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Electrical Drives of Machine Tools.ppt (Size: 1.31 MB / Downloads: 205)

Electrical Drives of Machine Tools

Introduction

The arrangement of the motor nearby the operative member and also the use of variable-speed electric motors make it possible to simplify mechanical transmissions as well as the construction of the machine.

This helps to improve machine-tool design and considerably reduce the physical force required to handle the machine.

Introduction

One or more electric motors driving the operative members of the machine tool make up the machine's essential components.

The great majority of machine tools are driven by alternating-current (AC) three-phase induction motors.

These motors are simple, reliable and inexpensive.

Direct-current (DC) motors are not so common and are used mainly to drive heavy machine tools.

Induction Motors

There are electric motors with speed-torque characteristics known as drooping, rigid and absolutely rigid.

With drooping speed motors, change of torque (i.e., load) results in substantial motorÃ‚Âspeed change.

If change of torque does not markedly affect the motor speed, such a motor is said to, have a rigid characteristic.

A motor possesses an absolutely rigid characteristic if its speed does not depend on load changes.

Induction Motors

The slippage of a motor is denoted by s, which represents the ratio between the fall in motor speed under load as compared with the speed of idling (when M = 0), and given by the following equation:

s = (n0 â€œ n)/n0

where

n0 = speed of rotating magnetic field (synchronous speed of electric motor), rpm

n= rotor speed (asynchronous), rpm

Slippage s is expressed as a percentage or decimal fraction.

Induction Motors

The torque developed by the induction motor can be roughly estimated by means of the following formula:

M = 2Mc /[(s/sc)+(sc /s)]

where

Mc =critical torque (maximum load) of motor

Sc= critical slippage corresponding to Mc

Induction Motors

Figure 53 depicts the induction motor speed-torque characteristics.

With n being equal to n0, M = 0, which corresponds to the idling of the motor.

When the motor is started and the rotor is still immovable (s = 1), the motor develops starting torque MS, which is higher than the rated (or nominal) torque Mr.

The values of Mc and sc determines the critical point (or the maximum) of the characteristic.

Fig. 53 Speed-torque characteristic of a.c. motors

Induction Motors

The interval between the idling point and the critical point of the characteristic is called the working interval.

It is this interval that is suitable for stable operation with the motor speed not depending on the torque change.

The rated value of slippage depends on the type of motor and its rated power, and is within the limits of 0.02-0.12.

The higher the motor power, the lower the slippage.

Induction Motors

In addition to plain induction motors, there are motors with higher starting torque and slippage (0.07-0.16).

While motors with normal slippage have a rigid speed-torque characteristic and are used in most machine tools, motors with higher slippage have a drooping characteristic and are used in machine-tool drives with frequent motor starts and considerable starting loads.

Induction Motors

Curve 1 shown in Fig. 53 is for a rigid characteristic motor, and curve 2 -for a drooping characteristic motor.

It is evident from the Figure that, all other things being equal, the drooping characteristic motor has lower rated speed and higher starting torque Ms.

Point A represents the rated value of load.

Induction Motors

The following induction motor specifications are usually given in catalogues:

Rated power Nr, kW;

rated speed n, rpm;

synchronous speed n0, rpm,

ratios Mc /Mr and Ms /Mr (where Mr and Ms are rated and starting torques, respectively).

Induction Motors

The value of Mr is determined from the formula:

Mr = 9550 Nr/n Nm (kgf. m)

Ratio Mc/Mr defines the value of permissible mechanical over load of the motor.

Mc /Mr = 1.7-2.5

Induction Motors

The value of Mc depends on mains voltage.

As the voltage value may change, the maximum permissible value of overload is assumed to be 0.85 Mc.

With general-purpose induction motors having cage rotors, ratio Ms /Mr = 0.8-2.

Principal movement drive motors are started up under no load, so the starting torque Ms< Mr /2 is sufficient.

Motors started up under load should develop higher starting torques.

Changing a.c. motor speed

The rotor speed of a cage-rotor induction motor is found from the formula

n = (60f/p)(1- s) rpm

where

f = alternating current frequency, Hz

p = number of pole pairs

s = slippage of rotor

It is evident from the formula that motor speed can be changed by:

changing current frequency,

slippage or

the number of pole pairs.

Changing a.c. motor speed

With the frequency of alternating current in the mains being constant, the first method can be applied only if there is a separate a.c. generator to feed the electric motor.

The second method of speed changing - by change of slippage - is accomplished by introducing effective resistance into the rotor circuit, which can be done only with wound-rotor induction motors.

The third method of speed changing - by changing the number of pole pairs - is the most widely used in machine tools. It involves the use of multi-speed pole-change motors.

Direct Current Motors

Direct current motors with shunted excitation (shunt-wound motors) are extensively used in heavy machine-tool drives.

They are connected according to the circuit diagram shown in Fig. 54.

The armature winding A is connected to the mains through starting rheostat 1, exciting (shunt) winding SW, and rheostat 2 used for speed variation.

Fig. 54. Circuit diagram of shunt-wound motor connection

Direct Current Motors

The torque and speed values for the d.c. motor are determined by means of the following formulas:

M= kIaF, Nm (kgf.cm);

n= [(V â€œ Iara)/cF] rpm

where M = torque developed by motor, Nm

n = motor speed, rpm

V = mains voltage, V

Direct Current Motors

Ia = current intensity in armature winding, A

ra = armature circuit resistance, ohm

c = constant of given motor

k = 0.05-0.12 - proportionality factor

F = magnetic flux of motor, Wb.s

The speed-torque characteristics of the motor are shown in Fig. 55.

Fig. 55 Speed-torque characteristics of shunt-wound dc motor.

Changing the speed of d.c. motors

Change of d.c. motor speed can be effected:

by changing the armature circuit resistance,

by changing the magnetic flux, and

by changing the input voltage.

The first method is explained in the next sheet though it is rarely used because it involves energy losses.

Direct Current Motors

Number 1 denotes the line corresponding to the rated speed torque characteristics.

The relatively small value of armature winding resistance determines a sufficiently rigid rated characteristic of the shunt-wound motor, as shown graphically by the modest slope of line 1.

With the motor in operation, the resistance of rheostat 1â„¢ can be increased; this will result in an increase of, the total armature circuit resistance leading to slopes 2, 3 and 4 of the characteristic line.

The second method-by changing the magnetic flux- is the most commonly used. The magnetic flux value is changed by rheostat 2 (Fig. 55).

Fig. 55 Speed-torque characteristics of shunt-wound dc motor.

Changing the speed of d.c. motors

The rheostat resistance being increased, the exciting current and magnetic flux are reduced, which results in an increase in idling motor speed and slope of motor speed-torque characteristics, represented by a number of straight lines (5, 6, 7, 8).

The number of these characteristics depends on the number of steps on rheostat 2â„¢.

Where the number of rheostat steps is large, motor speed changing becomes practically stepless.

Changing the speed of d.c. motors

Line 9 in Fig. 55 represents the speed-torque characteristic of motor with reversed armature polarity, in which the direction of motor rotation is reversed.

The third method of speed changing - by changing the input voltage - involves the use of special circuitry and is employed in generator-motor systems.

The Generator-Motor System

This system, known also as the Ward-Leonard system, is used in heavy and high-power machine tools with frequent motor reversal or where infinitely variable speed or feed are required.

This system also facilitates the starting of the machine tool.

The system (Fig. 56) consists of induction a.c. motor 2; d.c. generator 3 driven by motor 2; self-excited direct-current generator 1 for the excitation of generator 3 and motor 4.

The Generator-Motor System

The Generator-Motor System

d.c. motor 4 is used as the drive motor of the machine tool.

Motor 4 is started up by means of shunt-circuit rheostat 6 connected to the exciting winding of generator 3.

Rheostat 6 reduces the magnetic flux of generator 3.

The Generator-Motor System

This gives a very small value of voltage on the generator brushes, which is then gradually increased.

As motor 4 is sped up, it develops a back electromotive force (emf) and the shunt-circuit rheostat is gradually switched off, increasing the generator voltage.

The Generator-Motor System

The generator-motor system allows the speed of motor 4 to be varied in two ways:

(a) by changing the input voltage fed into motor 4 with the aid of rheostat 6 (by changing the magnetic flux of generator 3); and

(b) by changing the excitation magnetic flux of motor 4 by means of rheostat 5.

The reversal of motor 4 is accomplished by changing over the direction of current in the exciting winding of generator 3 with the aid of switch 7.