(notes) Electronics - Basic in Motors.pdf - Najnowsze ksiazki(50) - walbo48

BALDOR ELECTRIC COMPANY

SERVO CONTROL FACTS

A HANDBOOK EXPLAINING

THE BASICS OF MOTION

MN1205

TABLE OF CONTENTS

TYPES OF MOTORS . . . . . . . . . . . . . . 3

OPEN LOOP/CLOSED LOOP . . . . . 9

WHAT IS A SERVO . . . . . . . . . . . . . . 11

COMPENSATION . . . . . . . . . . . . . . . 13

TYPES OF CONTROLS . . . . . . . . . . . 15

TYPES OF FEEDBACK DEVICES . 17

TYPES OF ACTUATORS . . . . . . . . . . 22

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Servo Control Facts

TYPES OF MOTORS

The direct current (DC) motor is one of the first machines devised to convert electrical energy to

mechanical power. Its origin can be traced to machines conceived and tested by Michael Faraday,

the experimenter who formulated the fundamental concepts of electromagnetism. These concepts

basically state that if a conductor, or wire, carrying current is placed in a magnetic field, a force will

act upon it. The magnitude of this

force is a function of strength of the

magnetic field, the amount of current

passing through the conductor and

the orientation of the magnet and

conductor. The direction in which

this force will act is dependent on the

direction of current and direction of

the magnetic field.

Electric motor design is based on the

placement of conductors (wires) in a

magnetic field. A winding has many

conductors, or turns of wire, and the

contribution of each individual turn

adds to the intensity of the interac-

tion. The force developed from a

winding is dependent on the current

passing through the winding and the magnetic field strength. If more current is passed through the

winding, then more force (torque) is obtained. In effect, two magnetic fields interacting cause

movement: the magnetic field from the rotor and the magnetic field from the stators attract each

other. This becomes the basis of both AC and DC motor design.

FORCE

CURRENT

Fig. 1 - CONCEPT OF ELECTROMAGNETISM

AC MOTORS

Most of the world's motor business is addressed by AC motors. AC motors are relatively constant

speed devices. The speed of an AC motor is determined by the frequency of the voltage applied

(and the number of magnetic poles). There are basically two types of AC motors: induction and

synchronous .

INDUCTION MOTOR. If the induction motor is viewed as a type of transformer, it becomes

INDUCED VOLTAGE

AND CURRENT

ROTOR

FIELD

STATOR

FIELD

Fig. 2 - INDUCTION MOTOR

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Servo Control Facts

easy to understand. By applying a voltage onto the primary of the transformer winding, a current

flow results and induces current in the secondary winding. The primary is the stator assembly and

the secondary is the rotor assembly. One magnetic field is set up in the stator and a second magnet-

ic field is induced in the rotor. The interaction of these two magnetic fields results in motion. The

speed of the magnetic field going around the stator will determine the speed of the rotor. The rotor

will try to follow the stator's magnetic field, but will "slip" when a load is attached. Therefore

induction motors always rotate slower than the stator's rotating field.

Typical construction of an induction motor consists of 1) a stator with laminations and turns of cop-

per wire and 2) a rotor, constructed of steel laminations with large slots on the periphery, stacked

together to form a "squirrel cage" rotor. Rotor slots are filled with conductive material (copper or

aluminum) and are short-circuited upon themselves by the conductive end pieces. This "one" piece

casting usually includes integral fan blades to circulate air for cooling purposes.

STATOR LAMINATIONS

STATOR WINDINGS

SHAFT

FAN

BLADES

SQUIRREL CAGE

ROTOR

HOUSING

Fig. 3 - CUTAWAY OF INDUCTION MOTOR

The standard induction motor is operated at a "constant" speed from standard line frequencies.

Recently, with the increasing demand for adjustable speed products, controls have been developed

which adjust operating speed of induction motors. Microprocessor drive technology using meth-

ods such as vector or phase angle control (i.e. variable voltage, variable frequency) manipulates the

magnitude of the magnetic flux of the fields and thus controls motor speed. By the addition of an

appropriate feedback sensor, this becomes a viable consideration for some positioning applications.

Controlling the induction motor's speed/torque becomes complex since motor torque is no longer a

simple function of motor current. Motor torque affects the slip frequency, and speed is a function

of both stator field frequency and slip frequency.

Induction motor advantages include: Low initial cost due to simplicity in motor design and con-

struction; availability of many standard sizes; reliability; and quiet, vibration-free operation. For

very rapid start-stop positioning applications, a larger motor would be used to keep temperatures

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Servo Control Facts

within design limits. A low torque to inertia ratio limits this motor type to less demanding incre-

menting (start-stop) applications.

SYNCHRONOUS MOTOR. The synchronous motor is basically the same as the induction

motor but with slightly different rotor construction. The rotor construction enables this type of

motor to rotate at the same speed (in synchronization) as the stator field. There are basically two

types of synchronous motors: self excited ( as the induction motor) and directly excited (as with per-

manent magnets).

The self excited motor (may be called reluctance synchronous) includes a rotor with notches, or

teeth, on the periphery. The number of notches corresponds to the number of poles in the stator.

Oftentimes the notches or teeth are termed salient poles. These salient poles create an easy path for

the magnetic flux field, thus allowing the rotor to "lock in" and run at the same speed as the

rotating field.

A directly excited motor (may be called hysteresis synchronous, or AC permanent magnet synchro-

nous) includes a rotor with a cylinder of a permanent magnet alloy. The permanent magnet north

and south poles, in effect, are the salient teeth of this design, and therefore prevent slip.

In both the self excited and directly excited types there is a "coupling" angle, i.e. the rotor lags a

STATOR

SHAFT

ROTOR

SHAFT

STATOR LAMINATIONS

STATOR

WINDINGS

ROTOR

WITH TEETH

OR NOTCHES

HOUSING

Fig. 4 - CUTAWAY OF AC SYNCHRONOUS MOTOR

small distance behind the stator field. This angle will increase with load, and if the load is

increased beyond the motor's capability, the rotor will pull out of synchronism.

The synchronous motor is generally operated in an "open loop" configuration and within the limi-

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(notes) Electronics - Basic in Motors.pdf

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