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1Series-wound motor

With the series-wound motorthe armature and field coils are in

series (Figure 6.30(a)). Such a motor exerts the highest starting

torque and has the greatest no-load speed. However, with light loads

there is a danger that a series-wound motor might run at too high a

speed. Reversing the polarity of the supply to the coils has no effect

on the direction of rotation of the motor, since both the current inthe armature and the field coils are reversed.

2Shunt-wound motor

With the shunt-wound motor(Figure 6.30(b)) the armature and field

coils are in parallel. It provides the lowest starting torque, a much

lower no-load speed and has good speed regulation. It gives almost

constant speed regardless of load and thus shunt wound motors are

very widely used. To reverse the direction of rotation, either the

armature or field current can be reversed.

3 Compound motor

The compound motor(Figure 6.30(c)) has two field windings, one in

series with the armature and one in parallel. Compound-wound

motors aim to get the best features of the series and shunt-wound

motors, namely a high starting torque and good speed regulation.4Separately excited motor

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The separately excited motor(Figure 6.30(d)) has separate control

of tlie armature and field currents. The direction of rotation of the

motor can be obtained by reversing either the armature or the field

current.

Figure 6.31 indicates the general fonn of the torque-speed

characteristics of the above motors. The separately excited motor has a

torque-speed characteristic similar to the shunt wound motor. The speedof such d.c. motors can be changed by either changing the armature

current or the field current. Generally it is the armature current that is

varied. The choice of d.c. motor will depend on what it is to be used for.

Thus, for example, with a robot manipulator the robot wrist might use a

series-wound motor because the speed decreases as the load increases. A

shunt-wound motor might be used if a constant speed was required,

regardless of the load.

The speed of a permanent magnet motor can be controlled by varying

the current through the armature coil, with a field coil motor by either

varying the armature current or the field current though generally it is

the armature current that is varied. Thus speed control can be obtained

by controlling the voltage applied to the armature. Rather than just tiy to

directly vary the input voltage, a more convenient method is to use pulsewidth modulation (PWM). This basically involves taking a constant d.c.

supply voltage and using an electronic circuit to chop it so that the

average value is varied (Figiu"e 6.32).

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6.5.2 Brushless permanent magnet d.c. motor

A problem with the d.c. motors described in the previous section, is that

they require a commutator and brushes in order to periodically reverse

the current through each armature coil. Brushes have to be periodically

changed and the commutator resurfaced because the brushes make

sliding contacts with the commutator and suffer wear. Brushless d.c.

motors do not have this problem.A current-carrying conductor in a magnetic field experiences a force

and with the conventional d.c. motor the magnet is fixed and the

current-carrying conductors consequently made to move. However, as a

consequence of Newton's third law of motion, the magnet will

experience an opposite and equal force to that acting on the

current-carrying conductors and so, with the brushless permanent

magnet d.c. motor, the current carrying conductors are fixed and the

magnet moves. With just one current carrying coil, the resulting force on

the magnet would just cause it to deflect. In order to keep the magnet

moving, a sequence of current carrying coils have to be used and each in

turn switched on.

Figure 6.33 shows the basic form of such a motor. The rotor is a

ferrite or ceramic permanent magnet. The current to the stator coils AA,BB and CC is electronically switched by transistors in sequence round

them, the switching being controlled by the position of the rotor so that

there are always forces acting on the magnet causing it to rotate in the

same direction. Hall sensors (a magnetic field input to the sensor gives a

voltage output) are generally used to sense the position of the rotor and

initiate the switching by tlie transistors, tlie sensors being positioned

around the stator. Figure 6.34 shows the transistor switching circuits that

might be used with the motor shown in Figure 6.33.

To switch the coils in sequence we need to supply signals to switch the

transistors on in the right sequence. Tliis is provided by the outputs from

the three sensors operating through a decoder circuit to give the

appropriate base currents. Thus when the rotor is in the vertical position,

i.e. 0, there is an output from sensor c but none from a and b and this is

used to switch on transistors A+ and B-. When the rotor is in the 60

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position there are signals from the sensors b and c and transistors A+

and C- are switched on. Table 6.2 shows the entire switching sequence.

The entire circuit for controlling such a motor is available as a single

integrated circuit.

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Each pole is activated by a current being passed through the

appropriate field winding, the coils being such that opposite poles are

produced on opposite coils. The current is supplied from a d.c. source to

the windings through switches. With the currents switched through the

coils such that the poles are as shown in Figure 6.35, the rotor will move

to line up with the next pair of poles and stop there. This would be, for

Figure 6.35, an angle of 45"*. If the current is then switched so that the

polarities are reversed, the rotor will move a step to line up with the next

pair of poles, at angle 135"^ and stop there. The polarities associated with

each step are:

There are thus, in this case, four possible rotor positions: 45*^, 135*", 225*"

and 315.

Figure 6.36 shows the basic form of the variable reluctance type of

stepper motor. With this form the rotor is made of soft steel and is not a

permanent magnet. The rotor has a number of teeth, the number being

less than the number of poles on the stator. When an opposite pair of

windings on stator poles has current switched to them, a magnetic field

is produced with lines of force which pass from the stator poles through

the nearest set of teeth on the rotor. Since lines of force can beconsidered to be rather like elastic thread and always trying to shorten

themselves, the rotor will move until the rotor teeth and stator poles line

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up. This is termed the position of minimum reluctance. Thus by

switching the current to successive pairs of stator poles, the rotor can be

made to rotate in steps. With the number of poles and rotor teeth shown

in Figure 6.36, the angle between each successive step will be 30"*. The

angle can be made smaller by increasing the number of teeth on the

rotor.

There is another version of the stepper motor and that is a hybridstepper. This combines features of both tiie permanent magnet and

variable reluctance motors. They have a pennanent magnet rotor encased

in iron caps which are cut to have teeth. The rotor sets itself in the

minimum reluctance position in response to a pair of stator coils being

energised.

The following are some of the terms commonly used in specifying

stepper motors:

1Phase

This is the number of independent windings on the stator, e.g. a

four-phase motor. The current required per phase and its resistance

and inductance will be specified so that the controller switching

output is specified. Figure 6.35 is an example of a two-phase motor,

such motors tending to be used in light-duty applications. Figure

6.36 is an example of a three-phase motor. Four-phase motors tend

to be used for higher power applications.

2Step angle

This is the angle through which the rotor rotates for one switching

change for the stator coils.

3Holding torqueThis is the maximum torque tliat can be applied to a powered motor

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without moving it from its rest position and causing spindle rotation.

4Pull-in torque

This is the maximum torque against which a motor will start, for a

given pulse rate, and reach synchronism without losing a step.

5Pull-out torque

This is the maximum torque that can be applied to a motor, running

at a given stepping rate, without losing synchronism.6Pull-in rate

This is the maximum switching rate or speed at which a loaded

motor can start without losing a step.

7Pull-out rate

This is the switching rate or speed at which a loaded motor will

remain in synchronism as the switching rate is reduced.

8Slew range

This is the range of switching rates between pull-in and pull-out

within which the motor runs in synchronism but cannot start up or

reverse.

To drive a stepper motor, so that it proceeds step-by-step to provide

rotation, requires each pair of stator coils to be switched on and off in the

required sequence when the input is a sequence of pulses (Figure 6.38).

Driver circuits are available to give the correct sequencing and Figure

6.39 shows an example, the SAA 1027 for a four-phase unipolar stepper.

Motors are termed unipolarif they are wired so that the current can only

flow in one direction through any particular motor terminal, bipolarif

the current can flow in either direction through any particular motor

terminal. The stepper motor will rotate through one step each time the

trigger input goes from low to high. The motor runs clockwise when the

rotation input is low and anticlockwise when high. When the set pin is

made low the output resets. In a control system, these input pulses might

be supplied by a microprocessor.

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Some applications require very small step angles. Though the step

angle can be made small by increasing the number of rotor teeth and/or

the number of phases, generally more than four phases and 50 to 100

teeth are not used. Instead a technique known as mini-stepping is used

with each step being divided into a number of equal size sub-steps by

using different currents to the coils so that the rotor moves to

intermediate positions between normal step positions. For example, this

method might be used so that a step of 1.8 is subdivided into 10 equalsteps.

Example

A stepper motor is to be used to drive, through a belt and pulley

system (Figure 6.40), the carriage of a printer. The belt has to move

a mass of 500 g which has to be brought up to a velocity of 0.2 m/s

in a time of 0.1 s. Friction in the system means that movement of the

carriage requires a constant force of 2 N. The pulleys have an

effective diameter of 40 mm. Determine the required pull-in torque.

The forceFrequired to accelerate the mass is:

F=ma=^0.500 x (0.2/0.1) = 1.0 N.

The total force that has to be overcome is the sum of the above force

and that due to friction. Thus tlie total force that has to be overcomeis 1.0+ 2 = 3 N.

This force acts at a radius of 0.020 m and so the torque that has to

be overcome to start, i.e. tlie pull-in torque, is

torque = force x radius = 3 x 0.020 = 0.06 N m

Case Studies

The following are case studies designed to illustrate the use of correction

elements discussed in this chapter.

6.6.1 A liquid levelprocess control system

Figure 6.41 one method of how a flow control valve can be used to

control the level of a liquid in a container. Because there may be surfaceturbulence ti a result of liquid entering the container or stirring of the

liquid or perhaps boiling, such high frequency 'noise' in the system is

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often filtered out by the use of astilling well, as shown in Figure 6.41.

However, it must be recognised that the stilling well constitutes a U-tube

in which low frequency oscillations of the liquid level can occur.