What is the major difference between the BLDC and SRM motor? Which is better? Why?

difference between the BLDC and SRM

Before comparing difference between BLDC vs SRM, let’s look at what these two motors have in common.

Is Switched Reluctance Motor AC or DC?

Brushless DC Motor (BLDC) and Switched Reluctance Motor (SRM) both motors are EC (Electronically Commutated) Motors.

It means that both motors require a position sensor to run.

It also requires a power electronics converter to run and can not run directly on supply, neither AC nor DC.

That’s why it is not appropriate to compare the EC motors with the conventional AC or DC motors. Instead it is recommended to compare motor drives instead of just motors or compare for a particular application.

However, we are not going to compare EC motors with conventional AC or DC motors in this article.

We will compare the BLDC and SRM point to point without considering an application. So it will become a more general comparison of the two latest motors. To make it easy to understand I divided the comparison point to point.

Common features offered by both BLDC & SRM are

  • Higher Efficiency
  • Compact Size
  • Longer Life

So, the major difference is that the BLDC motor has a permanent magnet in the rotor while SRM neither has a magnet nor a winding.

Let’s first checkout the comparison of BLDC and SRM for each parameter in the table format and then understand each in detail.

Cost:

The BLDC motors are more expensive as they use permanent magnets.

The BLDC also required comparatively higher cooling arrangements compared to SRM as explained in the next section.

Temperature Effect:

Magnet does not add the cost only, but it is also responsible for the performance degradation of the motor at the higher temperature.

Nowadays neodymium magnets are used widely as it is the one of the most powerful magnets with the capacity to attract 1000 times their own weight. That means the size required for producing a desired magnetic field becomes very small when using a neodymium magnet.

But, unfortunately the neodymium magnets are the one which is affected most by the temperature.

It restricts the maximum operating temperature of the motor around 80⁰ C compared to which, alnico type magnet can operate up to 500⁰ C but not used due to low resistance to demagnetization and higher manufacturing cost.

It results in degradation in performance of BLDC motor at higher temperature in terms of efficiency and power output both.

Thus the cost of the BLDC is further increased as it requires more cooling arrangement to maintain the temperature below maximum limit.

High Power Applications:

Temperature effect also restricts use of BLDC motors for the high power applications.

The rotor of the SRM does not use a magnet and so is more preferable for the high power applications.

Requirement of a more powerful magnet for the high power BLDC creates the issue of the electromagnetic interference to the nearby sensitive electronics devices.

Thus BLDC motors are generally restricted to the lower power rating that is below 20kW, except taking special care in selecting a magnetic material and design.

Manufacture Process:

Stators of both the motors are salient pole type and made up of laminated steel so there is not much difference in the manufacturing process of the stator except winding type.

However, the process of manufacturing the rotor of BLDC is quite complex and challenging compared to SRM. This is because it requires a process to attach the magnets to the rotor steel with the ability to remain fixed in position even at high speed.

Interior magnet mounting is used in general which increases manufacturing complexity as it requires insertion of the magnets inside the slots provided in rotor stamping.

Thus the manufacturing process is more complex for BLDC than SRM.

It also increases manufacturing time as well as cost of the process.

Motor Design:

Motor design is the process of obtaining the physical dimensions of all parts of the motor for given power and performance before finalizing the manufacturing process.

It is assumed that the design process of the BLDC is more complex as compared to that of the SRM.

Designing a BLDC motor requires selecting the type and placement of the magnet from various options whereas that of SRM is more simple as flux produced by the stator pole only and there is no secondary flux and its resultant issue like a BLDC motor.

Thus predicting the performance of the SRM is easier than that of BLDC.

Torque Ripple & Noise:

The SRM is generally known for the higher torque ripple and acoustic noise than any other motor. This is because of its torque producing mechanism. It produces only an attraction force as there is no secondary magnetic field. Whereas BLDC can produce the torque by both attraction as well as repulsion effect. Simultaneous phase excitation techniques and higher numbers of phases are some of the techniques used to reduce the torque and acoustics noise of the SRM.

Thus torque ripple and noise issue is more in the SRM compared to the BLDC motor.

Industrial Acceptance:

Despite offering all the advantages by the SRM over BLDC, still the industrial acceptance of the BLDC motor is much higher.

This is mainly because the major countries manufacturing BLDC motors have a sufficient natural source to produce the magnet. So they want to use the magnet in the maximum products as much as they can. Besides, BLDC motors have proven the performance for the low power electric vehicles in the last decade. Whereas there is no major field review of the SRM drive except a few applications.

Conclusion:

  • BLDC motors are more costly and their performance is affected by the temperature.
  • SRM is more suitable for high power applications above 20kW.
  • Performance of the BLDC varies for different geographical conditions and temperature whereas that of SRM remains constant.
  • Industrial acceptance of the SRM is still very less. Fact is that countries having source of magnets do not want the magnetless motor to become a new trend.

Thank you for reading throughout.

Please share your view or any additional information in the comment section.

Also checkout my video courses on Switched Reluctance Motor which includes Fundamentals of SRM drive and Modelling and Simulation of SRM Drive.

You might like the similar article Scope of Research in the Field of Switched Reluctance Motor Drive

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By Dr. Jignesh Makwana

Dr. Jignesh Makwana, Ph.D., is an Electrical Engineering expert with over 15 years of teaching experience in subjects such as power electronics, electric drives, and control systems. Formerly an associate professor and head of the Electrical Engineering Department at Marwadi University, he now serves as a product design and development consultant for firms specializing in electric drives and power electronics.