In this article, we intend to explain what an electric motor is and what practical applications it has for you. Additionally, you will become familiar with different types of electric motors. Please read comprehensive and complete information about electric motors until the end of this article.
Firstly, let’s review the concept of an electric motor and provide a definition of it.
What is an Electric Motor?
Fundamentally, electric motors are divided into two main types: Totally Enclosed Fan Cooled (TEFC) motors and explosion-proof motors, as well as cooler motors. From the perspective of power consumption, they are divided into two types: single-phase and three-phase.
This device is a type of machine that converts electrical energy into mechanical motion. The principle of operation is based on the effect of a magnetic field on a current-carrying conductor. The magnetic field exerts a force on the conductor, causing it to move. Most electric motors are rotary, but linear motors also exist.
Every rotary electric motor consists of two components: a movable part and a stationary part. The movable part is called the rotor, and the stationary part is called the stator. In an electric motor, the rotor rotates around its axis due to the torque generated by the magnetic field of the stator. Electric motors are powered by direct current or alternating current, depending on their construction.
History of the Electric Motor
In 1882, Nikola Tesla introduced the principles of rotating magnetic fields, paving the way for harnessing the rotating field as a mechanical force. In 1883, he utilized these principles to design a two-phase induction motor. Concurrently, Galileo Ferraris independently began research in this field and presented his findings in a paper to the Academy of Sciences in Turin, Italy, in 1888.
The movement initiated by Nikola Tesla in 1888 is now regarded by some as the “Second Industrial Revolution.” This movement led to easier production of electrical energy and enabled its transmission over long distances.
Before Tesla’s invention of alternating current motors, motors operated with a permanent conductor moving within a stationary magnetic field. Tesla pointed out that motor collectors could be eliminated so that the motor could be driven by a rotating field.
Tesla succeeded in obtaining US Patent No. 416,194 for his invention. This motor, which is also seen in many of Tesla’s photographs, was a specific type of induction motor.
In 1890, Mikhail Dolivo-Dobrovolsky invented a three-phase cage rotor motor, which is widely used today in various applications.
Types of Electric Motors
In today’s advanced mechanical life, humans interact with electric motors or generators. Although it may go unnoticed, generally wherever there is movement in progress, an electric motor is at work.
DC Motors
1. Series Motor
2. Parallel Motor
3. Compound Motor
4. Self-Excited Motor
5. Permanent Magnet DC Motor (PMDC)
AC Motors
1. Induction Motor
2. Synchronous Motor
3. Special Motors
– Stepper Motor
– Brushless DC Motor
– Hysteresis Motor
– Reluctance Motor
– Universal Motor
Electric Definitions and Terminologies
1. Current: The ratio of changes in electric charge (q) over time (t), measured in Amperes (A) and measured by an ammeter installed in series in the circuit.
2. Voltage: A quantity that causes current flow in a circuit, measured in Volts (V), and measured by a voltmeter installed in parallel in the circuit.
3. Electrical Work Unit and Apparent Power Unit: The unit of electrical work is Watt per second (W/s), and the unit of apparent power is Volt-Ampere (VA) or kilo Volt-Ampere or Mega Volt-Ampere.
4. Rated Current: The current that an electrical device can pass under specified conditions without overheating or being subjected to excessive mechanical stress.
Ohm
If a voltage equivalent to 1 volt is applied across a resistance and a current of 1 ampere passes through it, the resistance value is 1 ohm. The unit of electrical resistance is Ohm (Ω).
Watt
They express the amount of energy produced in watts. For example, light bulbs are specified by their wattage, indicating how much light energy they can produce.
Ampere
An ampere is the amount of electric current flowing in a circuit. In fact, when a force causes electrons to move in a particular direction, electric current is generated. Current (I) is expressed in Amperes (A).
Volt
The amount of voltage or potential difference is a measure of electric charge. In fact, the force that brings free electrons into motion is called electric voltage. Electric potential difference (V) is expressed in volts (V).
Magnetic Field
The electric field at each point is equal to the force applied to the test charge q0. In other words, to determine the electric field E, we divide the force F by the charge q0.
Components of Electric Motors
First, let’s take a look inside a simple electric motor. A simple motor consists of 6 parts:
1. Armature
2. Commutator
3. Directional Power Switch
4. Shaft
5. Iron Core
6. DC Power Source
Electric motors are divided into two main categories in terms of current consumption:
1. Direct Current (DC) Motors
2. Alternating Current (AC) Motors
What are DC Motors?
A DC motor, or direct current motor, has an armature made of electrically conductive iron. A rotating switch called a commutator reverses the electric current twice in each cycle to create electrically conductive armatures.
In the armature, permanent magnets attract and repel. The speed of a DC motor depends on the braking torque, in other words, it depends on the input voltage and the torque of the input current.
Series DC Motor
Series DC motors have high starting torque and are therefore used in industries where high starting torque is required, such as cranes, hydraulic lifts, impact presses, and elevators.
They are also used in city locomotives (subways and trams), referred to as traction motors.
Shunt DC Motor
Shunt DC motors have maximum torque at nominal speed. Therefore, they are used in applications such as industrial blowers and fans. These types of motors should not be started under heavy loads because their armature current may exceed the limit and damage the motor.
Compound DC Motor
Compound motors have the characteristics of both series and shunt motors and are divided into two categories.
1. Compound DC Motor – Cumulative
This type of motor is used in cases where the characteristics of a series motor are required, but the motor should not run uncontrollably and its speed should not rise too high when the load is removed. For example, in lathes where the machine is unloaded in each work cycle and then loaded again.
In various industries, instead of using an additional compound dynamo, which is designed similarly to series electric motors, it is used.
2. Compound DC Motor – Differential
Compound differential motors are used for loads less than rated loads and cases where almost constant speed is required. These motors are usually used in laboratories to provide a constant speed.
What are AC Motors?
The most common dynamos used in industry and households are induction AC types.
Advantages of Using AC Motors
– Simple and robust design
– Affordable price
– Low maintenance costs
– Easy and complete connection to a power source
There are various types of induction AC motors in the industry. Different electric motors are suitable for different tasks. Although the design of AC motors is easier than DC electric motors, controlling the speed and torque in various types of AC motors requires a deeper understanding of the design and specifications.
Stator
The stator is made up of several thin aluminum or lightweight iron pieces. These pieces are assembled together in the form of a hollow cylinder and securely fixed. Wire coils with insulated wire are placed in these slots.
Each coil group, wrapped around a core, forms a magnetic pole (with two poles) for operation with AC power. The number of poles in an AC motor depends on the internal connection of the stator coils. The stator coils are directly connected to the power source. They are connected in a way that, when AC power is supplied, a rotating magnetic field is generated.
Rotor
The rotor is made up of several separate thin steel pieces with rods made of copper or aluminum embedded between them. In the most common type of rotor (squirrel-cage rotor), these rods are electrically and mechanically connected to each other at their ends through rings.
Nearly 90% of electric motors have squirrel-cage rotors. This is due to the robust and simple structure of this type of rotor. This rotor consists of a multi-piece cylindrical core with slots parallel to accommodate the conductors inside.
Each slot contains a copper, aluminum, or alloy rod. In these rods, a short circuit is permanently established through their end rings. This type of assembly, resembling a squirrel cage, hence the name squirrel-cage rotor.
The rotor rods are not exactly parallel to the axis. They are installed slightly inclined for two main reasons:
1. The motor operates silently by reducing magnetic noise and minimizing harmonics in the slots.
2. The rotor’s tendency to lock is reduced. The rotor teeth try to remain aligned with the stator teeth due to direct (pure) magnetic attraction. This occurs when the number of rotor and stator teeth are equal.
Types of AC Electric Motors
Synchronous AC Motor
In synchronous dynamos, both the rotor and stator are wound with wire coils. When the stator is connected to a power source, a rotating field is created in the motor, which rotates at synchronous speed. The rotor is also wound with wire coils and powered by a DC source.
By supplying DC current to the rotor, it starts rotating with the revolving field of the stator. To start a synchronous electric motor, initially, after disconnecting the initial exciter, the motor rotates at synchronous speed.
Application of Synchronous Electric Motor
This product is used to correct the power factor (Cosφ). In this case, no load is placed on the shaft of the synchronous motor, meaning the electric motor operates without a load. In this situation, the synchronous motor is also called a synchronous capacitor, which is used in factories.
These motors can be used both as synchronous generators and synchronous electric motors.
Due to the fact that the synchronous electric motor has a constant speed, it is used in equipment such as electric clocks that require this property.
Advantages of Synchronous Electric Motor:
1. Low sensitivity to voltage fluctuations.
2. Very high efficiency.
3. Ability to adjust power factor with appropriate values.
4. Ability to operate with high voltages directly.
Disadvantages of Synchronous AC Motor:
1. This type of motor has a fixed speed and cannot adjust to higher or lower speeds.
2. It cannot tolerate overload.
3. It requires constant current for the motor poles plus alternating current for the stator windings, which increases the cost compared to similar motors.
4. It requires an initial starting device, which may be an auxiliary motor.
Asynchronous AC Motor:
The rotor of this type of electric motor is formed using squirrel-cage rotors and wire coils.
What is an Asynchronous AC Motor with Squirrel-Cage Rotor?
The rotor of asynchronous motors with squirrel-cage rotors is designed cylindrically, with aluminum or copper bars inside peripheral slots made of iron or steel. These bars can be round or rectangular and can be connected or separated from each other. The inclined design of the rotor bars is to ensure that the stator and rotor fields interfere with each other and prevent vibration or locking at startup.
Advantages of Asynchronous Motor with Squirrel-Cage Rotor:
1. Affordable price and simplicity in design, making them suitable for many industries instead of other motors.
2. The rotational speed remains almost constant at different loads.
3. Changing the load does not destabilize the motor.
4. They have a better power factor compared to wound motors.
Disadvantages of Asynchronous Motor with Squirrel-Cage Rotor:
1. Reduction in power factor when working with light loads.
2. Inability to change speed with reduced voltage and the need for an AC Drive device for this purpose.
3. Sensitivity to voltage fluctuations, where a decrease in voltage leads to an increase in current.
4. Low torque.
5. At startup, it draws a high current from the grid, which is approximately 3 to 7 times the rated current.
Asynchronous AC Motor with Wound Rotor:
One of the significant issues in squirrel-cage motors is the very low resistance in their rotor. In this model, the rotor bars are short-circuited through rings located at their beginning and end. This situation, at startup when the rotor is not magnetic, leads to the absorption of a significant amount of flux to magnetize and increase the motor current. As a result, a very high current is drawn from the grid, reducing the starting torque. To address this issue, motors with wound rotors have been designed and produced, significantly mitigating this problem.
In these types of products, the rotor is covered with wire coils instead of bars, and the ends of these coils are connected to slip rings that are guided outside the motor by brushes. By using a resistance bank (starter or starter) in the path of these wire coils, we can increase or decrease the rotor resistance and change the motor current as desired.
Starter or Starter:
A three-step starter has three stages to bring the motor to its rated speed. In the first stage, when the motor reaches 30% of its rated speed, the resistance bank is disconnected from the circuit by the respective contactor, and the motor continues on its own. The next stages are executed at 50% and 75% of the rated speeds. After these rounds, the starter is generally removed from the circuit, and the motor is directly connected to the grid and reaches its rated speed.
Wound rotor asynchronous motors are used in mills, crushers, cranes, and in cement and steel industries.
Advantages of Asynchronous AC Motor with Wound Rotor:
1. Heat generation occurs outside the starter.
2. This dynamo can perform consecutive and under-load starts.
3. The motor start current is adjustable.
4. Wound rotor motors have maximum torque at startup.
AC Electric Motor:
AC motors, like most electric motors, consist of an external fixed part called the stator and a rotor that rotates inside it. Electric motors use a rotating magnetic field virtually to turn their rotor.
The three-phase AC motor is the only type in which the rotating magnetic field is naturally produced by the stator due to the nature of its power supply. Whereas in DC motors, an electrical or mechanical means is required to produce this rotating field. To start a single-phase AC motor, an external electrical device is needed to generate this rotating magnetic field. Inside every AC motor, two sets of magnetic poles are embedded.
Types of Electric Motors:
– Single-phase Electric Motor
– Three-phase Electric Motor
Usually, electric motors are classified based on the number of stator slots.
Single-phase Electric Motor:
Due to their affordability and low maintenance costs, single-phase dynamo motors have the highest consumption in the single-phase dynamo industry. In this model of electric motors, a coil or winding is used, and it operates with a single-phase alternating current power source. Single-phase motors are not self-starting, and in all types of single-phase electric motors, the rotor is of the squirrel-cage type.
When the motor is connected to a single-phase power source, the main coil becomes alternating current. This alternating current generates a pulsating magnetic field. Due to induction, the rotor is excited. However, because the main magnetic field is pulsating, the torque required for the electric motor to rotate is not generated, resulting in rotor vibration rather than rotation.
For this reason, the single-phase electric motor requires a starter device (starter) that can generate starting impulses to rotate the motor. The single-phase electric motor starter essentially consists of an additional coil in the stator (starter winding). The starter coil can have series capacitors or centrifugal switches.
When the power supply voltage is established, the current in the main coil, due to its resistance, decreases the supply voltage (voltage is converted to current). Meanwhile, the current in the start coil, depending on the resistance of the starter device, converts to increased supply voltage. The interaction between the magnetic fields generated by the main coil and the starter device (including the auxiliary coil and capacitor) creates a resultant field that rotates.
The axis of the electric motor initiates rotation in the direction of this resultant field. When the motor reaches 75% of its rated speed, a centrifugal switch in the starter coil circuit disconnects. From this point on, the single-phase electric motor can maintain sufficient torque to continue operating. Except for specific types, usually, all single-phase motors are used for applications requiring less than 3 horsepower. Based on the various starting techniques, single-phase AC electric motors are classified as follows:
Types of Single-phase Electric Motors:
– Single-phase AC Motor with Split Phase
– Single-phase AC Motor with Capacitor Start
– Single-phase AC Motor with Permanent Split Capacitor (PSC)
Single-phase AC Motor with Split Phase:
These types of motors have two windings. In the start coil of this dynamo, a thinner wire and fewer turns compared to the main coil are used to create greater resistance. This electric motor is known as an induction start motor or capacitor start motor. A different angle than the angle of the main coil is chosen in the field of the start coil, which initiates the rotation of the motor axis.
The main coil, made of a thicker wire, always keeps the electric motor in a rotating state.
Suitable applications for split-phase electric motors include small grinders, blowers, and small fans, and other devices requiring low starting torque and requiring electric motors with a power range of 1/20 to 1/3 horsepower. These motors are not suitable for applications requiring frequent on-off switching or requiring high torque.
Single-phase AC Motor with Capacitor Start:
This type of motor has a modification compared to the split-phase single-phase motor, which involves adding a capacitor to the circuit, improving the motor’s starting performance. Like conventional split-phase motors, this type of motor has a centrifugal switch that disconnects the start coil from the circuit when the motor axis reaches 75% of its rated speed.
The presence of a capacitor in the start circuit increases the starting torque, usually between 200% to 400% of the rated torque. The start current is also typically between 450% to 575% of the rated current, much less than that of a split-phase motor, due to the use of thicker wire in the start circuit. The modified version of this type of motor with a capacitor start is called a resistor start motor.
In this type of motor, the start capacitor is replaced with a resistor. The resistor start motor is used in applications requiring less starting torque compared to capacitor start motors.
Apart from cost, this type of motor does not have significant advantages over capacitor start motors. These motors perform well in various belt and pulley applications such as small conveyors, pumps, large blowers, and gear-driven devices.
Single-phase AC Motor with Permanent Split Capacitor:
In this type of motor, a capacitor is permanently connected in series with the start coil. This connection allows the start coil to act as an auxiliary coil until the motor shaft reaches its rated speed.
Since the capacitor must be designed for continuous use, it cannot generate the starting power equivalent to a capacitor start motor. The starting torque of a single-phase motor with a permanent split capacitor is usually lower, around 30% to 150% of the rated torque. Single-phase motors with a permanent split capacitor have a low starting current, typically less than 200% of the rated current, making them very suitable for applications with high-speed and frequent on-off cycles.
Single-phase motors have many advantages. Their design can be easily modified for use with speed controllers. They can also be designed for better efficiency and high power factor at rated loads.
These motors are known as the most reliable single-phase motors because they do not require a centrifugal switch. Depending on their design, they can perform well in various applications such as fans, low-torque blowers, and garage door openers.
Three-phase AC Motor:
For applications requiring higher power, three-phase AC (or multiphase) motors are used. These types of motors use the phase difference between the phases of the multi-phase power source to create a rotating electromagnetic field inside them.
Typically, the rotor of these motors consists of several copper bars embedded in steel. These bars create a rotating magnetic field through electromagnetic induction, which in turn induces current in them. This action creates a balanced magnetic field that causes the motor shaft to rotate.
To initiate motion in the shaft of a three-phase motor, the shaft must rotate at a speed lower than the frequency of its power source. Otherwise, a balanced field will not be created in the rotor. The use of these types of motors, especially in applications such as locomotives, where they are known as asynchronous traction motors, is increasingly on the rise.
The stator windings of the rotor are energized to create a continuous magnetic field that is synchronized with the rotating magnetic field generated by the three-phase AC motor. Synchronous motors can also be used as generators of electrical power.
The speed of an AC motor initially depends on the frequency of the power supply, and the amount of slip, or the difference in speed between the rotor and the stator magnetic field, determines the motor’s produced torque.
Speed variation in these types of motors can be achieved by having different sets of windings with different pole numbers in the stator section, which changes the speed of the rotating magnetic field. Additionally, by changing the frequency of the power supply, more uniform control over the speed of this type of electric motor can be attained.
Three-phase AC Motor:
Almost all types of three-phase AC motors are squirrel cage motors, with rotors of this type. Their power ranges from one-third to several hundred horsepower. Three-phase motors of this type, which are in the range of one horsepower and above, have lower costs compared to similar single-phase motors and can handle heavier loads during startup.
The squirrel cage three-phase induction motor with a wound rotor (slip ring) is an advanced type of motor. In this motor, the stator is similar to that of a squirrel cage induction motor, but with the difference that some windings are terminated on slip rings instead of being in the rotor. By adding resistors and external capacitors, these windings help improve the motor’s performance.
The amount of slip required to produce maximum torque is directly related to the rotor resistance. By adding external resistance between the slip rings, the effective rotor resistance decreases, resulting in greater slip, which increases maximum torque and improves performance at lower speeds.
This type of motor is highly suitable for applications with constant static loads, where maximum torque is required at speeds close to zero, and the motor needs to reach its rated speed with minimal current consumption and in the shortest time possible. Additional load can reduce the speed to 50% of synchronous speed.
Moreover, reducing the speed below 50% significantly reduces efficiency due to energy losses in the resistors. This type of motor is used in applications requiring rotation with varying torque and speeds, such as printing presses, compressors, conveyor belts, elevators, and lifts.
Linear Motor:
A linear motor is essentially a motor that deviates from the typical rotational mode and instead generates linear force. This linear force is created by establishing a traveling electromagnetic field along its length. Linear motors are often of the induction or stepper motor type.
Note: The speed of an electric motor is related to its frequency. Therefore, an electric motor that, for example, runs at 1500 rpm at a frequency of 50 Hz, does not have the same speed of 1500 rpm at a frequency of 60 Hz.
Date: Specifies the manufacturing date of the motor.
RPM: Indicates the rotational speed of the electric motor in revolutions per minute on the output shaft.
Kilowatts: Indicates the power rating of the electric motor.
Note: If it is written on the motor as 380/220 = volts, it means that this electric motor should be connected in delta form for a voltage of 110 volts (which some countries use), and in star form for countries with a voltage of 220 volts (the voltage between phase and neutral), such as Iran.
IP: The level of protection of the electric motor against dust and water according to the table below.
P.H: Types of protections according to the DIN standard 40050.
P00: Open without protection against contact with foreign objects and water. In this case, the motor must be kept in a covered space.
P10: Protected against hand contact and large foreign objects. Also, protected against water. The motor can operate in open spaces and under rain.
P11: Protected against hand contact and large foreign objects. Also, protected against water.
P20: Protected against finger contact and objects of medium weight without protection against water. A suitable cover must be provided for the motor.
P21: Protected against finger contact and objects of medium weight. Also, waterproof.
P22: Protected against finger contact and objects of medium weight. Also, protected against water leakage vertically or inclined at an angle greater than 30 degrees relative to the horizon.
P30: Protected against contact with lightweight tools, etc., and lightweight foreign objects without protection against water.
P31: Protected against contact with lightweight tools, etc., and lightweight foreign objects. Also, waterproof.
P32: Protected against contact with lightweight tools, etc., and lightweight foreign objects. Also, protected against water leakage vertically or inclined at an angle greater than 30 degrees relative to the horizon.
P40 and above: Protected against all external factors.
Other information may also be present on the motors. Users can refer to the motor catalog for further details.
Standard Defined for IP Electric Machine
The letter “W” is used after the two specified digits in some cases, indicating an electric machine designed with an open circuit system with air cooling and operating under specific weather conditions. It reduces and controls the penetration of rain and windborne particles into the electric machine, with the amount of ingress having no impact on its performance, such as IP13W.
When only one characteristic of the IP indices is important for the consumer, the insignificant index is indicated by the letter “X,” such as IP2X, which only specifies the degree of mechanical protection, or IPX4, which specifies the degree of protection against moisture.
Thermal Insulation Class
According to the standards set by the National Electrical Manufacturers Association (NEMA), motor insulation is classified into four classes (A, B, F, H) based on the motor temperature in different working environments, considering an ambient temperature of 40 degrees Celsius. The designated temperature is determined with a maximum efficiency of 10 degrees for the hottest point in the winding center (HOT SPOT).
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