Classification of Electric Motors - Part Two


In the previous topic” Classification of Electric Motors “ , I explained the different types of the Brushed DC motor (BDC) which as in the following diagram:


Today, I will explain the Brushless DC motor (BLDC) and the AC induction motors as follows.

You can review the following related topics for review and good following.



2-  Brushless DC motors 



Brushless DC motors


In brushes DC motors, the mechanical commutator and associated brushes are problematical for a number of reasons as follows:
  1. Brush wear occurs, and it increases dramatically in low‐pressure environment. 
  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. 

Brushless Direct Current (BLDC) motors are one of the motor types rapidly gaining popularity. BLDC motors are used in industries such as Appliances, Automotive, Aerospace, Consumer, Medical, Industrial Automation Equipment and Instrumentation.

As the name implies, BLDC motors do not use brushes for commutation; instead, they are electronically commutated.

BLDC motors have many advantages over brushed DC motors and induction motors, a few of these are:
  1. Better speed versus torque characteristics.
  2. High dynamic response.
  3. High efficiency.
  4. Long operating life.
  5. Noiseless operation.
  6. Higher speed ranges.

In addition, the ratio of torque delivered to the size of the motor is higher, making it useful in applications where space and weight are critical factors.


Construction


BLDC motors are a type of synchronous motor. This means the magnetic field generated by the stator and the magnetic field generated by the rotor rotates at the same frequency.

BLDC motors come in single-phase, 2-phase and 3-phase configurations. Corresponding to its type, the stator has the same number of windings. Out of these, 3-phase motors are the most popular and widely used.



1- Stator 



Stator of a BLDC Motor

The stator of a BLDC motor consists of stacked steel laminations with windings placed in the slots that are axially cut along the inner periphery.

Most BLDC motors have three stator windings connected in star fashion. Each of these windings is constructed with numerous coils interconnected to form a winding. One or more coils are placed in the slots and they are interconnected to make a winding. Each of these windings is distributed over the stator periphery to form an even numbers of poles.

Depending upon the control power supply capability, the motor with the correct voltage rating of the stator can be chosen. Forty-eight volts, or less voltage rated motors are used in automotive, robotics, small arm movements and so on. Motors with 100 volts, or higher ratings, are used in appliances, automation and in industrial applications.


2- Rotor

Rotor of a BLDC Motor


The rotor is made of permanent magnet and can vary from two to eight pole pairs with alternate North (N) and South (S) poles. 


BLDC Rotor Magnet Positions

Based on the required magnetic field density in the rotor, the proper magnetic material is chosen to make the rotor. Ferrite magnets are traditionally used to make permanent magnets.



3- Hall Sensors 



BLDC Hall Sensors


  • Unlike a brushed DC motor, the commutation of a BLDC motor is controlled electronically. To rotate the BLDC motor, the stator windings should be energized in a sequence. It is important to know the rotor position in order to understand which winding will be energized following the energizing sequence. Rotor position is sensed using Hall Effect sensors embedded into the stator. 
  • Most BLDC motors have three Hall sensors embedded into the stator on the non-driving end of the motor. 
  • Whenever the rotor magnetic poles pass near the Hall sensors, they give a high or low signal, indicating the N or S pole is passing near the sensors. Based on the combination of these three Hall sensor signals, the exact sequence of commutation can be determined. 
  • Based on the physical position of the Hall sensors, there are two versions of output. The Hall sensors may be at 60° or 120° phase shift to each other. Based on this, the motor manufacturer defines the commutation sequence, which should be followed when controlling the motor. 
Note: The Hall sensors require a power supply. The voltage may range from 4 volts to 24 volts. Required current can range from 5 to 15 mAmps. 


Theory of Operation


  • Each commutation sequence has one of the windings energized to positive power (current enters into the winding), the second winding is negative (current exits the winding) and the third is in a non-energized condition. 
  • Torque is produced because of the interaction between the magnetic field generated by the stator coils and the permanent magnets of the rotor. 
  • In order to keep the motor running, the magnetic field produced by the windings should shift position, as the rotor moves to catch up with the stator field. What is known as “Six-Step Commutation” defines the sequence of energizing the windings. 
  • In six-step commutation, only two out of the three Brushless DC Motor windings are used at a time. Steps are equivalent to 60 electrical degrees, so six steps make a full, 360 degree rotation. One full 360 degree loop is able to control the current, due to the fact that there is only one current path. Six-step commutation is typically useful in applications requiring high speed and commutation frequencies. A six-step Brushless DC Motor usually has lower torque efficiency than a sine-wave commutated motor.


Typical BLDC Motor Applications 


We can categorize the type of BLDC motor control into three major types:

  1. Constant load.
  2. Varying loads.
  3. Positioning applications.


1- Applications with Constant Loads:
These are the types of applications where a variable speed is more important than keeping the accuracy of the speed at a set speed. In addition, the acceleration and deceleration rates are not dynamically changing. In these types of applications, the load is directly coupled to the motor shaft.
For example, fans, pumps and blowers come under these types of applications. These applications demand low-cost controllers, mostly operating in open-loop.


2- Applications with Varying Loads:
These are the types of applications where the load on the motor varies over a speed range. These applications may demand high-speed control accuracy and good dynamic responses.
For example,


  • In home appliances: washers, dryers and compressors. 
  • In automotive, fuel pump control, electronic steering control, engine control and electric vehicle control. 
  • In aerospace, there are a number of applications, like centrifuges, pumps, robotic arm controls, gyroscope controls and so on. 
These applications may use speed feedback devices and may run in semi-closed loop or in total closed loop.


3- Positioning Applications:
Most of the industrial and automation types of application come under this category. The applications in this category have some kind of power transmission, which could be mechanical gears or timer belts, or a simple belt driven system. In these applications, the dynamic response of speed and torque are important. Also, these applications may have frequent reversal of rotation direction.
These systems mostly operate in closed loop.


Finally, a comparison between Brushed DC motor (BDC) and Brushless DC motor (BLDC) is as shown in the below image.





Second: AC Motors 


Alternating current (AC) motors use an electrical current, which reverses its direction at regular intervals.

The main advantage of DC motors over AC motors is that speed is more difficult to control for AC motors. To compensate for this, AC motors can be equipped with variable frequency drives but the improved speed control comes together with a reduced power quality.


Types of AC Motors:






AC motors in common use today may be divided into two broad categories:

  1. Induction (asynchronous) motors. 
  2. Synchronous motors. 
  3. Linear Motors.


These two types of motors differ in how the rotor field excitation is supplied as follows:

For induction motors, there is no externally-applied rotor excitation, and current is instead induced into the rotor windings due to the rotating stator magnetic field.

For synchronous motors, a field excitation is applied to the rotor windings. This difference in field excitation leads to differences in motor characteristics, which leads in turn to different protection and control requirements for each motor type.


1- Induction motor



Induction motors are the most common motors used for various equipments in industry.

Induction Motor: So called because voltage is induced in the rotor (thus no need for brushes), but for this to happen, the rotate than rotor must at a lower speed the magnetic field to allow for the existence of an induced voltage.

Therefore a new term is needed to describe the induction motor which is the slip.


The slip:



A driving torque can only exist if there is an induced current in the shading ring. It is determined by the current in the ring and can only exist if there is a flux variation in the ring. Therefore, there must be a difference in speed in the shading ring and the rotating field. This is why an electric motor operating to the principle described above is called an “asynchronous motor”.

The difference between the synchronous speed (Ns) and the shading ring speed (N) is called “slip” (s) and is expressed as a percentage of the synchronous speed.

S= (Nsyn – Nm)/ Nsyn

Where s is the slip. Slip is one of the most important variables in the control and operation of induction machines.

s = 0 : if the rotor runs at synchronous speed.

s = 1 : if the rotor is stationary.

s is –ve : if the rotor runs at a speed above the synchronous speed.

s is +ve : if the rotor runs at a speed below the synchronous speed.



Advantages:

  1. Simple design, rugged, low-price, easy maintenance.
  2. Wide range of power ratings: fractional horsepower to 10 MW.
  3. Run essentially as constant speed from no-load to full load.
  4. Its speed depends on the frequency of the power source.
  5. Most popular motor today in the low and medium horsepower range.
  6. Very robust in construction.
  7. Have replaced DC Motors in areas where traditional DC Motors cannot be used such as mining or explosive environments Of two types depending on motor construction; Squirrel Cage or Slip Ring.


Disadvantages:

  1. Not easy to have variable speed control.
  2. Requires a variable-frequency power-electronic drive for optimal speed control.
  3. Most of them run with a lagging power factor.


Principle of operation:



  • The stator is usually connected to the grid and, thus, the stator is magnetized. 
  • Stator magnetic field cuts the rotor windings and produces an induced voltage in the rotor windings. 
  • Due to the fact that the rotor windings are short circuited, for both squirrel cage and wound-rotor, and induced current flows in the rotor windings. 
  • The rotor current produces another magnetic field. 
  • A torque is produced as a result of the interaction of those two magnetic fields. 



Construction:



An induction motor has two main parts

1- Stator 



Induction Motor Stator

This is the immobile part of the motor. A body in cast iron or a light alloy houses a ring of thin silicon steel plates (around 0.5mm thick). The plates are insulated from each other by oxidation or an insulating varnish. The “lamination” of the magnetic circuit reduces losses by hysteresis and eddy currents.

The plates have notches for the stator windings that will produce the rotating field to fit into (three windings for a 3-phase motor). Each winding is made up of several coils. The way the coils are joined together determines the number of pairs of poles on the motor and hence the speed of rotation.



2- Rotor 



This is the mobile part of the motor. Like the magnetic circuit of the stator, it consists of stacked plates insulated from each other and forming a cylinder keyed to the motor shaft.


Types of Induction Motors



Types of Induction Motors

Induction motors are classified according to the Rotor Type as follows:

A- Squirrel-Cage Rotor: 



 



Squirrel-Cage Rotor

It consists of thick conducting bars embedded in parallel slots. These bars are short-circuited at both ends by means of short-circuiting rings.



B- Wound Rotor: 



Wound Rotor


It has a three-phase, double-layer, distributed winding. It is wound for as many poles as the stator. The three phases are wired internally and the other ends are connected to slip-rings mounted on a shaft with brushes resting on them.


Each of the two types of Induction motors above can be classified into two main groups as follows:


I- Single-phase induction motors:



These only have one stator winding, operate with a single-phase power supply, have a squirrel cage rotor, and require a device to get the motor started. This is by far the most common type of motor used in household appliances, such as fans, washing machines and clothes dryers, and for applications for up to 3 to 4 horsepower.

Single phase induction motors come also with wound rotor which has excellent starting and accelerating characteristics, and they are ideal for Value Operators, Farm Motor Applications, Hoists, Floor Maintenance Machines, Air Compressors, Laundry Equipment and Mining Equipment.


II- Three-phase induction motors:



The rotating magnetic field is produced by the balanced three-phase supply. These motors have high power capabilities, can have squirrel cage or wound rotors (although 90% have a squirrel cage rotor), and are self-starting. It is estimated that about 70% of motors in industry are of this type, are used in, for example, pumps, compressors, conveyor belts, heavy-duty electrical networks, and grinders. They are available in 1/3 to hundreds of horsepower ratings.


Now, let us see the first classification of induction motors based on the above types: 



1- Single Phase, Squirrel Cage, Induction Motor: 



This category have many types as shown in the below image.




A- Shaded-Pole Induction Motors 



Construction and operation principle: 



Shaded-Pole Induction Motors 

Shaded-pole motors have only one main winding and no start winding. Starting is by means of a design that rings a continuous copper loop around a small portion of each of the motor poles. This “shades” that portion of the pole, causing the magnetic field in the shaded area to lag behind the field in the unshaded area. The reaction of the two fields gets the shaft rotating.


Advantages:

  1. Because the shaded-pole motor lacks a start winding, starting switch or capacitor, it is electrically simple and inexpensive. 
  2. The speed can be controlled merely by varying voltage, or through a multi-tap winding. 
  3. Mechanically, the shaded-pole motor construction allows high-volume production. 
  4. These are usually considered as “disposable” motors, meaning they are much cheaper to replace than to repair. 

Disadvantages:

  1. It’s low starting torque is typically 25% to 75% of the rated torque. 
  2. It is a high slip motor with a running speed 7% to 10% below the synchronous speed. 
  3. Generally, efficiency of this motor type is very low (below 20%). 

Applications:
The low initial cost suits the shaded-pole motors to low horsepower or light duty applications. Perhaps their largest use is in multi-speed fans for household use. But the low torque, low efficiency and less sturdy mechanical features make shaded-pole motors impractical for most industrial or commercial use, where higher cycle rates or continuous duty are the norm.



In the next Topic, I will continue explaining other types of Single Phase, Squirrel Cage Induction Motor. 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.  




3 comments:

  1. I want to add a useful information , that the brushes is made from carbon ? why ????
    because carbon has high surface resistivity so the short circuit circulating currents are reduced and the second reason is that the carbon is a soft material so the commutator surface doesnot get damaged

    ReplyDelete
  2. thanks medo88 for the helpful info.

    ReplyDelete
  3. the brushes are made of carbon because its stand a high temperature. other material could melt due high temperature caused by friction between slip rings and brushes

    ReplyDelete

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