. Rotor has a constant current and hence constant magnetic field, which shows the inclination to follow rotating magnetic field of the stator coils, causing rotor to rotate. This particular image shows phase vectors change in time, the other one sums them using parallelogram theorem.
• #31: In practice however, the motor never runs at synchronous speed but at a lower “base speed”. The difference between these two speeds is the “slip”, which increases with higher loads. Slip only occurs in all induction motors. To avoid slip, a slip ring can be installed, and these motors are called “slip ring motors”. The following equation can be used to calculate the percentage slip % Slip = Ns – Nb x 100 Ns Where: Ns = synchronous speed in RPM Nb = base speed in RPM
• #33: The figure shows the typical torque-speed curve of a three-phase AC induction motor with a fixed current. When the motor: (Click once) Starts there is a high starting current and low torque (“pull-up torque”). (Click once) Reaches 80% of the full speed, the torque is at its highest level (“pull-out torque”) and the current begins to drop. (Click once) Is at full speed, or synchronous speed, the torque and stator current drop to zero.
• #43: More questions: How does the rotor of a dc motor maintain electrical contact with its commutation circuit? Name two motors that do not need brushes for their rotor windings.