Single-phase motors are more widely used than 3-phase motors because of two following basic reasons:
First for reasons of economy, most houses, offices and also rural areas are supplied with single-phase ac and the second factor is the economics of the motor and its branch circuit. Fixed loads requiring not more than 0.5 kW can generally be served most economically with single phase power and a single phase motor. Single-phase motors are simple in construction, reliable, easy to repair and comparatively cheaper in cost and, therefore, find wide use in fans, refrigerators, vacuum cleaners, washing machines, other kitchen equipment, tools, blowers, centrifugal pumps, small farming appliances etc.
Single-phase ac motors may be divided in three general classes namely:
(i) Induction motors
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(ii) Commutator motors and
(iii) Synchronous motors.
Induction motors are further classified as split-phase motors, shaded-pole motors and repulsion-induction motors according to the method of producing starting torque. The commutator motors are the series motors, universal (ac/dc) motors, the repulsion- induction motors, with various modifications and combinations of these types being in use.
Single-phase induction motors in very small sizes (1/400 to 1/25 kW) are used in toys, hair dryers, vending machines etc. The universal motor is widely used in portable tools, vacuum cleaners and kitchen equipment. The main drawbacks of single-phase motors are low overload capacity, poor efficiency, low pf and low output as compared to that of a 3-phase motor of the given frame size.
1. Single-Phase Induction Motors:
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A single-phase induction motor is similar to a 3-phase squirrel cage induction motor in physical appearance. The rotor of a single phase squirrel cage induction motor is essentially the same as that employed in 3-phase induction motors. There is uniform air gap between stator and rotor but no electrical connection between them (stator and rotor). Except for shaded-pole types, the stator core is also very similar. A single-phase motor can be wound for any even number of poles, two, four, and six being most common. Like three-phase machines, adjacent poles have opposite magnetic polarity and synchronous speed equation (Ns = 120f/P) also applies.
When the stator winding of a single-phase induction motor is connected to single-phase ac supply, a magnetic field is developed, whose axis is always along the axis of stator coils. With alternating current in the fixed stator coil the mmf wave is stationary in space but pulsates in magnitude and varies sinusoidally with time. Currents are induced in the rotor conductors by transformer action, these currents being in such a direction as to oppose the stator mmf.
Thus the axis of the rotor mmf wave coincides with that of the stator field, the torque angle is, therefore, zero, and no torque is developed at starting. However, if the rotor of such a motor is given a push by hand or by another means in either direction, it will pick up the speed and continue to rotate in the same direction developing operating torque. Thus a single- phase induction motor is not self-starting and needs special starting means.
Commercial single-phase induction motors employ the principle of ‘phase split’ and are, therefore, known as split- phase motors.
2. AC Series Motor:
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DC shunt or series motor rotates in the same direction regardless polarity of supply, i.e., if the line terminals be reversed, the motor continues to rotate in a direction it was rotating before the line terminals were reversed. From this it seems that any dc motor would operate satisfactorily when connected to single phase ac supply.
However, it is not true. Some modifications are necessary in a dc series motor that is to operate satisfactorily on single phase ac supply. So the construction of an ac series motor is very similar to a dc series motor except that some modifications (such as whole magnetic circuit laminated, series field with as few turns as possible, large number of armature conductors, use of high-resistance carbon brushes, numerous poles with lesser flux per pole, very short air gap etc.) are incorporated. The machine is provided with compensating winding and interpoles to improve commutation. A schematic diagram of a single-phase series motor with interpole and compensating windings is given in Fig. 1.42.
The average value of the torque on the motor shaft is given as
Where, I is the effective value of current, φmax is the peak value of the flux per pole and θ is the phase angle between phasors φ and I.
For a given value of torque T and applied voltage, the armature current is same but voltage drop in case of an ac series motor is much more than that in case of a dc series motor and so speed of an ac series motor for a given developed torque is less than that of a dc series motor, as illustrated in Fig. 1.43.
The single-phase ac series motor has practically the same operating characteristics as the dc series motor. The torque, or tractive effort, varies nearly as the square of the current and the speed varies inversely as the current. This is illustrated in Fig. 1.44.
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However, in case of an ac series motor (i) power factor is very low at starting and on overloads on account of high inductive nature of the series field and armature circuits (ii) efficiency is not as good as in a corresponding dc machine due to eddy current losses and effects of power factor and (Hi) starting torque is low due to poor power factor at starting.
For a given kilowatt rating ac series motor is 1.5-2 times in size and weight of the corresponding dc series motor. The construction cost of an ac series motor is much more than that of a dc series motor.
The speed of an ac series motor may be controlled efficiently by taps on a transformer, which is not possible in case of a dc series motor.
The torque-speed characteristic of the single phase series motor is similar to that of the dc series motor i.e., high starting torque and decrease in speed with increase in load making it to have self-relieving property from heavy excessive load, so such a machine is particularly useful for traction services.
3. Universal Motor:
A universal motor is a specifically designed series wound motor that operates at approximately the same speed and output on either dc or ac of approximately same voltage. Because of the difficulty in obtaining like performance on dc and ac at low speeds, most universal motors are designed to operate at speeds exceeding 3,500 rpm. Motors operating at speeds 8,000 to 10,000 rpm are common. Universal motor is constructed with few series field turns, laminated armature and field circuits, low-reluctance magnetic path, increased armature conductors and commutator segments and using low flux densities so as to minimize the adverse effects caused by high field reactances, eddy current and hysteresis losses.
Universal motors may be either compensated (distributed field) or uncompensated (concentrated field) type, the latter type being used for higher speeds and smaller output ratings (usually not exceeding 200 W) only.
The speed-torque characteristic of a universal motor is quite similar to that of a series wound dc motor, i.e., high starting torque and high no-load speed. Universal motors beings high speed motors, are smaller in size and lighter in weight as compared to other motors of the same output. Full-load pf is high (approximately 0.9) but it is poor at start and on overloads. The direction of rotation of any series motor can be reversed by reversing the direction of flow of current in either the field or armature circuit (but not through both). The speed of universal motor, for any given load, can be changed by changing either flux or applied voltage or both.
Very small power output rating universal motors, which usually do not exceed 5 or 10 watts are employed in such equipment as sewing machines, fans, portable hand tools, hair dryers, motion-picture projectors, and electric shavers. The higher rating (5-500 W) universal motors are used in vacuum cleaners, electric typewriters, food mixers and blenders, motion-picture projectors, cameras, adding machines, and calculating machines.
The small series motors are frequently furnished as motor parts, i.e., consisting of bare stators and rotors (with shaft) but without bearings or feet. They can then be compactly “built in” to the power using devices.
4. Repulsion Motors:
The characteristics of repulsion motors are similar to those of a series motor, i.e., high starting torque and high light-load speed. Its construction is also similar to that of a series motor except that armature is short circuited on itself instead of being connected in series with the stator. Simplified schematic diagram is shown in Fig. 1.46. A repulsion motor develops a torque in the direction in which the brushes are shifted from the field axis.
The torque developed by a repulsion motor should be maximum, theoretically, when the space angle between the pole axis and brush axis is 45°, but in practice the angle of inclination is about 15-25 electrical degrees.
The repulsion motor has better commutation than the series motor at speeds below synchronous speed and poor commutation at very high speeds. The direction of rotation of a repulsion motor can be reversed by shifting the brushes round the commutator, on the other side of the field axis. Speed control can be affected by varying the applied voltage to motor or by mounting the brushes on a rocker which can be rotated by a lever handle mounted on the motor end- shield.
The repulsion motor has high starting torque (about 3- 5 times full-load torque) and moderate starting current (about 3-4 times full-load current) but poor speed regulation. Shifting the brushes during operation gives a wide range of speed control, as high as 6:1 ratio, and yet provides a continuous variation. The upper speed is not limited by frequency. The motor is a reversing type, and the direction may be changed during rotation.
The drawbacks of repulsion motors are:
(i) speed variations with the variations in loads-dangerously high at no load
(ii) low power factor, except at high speeds,
(iii) tendency to spark at the brushes-sparking at the brushes is negligible at the rated speed which usually occurs near synchronous speed
(iv) higher cost and
(v) more attention and maintenance is required.
The repulsion motor has never enjoyed popularity. The motor is used where sturdy motor with large starting torque and adjustable speed is required. Most common use of this type of motor is in the coil winders in which the operator adjusts the speed by shifting the brushes; the motor is equipped with a special lever mechanism that shifts the brushes when a foot treadle is pressed.
The ratings in which the repulsion motors are built is limited due to commutation problems. The usual rating of the repulsion motor does not exceed 5 kW.
5. Synchronous Motors:
There are many timing-device applications where small motors having exact constant speed characteristics will be very advantageous. Very small motors with constant speed characteristics have been developed. They operate from a single phase supply. Because of their exact constant speed characteristics, they are called the single phase synchronous motors. They do not require dc source of supply for excitation. The principal applications of such single phase synchronous motors are for driving of electric clocks, phonographs, record-player turntables, tape and other timing devices.
The most commonly used types of single-phase synchronous motors are the reluctance motors and the hysteresis motors. The efficiency and torque developing ability of these motors is low. The output of most of the commercial motors available is only a few watts. It is practical to design hysteresis motors up to approximately 125 watts.
i. Reluctance Motor:
It is a split-phase induction motor with properly designed salient shaped poles. It consists of a stator carrying both the main and auxiliary windings for developing a synchronously rotating magnetic field. Rotor punching for a 4-pole reluctance type synchronous motor is shown in Fig. 1.50. Such motors have poor power factor, low efficiency and poor torque.
They cannot accelerate high inertia loads to synchronous speed. The pull-in and pull-out torques of such motors are small. Reversal of direction of rotation can be accomplished as in any single phase induction motor. Such motors are widely used for absolute constant speed applications, such as in timing devices, signaling devices, recording instruments, phonograph turntables, control apparatus etc.
ii. Hysteresis Motor:
It is a synchronous motor with uniform air gap but with no dc excitation. Its operation depends upon the effect of hysteresis. A 2-pole shaded-pole type hysteresis motor employed for driving ordinary clocks is shown in Fig. 1.51. Because of noiseless operation and ability to drive high-inertia loads, hysteresis motors are particularly well suited to drive timing devices, electric clocks, tape decks, turntables and other precision audio-equipment. Commercial motors, being 2-pole motors, run at 3,000 rpm and so for driving an electric clock and other indicating devices, gear train is connected to the motor shaft for reducing the speed. By changing number of stator poles through pole-changing connections, a set of synchronous speeds can be obtained for the motor.
iii. Permanent Magnet Synchronous Motor:
It is constructed by embedding permanent magnets in the rotor, as illustrated in Fig. 1.52. The rotor itself has a squirrel cage construction to provide starting torque. When the motor is connected to a single phase ac supply, it starts as an induction motor, attains nearly synchronous speed and locks into synchronism with the rotating stator field near synchronous speed. Such a motor is quieter in operation and has high pf and efficiency- approaching that of a polyphase motor. It is thus finding more use, even in the low-integral kW ratings (0.5-1.5 kW).
6. Stepper Motor:
The stepper motor is a form of synchronous motor which is designed to rotate through a specific number of degrees for each electrical pulse received by its control unit. Typical steps are 2, 2.5, 5, 7.5, and 15° per pulse. The stepper motor is used in digital control systems, where the motor receives open-loop commands as a train of pulses to turn a shaft or move a plate by a specific distance.
A typical application for the motor is positioning a worktable in two dimensions for automatic drilling in accordance with hole-location instructions on tape. With a stepper motor a position sensor and feedback system is normally not required to make the output member follow input instructions. Stepper motors are built to follow signals as rapid as 1,200 pulses per second and with equivalent power ratings up to several kilowatts.
Stepper motors are usually designed with a multi-pole, multiphase stator winding that is not unlike the windings of conventional machines. They typically use 3- and 4-phase windings, the number of poles being determined by the desired angular change per input pulse. The rotors are either of the variable reluctance or permanent magnet type. Stepper motors operate with an external drive logic circuit; as a train of pulses is applied to the input of the drive circuit, the circuit delivers appropriate currents to the stator windings of the motor to make the axis of the air-gap field step around in coincidence with the input pulses. Depending upon the pulse rate and the load torque, including inertia effects, the rotor follows the axis of the air-gap magnetic field by virtue of the reluctance torque and/or the permanent-magnet torque.
The elementary operation of a 4-pole stepper motor with a 2-pole, rotor is illustrated in the sequence of Fig. 1.53. The rotor assumes the angles θ = 0, 45°, 90° … as the windings are excited in the sequence Na, Na + Nb, Nb, …. The stepper motor shown in Fig. 1.53 may also be employed for 90° steps by exciting the coils singly. In the latter case, only the permanent-magnet rotor can be employed.
The characteristics of a stepper motor are frequently presented as the torque versus stepping rate of the pulses applied to the drive unit, as illustrated in Fig. 1.54. As the stepping rate is increased, the motor can provide less torque because the rotor has less time to drive the load from one position to the next as the stator-winding-current pattern is shifted.
The stepper motor is essentially a position control device and has the following advantages over a conventional machine:
1. The angular displacement can be precisely controlled without any feedback arrangement.
2. It can be readily interfaced with microprocessor/computer based controller.
Stepper motors have a wide range of applications—paper feed motors in typewriters and teleprinters, positioning of print heads, pens in X-Y graphical plotters, recording heads in computer disk drives and in positioning of worktables and tools in numerically controlled machining equipment.
The stepper motor is also employed to perform many other functions such as metering, mixing, cutting, blending, stirring etc. in many commercial, military and medical applications, usually along, with microprocessor and controlled switches.