ELECTRICAL DRIVES pps

ELECTRICAL DRIVES

CONTENT

1. Generators and motors

2. Actuators

3. DC motors

4. AC MOTORS

5. Stepper motors

6. Calculations

7. Power conversions

8. Power supplies.

9 Converter control of machines.

1. Generators and Motors

Basic Magnetic Field

Magnets are pieces of metal that have the ability to attract other metals. Every magnet has two poles: a

north and a south. Much like electrical charges, two similar magnetic poles repel each other; while

opposite magnetic poles attract each other. Magnets have a continuous force around them that is known

as a magnetic field. This field enables them to attract other metals. Figure 1 illustrates this force using

bar and horseshoe magnets.

Fig 1

The shape of the magnet dictates the path the lines of force will take. Notice that the force in Figure 1 is

made up of several lines traveling in a specific direction. It can be concluded that the lines travel from

the magnet's north pole to its south. These lines of force are often called the magnetic flux. If the bar

magnet is now bent to form a horseshoe magnet, the north and south pole are now across from each

other. Notice in the horseshoe magnet how the lines of force are now straight, and that they travel from

the north pole to the south. It will be revealed how generators and motors use these lines of force to

generate electricity, as well as mechanical motion.

Magnetic Fields Around Conductors

When a current flows through a conductor, a magnetic field surrounds the conductor. As current flow

increases, so does the number of lines of force in the magnetic field (Figure 2).

Fig2

The right hand rule helps demonstrate the relationship between conductor current and the direction of force. Grasp a

wire conductor in the right hand, put your thumb on the wire pointing upward, and wrap your four fingers around the

wire. As long as the thumb is in the direction that current flows through the wire, the fingers curl around the wire in

thedirection of the magnetic field.

A conductor can be twisted into a coil, which efficiently produces current when cutting the lines of

force in a magnetic field. The more turns in this coil, the stronger the magnetic field. Furthermore, if

the coil is wrapped around a piece of iron, the current becomes even stronger.

When needing to discover which poles are which in a conductor, it is important to notice which way

the coils turn in order to apply the right hand rule. In addition, one should always look at which side of

the coil is attached to the positive terminal of a power source such as a battery, and which side is

attached to the negative. Figure 3 illustrates four different scenarios and the appropriate poles.

Fig.3.

As a conductor cuts across the lines of force in a magnetic field, it generates a current. This method of

inducing a current is called induction. There are three rules for induction:

1. When a conductor cuts through lines of force, it induces an electromotive force (EMF), or voltage.

2. Either the magnetic field or the conductor needs to be moving for this to happen.

3. If the direction of the cutting across the magnetic field changes, the direction of the induced EMF also

changes.

Accordingly, Faraday's law states that induced voltage can be determined by the number of turns in a

coil, and how fast the coil cuts through a magnetic field. Therefore, the more turns in a coil or the

stronger the magnetic field, the more voltage induced.

In addition, current changes direction depending on which way it cuts across a magnetic field. As

depicted in Figure 4, a coil cutting through a basic magnetic field in a clockwise direction will at first

result in a current with positive polarity, but as it cuts across the same field in the opposite direction

during the second half of its turn, the polarity becomes negative.

Fig.4.

When current switches from positive to negative repeatedly, it is called alternating current, or A.C.

Alternating current will be explained in more detail later.

DC Current

When a current is direct (D.C.) rather than alternating (A.C.), the polarity of that current never changes

direction. Usually, when a coil turns in a clockwise direction, the first 180 degrees of the turn result in

the induced current going in a positive direction. As mentioned above, however, the second 180

degrees result in the induced current going in a negative direction. In direct current, the current always

travels in a positive direction. How is this possible? When inducing direct current, some mechanism

must be employed to make sure the coils only cut through the magnetic field in one direction, or that

the circuit only uses current from the coil cutting in that one direction. Devices such as D.C. generators

employ a mechanism called a commutator to keep current flowing in one direction. Figure 5 shows

direct current in the form of a sine wave. Notice that the current never has negative polarity, and is

therefore always flowing in a positive direction.

Fig.5

Alternating Current

Much like the process of producing direct current, the process of producing an alternating current

requires a conductor loop spinning in a magnetic field. As a matter of fact, the process is the same for

both types of current, except that the alternating current is never changed into direct current through the

use of a commutator. The conductor loop, or coil, cuts through lines of force in a magnetic field to

induce A.C. voltage at its terminals. Each complete turn of the loop is called a "cycle." The alternating

current wave is pictured in Figure 6.

Fig.6.

Notice what segment of the wave consists of one cycle, and which is the part of the wave from point A

to the next point A. If we divide the wave into four equal parts, the divisions happen at points A, B, C,

and D. We can read the turn of the coil and how it relates to the wave produced. From A to B is the first

quarter turn of the coil, from B to C is the second quarter turn, from C to D is the third quarter turn, and

from D to A is the final quarter turn.

It is important to note that degree markings on a horizontal axis refer to electrical degrees and are not

geometric. The example above is for a single pole generator. However, if this were a double pole

generator, then 1 cycle would happen at each 180 degrees rather than 360 degrees, and so on.

2. ACTUATORS

A mechanism that puts something into automatic action is called an actuator.In industrial control systems, an actuator

is a hardware device that converts a controller command signal into a change in a physical parameter. The change in

the physical parameter is usually mechanical, such as position or velocity change. An actuator is a transducer, because

it changes one type of physical quantity, say electric current, into another type of physical quantity, say rotational

speed of an electric motor. The controller command signal is usually low level, and so an actuator may also include an

amplifier to strengthen the signal sufficiently to drive the actuator.

A list of common actuators is presented in Table 1. Depending on the type of amplifier used, most actuators can be

classified into one of three categories: (1) electrical, (2) hydraulic, and (3) pneumatic. Electrical actuators are most

common; they include ac and dc motors of various kinds, stepper motors, and solenoids. Electrical actuators include

both linear devices (output is linear displacement) and rotational devices (output is rotational displacement or

velocity). Hydraulic actuators use hydraulic fluid to amplify the controller command signal. The available devices

provide both linear and rotational motion. Hydraulic actuators are often specified when large forces are required.

Pneumatic actuators use compressed air (typically “shop air” in the factory environment) as the driving power. Again,

both linear and rotational pneumatic actuators are available. Because of the relatively low air pressures involved, these

actuators are usually limited to relatively low force applications compared with hydraulic actuators.

TABLE 1 Common Actuators Used in Automated Systems

Actuator Description

DC motor Rotational electromagnetic motor. Input is direct current (dc). Very common

servomotor in control systems. Rotary motion can be converted to linear

motion using rack-and-pinion or ball screw.

Hydraulic piston Piston inside cylinder exerts force and provides linear motion in response to

hydraulic pressure. High force capability.

Induction motor (rotary) Rotational electromagnetic motor, Input is alternating current (ac).

Advantages compared with dc motor: lower cost, simpler construction, and

more-convenient power supply. Rotary motion can be converted to linear

motion using rack-and-pinion or ball screw.

Linear induction motor Straight-line motion electromagnetic motor. Input is alternating current (ac).

Advantages: high speed, high positioning accuracy, and long stroke

capacity.

Pneumatic cylinder Piston inside cylinder exerts force and provides linear motion in response to

air pressure.

Relay switch On-off switch opens or closes circuit in response to an electromagnetic force.

Solenoid Two-position electromechanical assembly consists of core inside coil of wire.

Core is usually held in one position by spring, but when coil is energized,

core is forced to other position. Linear solenoid most common, but rotary

solenoid available.

Stepping motor Rotational electromagnetic motor. Output shaft rotates in direct proportion to

pulses received. Advantages: high accuracy, easy implementation,

compatible with digital signals, and can be used with open-loop control.

Disadvantages: lower torque than dc motors, limited speed, and risk of

missed pulse under load. Rotary motion can be converted to linear motion

using rack-and-pinion or ball screw.

Three main types of actuation have been the core of motion and force power for all robotic systems.

They are Hydraulic, Pneumatic, and Electric motors. These three come from two main types of power

conversion. The first two are considered fluid machines in that they use fluid to create mechanical

motion whereas the electric motor converts electrical energy into mechanical energy.

Hydraulic Actuators

Principle of Operation

An actuator of this type works by changes of pressure. This system can be used in both linear and

rotary actuation. The general linear mechanism consists of a piston encased in a chamber with a piston

rod protruding from the chamber. The piston rod serves as the power transmission link between the

piston inside the chamber and the external world. There are two major configurations of this actuator:

single or double action. For the single action configuration, it can exert controllable forces in only one

direction and uses a spring to return the piston to the neutral or un-energized position. Figure 1 shows a

cut away view of a double action actuator which can be actively controlled in both directions within the

chamber. In the case of rotary actuation, the power unit is a set of vanes attached to a drive shaft and

encased in a chamber. Within the chamber the actuator is rotated by differential pressure across the

vanes and the action is transmitted through the drive shaft to the external world.

Figure 1: Drawing of Double Action Hydraulic Actuator

A representative closed loop position control for a linear hydraulic actuator is shown in Figure 2. The

open loop dynamics of the hydraulic actuator are usually approximated with a first order system where

the time constant depends on the piston area A. The desired piston position Xd is compared to the actual

position X. The obtained error E is processed by the controller which in most of the cases is a PID

controller and the drive current I which will be the input to the servo-valve is obtained. Then the

current I is multiplied with the flow gain Kq and the servo-valve no-load flow Q0 is obtained. The

Output Flow Q is the difference between Q0 and a flow Qd due to disturbance forces Fd which are

mainly friction and gravity. Flow Q is the input to the hydraulic system that will result in a piston

velocity V and a piston displacement X.

Figure 2: Closed Loop Position Control of a Hydraulic Actuator

It has to be noted that in Figure 2, the dynamics of the servo-valve have been approximated with very

simple linear relationships. This representation can be realistic in low frequency operations. However,

in high frequencies, electro-hydraulic servo-valves can exhibit highly non-linear dynamics. Hydraulic

manipulators are mainly used in applications where large robotic systems with high payload capability

are needed. Examples are nuclear and underwater applications. One of the main advantages of

hydraulic actuators is that these systems can deliver a great deal of power compared to their actuator

inertia. Other aspects, which make a hydraulic actuator useful are the low compressibility of hydraulic

fluids and, the high stiffness which leads to an associated high natural frequency and rapid response.

This means that the device using hydraulic actuators can execute very quick movements with great

force..

Pneumatic Actuators

These type of actuators are the direct descendents of the hydraulic systems. The difference between the

two is that pneumatic systems use a compressible gas (i.e. air ) as the medium for energy transmission.

This makes the pneumatic system more passively compliant than the hydraulic system. With pneumatic

actuators, the pressure within the chambers is lower than that of hydraulic systems resulting in lower

force capabilities. In Figure 4 there is a cut away view of the basic pneumatic actuator. It is quite

similar to the hydraulic counterpart however there are no return hydraulic lines for fluid. In a typical

actuator of this type the fluid, namely air, is simply exhausted through the outlet valve in the actuator.

Digital control of pneumatic systems is very similar to hydraulics with some exceptions to gains and

stiffness constants (see also Figure 2.)

.

Figure 3: Cutaway View of Pneumatic Actuator

Pneumatic actuators have less force capability than hydraulic actuators. Since, in contrast, the system

operates at a lower pressure than the hydraulics and does not require return lines for the fluid, the

support structure of the manipulator is much lighter than the other system. Pneumatics are cleaner and

nonflammable which makes its uses in certain environments ( i.e. – cleanrooms, operating rooms) more

desired. Additionally, installation, operation and maintenance is easier and cost is lower.

Rotary actuator

The rotary actuator is a device use to alternate the rotated position of an object. Just like

the human wrist the actuator enables the rotation of an object, except that rotary

actuators are available in a wide variety of models with different — Sizes, Torques,

Rotation angles. The energy for the rotation is delivered by pneumatic pressure. The

rotary actuator converts the air pressure from a linear motion to a rotating motion.

The rotary actuator converts the air

pressure from a linear motion to a

rotating motion. This is done by a rack

and pinion. Air pressure is supplied

pushing the piston in a linear motion,

attached to the piston is a straight set

of gear teeth called a "rack". The rack

is pushed in a linear motion as the

piston moves. The gear teeth of the rack are meshed with the circular gear teeth of a

"pinion" forcing the pinion to rotate. The pinion can be rotated back into the original

position by supplying air pressure to the opposite side of the air cylinder pushing the

rack back in the other direction. The pinion is connected to a shaft that protrudes from

the body of the rotary actuator. This shaft can be connected to various tools or grippers.

Electric Actuators

Of the three types of conventional actuator systems, electric motors have the largest variety of possible

devices such as: Direct Current (DC) motors and their variants (brushed and brushless, low inertia,

geared and direct drive, permanent magnet), Alternate Current (AC) motors, Induction Motors, and

Stepping Motors. By definition, the principle behind an electric motor is a simple one, which is the

application of magnetic fields to a ferrous core and thereby inducing motion.

A schematic of an AC motor and of a two-pole induction motor is shown in Figure 4 and of a DC

motor in Figure 5. An AC motor is comprised of an electromagnet positioned above a permanent

magnet which is mounted on a pivot. When current is sent through the coil, the permanent magnet

rotates so that opposite poles align. As the AC current switches direction, the permanent magnet

continues rotation to align with reversed poles.

Figure 4: Basic Operation of a DC induction motor

A two-pole induction motor, shown in Figure 6, has a rotor which is the inner part that rotates and a

stator of a permanent magnet composed of two or more permanent magnet pole pieces. The rotor is

composed of windings which are connected to a mechanical commutator. The opposite polarities of the

energized winding and the stator magnet attract and the rotor will rotate until it is aligned with the

stator. Just as the rotor reaches alignment, the brushes move across the commutator contacts and

energize the next winding. Notice that the commutator is staggered from the rotor poles. If the

connections of a motor are reversed, the motor will change directions. DC motors are very similar to

induction motors. The only difference is that a current is sent to the armature through contact between

brushes and commutator. Spinning commutator acts as a reversing switch that alternates magnetic field.

Figure 5 : Block diagram of a DC

A position control scheme, in the Laplace domaine, for a permanent magnet DC motor is shown in

Figure 6. The desired motor position Θd (and velocity) is compared to the actual position Θ (and

velocity) that is usually obtained using angular position sensor such as optical encoders or

potensiometers. This comparison yield the position (and velocity) error E that is processed by the

controller. Usually, a PID controller is used and is designed to achieve fast and accurate response with

no overshoot. The output of the controller is the voltage V that will be the input to the DC motor.

Voltage V will be reduced by the "back emf" voltage Vb which is created by the angular motion of the

motor shaft. The back emf voltage is proportional to the angular velocity Ω and the coefficient of this

linear relationship is called the "back emf constant" Kb. The open loop dynamics of the motor are

distinguished into the dynamics of the electrical part and of the mechanical part. The electrical circuit

dynamics are approximated using a first order system where L is the armature inductance and R is the

armature resistance. The armature current I which is the output of the electrical part, will result in the

motor torque τm. The relationship between I and τm is linear and the coefficient Ki of this linear

relationship is called the "motor constant". The sum of the motor torque τm with the disturbance torques

τl felt by the motor shaft will be the input to the mechanical part of a DC motor. The disturbance

torques τl are due to the load carried by the motor, friction and elastic effects at the payload level and

other dynamic effects that have not been taken into account by the model. The coefficient n is the gear

ratio and divides any torques due to the payload. The dynamics of the mechanical part are of second

order, including a motor inertia Jm and a friction term with friction coefficient Bm.

Figure 6: Position Control of a DC Motor

Since the energy medium for electric motors is easily stored and re-supplied by recharging batteries if

mobility is needed, this makes electric motors the best choice when it comes to portability.

Concurrently, as far as energy mediums, electric motors’ power source is more adaptable to

environments than hydraulics or pneumatics since volumetrically they take up less space. There are no

hydraulic return lines, air lines, high pressure pumps, or reservoir tanks as in the case of the previously

described systems. In tasking a robot to perform difficult maneuvers, the flexibility of control of the

mechanical system with electric motors is far greater. This is because of the energy medium can be

used by both the control system and the manipulator directly. They are also easy to install, clean (no

leaks) and relatively quiet when they are compared to the hydraulic and pneumatic actuators. The major

disadvantage of electrical motors is that they produce very small torques compared to their size and

weight. As the trend in robotics is to build smaller robots that are very powerful, electrical motors seem

to be not suitable for such applications.

We consider an electrical actuator. It consist of stators, rotors and windings (permanent magnets). See figure below.