6 Key Factors to Consider When Selecting an Electric Motor for Your Application

Introduction:

Electric motors are ubiquitous in modern society, powering everything from household appliances to industrial machinery. As the world moves towards greater sustainability, the importance of selecting the right electric motor for specific applications has become more critical than ever.

There are many different types of applications that utilize electric motors, each with its own unique requirements. For example, electric motors are commonly used in manufacturing processes to power conveyor belts, robots, and other equipment. They are also used in transportation applications, such as electric vehicles and trains, as well as in renewable energy systems, such as wind turbines and solar tracking systems.

When it comes to electric motors, there are a variety of types to choose from, including AC motors, DC motors, and stepper motors, each with its own strengths and weaknesses. For example, AC motors are commonly used in high-power applications due to their efficiency, while DC motors are often used in low-voltage applications where precise speed control is required. Stepper motors, on the other hand, are ideal for applications that require precise positioning, such as in 3D printing or robotics.

Given the range of applications and motor types available, selecting the right electric motor for a specific application can be a complex task. In this article, we'll discuss the six key factors you should consider when choosing an electric motor for your project.

Performance Requirements:

The first step in selecting an electric motor is determining the Performance requirements (power, torque, speed, and acceleration requirements) of your application. Understanding these parameters is crucial because they influence the performance and efficiency of the motor. Power is the rate at which work is done and is typically measured in watts or horsepower. It is a fundamental requirement that determines how much work the motor can do in a given time. It is critical in selecting the right motor to match the application's power requirements. Torque is the rotational force applied to a shaft and is typically measured in Newton-meters (Nm) or pound-feet (lb-ft). Speed is essential for applications that require a specific rate of rotation or motion, such as conveyors, fans, and pumps and it is typically measured in revolutions per minute (RPM). Finally, acceleration is the rate of change of velocity and is typically measured in meters per second squared (m/s^2). It is crucial for applications that require a fast response time or quick start-up, such as robotics, medical equipment, and vehicles.

• To determine the power requirement, it is necessary to have information about the amount of work to be done and the time frame in which it needs to be completed. For example, if you need to lift a certain weight a certain height in a given amount of time, you can calculate the power requirement using the formula:

Power (W) = (Force x Distance) / Time.

• In order to calculate the torque requirement, you must have data on the amount of force that needs to be applied and the distance from the center of rotation where the force is applied. You can then use the formula:

Torque (Nm) = Force (N) x Distance from the center of rotation (m).

• To figure out the speed requirement, you need to know how fast the object or load needs to rotate. This information can be used to calculate the required speed. For example, if you need to rotate a conveyor belt at a certain speed, you can calculate the speed requirement using the formula:

Speed (RPM) = (60 x Velocity (m/s)) / (2 x π x Radius (m)).

• If you need to accelerate an object to a certain speed in a specific amount of time, you must know the starting velocity, final velocity, and the duration of acceleration. You can then use the formula:

Acceleration (m/s^2) = (Final Velocity (m/s) - Initial Velocity (m/s)) / Time (s).

Once you have determined the power, torque, speed, and acceleration requirements for your application, you can use the motor manufacturers' data sheets to choose the right motor that meets those requirements. These data sheets typically contain detailed technical specifications, including the motor's performance characteristics and operational curves. The performance characteristics typically include information such as the motor's rated power, torque, speed, and efficiency at various operating conditions. The operational curves, such as speed-torque, power-speed, and efficiency-speed, help you understand the motor's performance across its entire operating range.

Selecting proper drive:

When selecting a motor for an industrial application, it's important to consider the drive as an integral part of the system. The drive is responsible for controlling the speed and torque of the motor, and can have a significant impact on the overall efficiency and performance of the system.

There are several factors to consider when selecting a drive, including:

1. Type of Drive: There are several types of drives available, including variable frequency drives (VFDs), servo drives, and stepper drives. Each type of drive has its own unique features and benefits, and the selection will depend on the specific application requirements.
2. Compatibility with the Motor: The drive must be compatible with the motor being used, including the voltage, current, and frequency ratings. Choosing a drive that is not compatible with the motor can result in reduced performance and efficiency, as well as potential damage to the motor.
3. Control Capabilities: The drive should have the necessary control capabilities to meet the requirements of the application. This may include features such as speed control, torque control, and position control.
4. Energy Efficiency: The drive should be energy efficient and able to operate at high levels of efficiency across a range of loads. This can help to reduce energy consumption and operating costs over time.

In addition to these factors, it's important to ensure that the motor and drive are properly sized for the application. This involves considering factors such as the load requirements, duty cycle, and operating environment, and selecting a motor and drive combination that can meet these requirements while operating at maximum efficiency.

By considering the drive as an integral part of the system, engineers can help to ensure that the motor and drive combination selected for an industrial application provides optimal performance, efficiency, and cost-effectiveness over time.

Size and Mounting:

When selecting an electric motor, it's crucial to consider the physical space available for the motor, as well as any weight or vibration limitations. Electric motors come in a wide variety of sizes and mounting options, from compact designs to larger, heavier models. It's essential to choose a motor that fits within the available space and meets any weight or vibration requirements.

For example, in applications where there are severe physical space limitations, such as in-wheel electric traction systems for electric vehicles or electric bicycles, axial flux topologies may be more suitable compared to radial flux motors.

Additionally, you'll want to consider the mounting options available for your application, such as flanges or brackets. Choosing the right mounting option can help ensure that the motor is securely and safely installed.

Environmental Conditions:

The environment in which your motor will operate can have a significant impact on its performance and lifespan. For example, motors used in harsh or corrosive environments may require special coatings or materials to protect against damage. Similarly, motors used in high-temperature environments may require additional cooling mechanisms.

There are several standards that engineers could refer to regarding the environmental conditions of motors. Two commonly used standards are:

1. NEMA MG 1-2016- This standard provides guidelines for motor enclosures, which protect motors from the surrounding environment. The NEMA standard categorizes motor enclosures based on their ability to protect against dust, moisture, corrosion, and other environmental factors.
2. IEC 60034-1 - This standard provides guidelines for motor performance and efficiency in various environmental conditions, including temperature, humidity, altitude, and vibration. The IEC standard also provides guidelines for motor enclosures and protection against environmental factors.

Efficiency:

Electric motors can vary widely in terms of efficiency, which can have a significant impact on both operating costs and environmental impact. To meet the required standards for motor efficiency, there are several things that engineers can do. Here are a few steps they can take:

1. Determine the specific efficiency requirements: Before selecting a motor, it's important to determine the specific efficiency requirements for the application. This can be done by consulting the relevant standards and regulations, as well as considering the energy efficiency goals of the organization.
2. Choose a motor with high efficiency rating: Look for motors with high efficiency ratings, such as those labeled as "premium efficiency" or "IE3". These motors have been designed to meet specific efficiency requirements and can save energy and reduce operating costs over time.
3. Consider the load factor: When selecting a motor, it's important to consider the load factor, which is the percentage of rated motor power that is required for the application. Motors that are oversized or undersized for the application can lead to reduced efficiency and increased energy costs.
4. Proper installation and maintenance: Proper installation and maintenance of the motor can also play a role in ensuring high efficiency. This includes things like ensuring proper alignment and tension of belts, and regularly checking and replacing worn or damaged parts.

By following these steps, engineers can help ensure that the motors they select meet the required efficiency standards and provide maximum energy savings and cost-effectiveness over time.

Cost:

Finally, you'll want to consider the cost of your motor and how it fits into your budget. While it may be tempting to opt for the cheapest motor available, keep in mind that a higher-quality motor may offer better performance and a longer lifespan, ultimately saving you money in the long run. Additionally, consider any ongoing maintenance or repair costs associated with your motor.

Conclusion:

In conclusion, selecting the right electric motor for your application is critical to achieving optimal performance and efficiency. When choosing an electric motor, you need to consider various factors, including the power, torque, speed, and acceleration requirements of your application. Once you have determined these parameters, you can use the manufacturer's data sheets to select the right motor that meets those requirements. Additionally, selecting a proper drive that is compatible with the motor and has the necessary control capabilities and energy efficiency is crucial for optimal performance. By considering these key factors, you can ensure that you choose the right electric motor for your application, which can result in increased productivity, reduced energy consumption, and overall cost savings.

Electric motors, both AC motors and DC motors, come in many shapes and sizes. Some are standardized electric motors for general-purpose applications. Other electric motors are intended for specific tasks. In any case, electric motors should be selected to satisfy the dynamic requirements of the machines on which they are applied without exceeding rated electric motor temperature. Thus, the first and most important step in electric motor selection is determining load characteristics - torque and speed versus time. Electric motor selection is also based on mission goals, power available, and cost.

Starting and running torque are the first parameters to consider when sizing electric motors. Starting torque requirements for electric motors can vary from a small percentage of full load to a value several times full-load torque. Starting torque varies because of a change in load conditions or the mechanical nature of the machine, in which the electric motor is installed. The latter could be caused by the lubricant, wear of moving parts, or other factors. Electric motors feature torque supplied to the driven machine which must be more than that required from start to full speed. The greater the electric motor’s reserve torque, the more rapid the acceleration.

Electric motor drive systems that use gear reducers have parts that rotate at different speeds. To calculate acceleration torque required for these electric motors, rotating components must be reduced to a common base. The part inertias are usually converted to their equivalent value at the drive shaft.

ELECTRIC MOTORS - POWER RATING

Electric motors offer the horsepower required to drive a machine, which is typically referred to as electric motor load. The most common equation for power-based electric motors on torque and rotational speed is: hp = (torque X rpm)/5,250. If the electric motor’s load is not constant and follows a definite cycle, a horsepower versus time curve for the driven machine is helpful. From this curve, both peak and rms the electric motor’s horsepower can be determined. Rms load horsepower indicates the necessary continuous electric motor rating. Peak load horsepower is not necessarily an indication of the required electric motor rating. However, when a peak load is maintained for a period of time, electric motors feature a rating, which usually should not be less than peak load horsepower.

DUTY CYCLE - ELECTRIC MOTORS

Continuous steady-running loads over long periods are demonstrated by fans and blowers. On the other hand, electric motors installed in machines with flywheels may have wide variations in running loads. Often, electric motors use flywheels to supply the energy to do the work, and the electric motor does nothing but restore lost energy to the flywheel. Therefore, choosing the proper electric motor also depends on whether the load is steady, varies, follows a repetitive cycle of variation, or has pulsating torque or shocks. For example, electric motors that run continuously in fans and blowers for hours or days may be selected on the basis of continuous load. But electric motors located in devices like automatically controlled compressors and pumps start a number of times per hour. And, electric motors in some machine tools start and stop many times per minute.

Duty cycle is a fixed repetitive load pattern over a given period of time which is expressed as the ratio of on-time to cycle period. When operating cycle is such that electric motors operate at idle or a reduced load for more than 25% of the time, duty cycle becomes a factor in sizing electric motors. Also, energy required to start electric motors (that is, accelerating the inertia of the electric motor as well as the driven load) is much higher than for steady-state operation, so frequent starting could overheat the electric motor. For most electric motors (except squirrel-cage electric motors during acceleration and plugging), current is almost directly proportional to developed torque.

At constant speed, torque is proportional to horsepower. For accelerating loads and overloads on electric motors that have considerable droop, equivalent horsepower is used as the load factor. The next step in sizing the electric motor is to examine the electric motor’s performance curves to see if the electric motor has enough starting torque to overcome machine static friction, to accelerate the load to full running speed, and to handle maximum overload. 

ELECTRIC MOTORS - SERVICE FACTORS

A change in NEMA standards for electric motor service factors and temperature rise has been brought about because of better insulation used on electric motors. For instance, a 1.15 service factor - once standard for all open electric motors - is no longer standard for electric motors above 200 hp. Increases in electric motor temperature are measured by the resistance method in the temperature rise table. Electric motors feature a nameplate temperature rise which is always expressed for the maximum allowable load.

That is, if the electric motor has a service factor greater than unity, the nameplate temperature rise is expressed for the overload. Two Class-B insulated electric motors having 1.15 and 1.25 service factors will, therefore, each be rated for a 90°C rise. But the second electric motor will have to be larger than the first in order to dissipate the additional heat it generates at 125% load. Electric motors feature a service factor, which indicates how much over the nameplate rating any given electric motor can be driven without overheating. NEMA Standard MGI-143 defines service factor of an ac motor as “...a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower loading which may be carried under the conditions specified for the service factor...” In other words, multiplying the electric motor’s nameplate horsepower by the service factor tells how much electric motors can be overloaded without overheating.

Generally, electric motor service factors: * Handle a known overload, which is occasional. * Provide a factor of safety where the environment or service condition is not well defined, especially for general-purpose electric motors. * Obtain cooler-than-normal electric motor operation at rated load, thus lengthening insulation life.

From: Electric Motors and VFDs Handbook, Vol 3, The Electricity Forum