Types and Characteristics of Load Torque

Classification of load torques :

1.Active Load torques

2.Passice Load torques

Active Load Torques:

Load torques which have the potential to drive the motor under equilibrium conditions are called active load torques.

Load torques usually retain sign when the drive rotation is changed.

Active Load Torques

Passive Torque:

Load torques which always oppose the motion and change their sign on the reversal of motion are called passive load torques.

Torque due to friction cutting – Passive torque.

Components of load torques:

1.Friction Torque (TF)

The friction torque (TF) is the equivalent value of various friction torques referred to the motor shaft.

2.Windage Torque (Tw)

When a motor runs, the wind generates a torque opposing the motion . This is known as the winding torque.

3.Torque required to do useful mechanical work ( Tm)

Nature of the torque depands of type of load.

It may be constant and indeoendent of speed, Some function of speed, may be time invariant or time variant.

The nature of the torque may change with the change in the loads mode of operation.

Characteristics of different types of load:

In electric drives the driving equipment is an electric motor.

Selection of particular type of motor driving a m/c is the matching of speed-torque charal of the driven unit and that of the motor.

· Different types of loads exhibit different speed torque charal.

· Most of the industrial loads can be classified into the following 4 general categories:

1. Constant torque type load.

2. Torque proportional to speed (generator type load)

3. Torque proportional to square of the speed (fan type load)

4. Torque inversely proportional to speed (const power type load)

1.Constant Torque Characteristic :

The speed – torque characteristic of this type of load is given by T=K.

Working motor have each mechanical nature of work like shaping , cutting, grinding or sharing, require constant torque irrespective of speed. Similarly cranes during the hoisting.

Similarly cranes during the hoisting and conveyors handling constant weight of material / unit, time also exhibit this type of characteristics.

Torque proportional to speed:

Separately excuted dc generators connected to a constant resistance load, eddy current brakes and calendaring m/cs have a speed torque characteristics m/cs have a speed – torque characteristics given by T= Kw.

Torque propositional to square of the speed :

Load Torque Square of speed

Example : Fans , Rotary pumps , compressors , ship propellers. The speed – torque characteristics of this type of load is given by

Torque inversely propositional to speed:

· In such types of loads , torque is inversely proportional to speed or load power remains constant.

· Eq: Lathes, boring m/cs, milling m/cs , steel mill colier and electric traction load.

· This type of characteristics is given by

· Most of the load require extra effort at the time of starting to overcome static friction. In power application it is known as brake away torque and load control engineers call it “stiction” . Because of slition , the speed torque characteristics of the load is modified near to zero speed.

Classification of Electric Drive Based on Power Supply

Classification of Electric Drive:

Main classification of electric drives are :

1. DC drives and

2. AC drives

 

DC drives :

A dc motor speed can be controlled by using a power converter. Here motor is coupled with load.

 

AC drives:

An AC motor speed can be controlled by power converter. Here motor is coupled to the load.

The main purpose is to vary the motor speed.

 

Comparison between DC and AC drives:

                    DC drives                                                                  AC drives

1. Power circuit and control circuit is simple 2. Frequent Maintenance. 3. Commulator makes bulky , costly and heavy. 4. Speed and design rating are limited due to commutation. 5. This is used in certain location 6. Fast response and wide speed range smooth achieved by conventional and solid state control 7. Poor PF, harmonic distortion of the current. 8. Power / weight ratio is small.

1. Power and Control circuit is complex. 2. Less Maintenance. 3. Problems are not there, particularly squirrel cage motor. 4. Ratings have no upper limits. 5. Used in all location 6. In solid state control speed range is wide and conventional method it is stepped and limited. 7. For Regenerative drives the line pf is poor, for non-regenerative drives the line PF is better. 8. Power / weight ratio is large.

Solid State Drives – Parts of Electric Drives

Parts of Electric Drives:

1. Electrical motors and load

2. Power modulator

3. Source

4. Control unit

5. Sensing unit.

Electric Motors:

Most commonly used electric drives are

DC motos – Shunt , Series. Compound and permanent magnet.

Induction Motors – Squirel cage , wound rotor and linear

Synchronous motor - Wound field and permanent magnet

Brushless Dc motors , stepper motors, and switched reluctance motors can be used.

Power Modulator

It can classified as

1. Controlled rectifier (AC to DC converters)

2. Inverters (DC to AC converters)

3. AC voltage controllers (AC to AC converters)

4. DC choppers (DC to DC converters)

5. Cyclo converters (Frequency conversion)

Electrical Source:

Single Phase and Three Phase , 50Hz readily available in most locations.

Very low power drives fed from single phase source. Rest of the drives from three phase source.

Low and medium power motor are fet from 400v supply.

Higher rating , motors are fed from 3.3kv , 6.6 kv , 11kv.

Some drives are power from a battery , battery voltage may be 24v, 48v and 110v.

Sensing unit :

1. Speed sensing is required for implementation of closed loop speed control schemes.

2. Speed sensing by tachometers , high speed accuracies required digital tachometers are used.

3. Current sensing methods are

· Use of current sensor employing hall effect.

· Involves the use of a non-inductive resistance shunt in conjunction with an isolation amplifier which has an arrangement for an amplification and isolation bet power and control units.

Control Units :

· Control for power modulator is provided in the control unit.

· When semi conductor converts are used, the control unit will consist of firing circuits, which employ linear and digit integrated circuits and transistor and a up when sophisticated control is required.

Power Modulator and Simple Electric Drive

Block Diagram of a simple electric drive:

· System employed for motion control are called Drives.
· Drives employed electric motors are known as “Electric Drives”
· Load is usually a machinery designed to accomplish a given task eg: fans, pumps, robots, trains and drills.


Power Modulator:
· Modulates the flow of power from the source to the motor in such a manner that the motor will modulate the speed – torque characteristics required by the load.
· During transient operation , such as starting , breaking and speed reversal , it restricts source and motor currents within permissible values.
· Converts electrical energy of the source in the form of suitable to the motor.
· Select the mode of operation of the motor (i.e) Motoring or Braking.

Classification of Electric Drive with Advantage and Disadvantage

Classification of Electric Drive

Generally classified into 3 categories:

I. Group drive

II. Individual Drive

III. Multimotor Drive

 

Group Drive :

If several group of mechanisms or machines are organized on one shaft and driven or actuated by one motor, the system is called a group drive or shaft drive.

 

Advantage :

Most Economical

 

Disadvantage:

1. Any Fault that occurs in the driving motor renders all the driving equipment idle.

2. Efficiency low because of losses occurring in the energy transmitting mechanisms (Power loss)

3. Not safe to operate.

4. Noise level at the working spot is high.

5. Flexibility.

Single motor drives a no of machines through belt form common shaft.

 

Individual Drive:

1. If a single motor is used to drive or actuate a given mechanism and it does all the jobs connected with this load , the drive is called individual drive.

2. All the operations connected with operating a lathe may be performed by a single motor.

3. Each motor is driven by its own separated motor with the help of gears , pulleys etc.

 

Disadvantage:

Power loss occurs.

 

Multi Motor Drive:

· Each operation of the mechanism is taken care of by a separate drive motor.

· The System contains several individual drives each of which is used to operate its own mechanism.

· Separate motors are provided for actuating different parts of the driven mechanism.

 

Advantage :

1. Each Machine is driven by a separated motor it can be run and stopped as desired.

2. Machines not required can be shut down and also replaced with a minimum of dislocation.

3. There is a flexibility in the installation of different machine’s.

4. In the case of motor fault, only its connected machine will stop where as others will continue working undisturbed.

5. Absence of belts and line shafts greatly reduces the risk of a accidents to the operating personnel.

 

Disadvantage:

Initial high cost.

TANCET 2013 - Entrance Exam Dates and Timings

Anna University Chennai had announced the TANCET 2013 Entrance Exam Dates and Timings. The Exam will be held on 6th April for M.B.A and M.C.A , for others its on 7th April. The Timings are as follows :

M.B.A

Date : 6/4/2013 (Saturday)

Time : 10am to 12pm

M.C.A

Date : 6/4/2013 (Saturday)

Time : 2.30pm to 4.30pm

M.E/M.Tech/M.Arch/M.Plan

Date : 7/4/2013(Sunday)

Time : 10am to 12pm

All the Best...!!