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Motors are used when there is a need to move certain objects. And the efficiency of the motor determines the cost-effectiveness of the movement. A motor typically generates some rotational motion and transmits it to the end compartment where that motion is used. Some of the power generated by the motor is lost along the way due to factors such as friction or heat loss. Motor efficiency is therefore the ratio of the actual energy used to the energy put into the mechanical system. Obviously, more efficient motors guarantee lower operating costs, which is what every motor user is looking for.
In this article, we will attempt to explain how you can make a more informed decision as to which motor may be better suited to your application.
Some Terms
Before going further into motor efficiency and its estimation, let’s look at some important terms:
Work
Work is the amount of energy required to move a mass a distance by applying a certain force.
Power
Power is the amount of work done per unit of time.
Efficiency
A measure of how much of the energy input to a system can be converted into output energy or work.
Rated load
The maximum load a machine can deliver with sufficient efficiency.
Full load
The maximum load a machine can provide.
Power Factor
The ratio of real power to apparent power in a circuit.
Actual power is the electrical power actually used, apparent power is the total electrical power delivered to the circuit.
Rotor
The moving part of a motor that rotates the shaft to produce mechanical energy.
Stator
The fixed part of the motor with some windings or fixed magnets with cores made of different metal sheets or laminated materials.
Familiarizing yourself with these terminologies will help you to better understand the structure of a motor and the basic principles of its operation.
Calculating Motor Efficiency
To calculate the efficiency of an electric motor, it is necessary to determine the input power and output power of the motor. Efficiency is defined as the ratio of output power to input power, expressed as a percentage. The following are step-by-step instructions:
Determine the input power (Pin): This is the electrical power supplied to the motor. The formula is Pin = V x I, where V is the voltage applied to the motor and I is the current flowing through the motor. Be sure to use consistent units (e.g., volts and amps).
Measure or Calculate Power Output (Pout): This is the mechanical power produced by the motor. It can be determined in different ways depending on the application. For example
If the torque (T) and angular velocity (ω) produced by the motor are known, the formula Pout = T x ω can be used.
If the motor’s speed (N) and power rating (Prated) are available, the power output can be estimated as Pout = Prated x (N/Nrated) (where Nrated is the motor’s rated speed).
Calculating Efficiency (η): Efficiency is calculated as η = (Pout / Pin) x 100%. This will give the efficiency as a percentage.
For example, assume a motor with an input power of 1000 watts (Pin) and an output power of 800 watts (Pout). The efficiency is calculated as follows
η = (800 / 1000) x 100% = 80%.
In this case, the efficiency of the motor is 80%, indicating that 80% of the input electrical energy is converted into useful mechanical power.
It is important to note that motor efficiency varies depending on factors such as load conditions, temperature and design. Therefore, it is recommended that measurements be taken or that the motor specification be consulted in order to accurately calculate the efficiency in a practical situation.
Motor efficiency and its classes
To measure the efficiency of a motor, two types of tests are usually performed: a no-load test and a loaded test. A no-load test is when the motor is run without a load, while a load test is when the motor is run with a known load. In these tests, the motor’s input power and output power are measured using specialised instruments and the efficiency is calculated using the previously mentioned formula. It is important to note that motor efficiency varies depending on load conditions, so it is important to test motors under typical operating conditions to obtain accurate measurements.
The International Electrotechnical Commission (IEC) and the National Electrical Manufacturers Association (NEMA) have designated several motor efficiency classes. These ratings are based on the maximum allowable losses of the motor and are determined by the design and size of the motor. The higher the efficiency rating, the lower the allowable losses and the more efficient the motor will be overall.
The following are the main motor efficiency classes:
Standard Efficiency (IE1, NEMA Design A): This is the lowest efficiency rating for motors and is typically used for older or lower cost motors. These motors range from 50-90% efficiency, depending on their size and application.
High Efficiency (IE2, NEMA Design B): The minimum efficiency of this class is 3-6% higher than the standard efficiency class. These motors are more efficient and are typically used in applications where energy conservation is a priority.
Premium Efficiency (IE3, NEMA Design C): The minimum efficiency of this class is 2-3% higher than the high efficiency class. These motors are the most efficient and are typically used in high performance applications where energy efficiency and reliability are important.
Ultra High Efficiency (IE4, NEMA Design D): The minimum efficiency level for this class is 1-2% higher than the High Efficiency class. These motors are the most advanced and efficient motors available, allowing for the lowest losses and highest overall efficiency.
Selecting a motor with a higher efficiency rating can reduce energy consumption and operating costs while also improving performance and reliability. It is important to consult motor specifications and take measurements to get an accurate picture of motor efficiency in real-world applications.
Maintaining Motor Performance and Improving Motor Efficiency
Every system has losses that cause the output to be lower than the input. That’s why we need to consider system efficiency, one of which is motor efficiency. There are various reasons for motor losses, such as friction, motor magnetic energy dissipation, and resistive losses. Obviously, reducing motor losses will increase motor efficiency.
One thing to keep in mind is to continuously monitor the motor. This also means keeping track of the age of the motor, the output power provided, and the power rating of the motor to calculate the long-term efficiency of the motor. In addition, you need to keep an eye on voltage unbalance, current unbalance and power factor (PF). Obviously, you want a power factor close to 100 per cent.
There are several ways to improve motor efficiency:
1.The motor stator is the main cause of motor losses. Stator losses have been reported to account for about 30% or more of the overall motor losses. In order to reduce its corresponding resistive losses, the mass of the stator windings needs to be kept at a large value.
2.Another important cause of motor losses is hysteresis. The use of higher quality materials (e.g., silicon-containing steel) eliminates core magnetic losses and thus reduces losses. Increasing the length of the laminations while reducing the thickness of the laminations also reduces the magnetic flux density and thus reduces the core losses.
3.There is a phenomenon called slip, which is the difference between the magnetic field speed and the actual speed of the rotor and shaft under a given load. One needs to reduce slip, which means that the conductivity of the rotor needs to be increased. This can be achieved by using a number of highly conductive materials, such as moulded case copper for the rotor.
4.You need to ensure that the performance of the cooling system is not affected. To do this, you can clean the motor surroundings on a regular basis and check that the air flow to the frame and corresponding compartments is not likely to be obstructed. When it comes to the motor frame, it should be in good condition so that it not only holds the motor in its place, but also ensures that heat is properly transferred from the inside to the outside surface, where it is cooled by air.
5.The use of better insulating material between the laminates or sheets will reduce the losses caused by eddy currents.
6, Motor lubrication is another important factor in the efficient operation of the motor. Ensure that different types of grease are not used at the same time and that lubrication intervals are based on the motor’s installed speed rating, bearing size, grease type and temperature rise.
In summary, understanding motor efficiency is critical to optimising energy use and reducing operating costs. By choosing motors with higher efficiency ratings, businesses and individuals can make significant strides towards sustainability and cost-effectiveness. Whether in industrial environments, transport systems or everyday appliances, the impact of high-efficiency motors cannot be overstated. So let’s work together to improve the efficiency of our motors and pave the way for a greener, more efficient future. Together, we can power the world while minimising energy waste.
