Torque Speed Curve of Induction Motor Induction motors are the workhorse of modern industry. Three-phase induction motors are the most common and frequently encountered machines in industry because of their simple design, ruggedness, low-price, easy maintenance and wide range of power ratings ranging from fractional horsepower to 10 M. Induction motor runs essentially at constant speed from no-load to full load. Its speed depends on the frequency of the power source. It!s not easy to ha"e "ariable speed control and requires a "ariablefrequency power-electronic dri"e for optimal speed control. There are two types of rotor constructions in three phase induction motors, squirrel cage and wound rotor.
Fig 1 Cut Section of Three phase Induction Motor The most common motor type is the squirrel-cage motor, which has rotor windings consisting of copper or cast-aluminium bars solidly connected to conducting end rings on each end, forming a structure which resembles a squirrel cage. #ue to the simple rotor construction, the squirrel cage motor is rugged and durable, and is the most common type. ound-rotor motors are also a"ailable, usually for special application where e$ternal resistance is applied to the rotor for speed control. Induction motor works on simple principal of interaction of two magnetic fields theory. hen e$cited by %& supply, stator produces rotating magnetic field and induces %& "oltage in the rotor and this produces rotor magnetic field which tries to align with rotating stator magnetic field and hence rotor rotates. 'peed at which stator and rotor magnetic field rotates is often called as synchronous speed and gi"en by the equation
------------(1) where n s is in *+M, f is supply frequency in and p is no. of poles of motor, where as rotor or shaft speed is gi"en by the following equation or an induction motor, the speed will always be less than synchronous speed by a factor known as the slip of the motor. The motor speed can be e$pressed as
n = (1 –s)ns ----------(/)
where n is rotor or shaft *+M and s is the slip between synchronous and rotor speed.
Fi 2 Tor ue S eed Curve of
Fig 3 Torque speed Curve of
Typical torque speed cur"e of three phase induction motor is shown abo"e. %cceleration torque is the difference between motor torque and load torque and it accelerate the motor speed until motor torque is equal to load torque and this speed is called as steady state speed of motor for that particular load condition. In order to determine the acceleration time of a motor dri"ing a particular load, the torque vs. speed curv es of both the motor and load must be pro"ided, in addition to the load inertia and the power system information. The torque-speed cur"e for a gi"en load is a function of the specific nature of the load. Torque refers to the turning effort e$erted by the motor shaft. M% defines "arious torque characteristics for motors, which are designated as #esigns %,2,&, and #. with #esign 2 being by far the most common design used in industry. or each design classification, M% specifies performance parameters such as locked rotor torque, pull-up torque, breakdown torque (e$cept for #esign #), inrush current, and slip. The M% required minimum "alues are dependent upon the motor sie and speed. Three "alues of torque are generally of particular interest, and are as follows3 (i)
4ocked *otor Torque (4*T) - the torque de"eloped by the motor at stand still, also known as starting torque. (ii) +ull-up Torque (+5T) - the minimum torque de"eloped by the motor as it accelerates from ero speed to the speed at which breakdown torque occurs. (iii) 2reakdown Torque or +ull-out Torque (2#T or +6T) - the ma$imum torque that the motor is capable of de"eloping.
Table 1 Typical characteristics and applications of !edfrequency "ediu" #C squirrel
or instance, centrifugal loads such as centrifugal pumps and fans follow a square law relationship of torque "s. speed. That is, at ero speed, "irtually ero torque is required, but the torque requirement increases as the square of the speed (to 1007 torque at rated speed) as the load accelerates. 4oads of this nature are generally referred to as 89ariable Torque8 (9.T.). 4oads such as con"eyors, screw pumps, etc. are generally referred to as 8&onstant Torque8 (&.T.) loads, as they can require 1007 torque (current) at any or all speeds. 9.T. loads are therefore less demanding on motor starting performance, from the standpoints of torque and motor heating at less than full load speed. 4*T and 2#T are interdependent - each can be increased, but at the e$pense of the other. It is relati"ely easy to design a motor for a particularly high 4*T, or a high 2#T, but this would be at the sacrifice of the other. % thorough design should attempt to optimie both the 4*T and 2#T, and both should be considered when comparing torque.
The cur"es below shows typical torque speed requirement for each type of motor design. Therefore it is important to know the torque speed characteristics of a motor to be used for a particular dri"e application. There are three methods for plotting torque "s speed of motor. In first method motor is loaded with ad:ustable constant load and motor torque is measured along with other parameters such as speed, motor "oltage, motor supply current, pf, etc.. In second method, motor is loaded with inertia load such as fly-wheel of known inertia and then speed is recorded from starting to no-load condition. This allows acceleration to be computed using personal computer or +&. 'peed is measured by a rotary encoder and microcontroller circuit o"er complete range of speed "ariation. rom the following equation torque is determine at any gi"en speed. Fig $ Torque Speed Curve Characteristics of Type #% &% C and ' type Induction
---------(;) ---------(<)
Te = Ta + Tf
where Te is motor electric torque produced and measured in -M Ta is net acceleration torque and measured in -M Tf is torque equi"alent of friction plus windage losses and measured in -M /
J is total inertia of fly-wheel and rotor mass in =g-m . ω is rotor speed measured in rad>sec. Third method in"ol"es using motor parameters and motor equations to compute complete motor performance such as graphical method called circle diagram. Most of the time motor parameters are e$tracted by carrying out block rotor and no-load tests. #raw backs of first method is though this method is accurate for determining torque at gi"en load condition but it is time consuming due to coupling, "arying load condition and carrying out measurement. %s during loading normally all the energy is wasted (unlike regenerati"e loading which is "ery costly). %lso as the load is, though "ariable, it is constant type and hence can!t measure torque in unstable region ?fig 1@ of torque speed cur"e. Third method is an appro$imate andspace high limitation accuracy can!t be guaranteed and mostly preferred by re-winders of motormethod who ha"e to carry out full loading of motor and also by motor manufactures to compute torque o"er the full speed range. #uring no-load or short circuit test, small amount of energy is wasted as test is carried out quickly. 'econd method has ad"antages o"er first and third as it gi"es accurate torque speed cur"e plot in fraction of time compared to other two methods. This instrument can be used from fractional horsepower motor to Megawatt ratings with only changing fly-wheel and also a"ailable at affordable price. It!s +& user interface is user friendly. Ma:or #isad"antage is it!s
a"ailability in Indian market at affordable price. ortunately now it is sold by a local company, mbedded +ower 'olutions at "ery affordable price. %part from plottin g torque speed cur"e of a motor, it also draws circle diagram and display all important motor parameters on +& screen. Torque 'peed data measurement is e$tremely important from the application point of "iew as well as design and de"elopment or any modification in the e$isting motor design. These instruments ha"e been claimed to been "ery useful on production line also for quality control as testing is done in "ery short time and can re"eal manufacturing defect in motor under test and ser"e as a useful quality control tool.
&ourtesy of www.embeddedpowersolutions.com
&ourtesy of www.embeddedpowersolutions.com
