Single-Phase Motors (part 4)

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Repulsion-Start Induction-Run Motors

++++ Axial commutator.

++++ Short-circuiting ring for brush-riding-type repulsion-start induction-run motor. Shell End plate Spring Segments

++++A radial commutator is used with the brush-lifting-type motor. Centrifugal weight Radial commutator Push rod Wire slots

++++ Brush-lifting-type repulsion-start induction-run motor. Centrifugal weights; Short-circuiting necklace; Commutator; Spring barrel; Brushes; Brush holder; Push rods.



The repulsion-start induction-run motor starts as a repulsion motor but runs like a squirrel-cage motor. There are two types of repulsion-start induction-run motors:

1. The brush-riding type

2. The brush-lifting type

The brush-riding type uses an axial commutator. The brushes ride against the commutator segments at all times when the motor is in operation. After the motor has accelerated to approximately 75% of its full-load speed, centrifugal force causes copper segments of a short-circuiting ring to overcome the force of a spring. The segments sling out and make contact with the segments of the commutator. This effectively short circuits all the commutator segments together, and the motor operates in the same manner as a squirrel-cage motor.

The brush-lifting-type motor uses a radial commutator. Weights are mounted at the front of the armature. When the motor reaches about 75% of full speed, these weights swing outward due to centrifugal force and cause two push rods to act against a spring barrel and short-circuiting necklace. The weights overcome the force of the spring and cause the entire spring barrel and brush holder assembly to move toward the back of the motor. The motor is so designed that the short-circuiting necklace will short-circuit the commutator bars before the brushes lift off the surface of the radial commutator. The motor will now operate as a squirrel-cage induction motor. The brush-lifting motor has several advantages over the brush-riding motor. Because the brushes lift away from the commutator surface during operation, wear on both the commutator and brushes is greatly reduced. Also, the motor does not have to overcome the friction of the brushes riding against the commutator surface during operation. As a result, the brush-lifting motor is quieter in operation.

Squirrel-cage winding; Slots for armature winding; T1, T2

++++ Schematic symbol for a repulsion motor.

++++ Repulsion-induction motors contain both armature and squirrel-cage windings.

Repulsion-Induction Motors

The repulsion-induction motor is basically the same as the repulsion motor except that a set of squirrel-cage windings are added to the armature. This type of motor contains no centrifugal mechanism or short-circuiting device.

The brushes ride against the commutator at all times. The repulsion-induction motor has very high starting torque because it starts as a repulsion motor. The squirrel-cage winding, however, gives it much better speed characteristics than a standard repulsion motor. This motor has very good speed regulation between no load and full load. Its running characteristics are similar to a DC compound motor. The schematic symbol for a repulsion motor is shown.

Single-Phase Synchronous Motors

Single-phase synchronous motors are small and develop only fractional horsepower. They operate on the principle of a rotating magnetic field developed by a shaded-pole stator. Although they will operate at synchronous speed, they don’t require DC excitation. They are used in applications where constant speed is required such as clock motors, timers, and recording instruments. They also are used as the driving force for small fans because they are small and inexpensive to manufacture. There are two basic types of synchro nous motor: the Warren, or General Electric motor, and the Holtz motor. These motors are also referred to as hysteresis motors.

++++ A Warren electric motor.

Warren Motors

The Warren motor is constructed with a laminated stator core and a single coil. The coil is generally wound for 120-VAC operation. The core contains two poles, which are divided into two sections each. One half of each pole piece contains a shading coil to produce a rotating magnetic field. Because the stator is divided into two poles, the synchronous field speed is 3600 rpm when connected to 60 hertz.

The difference between the Warren and Holtz motor is the type of rotor used. The rotor of the Warren motor is constructed by stacking hardened steel laminations onto the rotor shaft. These disks have high hysteresis loss. The laminations form two crossbars for the rotor. When power is connected to the motor, the rotating magnetic field induces a voltage into the rotor and a strong starting torque is developed causing the rotor to accelerate to near- synchronous speed. Once the motor has accelerated to near-synchronous speed, the flux of the rotating magnetic field follows the path of minimum reluctance (magnetic resistance) through the two crossbars. This causes the rotor to lock in step with the rotating magnetic field, and the motor operates at 3600 rpm. These motors are often used with small gear-trains to reduce the speed to the desired level.

Holtz Motors:

++++ A Holtz motor. Coil -- Salient poles Squirrel-cage winding

The Holtz motor uses a different type of rotor. This rotor is cut in such a manner that six slots are formed. These slots form six salient (projecting or jutting) poles for the rotor. A squirrel-cage winding is constructed by inserting a metal bar at the bottom of each slot. When power is connected to the motor, the squirrel-cage winding provides the torque necessary to start the rotor turning. When the rotor approaches synchronous speed, the salient poles lock in step with the field poles each half cycle. This produces a rotor speed of 1200 rpm (one-third of synchronous speed) for the motor.

Stepping Motors

Stepping motors are devices that convert electric impulses into mechanical movement. Stepping motors differ from other types of DC or AC motors in that their output shaft moves through a specific angular rotation each time the motor receives a pulse. The stepping motor allows a load to be controlled as to speed, distance, or position. These motors are very accurate in their control performance. There is generally less than 5% error per angle of rotation, and this error is not cumulative regardless of the number of rotations. Stepping motors are operated on DC power but can be used as a two-phase synchronous motor when connected to AC power.

++++The rotor could turn in either direction.

++++ The direction of rotation is known.

++++ The magnet aligns with the average magnetic pole.

Average north pole -- Average south pole

Theory of Operation:

Stepping motors operate on the theory that like magnetic poles repel and unlike magnetic poles attract. In this illustration, the rotor is a permanent magnet and the stator windings consist of two electromagnets. If current flows through the winding of stator pole A in such a direction that it creates a north magnetic pole and through B in such a direction that it creates a south magnetic pole, it’s impossible to determine the direction of rotation. In this condition, the rotor could turn in either direction.

Now consider the circuit shown: the motor contains four stator poles instead of two. The direction of current flow through stator pole A is still in such a direction as to produce a north magnetic field, and the current flow through pole B produces a south magnetic field. The current flow through stator pole C, however, produces a south magnetic field and the current flow through pole D produces a north magnetic field. In this illustration, there is no doubt as to the direction or angle of rotation. In this example, the rotor shaft turns 90 dgr. in a counterclockwise direction. In this example, the current flow through Poles A and C is in such a direction as to form a north magnetic pole, and the direction of current flow through Poles B and D forms south magnetic poles. In this illustration, the permanent magnetic rotor has rotated to a position between the actual pole pieces.

To allow for better stepping resolution, most stepping motors have eight stator poles, and the pole pieces and rotor have teeth machined into them. In actual practice, the number of teeth machined in the stator and rotor determines the angular rotation achieved each time the motor is stepped. The stator-rotor tooth configuration produces an angular rotation of 1.8 degree per step.

++++ Construction of a stepping motor.

++++ A standard three-lead motor.

Windings:

There are different methods of winding stepping motors. A standard three lead motor is shown. The common terminal of the two windings is connected to ground of an above- and below-ground power supply.

Terminal 1 is connected to the common of a single-pole double-throw switch (Switch 1), and Terminal 3 is connected to the common of another single-pole double-throw switch (Switch 2). One of the stationary contacts of each switch is connected to the positive, or above-ground, voltage, and the other stationary contact is connected to the negative, or below-ground, voltage. The polarity of each winding is determined by the position setting of its control switch.

Stepping motors can also be wound bifilar. The term bifilar means that two windings are wound together. This is similar to a transformer winding with a center-tap lead. Bifilar stepping motors have twice as many windings as the three-lead type, which makes it necessary to use smaller wire in the windings. This results in higher wire resistance in the winding, producing a better inductive-resistive (LR) time constant for the bifilar-wound motor.

++++Bifilar-wound stepping motor.

The increased LR time constant results in better motor performance. The use of a bifilar stepping motor also simplifies the drive circuitry requirements. Notice that the bifilar motor does not require an above- and below-ground power supply. As a general rule, the power supply voltage should be about five times greater than the motor voltage. A current-limiting resistance is used in the common lead of the motor. This current-limiting resistor also helps to improve the LR time constant.

=== Four-Step Switching Sequence

++++ Eight-step switching.

Four-Step Switching (Full-Stepping)

The switching arrangement can be used for a four-step switching sequence (full-stepping). Each time one of the switches changes position, the rotor advances one-fourth of a tooth. After four steps, the rotor has turned the angular rotation of one "full" tooth. If the rotor and stator have 50 teeth, it will require 200 steps for the motor to rotate one full revolution.

This corresponds to an angular rotation of 1.8 degrees per step (360 dgr./200 steps 5 1.8 degrees per step). The chart shown illustrates the switch positions for each step.

Eight-Step Switching (Half-Stepping)

++++ the connections for an eight-step switching sequence (half-stepping). In this arrangement, the center-tap leads for Phases A and B are connected through their own separate current-limiting resistors back to the negative of the power supply. This circuit contains four separate single-pole switches instead of two switches. The advantage of this arrangement is that each step causes the motor to rotate one-eighth of a tooth instead of one fourth of a tooth. The motor now requires 400 steps to produce one revolution, which produces an angular rotation of 0.9 degrees per step. This results in better stepping resolution and greater speed capability. The chart illustrates the switch position for each step. A stepping motor is shown.

===2 Eight-Step Switching Sequence: Step, Switch 1, Switch 2, Switch 3, Switch 4

++++ Stepping motor.

++++ Cutaway of a stepping motor.

++++Phase-shift circuit converts single-phase into two-phase. Forward Off Reverse AC input

AC Operation:

Stepping motors can be operated on AC voltage. In this mode of operation, they become two-phase, AC, synchronous, constant-speed motors and are classified as permanent magnet induction motors. Refer to a cutaway of a step ping motor. Notice that this motor has no brushes, sliprings, commutator, gears, or belts. Bearings maintain a constant air gap between the permanent magnet rotor and the stator windings. A typical eight stator-pole stepping motor will have a synchronous speed of 72 rpm when connected to a 60-hertz, two-phase AC powerline.

A resistive-capacitive network can be used to provide the 90 dgr. phase shift needed to change single-phase AC into two-phase AC. A simple forward off-reverse switch can be added to provide directional control. A sample circuit of this type is shown. The correct values of resistance and capacitance are necessary for proper operation. Incorrect values can result in random direction of rotation when the motor is started, change of direction when the load is varied, erratic and unstable operation, and failure to start.

The correct values of resistance and capacitance will be different with different stepping motors. The manufacturer's recommendations should be followed for the particular type of stepping motor used.

Stepping Motor Characteristics:

When stepping motors are used as two-phase synchronous motors, they can start, stop, or reverse direction of rotation virtually instantly. The motor will start within about 1-1/2 cycles of the applied voltage and will stop within 5 to 25 milliseconds. The motor can maintain a stalled condition without harm to the motor. Because the rotor is a permanent magnet, there is no induced current in the rotor. There is no high inrush of current when the motor is started. The starting and running currents are the same. This simplifies the power requirements of the circuit used to supply the motor.

Due to the permanent magnetic structure of the rotor, the motor does pro vide holding torque when turned off. If more holding torque is needed, DC voltage can be applied to one or both windings when the motor is turned off. An example circuit of this type is shown. If DC is applied to one winding, the holding torque will be approximately 20% greater than the rated torque of the motor. If DC is applied to both windings, the holding torque will be about 1 to 1/2 times greater than the rated torque.

++++ Applying DC voltage to increase holding torque.

Phase A Phase B Forward Off Reverse R C BR AC input + BR

++++ Armature and brushes of a universal motor.

++++ The compensating winding is connected in series with the series field winding. Armature Series field Compensating winding

++++ Conductive compensation. Armature Series field; Compensating winding

++++ Inductive compensation. Armature; Series field; Compensating winding

 

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