Wednesday, January 7, 2015

Mechanical motion from electrical energy : basic motor action, torque and rotary motion and the need for commutation

Mechanical Motion from Electrical Energy
20–1 BASIC MOTOR ACTION

The term motor action was described briefly in Section 16–4. The sketch accompanying this explanation is reproduced for your convenience as Figure 20–1.

Figure 20–1 illustrates that a wire carrying electrons across a magnetic field will be pushed sideways. The current in the wire has a magnetic field of its own. The wire’s magnetic field combines with the externally applied magnetic field. The result of these combined fields causes the closely bunched, distorted lines of force to push the wire into the area that is less densely occupied by flux lines. This same idea is demonstrated, in a slightly different manner, by the sketch in Figure 20–2.

To apply this effect in an electric motor, examine the effect of an external magnetic field on a rectangular loop of wire. Assume that the wire is supplied with DC from a battery and that the external magnetic field is supplied by a permanent magnet, Figure 20–3. The sections of the loop that lie parallel to the field are not affected by the field. The side

image

of the loop marked A is pushed upward; the side marked B is pushed downward. Let us see why this happens.

Figure 20–4 represents a vertical cross-sectional view of the loop in the magnetic field shown in Figure 20–3. The heavy circles represent wires A and B in Figure 20–3. The X in the A circle represents electrons moving away from the observer (like the tail of an arrow flying away). The dot in the B circle represents electrons moving toward the observer (the point of an approaching arrow). The circular patterns around A and B represent the magnetic field of the current in these wires. (Using the left-hand rule, check the correctness of the direction arrows.)

Underneath A, the N S S magnetic field combines with the field of the wire, making a strong field under the wire. Above A, the field of the magnet and the field of the wire are in opposite directions. They cancel each other, making the weak field represented by the less-concentrated lines above A. Wire A is lifted by the strong magnetic field beneath. To account for this lifting effect, think of the N S S lines of force as both repelling each other and attempting to straighten themselves. A similar effect at B pushes wire B downward, much as a round stick is pushed down by a string, as shown in Figure 20–5.

The direction of motion of the wire can be found quickly by using a three-finger right-hand rule: With first finger, middle finger, and thumb at right angles to each other, forefinger 5 f ield, center finger 5 current, thumb 5 motion; see Figure 20–6.

image

Notice the similarity with Fleming’s left-hand rule for generators, as described in Section 18–3. The two rules may appear identical to the casual observer, but note the important difference:

The left-hand rule is used with generators, and the right-hand rule applies to motors.

This statement is, of course, based on the theory that defines current flow as a motion of electrons from negative to positive.

20–2 TORQUE AND ROTARY MOTION

Torque, also known as a moment of force, is a measure of the twisting effect that produces rotation about an axis. Torque is measured in pound-feet (or in similar units) and is calculated by multiplying the applied force by the radius of the turning circle.

image

Figure 20–7 shows examples of torque. Figure 20–7A represents a wagon wheel with a 4-foot diameter and the weight of a person (150 pounds) pressing downward on one of the spokes. Applying what we know about calculating torque,

2 ft x 150 lb = 300 lb-ft of torque

Note: Remember, radius is one-half the diameter.

Figure 20–7B shows two hands on a steering wheel with an 8-inch radius applying 5 pounds of pressure. Thus,

2⁄3 x 5 lb = 31⁄3 lb-ft of torque

The torque delivered by a motor is a more useful quantity than the single force pushing against the two vertical sides of the coil. The torque of an electric motor is

image

actually caused by the interaction of two magnetic fields, namely the main magnetic field (in our examples provided by the permanent magnet) and the magnetism created by the current flowing through the armature. Torque is a necessary quantity in the calculation of a motor’s horsepower, but we will discuss more about that later.

A Word of Caution:

Many students mistakenly confuse the term torque with work because of its similarity of calculation, the result being the mathematical product of force 3 distance.

Torque and work are not the same thing, and can exist independently of one another. To differentiate between the two terms, it is customary to call the unit of work the foot-pound and the unit of torque the pound-foot.

The lifting of one side of the loop and the pushing down of the other side creates a turning effect, or torque, on the loop of the wire. This combination of forces is the

image

torque that turns the armature of electric motors. This same method also explains the turning effect on the moving coil in a voltmeter or ammeter.

20–3 THE NEED FOR COMMUTATION

Imagine that the loop has rotated clockwise 90° from the position shown in Figure 20–3, until wires A and B are vertically aligned, as shown in Figure 20–8. Continued lifting of wire A and pushing down on wire B are useless. The loop is now perpendicular to the magnetic field and is said to be in the neutral plane.

When the loop is in the neutral plane, no torque is produced. To achieve continued rotation, the current direction in the loop must be reversed just as the loop enters the neutral plane. Since the loop is in motion, its inertia will carry it past the neutral plane position and, thus, continued rotation is assured.

This necessary reversal of the current is accomplished, for a single coil, by a two-segment commutator; see Figure 20–9. At the instant shown, electrons flow from the negative brush through A and return to the positive brush through B. As A is lifted to

image

the top of its rotation, the commutator segment that supplied electrons to A slides away from the negative brush and touches the positive brush; thus, the current in the loop is reversed. Momentum carries the loop past the vertical position. A is then pushed downward to the right, B is lifted to the left, and another half-turn of rotation continues. This description may help explain the need for commutators to reverse the current through the loop each time the loop passes through the neutral plane.

An additional function of the commutator is, of course, to carry the current from the supply line, via the brushes, into the rotating armature.