TORQUE PRODUCTION
Torque is produced by the interaction between the axial current- carrying conductors on the rotor and the radial magnetic Flux produced by the stator. The Flux or ‘excitation’ can be furnished by permanent magnets (Figure 3.2(a)) or by means of Weld windings (Figures 3.1 and 3.2(b)).
Permanent magnet versions are available in motors with outputs from a few watts up to a few kilowatts, while wound-Weld machines begin at about 100 W and extend to the largest (MW) outputs. The advantages of the permanent magnet type are that no electrical supply is required for the Weld, and the overall size of the motor can be smaller. On the other hand, the strength of the Weld cannot be varied, so one possible option for control is ruled out.
Ferrite magnets have been used for many years. They are relatively cheap and easy to manufacture but their energy product (a measure of their eVectiveness as a source of excitation) is poor. Rare earth magnets neodymium–iron–boron or samarium–cobalt) provide much higher energy products, and yield high torque/volume ratios: they are used in high-performance servo motors, but are relatively expensive and
diYcult to manufacture and handle. Nd–Fe–B magnets have the highest energy product but can only be operated at temperatures below about 1508C, which is not suYcient for some high-performance motors.
Although the magnetic Weld is essential to the operation of the motor, we should recall that in Chapter 1 we saw that none of the mechanical output power actually comes from the Weld system. The excitation acts like a catalyst in a chemical reaction, making the energy conversion possible but not contributing to the output.
The main (power) circuit consists of a set of identical coils wound in slots on the rotor, and known as the armature. Current is fed into and out of the rotor via carbon ‘brushes’ which make sliding contact with the ‘commutator’, which consists of insulated copper segments mounted on a cylindrical former. (The term ‘brush’ stems from the early attempts to make sliding contacts using bundles of wires bound together in much the same way as the willow twigs in a witch’s broomstick. Not surprisingly these primitive brushes soon wore grooves in the commutator.)
The function of the commutator is discussed below, but it is worth stressing here that all the electrical energy which is to be converted into mechanical output has to be fed into the motor through the brushes and commutator. Given that a high-speed sliding electrical contact is involved, it is not surprising that to ensure trouble-free operation the commutator needs to be kept clean, and the brushes and their associated springs need to be regularly serviced. Brushes wear away, of course, though if properly set they can last for thousands of hours. All being well, the brush debris (in the form of graphite particles) will be carried out of harm’s way by the ventilating air: any build up of dust on the insulation of the windings of a high-voltage motor risks the danger of short circuits, while debris on the commutator itself is dangerous and can lead to disastrous ‘flashover’ faults.
The axial length of the commutator depends on the current it has to handle. Small motors usually have one brush on each side of the com- mutator, so the commutator is quite short, but larger heavy-current motors may well have many brushes mounted on a common arm, each with its own brushbox (in which it is free to slide) and with all the brushes on one arm connected in parallel via their Fluxible copper leads or ‘pigtails’. The length of the commutator can then be comparable with the ‘active’ length of the armature (i.e. the part carrying the conductors exposed to the radial Flux).
Function of the commutator
Many diVerent winding arrangements are used for d.c. armatures, and it is neither helpful nor necessary for us to delve into the nitty-gritty of winding and commutator design. These issues are best left to motor designers and repairers. What we need to do is to focus on what a well- designed commutator-winding actually achieves, and despite the apparent complexity, this can be stated quite simply.
The purpose of the commutator is to ensure that regardless of the position of the rotor, the pattern of current flow in the rotor is always as shown in Figure 3.3.
Current enters the rotor via one brush, Xows through all the rotor coils in the directions shown in Figure 3.3, and leaves via the other brush. The Wrst point of contact with the armature is via the commutator segment or segments on which the brush is pressing at the time (the brush is usually wider than a single segment), but since the interconnections between the individual coils are made at each commutator segment, the current actually passes through all the coils via all the commutator segments in its path through the armature.
We can see from Figure 3.3 that all the conductors lying under the N pole carry current in one direction, while all those under the S pole carry current in the opposite direction. All the conductors under the N pole will therefore experience a downward force (which is proportional to the radial Xux density B and the armature current I) while all the conductors under the S pole will experience an equal upward force. A torque is thus produced on the rotor, the magnitude of the torque being proportional to the product of the Xux density and the current. In practice the Xux density will not be completely uniform under the pole, so the force on some of the armature conductors will be greater than on others. How- ever, it is straightforward to show that the total torque developed is given by
where F is the total Xux produced by the Weld, and KT is constant for a given motor. In the majority of motors the Xux remains constant, so we see that the motor torque is directly proportional to the armature current. This extremely simple result means that if a motor is required to produce constant torque at all speeds, we simply have to keep the armature current constant. Standard drive packages usually include provision for doing this, as will be seen later. We can also see from equation (3.1) that the direction of the torque can be reversed by reversing either the armature current (I ) or the Xux (F). We obviously make use of this when we want the motor to run in reverse, and sometimes when we want regenerative braking.
The alert reader might rightly challenge the claim made above – that the torque will be constant regardless of rotor position. Looking at Figure 3.3, it should be clear that if the rotor turned just a few degrees, one of the Wve conductors shown as being under the pole will move out into the region where there is no radial Xux, before the next one moves under the pole. Instead of Wve conductors producing force, there will then be only four, so won’t the torque be reduced accordingly?
The answer to this question is yes, and it is to limit this unwelcome variation of torque that most motors have many more coils than shown in Figure 3.3. Smooth torque is of course desirable in most applications in order to avoid vibrations and resonances in the transmission and load, and is essential in machine tool drives where the quality of Wnish can be marred by uneven cutting if the torque and speed are not steady.
Broadly speaking, the higher the number of coils (and commutator segments) the better, because the ideal armature would be one in which the pattern of current on the rotor corresponded to a ‘current sheet’, rather than a series of discrete packets of current. If the number of coils was inWnite, the rotor would look identical at every position, and the torque would therefore be absolutely smooth. Obviously this is not practicable, but it is closely approximated in most d.c. motors. For practical and economic reasons the number of slots is higher in large motors, which may well have a hundred or more coils, and hence very little ripple in their output torque.
Operation of the commutator – interpoles
Returning now to the operation of the commutator, and focusing on a particular coil (e.g. the one shown as ab in Figure 3.3) we note that for half a revolution – while side a is under the N pole and side b is under the S pole, the current needs to be positive in side a and negative in side b in order to produce a positive torque. For the other half revolution, while side a is under the S pole and side b is under the N pole, the current must Xow in the opposite direction through the coil for it to continue to produce positive torque. This reversal of current takes place in each coil as it passes through the interpolar axis, the coil being ‘switched- round’ by the action of the commutator sliding under the brush. Each time a coil reaches this position it is said to be undergoing commutation, and the relevant coil in Figure 3.3 has therefore been shown as having no current to indicate that its current is in the process of changing from positive to negative.
The essence of the current-reversal mechanism is revealed by the simpliWed sketch shown in Figure 3.4. This diagram shows a single coil
fed via the commutator, and brushes with current that always Xows in at the top brush.
In the left-hand sketch, coil-side a is under the N pole and carries positive current because it is connected to the shaded commutator segment which in turn is fed from the top brush. Side a is therefore exposed to a Xux density directed from left (N) to right (S) in the sketch, and will therefore experience a downward force. This force will remain constant while the coil-side remains under the N pole. Conversely, side b has negative current but it also lies in a Xux density directed from right to left, so it experiences an upward force. There is thus an anti-clockwise torque on the rotor.
When the rotor turns to the position shown in the sketch on the right, the current in both sides is reversed, because side b is now fed with positive current via the unshaded commutator segment. The direction of force on each coil side is reversed, which is exactly what we want in order for the torque to remain clockwise. Apart from the short period when the coil is outside the inXuence of the Xux, and undergoing commutation (current reversal), the torque is constant.
It should be stressed that the discussion above is intended to illustrate the principle involved, and the sketch should not be taken too literally. In a real multi-coil armature, the commutator arc is much smaller than that shown in Figure 3.4 and only one of the many coils is reversed at a time, so the torque remains very nearly constant regardless of the position of the rotor.
The main diYculty in achieving good commutation arises because of the self inductance of the armature coils and the associated stored energy. As we have seen earlier, inductive circuits tend to resist change in current, and if the current reversal has not been fully completed by the time the brush slides oV the commutator segment in question, there will be a spark at the trailing edge of the brush.
In small motors some sparking is considered tolerable, but in medium and large wound-Weld motors small additional stator poles known as
interpoles (or compoles) are provided to improve commutation and hence minimise sparking. These extra poles are located midway between the main Weld poles, as shown in Figure 3.5. Interpoles are not normally required in permanent magnet motors because the absence of stator iron close to the rotor coils results in much lower armature coil inductance.
The purpose of the interpoles is to induce a motional e.m.f. in the coil undergoing commutation, in such a direction as to speed-up the desired reversal of current, and thereby prevent sparking. The e.m.f. needed is proportional to the current (armature current) which has to be commutated, and to the speed of rotation. The correct e.m.f. is therefore achieved by passing the armature current through the coils on the interpoles, thereby making the Xux from the interpoles proportional to the armature current. The interpole coils therefore consist of a few turns of thick conductor, connected permanently in series with the armature.