MOTOR GENERATOR
CAT NO. PH1241

Instruction Manual

Description

The Motor Generator can operate as a motor, using electrical energy to perform mechanical work, or as a generator, converting mechanical work to electrical energy. Depending on the parts used, the electrical energy may be in the form of alternating (AC) or direct.

• Nomenclature and function of parts of both motor and generator
• Field magnet
• Armature
• Commentator and slip rings
• Electromagnetic field

For Better Working

• Experiment with brush shape, pressure and angle to obtain best performance.
• Remove unused brushes to reduce friction and wear.
• Lubricate bearings and sliding contact areas of brushes, commutator and slip rings with light oil periodically.

Basic DC Motor

Mount the round magnet pole pieces to the base by simply sliding through slits in the circular base. Connect your power supply across the pair of brushes that touch the commutator, as shown in Picture 1.

How the Basic Motor Operates

Current flows down one brush into the commutator segment and through the armature windings. It then flows back by means of the second commutator segment and brush to the battery.

As the current passes through the windings of the wire coil of the armature, it magnetizes the soft iron in the armature.

One end of the armature becomes a north pole, the other end south. Because like poles repel and unlike poses attract each other, the north pole of the armature will be pulled toward the south pole of the field magnet. The armature and commutator rotate.

Once the armature poles are close to the field pole, the brushes lie across the gaps between commutator segments. The armature is then short circuited briefly. The armature freewheels until the short circuit is broken. Each brush then touches the commutator segment opposite to the one on which it began. Current again flows through armature windings but in the opposite direction. The polarity of the armature is reversed; the north pole has become a south and is pushed toward the other (north) pole. Every half revolution, the poles become aligned, the polarity is reversed, and the armature moves.

If connections to the battery are reversed, current direction and armature polarity are reversed. The motor then runs backward.

This motor will not run on alternating current. When AC is used, the armature merely vibrates backward and forward in sympathy with the current reversals. Compare the rotation direction of your motor. Take into account the polarity of the battery, the polarity of the magnet, and the winding direction on the armature.

Basic DC Generator

To show DC generator disconnect the connections of Picture 1.

Turn the armature by external means-spin by hand or devise a pull string of fine cord or fishing line onto unused slip rings. The iron core of the armature is magnetized by the field magnet. The closer each armature end is to a field pole, the more strongly it takes on the polarity opposite to that of the field pole. As the armature rotates, it is magnetized first in one sense, then, half a revolution later, in the opposite sense.

According to the law of induction formulated by Michael Faraday, this change in magnetization induces an electromotive force (EMF) in the windings around the armature. The faster and larger the change, the larger the induced EMF, which is experienced as a voltage between the ends of the windings. Connecting the brushes to an external circuit allows current to flow through both that circuit and the armature winding.

Lenz’s Law states that the direction of the induced EMF is such that it tries to cause a current that would oppose the change causing it. As the iron is magnetized in one direction, the current generated is such that, as it flows in the armature windings, it tries to magnetize the iron in the opposite direction. Therefore the EMF must alternate in direction as the armature is magnetized in alternate senses.

However, because the commutator reverses connections to the brushes every half revolution, it can correct for the alternations of the EMF and produce a unidirectional voltage at the brush terminals. Connect a galvanometer to the brushes to observe a series of pulses in the same direction.

Basic AC Generator

Alternating current -AC

The motor generator has a second set of brushes sliding on two continuous rings, known as slip rings. Each ring is connected to one end of the armature winding. By connecting to the slip ring brushes instead of the commutator brushes, the reversing action of the commutator is bypassed, and the generator provides AC, as can be verified by connecting the galvanometer.

Because the same machine acts as both motor and generator, the two functions are not completely separable. The generator will act partially as a motor and vice versa.

Take Lenz’s Law as applied to the DC generator. The current generated tends to oppose that which caused it. When an armature end becomes a North pole as it approaches the field’s south pole, the induced current suppresses development of the North pole. If that current were driving the machine as a motor, it would produce a South pole to cause the motor to rotate in the opposite direction.

In the DC generator, the induced current will produce a torque that opposes whatever drives the generator. The higher the current drawn from the generator, the more difficult it is to turn. This is to be expected from conservation of energy principles since the extra electrical energy must come from mechanical work done in the generator; otherwise, Lenz’s Law would not be true. Verify this by connecting the brushes together. Note that the generator is harder to spin.

On the other hand, consider the basic motor. As it increases speed after being switched on, it behaves like a generator, inducing a “back EMF” – a voltage in opposition to the battery voltage. Once this voltage, plus the voltage dropped across the resistance in the wires, equals the applied battery voltage, the speed settles to a constant. If the mechanical load on the motor is increased, the motor slows, and the induced voltage drops. The battery is then able to supply more current and the motor more torque. This can be demonstrated by slowing the motor down with your fingers.

Because of the symmetry between motor and generator, you may wonder why a motor version of the AC generator using the slip ring brushes is not presented above. Although it is a possibility, in practice it requires precise timing of the current reversals in relation to the armature position under varying load conditions. This type of motor, called a synchronous motor requires that speed of rotation be synchronized with the speed of reversal of the AC source to supply a continuous torque in the same direction as that of the rotation. For this reason, the motor generator is not a workable synchronous motor.

High Power Motors

Electromagnet Fields

The development of strong rare earth magnets (such as neodymium, iron, and boron) greatly enhanced the power output and the usefulness of motors with permanent field magnets. Nonetheless, for high power applications, the permanent field magnet is best replaced by an electromagnet in which a core material such as iron is magnetized by a current-carrying coil.

Replace the 2 permanent magnets in the basic motor with the wound field coil/electromagnet pair to demonstrate two such motors – the series and the shunt motor – which differ only in their electrical connections. It should be noted that a motor designed to operate in two modes is less efficient or as powerful than one designed to operate in just one of these modes.

Slide the electromagnet attachments as far into the slit in the circular clip as possible without the armature hitting the windings. In both series and shunt motors, the voltage drop across the windings times the coil current is approximately equal. Likewise, the coil current times and the number of turns also remain the same at normal operations.

Series Motor

Connect the electromagnet attachment/field coil in series with the commutator brushes so the same current passes through the armature and the field coil. Increasing the mechanical load slows the motor and lowers the back EMF resulting in a higher current. Although this occurred in the permanent field motor, now, in the series motor, the strength of the field magnet is also increased.

The resultant torque is, therefore, automatically increased when needed. Series motors are for high torque, low-speed applications such as the starter motor for a car. They are characterized by few turns, heavy wire, and low resistance.

Shunt Motor

Connect the field magnet so that the field current is independent of the armature, having been diverted (“shunted”) through its own separate circuit. A 10-ohm variable resistor may be included to facilitate adjustment to the field that, in most shunt motors, increasing the field current causes the motor to slow down. Remember, the speed of a motor becomes steady when the induced back EMF reaches a certain level. That same level is reached at a lower speed if the field magnet has been strengthened by increasing its current. Although the speed is lower, the torque and power developed are higher, as your intuition may imply.

In this demonstration motor, you may find that the speed remains fairly constant as the field current is increased. This is because – with only two poles – the speed is determined by several factors, not just the back EMF. At any rate, the speed does not increase in proportion to the field current.

Shunt motors are characterized by many turns, small (fine) wire, and high resistance. The speed of the shunt motor varies less than that of the series motor under varying load conditions.

Reversing the battery connections reverses both armature and field currents and therefore does not change the motor direction. Motor direction can only be achieved by reversing either the brush or field connections. Both shunt and series motor will therefore run on AC but not for long periods. The alternating magnetic fields induce currents (“eddy currents”) within the iron cores, which result in overheating. Ten or twenty seconds is safe for testing the use of AC with a source of about 10 volts.

Most motors and generators are constructed from “laminations.” The field cores and armatures are made of thin layers of iron stacked together with insulation between layers. The laminations reduce the eddy currents and hence the overheating. They also reduce the power loss that core heating represents. Proper laminated construction is expensive and therefore has been omitted from this demonstration model.

U.S. Distributor :
Eisco Scientific
850 St Paul St, Suite 15, Rochester, NY 14605
Website :www.eiscolabs.com

EISCO SCIENTIFIC instructions, content and design is intellectual property of EISCO

References

• Eisco Labs

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