ELECTRIC DRIVES. Prepared by:- Er . Varinder Kaur. •While selecting a motor for a particular drive, the mechanical characteristics are also taken into account. •The following features determine the suitability of the motor. 1.Types of enclosures 2.Bearings 3.Noise 4.Transmission of drive
Er. Varinder Kaur
•While selecting a motor for a particular drive, the mechanical characteristics are also taken into account.
•The following features determine the suitability of the motor.
1.Types of enclosures
4.Transmission of drive
1. Types of enclosures :-
•All the major parts of the motors such as windings, bearings, insulation etc are to be protected from the surroundings contaminated air.
•In an industry the air surrounding the motors may contain metal, dust, oil, mist, water, dust inflammable fumes etc. Also accidents may occur to persons coming in contact with the moving parts. Therefore it is necessary to provide proper enclosures.
The different types of enclosures are as follows:-
•This type can only be used where the atmosphere and surroundings are free from all contaminations and surrounding air completely dry.
•The advantage of this type of motor is that the cost of cooling is very low.
•But this type is rarely used since there is no protection to the motor parts.
b)Screen protected type
In this type of machines openings provided for ventilation are covered with wire mesh screen.
•This type of enclosures does not protect the motor against dirt and dust.
c)Drip proof type
•This motor has ventilating opening provided in such a way that drops of liquid or solid falling on it vertically are prevented to enter inside.
•This type of motor cannot be used where inflammable dust particles are present in the surrounding air.
•Such motors are used in damp atmosphere. E.gPumpsets.
d)Totally enclosed type
•This type of motors has solid frames and end shields but no opening for ventilation.
•They are cooled by surface radiation only. In this type machines no dirt or foreign matter can enter and block the air passage.
•These machines are used for very dusty atmosphere.
•E.g saw mills, coal handling plants and stone crushing quarries.
e)Splash proof type
•In this type, the ventilation ducts are provided in such a way that drops of liquid or solid particles reaching the machine at any angle between vertical and 100˚ from it cannot enter the machine.
•These enclosures do not communicate an internal fire to the external environment.
•Hence these motors are used in coal mines, gas plants, oil refineries etc.,
•where the risk of fire is more.
g)Pipe ventilated type
•Large sizes of totally enclosed motor employ pipe ventilation.
•Air is drawn through pipe from outside the building, where clean air is available and forced to cool the motor.
It consist of an inner and outer race and cage containing steel roller or balls.
•The outer race is attached to the housing(end cover) and the inner race is attached to the shaft.
•When the shaft rotates, the steel ball also rotates. Hence the friction of the shaft is minimized.
•It has a longer life and maintenance costs are low.
•It occupy less space. But the initial cost of ball and roller bearings is high. It is used in three phase induction motor where smaller air gap is possible.
•It is used for chain, belt and gear drives.
2) Sleeve or brush bearing
•Sleeve or brush bearings are normally made of bronze.
•The rotating shaft is supported by bearing component and is rigidly fixed to the frame
of the machine.
•Various methods employed for transmission of mechanical power are described below.
4.Chain drive and
1. Direct drive
•In direct drive, motor is coupled directly to the driven machine with the help of solid or flexible coupling.
•Flexible coupling protects the motor from jerks.
•It is more efficient and requires minimum space and it is the simplest method.
•It can be used where driven and driving machine speed are same.
2. Belt drive
•In belt drive, belt is used to transmit the power from motor to driven machine through pulley system.
•The mechanical power wasted due to slip is about 3 to 4 percentage.
•Maximum power of 300 H.P can be transmitted through this drive .
•This method for transmission for power is used, when it is not possible to employ belt drive.
•A number of ropes run in V-grooves over pulleys.
•The advantages of rope drive are negligible slip and ability of taking sudden loads.
•It is mainly used in lift and cranes.
•Chain drive is very costly in comparison to belt and rope drive.
•It can be used for high speed ratio (upto 6:1).
•It is more efficient and transmits large amount of power.
•It is noiseless, sliplesss and smooth in operation
1.Class Y:- Cotton yarn and fabrics, paper and fibrous materials of cellulose or silk neither impregnated with nor immersed in liquid dielectric. The limiting temp is 90°C.
2. Class A:- The same material as mentioned above but impregnated with or immersed in liquid dielectric . Limiting temp. is 105°C.
3. Class E:- Certain synthetic organic films and other materials having the same thermal stability. Limiting temp is 120°C.
4. Class B :- Mica , glass fibre base materials combined with an organic binding agent. Limiting temperature is 130°C.
5. Class F :- Limiting temperature is 155°C.
6. Class H:- Limiting temperature is 180°C.
7.Class C :- Mica , glass or quartz .Limiting temperature is 155°C.
A machine rating specifies the voltage ,current, speed, excitation, power factor, efficiency, power output etc. The rating of a machine should give all information necessary so that if the machine is operated within the limits of all factors specified in its rating ,the machine will operate satisfactory and safely and will give reasonable length of service. Rating of machine must give necessary information to safeguard the application of the machine from conditions of operation for which
would result in unsafe mechanical or electrical strains upon any part of its structure.
would result in excessive deterioration of the mechanical or electrical characteristics of the materials of which the machine is constructed.
Generators & motors are rated in terms of kW output at a given speed and voltage. The size and rating of electric machine for any service is mainly governed by the factor ‘temperature rise’. The maximum temperature to which an electric machine is allowed to reach is limited by the type of insulation used.
•The rating of motor is the amount of power which it can deliver without becoming unduly hot.
The rating of a motor is classified as follows.
2.Intermittent rating or short time rating.
•This is the rating or the output of a motor which can be delivered continuously for long periods without exceeding the permissible temperature.
•This rating is applicable to drives like fans, pumps, textile, mills etc. which operate continuously for long periods.
Intermittent rating or short time rating
•This is an output that a motor can give for specified short time without exceeding the permissible temperature rise.
•Such motor is loaded for short period of time and is then put off for sometime.
•During that period the motor cools off as in mixies.
(i)Speed of driving and driven machines
(iv)Clutching arrangement required
•The choice of motor speed is the most important factor as it not only affects the performance of motor but also overall cost The dimension and, therefore, the first cost of a motor for a given output are approximately inversely proportional to the speed, so for the some output kW the cost of a high speed motor is less than that of a slow speed motor.
•In case of induction motor, the efficiency and power factor decreases with decrease in speed.
•Thus for a low-speed drive high speed motor using a reduction gear is usually found cheaper than a low-speed direct-coupled motor.
1.Continuous duty and constant load:
•For most of the applications, the rating can be determined from the equation given as under:
P = TN/975η kW---------------------------------- (1)
Where, T = Load torque, kg-m, N = Speed, r.p.m., and η = Product of the efficiency of the driven equipment and that of transmitting device.
•In case of linear motion, the rating of the motor required is given by,
P = F* v/2 * 102 η kW------------------------------------ (2)
Where, F = force caused by the load, Kg, and v = velocity of motion of the load, m/s.
•Equation (2) is directly applicable in case of hoisting mechanisms. It is also suitable for lifts or elevators, it should be modified as follows: P = F* v/2 * 102 η kW The velocity of normal passenger lift cabins vary from 0.5 to1.5 m/s.
•In case of pumps, the rating can be determined from the following relation:
P = ρQH/102 η Kw
Where, ρ = Density of liquid pumped, kg/m3,
Q = Delivery of pumps, m3/s, and H = Gross head (static head + friction head), m.
η varies from 0.8 to 0.9 for reciprocating pumps and from 0.4to 0.8 for centrifugal pumps.
• The rating of a fan motor is given by,
P = Qh/102 η kW
Where, Q = volume of air or any other gas, m3/s, and h = Pressure in mm of water or kg/m2.For small power fans, the efficiency η may be taken as 0.6 and for large power ones it may reach a value up to 0.8.
• The rating of a motor used in metal shearing lathes can be found from the relation.
P =F* v/ 102 * 60 η kW
Where, F = shearing force, kg,
v = Velocity of shearing, m/min, and η = Mechanical efficiency of the lathe.
The following are the commonly used methods for determination of motor rating for variable load drives
(i)Method of average losses
(ii)Equivalent current method
(iii)Equivalent torque method
(iv)Equivalent power method.
(i) Method of average losses (Qav)
This method is based on the assumption that the actual variable current may be replaced by an equivalent current Ieq which produces the same losses in the motors as the actual current
Ieq =√(I12t1+ I22t2+ I32t3+ ……+ In2tn) / ( t1+ t2+ t3+ ……+ tn)
•The heating and cooling conditions in self ventilated machines depend upon its speed. At low speed the cooling conditions are poorer than at normal speeds.
•The equivalent current as found from eqn should be compared with the rated current of the motor selected and the conditions Ieq< Inom should be met.
•The machine selected should also be checked for its overload capacity.
•For D.C motors………….. Imax/ Inom< 2 to 2.5
•For induction motors ……… Imax/ Inom< 1.65 to 2.75
•In case the overload capacity of the motor selected is not sufficient it becomes necessary to select a motor of high power rating.
•The equivalent current in such cases is calculated from the following expression:
Ieq= √[(1/1∑n) t∫∑ti2dt]
•The value of the integral may be found with the help of integral.
•The current values obtained by this method are sufficiently accurate for practical purposes.
•For the selection of suitable capacity of the motor it often becomes necessary to use torque or power load diagrams.
•The equivalent torque or power is found in the same manner as the equivalent current.
•Assuming constant flux and constant power factor, the torque is directly proportional to current and, therefore, the equivalent torque is:
Teq= √ [T12t1+ T22t2+ T32t3+…. + Tn2tn/ t1+ t2+ t3+……..+ tn]
•The equation for equivalent power follows directly from above eqn. as power is directly proportional to the torque.
•At constant speed or where the changes in speed are small, the equivalent power is given by:
Peq= √ [P12t1+ P22t2+ P32t3+…. + Pn2tn/ t1+ t2+ t3+……..+ tn]
•The “equivalent current method” is the most accurate out of all the above methods discussed above.
•This method may be used to determine the motor capacity for all uses except where it is necessary to take into account the changes in so ‘constant losses’ i.e. the iron and mechanical losses.
•The “equivalent torque method” cannot be used for cases where equivalent current method cannot be applied.
1.Paper mill- Synchronous motor A paper mill requires a drive which must fulfill the following requirements
•To manufacture different thickness of papers it is required to vary the speed of entire series of rolls.
•Relative speed of rolls should be constant otherwise the paper may be tearing.
•It is required to adjust the speed at any one group of rolls relative to other in order to draw the paper.
2.Rolling mills or steel mills – separately excited DC motor
•Separately excited DC motor is mainly used in rolling mills.
•The motor required for these mills should have high starting torque about 2 to 2.5times the rated torque.
•It should have strong construction.
•The ward leonard speed control of D.C motors or slip ring induction motors are used.
3.Textile mills – Double cage induction motor
•In textile mills group drive is employed.
•The motors employed must have high starting torque with constant speed.
•The motors used must be totally enclosed and moisture proof to prevent entry of dust and moisture enter into machine.
•Hence totally enclosed, fan cooled, high torque double cage induction motors are used.
6.Lift and hoists – DC compound motor or 3Ф slip ring induction motor
•The essential requirements for a lift are high overload capacity, high smooth accelerating torque of 2 to 2.5 times the full load torque at starting and maximum degree of silence.
•D.C compound motor and three phase slip ring induction motor are used for lifts and hoists.
7.Belt conveyor – Double squirrel cage induction motor
•The conveyors are required to transport bulk materials like coal, are sand on either flat belt or bucket system.
•It requires a high starting torque so as to accelerate the load for transport.
•Hence totally enclosed surface cooled motors are used.
•Double squirrel cage induction motors are used in belt conveyors.
8.Ship – Synchronous motors Three phase induction motors and synchronous motors are used for very big ships.
•A three phase alternator gives the supply to the synchronous motor.
•The prime mover used for the alternator is steam turbine by varying the voltage and frequency of alternator the speed of motor is controlled.
9.Air compressor - 3 Ф Induction motor
•Air compressors are used for pneumatic drill, 3Ф induction motors are used to drive compressors.
•Repulsion motor is used for various industrial machinery air compressors.
•Single phase induction motor is used for small air compressors.
10.Punches and shears
•For punches and shears D.C cumulative compound motors and
•A.C 3 Ф slip ring induction motors provided with fly wheel are used.
•The rotary printing machinery requires variable speed motor.
•D.C compound motors or A.C 3 Ф induction motors with rotor resistance control are used for printing machineries.
•The load torque varies as square of the speed in a centrifugal pump.
•At starting the torque required is less.
•Hence 3 Ф squirrel cage induction motor is used for centrifugal pump.
•The liquid handled by the pump does not enter the motor.
•Hence totally enclosed motor is preferred.
•A reciprocating pump requires two times the full load torque at starting.
•A double cage induction motor is suitable for reciprocating pump.
•3 Ф slip ring induction motor is also used for this type of pump.
•D.C shunt motor is used where D.C supply is available.
•The single phase split phase induction motors are used for draught fan.
•The single phase split phase induction motor has shunt characteristics and so the operating speed is almost constant.
•Single phase capacitor start and run motors are used for ceiling fan.