machine tool drives

8/10/2019 machine tool drives

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Machine tool DrivesTanuj Kum

Sidharth Sing


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Overview

Introduction

Reversing

Speed Control, Changepole Motors

Stepless Speed variation, A.C motors

Stepless Speed variation, D.C motors

Stepless Speed variation, Ward Leonard method


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Introduction

Machining is any of various processes in which a piece of raw materiacut into a desired final shape and size by a controlled material-removalprocess.

To obtain a machined part by a machine tool, coordinated motions mustbe imparted to its working members. These motions are either primarmovements, which removes the chips from the work piece or auxiliarmotions that are required to prepare for machining and ensure thesuccessive machining of several surfaces of one WP or a similar surface odifferent WPs.

Principal motions may be either rotating or straight reciprocating. In somemachine tools, this motion is a combination of rotating and reciprocatingmotions. Feed movement may be continuous (lathes, milling machine,drilling machine) or intermittent (shapers, planers).


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Reversing

Reversing is a logical step further in the function of electrics applied tmachine tools.

In case of D.C motor the polarities of the input are changed to reverse thedirection of rotation.

While in case of 3-phase A.C motor, the sequence of the input is changedto reverse the direction of rotation.


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Speed Control, Changepole Motors

One of the fundamental problem associated with the machine tool drive isthat of speed control.

In case only a single speed is required, the squirrel cage induction motor isthe obvious choice of drive.

In this case:

revolutions/sec = (frequency) / (No. of pair of poles)

Unfortunately, No method is available to change the frequencconveniently, so that speed variation is only possible in steps by pochanging.


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Stepless Speed variation, A.C motors

Slip ring induction motor can give stepless speed variation by reducing therotor voltage which is possible in a simple manner by variable resistance inthe rotor.

But this method is not generally recommended because it introduces thelosses and unless the load is held constant the speed varies with the loadfor a given setting.

The A.C. commutator motor is designed to overcome the inheren

difficulties of speed control associated with a slip ring induction motwhich works on the principle of injecting voltage regulation.

The former machine has its speed varied by altering the position of thebrushes, and the latter machine has its speed varied by movement of anexternal regulator, Either constant torque or constant horse powecharacteristics can be obtained


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The main current passes through the armature circuit and a very smalcurrent passes through the branch circuit which supplies the field coils.

Speed increase of 3 to 1 is possible by varying this small current by means oa variable resistance (i.e., shunt regulation).

Speed decrease can be obtained by inserting a resistance in the armaturecircuit, but this has the same objections referred to in connection with theslip ring motor.

Stepless Speed variation, D.C motors


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This method is used when a large speed range is required, say in the orderof 10/1.

Ward Leonard method of speed control is used for controlling the speed ofa DC motor. It is a basic armature control method. This control system isconsisting of a dc motor M 1 and powered by a DC generator G. In thismethod the speed of the dc motor (M 1) is controlled by applyingvariable voltage across its armature. This variable voltage is obtained usinga motor-generator set which consists of a motor M 2(either ac ordirectly coupled with the generator G. It is a very widely used methodof speed control of DC motor.

Ward Leonard method


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Principle of Ward Leonard Method


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speed of motor M 1 is to be controlled which is powered by the generator G.The shunt field of the motor M 1 is connected across the dc supply lines.

Now, generator G is driven by the motor M 2.

The speed of the motor M 2 is constant. When the output voltage of the generator is fed to motor M 1 then the motor starts to rotate. When the output voltage of the generator varies thenthe speed of the motor also varies. Now controlling the output voltage of the generator thespeed of motor can also be controlled. For this purpose of controlling the output voltage, afield regulator is connected across the generator with the dc supply lines to control the fieldexcitation. The direction of rotation of the motor M 1 can be reversed by excitation cthe generator and it can be done with the help of the reversing switch R.S. But the motor-generator set must run in the same direction.


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Advantages of Ward Leonard System

1. It is a very smooth speed control system over a very wide range (from zeroto normal speed of the motor).

2. The speed can be controlled in both the direction of rotation of the motoreasily.

3. The motor can run with a uniform acceleration.

4. Speed regulation of DC motor in this ward Leonard system is very good.


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Disadvantages of Ward Leonard System

1. The system is very costly because two extra machines (motor-generatorset) are required.

2. Overall efficiency of the system is not sufficient especially it is lightlyloaded.


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Application of Ward Leonard System

This Ward Leonard method of speed control system is used where a verywide and very sensitive speed control is of a DC motor in both the directionof rotation is required. This speed control system is mainly used in collierywinders, cranes, electric excavators, mine hoists, elevators, steel rolling millsand paper machines etc


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Electrical Stepless Speed Drive

Figure shows the Leonard set, which consists of an induction motor thatdrives the direct current generator and an exciter (E). The dc generatorprovides the armature current for the dc motor, and the exciter providesthe field current; both are necessary for the dc motors that drive themachine tool.


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Electrical Stepless Speed Drive(contd.):

The speed control of the dc motor takes place by adjusting both thearmature and the field voltages by means of the variable resistances A andF, respectively. By varying the resistance A, the terminal voltage of the dcgenerator and hence the rotor voltage of the dc motor can be adjustedbetween zero and a maximum value. The Leonard set has a limitedefficiency: it is large, expensive, and noisy. Nowadays, dc motors andthyrestors that permit direct supply to the dc motors from alternating

current (ac) mains are available and, therefore, the Leonard set can becompletely eliminated. Thyrestor feed drives can be regulated such thatthe system offers infinitely variable speed control.


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Solid State Drive

SOLID-STATE ac drives for machine tools have seen ever-increasing use in recent

years. They have found a wide range of applications in turning, milling, andgrinding. Fig. 1 shows the horsepower- speed areas covered by theseapplications.


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Given drive has a rectifier input to generate a constant-voltage dc bus, a chopper toobtain a variable-voltage dc bus, and an inverter section to produce variable-voltagevariable-frequency power to operate an ac induction motor. Solid-state dynamic brakingcan dissipate power that is transiently returned from the motor to the drive whendecelerating.


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As in most cutting applications, high torque is required at low speed, andonly low torque is needed at high spindle speed. Therefore, most ac drivesare designed for a constant torque range at lower speeds, with motorvoltage and frequency increasing proportionally (constant V/Hz) up tobase speed. Above base speed only motor frequency is increased.


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AC vs DC Drives

Compared with dc drives, ac drives can go to higher motor speeds,because of the lack of a commutator and the rugged squirrel-cage rotor.DC drives require a field regulator and a tachometer feedback, adding tocost and complexity, and run into commutation limitations. Normally, thespeed linearity and speed regulation requirements for milling machines canbe met with a standard ac drive, without tachometer feedback. If betterspeed regulation is needed, slip compensation can be employed. Thedrive load is sensed and the motor frequency is increased to compensatefor speed drop due to motor slip.


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Dynamic Braking

Braking torque is develoelectronically controlledstate dynamic braking, dpower returned to the drresistor. While dynamic with dc drives is possiblpractical for tapping becbraking torque decreases

speed, resulting in the ndeceleration curve show6. DC drives therefore mregenerate power back inac line, which takes a mcomplicated and costly d


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machine tool drives

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