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Brushless DC (BLDC) Motors vs. AC Motors and DC Brushed Motors - Everything You Need To Know

August 26, 2022

What is an Electric Motor

Electric motor terminology can get very convoluted. There are AC and DC motors, and then each of these consist of different types, and types of types, and then two types could be the same, but named after different characteristics etc. As convoluted as this terminology can get, we live in a world filled with electric motors, so it can benefit anyone to be able to differentiate between them.

When you’re considering a new power tool, for example, you’ve definitely come across the term “brushless DC electric motor” as a major feature. But what benefit does this actually provide? How are these motors different from other electric motors? And how do they work? If you’ve asked yourself any of these questions, you’ve come to the right place.  

To start with the basics, an electric motor is used to convert electrical energy into mechanical energy. There are two types of electricity in general (alternating current and direct current). AC motors convert alternating current electricity into mechanical energy, and DC motors do so with direct current electricity. Both AC motors and DC motors use electrical current to produce a rotating magnetic field that, in turn, rotates the armature of a motor. In a nutshell, all motors work by rotating a magnetic field, the difference lies in how the design of the motor does this. AC motors are typically more powerful than DC motors, whilst DC motors can be 30% more efficient

With markets shifting towards the uptake of brushless DC motors due to their many benefits, maybe you’re wondering what this type of motor is, and what makes it stand out from AC motors and DC brushed motors. In this article, we’ll break down these differences, then dive into what makes a brushless DC motor so revolutionary in the world of electric motors. We’ll also touch on the even more recent evolution to DC brushless motors: the electronically commutated (EC) motor.

The Different Types of Electric Motors

The types of electric motors, a flow chart
Types of Electric Motors Copyright Argentum

Terminology

Power vs Torque Formula

Before we compare the different types of electric motors, it’s necessary to understand some terminology. For example, a powerful “torque” can provide a great benefit to certain motors. But there’s a difference between “torque” and “power”; torque is the total mechanical energy (or force), and power is the total electrical energy (wattage). 

Power formula for electric motors
Power Formula for Electric Motors

Mathematic formula for torque
Torque Formula Copyright Argentum

Asynchronous vs. Synchronous Motors

You’ll also often encounter the words “asynchronous” and “synchronous” in reference to different types of motors when diving into this topic. Here’s a simple definition of each of these terms in relation to motors: 

Asynchronous: The stator and rotor’s magnetic fields rotate at different speeds to generate torque. The rotor’s magnetic field is generally slower than that of the stator in asynchronous motors.  

Synchronous: The stator and rotor magnetic fields rotate at the same speed to generate torque. 

AC Motors 

Although other electric motors had been invented previously, when the AC induction motor was invented in 1887, it was still a revolution. Nikola Tesla invented it in 1887, and patented it in 1888. The induction motor is an asynchronous motor and has been given credit for kick starting the second industrial revolution by “drastically improving energy generation efficiency and making the long-distance distribution of electricity possible”. In 1891, AC motors were taken to the next level when General Electric started developing three-phase induction motors. Now the most common type of AC motor is a newer variation of the induction motor called the three phase squirrel cage motor. So, when we're comparing motors to AC motors, this is the AC motor we're talking about.

How an AC Induction Motor Works 

In a Squirrel Cage Motor, AC current flows to the stator (the ring of steel containing the other components of the motor). The stator stays stationary, but the current creates a magnetic field that ebbs and flows depending on the frequency of the AC electricity. The magnetic field induces a current to the rotor (this is called induction), and the interaction of the magnetic fields on the stator and rotor rotate the armature of the motor, producing torque. See below for a diagram of an AC motor. 

How a three phase induction motor works
How a 3 phase AC induction motor works
How an AC induction motor works according to the Engineering Mindset
Image Credit to the Engineering Mindset

Advantages of AC Motors: 

1. More powerful than DC brushed motors

They can generate a more powerful torque because they can supply a higher electrical current. 

2. Asynchronous motors, like the Squirrel Cage Motor, generally have a higher operational speed than synchronous motors. Side note: DC motors are synchronous.

This type of AC motor usually operates at around 1500 rpm, whereas synchronous AC motors operate smoothly and consistently at lower speeds. However, even though 1500 rpm is pretty good, keep in mind that DC brushless motors can sometimes operate at up to 100,000 rpm

3. Longer lifetime than DC brushed motors 

All AC induction motors are brushless. Less moving parts, means less friction and heat, and this reduces wear on the components of the motor. The absence of brushes, commutators and slip rings also makes them cheaper.

4. Power loads are more protected

AC motors have a lower start-up power. 

5. Resists changes in speed even when the load changes 

Disadvantages to AC Motors

Efficiency 

A major disadvantage to AC motors is that they are generally less efficient the DC brushless motors. Two of the things that contribute to this inefficiency are: 

  1. AC motors use electromagnets in the stator, which consume electricity. Brushed and BLDC motors typically use permanent magnets instead
  2. Secondly, AC motors experience a phenomenon known as “slip”, which is “the difference between the rotor's speed and the speed of the revolving magnetic field in the stator”. Although slip is bound to occur when producing torque in an AC motor, it’s directly related to power losses from heat.
  3. Thirdly, all AC motors suffer from reactive power losses, and DC motors do not

Installation

  • DC motors are, in general, easier to install than AC motors.

Applications for AC Motors: 

  • Water pumps 
  • Elevators
  • Cranes 
  • Cooling nuclear power stations
  • Ideal for situations where the motor speed can be low to medium, and needs to remain constant, even when the load on the motor varies. This is why AC motors are often found in heavy-duty, continuous-speed, industrial applications.

DC Motors 

There are two types of DC motors: brushed and brushless (BLDC) motors. Brushless DC motors can also be broken down into two types: single-phase and three phase. Between AC motors, brushed DC motors and brushless DC motors, brushless DC motors seem to provide the most benefits: they have no “excitation loss” when compared to AC motors, and no brush friction and heat/energy loss when compared to brushed DC motors.

The first BLDC motor was developed in 1962 alongside advancements in solid state technology. Although these early BLDC motors were durable, they were not able to generate much power. Robert E. Lordo developed the first large-scale brushless DC motor in the late 1980s, which was 10 times more powerful than previous BLDC motors. Brushless DC motors are increasingly taking over where AC motors used to be used. Because of their speed, precision, and efficiency, they can now replace AC motors for industrial applications, like water pumps and HVAC systems.

Brushed vs. Brushless DC Motors

As the name implies, brushless DC motors don’t have brushes, while brushed DC motors do. Brushes in brushed DC motors don’t have bristles like a toothbrush, they are usually made out of carbon and they act like springs. They are used to provide the coils (wrapped around the rotor) with current, causing the motor to spin. The simple difference of a motor either having brushes, or not having them, has many implications for the efficiency, applications, cost and complexity of the equipment.

Brushed DC motor diagram
Image Credit for Brushed DC Motor Diagram (Engineering Stuff) 

Brushless DC motor diagram
Brushless DC Motor Diagram (Image credit) 

Differences in Operation Between Brushed and Brushless DC Motors

To explain these diagrams, and the differences in the operation of brushed vs. BLDC motors, a BLDC motor is essentially flipped inside out; BLDC motors have the permanent magnets on the rotor, and the electromagnets on the stator. This design removes the need for brushes because it eliminates the need for brushes to flip the electromagnetic field (which is what’s done in a brushed motor). In BLDC motors, a digital controller is used instead to charge the electromagnets in the stator to rotate the rotor a full 360-degrees.

Image credit to Sontian Motor

Advantages of Brushless DC Motors (BLDC)

1. Lifetime

  • The lifetime of brushless motors is much longer than that of brushed motors because the brush and commutator don’t rub against each other and get worn down (like they do in brushed motors). In fact, brushed DC motors last about 1000 - 3000 service hours, whereas BLDC motors last about 20,000 service hours. 

2. Output Speed and Precision

  • Brushless motors also have a higher output speed, they can typically go about six times faster than brushed motors. 
  • The speed of BLDC motors is also thought to be more consistent. This is because there could easily be discrepancies between the speed of the rotor, and the speed of the revolving magnetic field of the stator in an AC motor. As we mentioned earlier in this article, this phenomenon is called “slip”, and it’s directly related to power losses from heat, which also occur in brushed DC motors. Slip does not occur in BLDC motors, which eliminates discrepancies in speed that would be caused by it. 
  • DC motors are able to immediately start, stop, and reverse, which is essential when it comes to controlling and operating production equipment. Many “motion control applications” use permanent magnet direct current (DC) motors for this reason. 

3. Lower Operational Cost and Less Maintenance

  • Brushed motors have a lower capital cost than brushless motors because brushless motors require a controller to supply a “voltage sequence between the three phases”. However, brushed motors do wear down more quickly; sometimes the brushes must be replaced, and this brings up maintenance costs. If you consider BLDC motors more of an investment, they can be less expensive in the long term because they require practically zero maintenance, and last much longer.

4. Efficiency

  • Brushless motors are more efficient than brushed motors because, again, the lack of brushes means that no brushes will be constantly in contact with the commutator (like they are in brushed motors). This lack of contact, means there’s also a lack of friction and heat created in brushless motors. Less energy lost in the form of heat, makes a brushless motor more efficient. In fact, BLDC motors typically have an efficiency of 85-90%, while brushed motors are usually only 75-80%. This means that for applications like power tools, for example, this improved efficiency extends battery life (meaning more can be done between charges). 

Image Source for “Efficiency of EC Motors”

5. Size

6. Torque and Control

Disadvantages of Brushless DC Motors

Despite their many advantages, we would be remiss if we didn’t cover the disadvantages of BLDC motors. These really come down to cost and complexity; BLDC motors have a more complex design because they involve an electronic communication device, and often involve sensors. These complexities not only add to the cost of BLDC motors, but it’s also assumed that “more complicated equipment is more likely to fail”. Therefore, if you’re looking for a motor to use in a less demanding, cost-sensitive, application, the brushed DC motor or an AC motor will probably do just fine.

Applications

Brushless DC (BLDC) Motors

  • Power tools 
  • Computer fans and hard drives    
  • Used when fine speed control is necessary, and the load does not vary significantly, such as in many domestic appliances or robotics.
  • Electric Vehicles (EVs)
  • Motion control applications 
  • Dishwashers
  • Fridges and freezers 
  • And now...HVAC! In fact, DC brushless motors in HVAC systems do the same job as AC motors, but DC motors allow HVAC systems to "operate at least 50% more efficiently than [with] AC motors".

Brushed DC Motors

These motors are still used in simple applications where motors don’t need to be constantly running, such as in windshield wipers, car windows, and seat position motors in cars. 

Additionally, they're often still used in: 

The Electronically Commutated (EC) Motor

This article has focused so far on AC motors, brushed DC motors, and BLDC motors, but there’s another type of motor that you’ll hear about if you read a few articles on BLDC motors: the electronically commutated (EC) motor.

We’ve come across this type as well, and we were not able to determine the difference between brushless DC motors, and EC motors. Actually, EE Power states that “An electronically commutated motor is a three-phase, brushless DC motor. It comprises three major components: a circuit board, an electronic control module, and a three-phase motor with a permanent magnet rotor”. It seems that an EC motor is just a BLDC motor that’s being characterized by its method of commutation rather than by whether or not it involves brushes. EC motors also differ from BLDC motors of the past because they “can connect directly to AC mains power supplies with their integrated electronics.” So, for the purposes of this article, we’ll treat them as a new evolution in BLDC motors, but not separate from them. 

According to Enervex, there are currently two types of EC motors:

  1. Internally controlled: This EC motor often consumes AC power, and converts it to DC power within its circuitry. You would control this motor with integrated electronics. 
  2. Externally controlled: This motor can also consume AC power, but also converts AC to DC power. Because the circuit board or variable frequency drive (VFD) that controls this motor is located externally, this type “offers an expanded speed control range and a variety of speed control options.” The additional speed control and options come from the fact that there’s “no internal electronics that can be damaged” in this type of EC motor. This also allows externally controlled EC motors to be used in high-heat applications.

If we were to add these two types of motors to our flow chart (from above), it might look like this: 

Types of Motors including electronically commutated motor (EC motor)
Copyright Argentum

We were surprised when we read that EC motors were powered by "AC power", considering they are brushless DC motors. But, after some digging, we were able to clear that up, too. EC motors use DC voltages, and their control systems convert DC power into precise pulses to make them run. Even though DC motors use a DC power supply, the current is switched to AC (or rather pulsed DC) in order to feed the motor windings, which needs to be done with high-frequency AC power. EC motors have an expanding array of applications, including: small fans, servo motors, motion-control systems, small appliances, and even conveyer belts and condenser units. 

What We Learned

Electric motors are used in just about every industry, including medicine, agriculture, automotive and indoor environmental control (heating, cooling, ventilation, etc.). Because of this, even if you’re not an electrical or mechanical engineer, it’s still helpful to know electric motor basics (especially if you’re in the market for a new power tool, and wondering what in the world a brushless DC motor is). 

Brushless DC (BLDC) motors have been gaining popularity for decades due to their many benefits in comparison to AC and brushed DC motors. As we covered in this article, these benefits mainly include their longer lifespans, better efficiency, and less maintenance requirements and costs. It’s often determined that these advantages save money in the long-run, and therefore outweigh the BLDC motor’s higher up-front cost. As technologies associated with BLDC and EC motors develop, we hope that these motors will also become more reliable, compact, and cost effective.   

As a side note from the author, this has been a complicated topic to cover. So, if you have anything to add, including questions or comments, we encourage you to contact us through our website and let us know! Especially if you think there’s anything crucial that we missed. 

To Wrap Up

The transition from AC motors, to motors powered by DC electricity, is just one way that more and more of our everyday devices are being powered by direct current (DC) electricity. With the number of devices and building systems powered by DC electricity growing, it’s becoming more clear that we’re going through an electrical revolution. In fact, DC consumption in buildings could climb as high as 74% in buildings that use electric vehicle chargers and HVAC equipment with DC motors. With all of these devices (and now an increasing number of motors) requiring DC electricity, it makes sense to power our DC world with DC electricity. Currently, our power lines supply alternating current (AC) electricity, but you can choose to reduce wasted energy in your building by implementing a DC power distribution system. If you’d like to learn more about this, you can visit Argentum’s website to read about their plug-and-play DC power system for commercial buildings.

blog author image

Erin Kelly

Erin is the Creative Director at Argentum Electronics. She has a New Media degree from the University of Toronto and 5 years of experience in the communications field. From 2017 - 2019 she worked with a manufacturing company on their YouTube content and strategy, and has done digital content creation for dozens of clients through her own business called Story Unlocked. Erin has built two computers, and loves technology, especially when it makes the world a better place.

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