Classification of Electric Motors

In the previous topic” Electrical Motors Basic Components “, I explained the construction and basic components of the main motor types of motors; AC and DC motors.

Today, I will explain the different types in electric motors world as follows.

Main Types of Motor 

Electric motors are broadly classified into two categories as follows:

1. AC Motors.
2. DC Motors.

Within those two main categories there are subdivisions as shown in the below image.

Motor Types

Notes: Recently, with the development of economical and reliable power electronic components, there are numerous ways to design a motor and the classifications of these motors have become less rigorous and many other types of motor have appeared. Our classification of motors will be comprehensive as can as possible.


First: DC motors

DC motors

DC power systems are not very common in the contemporary engineering practice. However, DC motors have been used in industrial applications for years Coupled with a DC drive, DC motors provide very precise control DC motors can be used with conveyors, elevators, extruders, marine applications, material handling, paper, plastics, rubber, steel, and textile applications, automobile, aircraft, and portable electronics, in speed control applications.


Advantages of DC motors:

1. It is easy to control their speed in a wide range; their torque-speed characteristic has, historically, been easier to tailor than that of all AC motor categories. This is why most traction and servo motors have been DC machines. For example, motors for driving rail vehicles were, until recently, exclusively DC machines. 
2. Their reduced overall dimensions permit a considerable space saving which let the manufacturer of the machines or of plants not to be conditioned by the exaggerated dimensions of circular motors. 

Disadvantages of DC motors

1. Since they need brushes to connect the rotor winding. Brush wear occurs, and it increases dramatically in low‐pressure environment. So they cannot be used in artificial hearts. If used on aircraft, the brushes would need replacement after one hour of operation. 
2. Sparks from the brushes may cause explosion if the environment contains explosive materials. 
3. RF noise from the brushes may interfere with nearby TV sets, or electronic devices, Etc.
4. DC motors are also expensive relative to AC motors. 

Thus all application of DC motors have employed a mechanical switch or commutator to turn the terminal current, which is constant or DC, into alternating current in the armature of the machine. Therefore, DC machines are also called commutating machines.


Types of DC motors:

Types of DC motors

The DC motors are divided mainly to: 

1. Brush DC motors (BDC).
2. Brushless DC motors (BLDC).

1. A Brush DC motors

Brush DC motors

A brushed DC motor (BDC) is an internally commutated electric motor designed to be run from a direct current power source.


Applications:
Brushed DC motors are widely used in applications ranging from toys to push-button adjustable car seats.


Advantages:
Brushed DC (BDC) motors are inexpensive, easy to drive, and are readily available in all sizes and shapes


Construction: 

Brushed DC motor Construction

All BDC motors are made of the same basic components: a stator, rotor, brushes and a commutator.


1- Stator
The stator generates a stationary magnetic field that surrounds the rotor. This field is generated by either permanent magnets or electromagnetic windings.


2- Rotor

Rotor (Armature)

The rotor, also called the armature, is made up of one or more windings. When these windings are energized they produce a magnetic field. The magnetic poles of this rotor field will be attracted to the opposite poles generated by the stator, causing the rotor to turn. As the motor turns, the windings are constantly being energized in a different sequence so that the magnetic poles generated by the rotor do not overrun the poles generated in the stator. This switching of the field in the rotor windings is called commutation.


3- Brushes and Commutator 

Commutator Example

Segments and Brushes

Unlike other electric motor types (i.e., brushless DC, AC induction), BDC motors do not require a controller to switch current in the motor windings. Instead, the commutation of the windings of a BDC motor is done mechanically. A segmented copper sleeve, called a commutator, resides on the axle of a BDC motor. As the motor turns, carbon brushes (ride on the side of the commutator to provide supply voltage to the motor) slide over the commutator, coming in contact with different segments of the commutator. The segments are attached to different rotor windings, therefore, a dynamic magnetic field is generated inside the motor when a voltage is applied across the brushes of the motor. It is important to note that the brushes and commutator are the parts of a BDC motor that are most prone to wear because they are sliding past each other.


How the Commutator Works:

How the Commutator Works 

As the rotor turns, the commutator terminals also turn and continuously reverse polarity of the current it gets from the stationary brushes attached to the battery.



Types of BDC motors:

Types of DC motors

The different types of BDC motors are distinguished by the construction of the stator or the way the electromagnetic windings are connected to the power source. These types are:

1. Permanent Magnet.
2. Shunt-Wound.
3. Series-Wound.
4. Compound-Wound.
5. Separately excited DC motor.
6. Universal Motor.
7. Servo Motors.

A- Permanent Magnet 

Permanent Magnet Motor

A permanent magnet DC (PMDC) motor is a motor whose poles are made out of permanent magnets to produce the stator field.

Advantages:

1. Since no external field circuit is needed, there are no field circuit copper losses.
2. Since no field windings are needed, these motors can be considerable smaller. 
3. Widely used in low power application.
4. Field winding is replaced by a permanent magnet (simple construction and less space).
5. No requirement on external excitation.

Disadvantages:

1. Since permanent magnets produces weaker flux densities then externally supported shunt fields, such motors have lower induced torque. 
2. There is always a risk of demagnetization from extensive heating or from armature reaction effects (Some PMDC motors have windings built into them to prevent this from happening). 

B- Shunt-Wound 

Shunt-Wound Motor

Shunt-wound Brushed DC (SHWDC) motors have the field coil in parallel (shunt) with the armature.

The speed is practically constant independent of the load and therefore suitable for commercial applications with a low starting load, such as centrifugal pump, machine tools, blowers fans, reciprocating pumps, etc.

Advantages:

1. The current in the field coil and the armature are independent of one another. as a result, these motors have excellent speed control. 
2. Loss of magnetism is not an issue in SHWDC motors so they are generally more robust than PMDC motors. 
3. Speed can be controlled by either inserting a resistance in series with the armature (decreasing speed) or by inserting resistance in the field current (increasing speed).

Disadvantages:

1. Shunt-wound Brushed DC (SHWDC) motors have drawbacks in reversing applications, however, because winding direction relative to the shunt winding must be reversed when armature voltage is reversed. Here, reversing contactors must be used. 

C- Series-Wound 

Series-Wound Motor

Series-wound Brushed DC (SWDC) motors have the field coil in series with the armature. These motors are ideally suited for high-torque applications such as traction vehicles (cranes and hoists, electric trains, conveyors, elevators, electric cars) because the current in both the stator and armature increases under load.

Advantages:

1. The torque is proportional to I2 so it gives the highest torque per current ratio over all other dc motors. 

Disadvantages:

1. A drawback to SWDC motors is that they do not have precise speed control like PMDC and SHWDC motors have. 
2. Speed is restricted to 5000 RPM. 
3. It must be avoided to run a series motor with no load because the motor will accelerate uncontrollably. 

D- Compound-Wound

Compound-Wound Motor

Compound Wound (CWDC) motors are a combination of shunt-wound and series-wound motors.

CWDC motors employ both a series and a shunt field. The performance of a CWDC motor is a combination of SWDC and SHWDC motors. CWDC motors have higher torque than a SHWDC motor while offering better speed control than SWDC motor.

It is used in Applications such as Rolling mills, sudden temporary loads, heavy machine tools, punches, etc.


Advantages:

1. This motor has a good starting torque and a stable speed. 

Disadvantages:

1. The no-load speed is controllable unlike in series motors. 

E- Separately excited DC motor 

Separately excited DC motor

In a separately excited DC motor the field coils are supplied from an independent source, such as a motor-generator and the field current is unaffected by changes in the armature current. The separately excited DC motor was sometimes used in DC traction motors to facilitate control of wheel slip.


F- Universal Motor

Universal Motor

The universal motor is a rotating electrical machine similar to DC series motor, designed to operate either from AD or DC source. The stator & rotor windings of the motor are connected in series through the rotor commutator. The series motor is designed to move large loads with high torque in applications such as crane motor or lift hoist.



G- Servo Motors

Servo Motors

Servo Motors are mechanical devices that can be instructed to move the output shaft attached to a servo wheel or arm to a specified position. Servo Motors are designed for applications involving position control, velocity control and torque control. 

Servo Motors Components

A servo motor mainly consists of a DC motor, gear system, a position sensor which is mostly a potentiometer, and control electronics.

Servo Motors Applications

In the next Topic, I will explain the Brushless DC Motor (BDLC) & AC Motors Types. So, please keep following.


Note: these topics about Motors in this course EE-1: Beginner's electrical design course is an introduction only for beginners to know general basic information about Motors and Pumps as a type of Power loads. But in other levels of our electrical design courses, we will show and explain in detail the Motor and Pumps Loads calculations.