Choosing the right electric motor
An electric motor is an electromechanical device that converts electrical energy into mechanical energy. Most often, the movement is rotary, with mechanical energy characterized by rotational speed and motor torque.
This buying guide will only cover this type of motor. Linear motors will be covered in another buying guide.
It is estimated that electric motor systems account for 46% of the world’s electricity consumption. In industry, electric motors are everywhere. They run pumps, drive compressors and conveyors and operate fans, blowers, drills and mixers. Electric motors are at the very heart of industry.View electric motors
What are the selection criteria for choosing an electric motor?
Electric motors make it possible to perform different types of movement: fast, precise, continuous, with or without gear shifting, etc. All of these applications require their own motor technology.
- First, you must choose between three main groups of electric motors:
- In order to choose between these three groups, it is necessary to determine the type of application required as this will influence your choice:
- If you want your motor to run continuously and with few gear changes, you should choose an asynchronous motor.
- For dynamic applications, it is essential to have a synchronous motor.
- Lastly, if you require precise positioning you should choose a stepper motor.
- Depending on the movement you require, you will also need to determine the technical specifications and size of the motor:
- To determine the technical specifications, it will be necessary to determine the power, torque and speed of the motor.
- In order to determine the size, you must know how much space the motor will take up and how it is mounted (i.e.how it will be fixed in the system).
- When choosing the dimensions and solidity of the motor you must also take into account the industrial environment that the motor will operate in:
- There is a format adapted for every type of environment (explosive, damp, corrosive, high temperature, etc.)
- For harsh environments there are motors with reinforced, waterproof, shock-resistant or dirt-resistant casings.
- Finally, in recent years, energy efficiency has become an important factor to take into account when choosing a motor:
- An electric motor that uses less energy will have a low energy impact, which will reduce its energy cost.
How to choose between an AC motor and a DC motor?
These two types of motors are built differently:
- The most fundamental difference is the power source: alternating current (single-phase or three-phase) and direct current (DC), for batteries for example.
- Speed is another difference. The speed of an AC motor is controlled by varying the current in the motor while that of a DC motor is controlled by varying the frequency, usually with a frequency converter. As such, AC motors run faster than DC motors.
1. AC motors:
AC motors are the most popular in the industry as they have several advantages:
- They are simple to build
- They are more economical due to lower start-up consumption
- They are also sturdier and therefore generally have a longer lifespan
- They require little maintenance
Because of how they operate, which involves synchronization between the rotor rotation and current frequency, the speed of AC motors remains constant. They are particularly suited for applications requiring continuous movement and few gear changes. This type of motor is therefore perfectly adapted for use in pumps, conveyors and fans.
They can also be integrated into systems that do not require high accuracy if they are used with variable speed.
On the other hand, speed control functions make them more expensive than other motors.
There are two types of AC motors: single-phase and three-phase.
- Single-phase motors are characterized by:
- The electrical power (in kW), which will determine the torque
- The number of poles, which will give the rotational speed
- The attachement method: flange (B14, B5) or brackets (B3)
- Less industrial as they are less powerful
- They can be used on the domestic electricity grid.
- Three-phase motors are characterized by:
- An architecture that allows much greater electrical power to be transmitted than a single-phase voltage motor
- Their use in industrial environments (about 80%)
- Their use for infrastructure and equipment requiring high electrical power
2. DC motors:
DC motors are also very common in industrial environments because they have significant advantages depending on the format (see brushless motor question):
- They are accurate and fast
- Their speed can be controlled by varying the supply voltage
- They are easy to install, even in mobile (battery-powered) systems
- The starting torque is high
- Starting, stopping, accelerating and reversing are done quickly
They are very well suited for dynamic applications requiring high accuracy particularly in terms of speed, as in the case of elevators, or in terms of position as in the case of robots or machine tools.
They can also be advantageous for applications requiring high power (10,000 kW for example).
However, they have certain disadvantages depending on their structure compared to AC motors:
- They are less common because they are less suitable for applications requiring high power
- They are made up of many parts that wear out and are expensive to replace
3. What are the market trends today?
- Brushed DC motors are used less and less frequently in industrial applications. For lower power, AC motors are used instead as they require little maintenance for equivalent use.
- Since DC motor parts are too expensive to replace, some manufacturers opt for AC motors in which they integrate an electric controller.
>The combination of an AC motor and a frequency inverter has become a cost-effective solution for most applications requiring speed variations.
Should you choose a brushless motor or a brushed motor?
The two most common types of DC motors are brushed motors and brushless motors.
Brushed motors are the simplest and most frequently used motors, especially for basic industrial equipment and low-budget applications.
- Brushed motors have certain advantages:
- They are easy to control
- The torque at low revs is very good
- They are inexpensive
There are four types of brushed motors depending on your intended use.
- In this type of motor, the stator is connected in series with the rotor and the speed is controlled by varying the supply voltage.
- However, this type of speed control is rather poor: the speed drops as soon as the torque to the engine increases.
- This type of engine is useful for applications requiring high starting torque such as cars or cranes.
- In this type of motor, the stator is connected in parallel to the rotor, which allows for a higher torque without reducing the speed as the motor current increases.
- This type of motor is suitable for applications with constant speeds such as vacuum cleaners or conveyors.
- Compound wound:
- This type of motor combines the structure of series-wound motors and shunt-wound motors.
- As a result, it offers a high starting torque as well as a higher speed variation.
- This type of motor is perfect for rotary presses, elevators, baggage carousels, centrifugal pumps and compressors.
- Permanent magnet:
- This type of motor contains a permanent magnet that enables low torque.
- This is useful for applications requiring precise control such as robotics or servo systems.
All brushed motors have significant disadvantages however:
- They are less efficient than brushless motors (75-80% compared to 85-90% for brushless motors).
- Their service lifespan is short because the brushes, due to regular friction, wear out more quickly over time (between 1,000 and 10,000 operating hours depending on the frequency of use, power, speed, vibration, etc.)
- The arc of the brushes and collector can generate electromagnetic noise that can create fires.
- Similarly, the risk of sparks due to friction makes using this type of motor in explosive environments undesirable.
- The speed is generally limited due to the heating of the brushes.
- Brushes made of graphite generate dust that can damage other devices, such as optical devices.
- They need to be lubricated, which makes it impossible to use them in vacuum cleaners.
Brushless motors make up for some of the weak points of brushed motors, such as the presence of brushes. But these motors also have other advantages:
- They can operate at higher speeds (up to 100,000 rpm compared to 20,000 rpm for brushed motors).
- They have a longer service life (over 10,000 hours of operation)
- They are more reliable and efficient.
- There are no wearing parts except for the bearings, which reduces maintenance operations.
The fact that these motors are able operate at very high speeds makes them particularly suitable for grinders, fans or saws.
Brushless motors are systematically equipped with an encoder, a sensor that allows for electronic switching and determination of the rotor position. These motors are therefore perfect for servomotor precision applications.
However, they have certain disadvantages:
- The initial cost is high because it is necessary to integrate a dedicated switching device (controller).
- They also generally require a gear reducer in drive applications.
Is this the end of brushed motors? Not according to the Opportunity rover
It might seem that in the match between brushed motors and brushless motors, the brushed ones would lose and have no choice but to go back to the closet. This is not the case, however, as brushed motors remain popular in industry and even in space. Maxon, a Swiss manufacturer of electric motors, is constantly developing new technologies for brushed motors and their brushed motors equipped Opportunity, NASA’s rover sent to Mars in 2003.
Opportunity took 34 DC brushed motors onbord which have been able to operate successfully under extreme conditions with wide temperature variations. NASA’s choice was simple: to benefit from the simplicity of brushed motor control and therefore have the ability to control all 34 motors from a single controller. Brushless motors would have required one controller per motor with risks of costs and complications.
Why choose a stepper motor?
A stepper motor converts an electrical pulse into an angular motion. It is useful for applications requiring open loop position control.
There are three categories of stepper motors:
- The variable reluctance motor: With the same electrical characteristics, this type of motor is less powerful but faster than a permanent magnet motor.
- The permanent magnet motor: it is a low cost, low resolution motor
average (up to 100 steps/revolution).
- The hybrid motor: this type of motor combines the two previous types of technology but is more
expensive. It has the advantage of better torque and higher speed. It has a resolution of 100 to 400 steps/revolution.
Permanent magnet and hybrid motors are the most commonly used motors because they have certain advantages:
- They are accurate
- They are inexpensive
- They are sturdy
- Simple construction
- The torque is high at start-up and at low speeds
However, they also have some disadvantages:
- The speed and torque are relatively low
- Torque decreases sharply as speed increases
- They generate vibrations that can create resonance problems
- There is a risk of overheating
When selecting a stepper motor, it is important to take into account:
- Torque and load
- The number of steps
- The dimensions of the motor (weight, mounting flanges, etc.).
What are the energy efficiency standards for electric motors?
Manufacturers are increasingly contemplating the question of energy efficiency. A greener and more environmentally friendly economy is one of the objectives of the 2015 United Nations Climate Change Conference which many States committed to. But it is above all in order to limit consumption and for savings that industry has been acquiring more energy-efficient equipment in recent years. According to a study by the European Commission, motors account for 65% of industrial energy consumption in Europe. Taking action when it comes to motors is therefore an important step in order to reduce CO2 emissions. The Commission even predicts that it is possible to improve the energy efficiency of European-made motors by 20 to 30% by 2020. The result would be 63 million tonnes less CO2 in the atmosphere and 135 billion kWh saved.
If you also want to integrate energy-efficient motors and get savings while contributing to the planet, you will first need to look at the energy efficiency standards for motors in your country or geographical area. But be careful, these standards do not apply to all motors, only to asynchronous AC electric motors.
- The International Electrotechnical Commission (IEC) has defined energy efficiency classes for electric motors placed on the market, known as the IE code, which are summarized in the international IEC standard
- The IEC has identified four levels of energy efficiency that define a motor’s energy performance:
- IE1 refers to STANDARD efficiency
- IE2 refers to HIGH efficiency
- IE3 refers to PREMIUM efficiency
- IE4, still under study, promises SUPER PREMIUM efficiency
- The IEC has also implemented the IEC 60034-2-1:2014 standard for testing electric motors. Many countries use national test standards, while also referring to the international IEC 60034-2-1 standard.
The EU has already adopted several directives aimed at reducing the energy consumption of motors, including the obligation for manufacturers to place energy-efficient motors on the market:
- Class IE2 has therefore been mandatory for all motors since 2011
- Class IE3 has been mandatory since January 2015 for motors with a power of 7.5 to 375 kW (or IE2 if these motors have a frequency inverter)
- Class IE3 has been mandatory since January 2017 for motors with a power of 0.75 to 375 kW (or IE2 if these motors have a frequency inverter)
In the United States
In the United States, the standards defined by the American association NEMA (National Electrical Manufacturers Association) are in force. Since 2007, the minimum level required has been set at IE2.
The same classification applies to Australia and New Zealand.
In China, the Korean MEPS (Minimum Energy Performance Standard) standards have been applied to small and medium-sized three-phase asynchronous motors since 2002 (GB 18693). In 2012, MEPS standards were harmonized with IEC standards, moving from IE1 to IE2 and now to IE3.
Japan has harmonized its national regulations with IEC efficiency classes and included IE2 and IE3 electric motors in its Top Runner program in 2014. Introduced in 1999, the Top Runner program forces Japanese manufacturers to constantly offer new models on the market that are more energy efficient than previous generations, thus forcing emulation and energy innovation.
India has had a comparative efficiency label since 2009 and a national standard at an IE2 level since 2012.