Introduction to Industrial Automation

Figure 2.2  Internal view of an AC motor (a) and step motor (b).

R

S

T

R

S

T

Three-phase

motor

Three-phase

motor

Forward rotation

Reverse rotation

Figure 2.3  Forward-reverse rotation of a three-phase motor.

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  Introduction to Industrial Automation

rotary motion of an electric motor can be converted to a linear one by using a ball screw pair and 

guide rails. The AC induction motors are the most widely used motors in the automation industry 

compared with the DC ones, mainly because of their efficiency and less maintenance required. It 

is the simpler solution in applications such as machine tools, fans, pumps, compressors, conveyors, 

extruders, and various other complex machines.

Stepper motors base their operation on a working principle similar to that of DC motors and 

can rotate in very small discrete steps. The steps of a stepper motor represent discrete angular move-

ments in the vicinity of 2° or 1° or even less, which are performed successively due to a series of 

digital impulses. It is obvious that a stepper motor can perform any number of rotation steps with 

the same precision by applying an equal number of electrical pulses to its phases. Regarding their 

internal structure, there are many types of stepper motors (such as unipolar, bipolar, single-phase, 

two-phase, multi-phase, etc.) which usually have multiple coils that are organized in groups called 

“phases”. Stepper motors are controlled by a driver electronic circuit accepting four different pulse 

digital control signals and applying the required electric pulses to the motor windings. The one-step 

function signal defines the direction of rotation, a second one defines the enable or disable state 

of the motor operation, and a third signal defines the half-step or full-step rotation of the motor. 

Finally, a pulse train signal causes the rotation of the motor. Each control pulse causes the motor to 

rotate by one step, while the speed of the rotation is determined by the frequency of the pulse train. 

These control signals may be produced by a programmable logic controller that will be described 

in Chapter 10, where step motor applications will be examined. In general, stepper motors provide 

precise speed, position, and direction control in an open-loop fashion, without requiring encoders 

or other types of sensors which conventional electric motors require. A stepper motor does not lose 

steps under normal conditions of mechanical load, while the final position of the stepper motor’s 

rotor is determined by the number of performed steps and expresses the total angular displacement. 

This position is kept until a new pulse train is applied. These properties make the stepper motor 

an excellent actuator for open-loop control applications, for low to medium power requirements.

When higher torque demands precise control, servomotors are then the best solution to be 

used. Servomotors are not a specific class of motors and the term “servomotor” is often used to 

refer to a motor suitable for use in a closed-loop control system. A servomotor consists of an AC or 

DC electric motor, a feedback device, and an electronic controller. In the case of a DC motor, this 

can be either a brushed or brushless type. Typically, the feedback device of a servomotor is some 

type of encoder built into the motor frame to provide position and speed feedback of the angular 

or linear motion. The electronic controller is a driver, supplying only the required power to the 

motor, in the simplest case. A more sophisticated controller generates motion profiles and uses the 

feedback signal to precisely control the rotary position of the motor and generally to control its 

motion and final position, thus accomplishing the closed-loop operation. Since the servo motors 

are driven through their electronic controllers, it is quite easily interfaced with microprocessors or 

other high level programmable controllers.

Figure 2.4 provides some fundamental torque-speed curves regarding the selection of the AC 

(a) and stepper (b) motors, characteristics that can be found in each motor’s manual and contrib-

ute also in the comprehension of its respective operation. For AC motors, at rest the motor can 

appear just like a short-circuited transformer and, if connected to a full supply voltage, draw a 

very high current known as a locked rotor current (LRC). The motors also produce torque that 

is known as locked rotor torque (LRT). As the motor accelerates, both the torque and the cur-

rent will tend to alter with the rotor speed if the voltage is kept constant. The starting current of 

a motor with a fixed voltage will drop very slowly as the motor accelerates, and will only begin to 

fall significantly when the motor has reached at least 80% of the full speed. The actual curves for 

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Hardware Components for Automation and Process Control 

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induction motors can vary considerably between different types of motors, but the general trend is 

for a high current, until the motor has almost reached full speed. The LRC of a motor can range 

from 500% of full-load current (FLC) to as high as 1400% of FLC. Typically, good motors fall in 

the range of 550% to 750% of FLC. The starting torque of an induction motor with a fixed voltage 

will drop a little to the minimum torque, known as the pull-up torque; when the motor accelerates 

it will then rise to a maximum torque, known as the breakdown or pull-out torque, at almost full 

speed; and then it will drop to zero at the synchronous speed. The curve of the start torque against 

the rotor speed is dependent on the terminal voltage and the rotor design. In the case that the load 

curve is added, the intersection of the load curve with the torque and voltage curves will define the 

operational point of the motor.

For the case of a stepper motor, the characteristic torque speed curves are the following ones. 

The pull-out torque curve is the curve that represents the maximum torque that the stepper motor 

can supply to a load at any given speed. Any torque or speed required that exceeds this curve will 

cause the motor to lose synchronism. Holding torque is the torque that the motor will produce 

when the motor is at rest and rated current is applied to the windings. Slew range is the region 

where the stepper motors are usually operated. A stepper motor cannot be started directly in the 

slew range. After starting the motor somewhere in the self-start range, the motor can be acceler-

ated or loaded while remaining within the slew range. The motor should then be decelerated or the 

load should be reduced back into the self-start range before the motor can be stopped. As in the 

previous case, the intersection of the motor’s characteristic curves with the load curve will indicate 

if the size of the selected motor is sufficient for the envisioned application.

At this point, it should be highlighted that it is beyond the scope of this textbook to present 

the details of any type of electric motors, since the objective is to get an understanding of the 

basic principles of operation of the various actuators and to study their use in automation systems. 

Therefore, the objective of this chapter is to provide an overview of the basics of other common 

actuators (particularly the non-electrical ones) such as pneumatic actuators. Finally, regarding 

electric motors, we will need to distinguish their control task from their automation task. The con-

trol task refers to a closed-loop control scheme for the regulation of their speed, angular position, 

and torque output. The automation task refers to the sequential steps of power relays (energizing 

or de-energizing) in order for an electric motor to change the direction of rotation or to startup 

according to a star-delta configuration.

Cu

rr

ent (% of motor f

ull-load-current

)

To

rque (% of motor f

ull-load-

torque)

Full voltage

Stator current

Pull-out torque

Full voltage

Start torque

Pull-up torque

7×FLC

6×FLC

5×FLC

4×FLC

3×FLC

2×FLC

1×FLC

1×FL

T

2×FL

T

0%

25%

50%

75%

100%

(a)

(b)

Rotor speed (% of full speed)

To

rque

Holding

torque

Pull-out torque

Pull-in torque

Start/stop

region

Slew range

Speed

Max. no load

starting speed

Max. running

speed