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SEE-Tech Solutions Pvt. Ltd
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Motors
Terms related to Motors
SLIP :
Slip is the difference between the speed of the magnetic field and the speed of the rotor.
TORQUE
Torque is a measure of the rotational force that a motor can produce.
STARTING TORQUE
It is the torque developed at the moment the motor is started.
PULL UP TORQUE
It is the speed range where minimum torque occurs.
BREAK DOWN TORQUE:
It is the maximum value of torque exerted by a squirrel cage induction motor with out stalling.
FULL LOAD TORQUE
It is the torque that the motor develops in producing rated horse power at rated full load speed.
ACCELERATING TORQUE
It is the difference at any speed between the torque required by the external load and that developed by the motor.
Standard designs of Motors
STANDARD DESIGNS BY NEMA:
DESIGN A:
Locked rotor current 6 to 10 times full load current.
Good running efficiency & power factor.
High pull out torque
Low rated slip(= 200% full load torque).
DESIGN B:
Higher reactance than DESIGN A.
Starting current = 5 times full load current
Starting torque, slip, efficiency are nearly the
same as DESIGN A.
Power factor & pull out torque are some what less.
DESIGN C:
High starting torque than either design A & B.
Break down torque lower than design A & B.
Full load torque same as design A & B.
DESIGN D:
High starting torque (=275% full load torque)
Low starting current.
High slip.
Low efficiency.
Types of Motors
There are several major classifications of motors in common use, each with specification characteristics that suit it to particular applications.
MAIN CLASSIFICATIONS
Alternating Current (AC) Motors
Direct Current (DC) Motors
Motor Losses
P
ower Losses in a motor are that portion of the input power that becomes heat rather than driving the load. These losses can be divided into two categories-Fixed losses are assumed to be constant at all conditions of motor loading from no load to full rated load. This is not exactly true, but it is nearly so, and little significant error is created by this approximation. Fixed losses include magnetic core losses (hysteresis and eddy current) and mechanical friction losses(bearing friction, brush friction, and air friction or windage).
Variable Losses are those that vary with the load on the motor and thus with the motor current. These losses increase as the load on the motor ,and therefore the current drawn by the motor, increase. They are primarily the power lost in the resistance of the motor windings and are often called copper losses, or I2R losses.
Variable losses also include stray load losses such as minor variations in fixed losses with load and speed and other small miscellaneous losses. Variable losses are approximately proportional to the square of the motor load current.
Motor Efficiency is the output of the motor divided by the electrical input to the motor, usually expressed as a percentage .power or work output is input less losses.
Efficiency(%) = Watts output x 100
watts input
= 746 x HP x 100
E x I x PF
= Input - Losses x 100
Input
Why Motor Fails
Many Motor failures can be averted ,or at least the useful life of a motor can be extended , if proper preventive measures are taken.
The sources of motor troubles generally falls into one of the following categories-
Equipments to read Motor Parameters
The following are the various instruments used for measuring the motor parameters :
Economic Evaluation Methods
The following are the various methods used for the economic evaluation of a motor when it is required to install energy conserving device to improve its performance :
The following formulas can be used to evaluate the above mentioned methods :
NAS x (1- (1+IRR)-EL) / IRR = FC.
Where
FC : First Cost.
AES : Annual Electricity Saving.
PEP : projected Electricity Price. EL Estimated Life time.
PV : Present Value.
NAS : Net Annual Saving.
Features of Energy Efficient Motor
Energy Conservation in Motors
The following formulas can be used to find out % Loading, Motor Losses and Efficiency of the motor :
% Loading of Motor = Actual KW Consumption / Rating of Motor (KW) x 100
The following method has been used to find out motor losses and efficiency
Losses at Full Load (L100) = KW of motor x [(1/full load efficiency) - 1]
Losses at 75 % Load (L75)= KW of motor x [(1/75% load efficiency) - 1]
L100 = (%Load) x (%Load) x A + B
L75 = (%Load) x (%Load) x A + B
Where
A - Variable Losses (KW)
B - Fixed Losses (KW)
Total Losses (neglecting windage and frictional losses) = A + B
Efficiency = Output / input = Output(KW) / (Output(KW) + losses)
Energy conservation measures available for an Induction motor are as follows :
With lower rating (HP) Standard Motor to reduce under loading
With lower rating (HP) energy efficient motors to reduce under loading
Replacing older motors with higher efficiency modern designs. Replacing a standard induction motor with a high-efficiency model has many other advantages. In addition to cutting down electricity costs, the replacement will probably have a longer life because it runs cooler and has better bearings, will need fewer capacitors to boost the motor's power factor and will work better with adjustable speed drives.
With lower rating Multi Speed motors to meet the requirement
Incorporation of electronic speed controls for motors driving pumps/fans/compressors requiring variations in throughout on a continuous basis to match production needs. Electronic adjustable speed drives can typically save 14 to 27% of energy.
Replacement of V-belts with "synchronous" belts which have teeth that engage sprocket lugs on the pulley can typically save 5 to 15% of the transmitted energy.
Improved maintenance practice ranges from the simplest task of using clean hands during lubrication, and to the more complex task of replacing windings in a manner which results in no loss in efficiency.
By knowing the % loading and P.F of compressor motor during full load and off load, it is possible to estimate operating efficiency from motor characteristic curves. If the efficiency is low, the motor may be replaced by a higher efficiency motor after calculating the pay back period.
Energy Conservation Analysis
Conversion of Delta connection in to Star Connection
The induction motor with a percentage loading below 50% would operate at lower efficiency in delta mode. This efficiency at low loading can be improved by converting delta connection into star connection. The reported savings due to this conversion varies from around 3% to 10% bacause the rated output of motor drops to 1/3rd of delta configuration without affecting performance and the percent loading increases as compared to delta mode. This option does not require any capital investment and is one of the least cost options available for the energy conservation in induction motors.
Though the margin of saving due to this option is low, but as the plant installations normally have hundreds of motors, converting most of the under-loaded motos in the plant would result into considerable savings.
Some motors operate on step loading and some on continuously variable load. The motors which operate on step loading, techno-economic feasibility of Delta-Star Automatic Change-over Switch is to be worked out (e.g. a machine with an induction motor performs three operations in its operating cycle resuling into motor loading of 25%, 40% & 80%; in such cases permanent delta to star conversion is not possible. A n automatic delta-star change-over controller could be installed there. It will connect the motor in star mode in 25% & 40% motor load operations; and in delta mode in 80% load operation). For the applications where starting torque requirement is high but otherwise the load is low, Automatic Delta to star Convertor can give significant energy savings.
The motors which operate on contenuously variable load, feasibility of installing Soft-Starter/Energy Saver is to be worked out.
This option of permanent Delta to Star conversion can not be implemented for the loads where starting torque requirement is very high. While implementing permanent Delta to Star connversion, care should be taken to decrease the setting of over load protection relay to 2/3 rd of the delta setting.
Conversion of Standard Motor with Energy Efficient Motor
As the eficiency of standard motor at less loading is low, its operating performance get reduces considerably. If the delta to star change over option is not suitable for improving the efficiency, replacement of existing standard motor with energy efficient motor could be very viable. The conditions which increases viability of installing energy efficient motors are as follows :
The efficiecy of the Energy efficient motor is almost constant at all percentage loadings. Due to its flat efficiency characterstics, it maintains efficiency almost constant at all loads. Normally, this option is suitable for the motors with rated capacity below 50 HP. The efficiencies of standard motors above 50 HP rating are almost similar to that of energy efficient motors. In many cases, though the initial cost of energy efficienct motor is 15 to 20% higher than the standard motor, the simple payback period is less due to the savings.
Conversion of V-Belts with Efficient Flat Belts
With conventional V-belts the efficiency for power transmission is low as high frictional engagement exists between the lateral wedge surfaces of the belts which causes less power transmission and hence higher power consumption for the same work to be done by the load; but with Flat-belts, this frictional engagement is on the outer pulley diameter only. V-belts contain higher bending cross section and large mass which cause higher bending loss. Also, as each groove of the pulley contains individual V-belt, the tension between the belt and the pulley distributes unevenly which causes unequal wear on the belt. This leads to vibrations and noisy running and hence reduces power transmission further. The consequences could be bearing damage also. This problem can be solved by using energy efficient Flat-belt.
It has been observed that the percentage of energy savings achieved during practical trails fairly match the gain in efficiency represented in above graph. As with the flat belt drive, the frictional engagement and dis-engagement is on the outer pulley diameter, not on the lateral wedge surface as in the case of the V-belt, wear on the belt is less and hence the life of the flat belt drive is higher than V-belt.
Some of the applications where conversion of V-belts with Flat belts is much effective are Compressors, Milling machines, Sliding lathes, Rotary printing presses, Stone crushers, Fans, Generators in Hydroelectric power plants etc.
Using Soft Starters
Soft starters, which have solid state electronic components, are used to control the input voltage according to the torque required by the driven equipment. Thus at almost all the load the motor operates at same efficiency and power factor.
This results in smooth starting of the motors by drawing lower current and thus avoiding the high instantaneous current normally encountered. Starting current and torque are directly related to the voltage applied when starting the motor. By reducing the line voltage when the motor is started, soft starter reduces the starting inrush current and eliminates the high impact or jerk starts that causes mechanical wear and damage. Soft starters are useful in cases where motors operate with high impact loads. Some of the applications are Cranes, Conveyors, Hoists, Compressors, Machine tools, Textile machinery, Food processing machinery etc.
Energy Conserving devices for Motors
The following are the various energy conserving devices for motors, also improves the motor performance considerably :
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