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A BLDC motor is a Permanent Magnet Synchronous Motor which is powered by a DC supply via an inverter stage. Since the motor is externally commutated by the inverter and the supply is DC, it is called "brushless DC".

If the same scheme is applied to an Induction Motor, i.e An induction motor powered by a DC source via an inverter stage, will it be called a BLDC motor?

If yes, then why do BLDC like computer cooling fans and bicycle hub motors use Permanent Magnet Synchronous Motors instead of Induction? Permanent magnets are expensive and degrade over time. Wouldn't Induction motor be more economical in the case?

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  • \$\begingroup\$ BLDC does not need to be permanent magnet nor synchronous. \$\endgroup\$ – winny Mar 17 '18 at 8:47
  • \$\begingroup\$ @winny You do realize that you could call pretty much any electric motor "BLDC" if that's true? \$\endgroup\$ – jms Mar 17 '18 at 9:48
  • \$\begingroup\$ Induction motors are often driven from VFDs (Variable Frequency Drives) which have a DC supply internally. They are still called induction motors. \$\endgroup\$ – Brian Drummond Mar 17 '18 at 10:10
  • \$\begingroup\$ @BrianDrummond But still we are supplying AC to VFD which only gets converted to DC internally. \$\endgroup\$ – Rupesh Routray Mar 17 '18 at 10:24
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    \$\begingroup\$ @winny It may be misused by marketing, but AFAIK there is a consensus that the term BLDC refers to permanent magnet synchronous motors. \$\endgroup\$ – jms Mar 17 '18 at 11:58
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You could call an induction motor driven from an inverter a BLDC motor, because, taking the meaning of each word, it is.

However, once you'd done that, few people would understand you, because BLDC is usually used in the context of permanent magnet motors.

So, do you want to be pedantically correct, or do you want to be widely understood?

The history of the term BLDC is that it follows on from the Brushed DC Motor, which, since permanent magnets got good around 50 years ago were invariably PM rather than wound field machines. So the acronym has inherited the permanent magnet baggage. It's just the way it evolved.

There are many things where taking a literal meaning of the words results in nonsense, and I'll edit in a few examples later. Any interesting suggestions for them welcomed in comments.

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  • \$\begingroup\$ But the removal of permanent magnet would make them cheaper. Still Induction based BLDC are nowhere to be found \$\endgroup\$ – Rupesh Routray Mar 17 '18 at 10:06
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    \$\begingroup\$ @Goku Musk uses dc-inverter-induction motors in his Tesla range of cars. I'd hardly call that 'nowhere to be found'. They have different cost/efficiency compromises than BLDC motors, and in big applications like that, it's difficult to call a winner. In smaller apps like drones, the PMBLDC seems to be used preferrentially. \$\endgroup\$ – Neil_UK Mar 17 '18 at 10:45
  • \$\begingroup\$ By a similar argument, a typical wound field brush motor will run on DC, but it is usually termed a "universal motor" rather than "DC motor" \$\endgroup\$ – Chris Stratton Mar 17 '18 at 15:17
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The answer the main question is no, as explained by Neil_UK, but the remainder of the following question is still valid.

If yes, then why do BLDC like computer cooling fans and bicycle hub motors use Permanent Magnet Synchronous Motors instead of Induction? Permanent magnets are expensive and degrade over time. Wouldn't Induction motor be more economical in the case?

The price of permanent neodymium magnets has been quite high, but it has varied a lot and other formulations are available. Price may or may not strongly discourage their use.

Permanent magnets do not degrade over time to any extent that discourages their use. They may be susceptible to excessive temperature and demagnetizing by excess armature current, but those are design challenges, not unsurmountable barriers.

Permanent magnets offer the possibility of higher efficiency with higher torque per mass. That makes them very attractive for battery-powered vehicles. If the cost is a little higher, it may be justified by providing longer driving distance per charge.

Induction motors can have efficiency and torque/mass increased by using copper instead of aluminum in the rotor and enhancing the cooling and as done for the first Tesla auto models. However that kind of motor is significantly more expensive than a typical induction motor.

The most economically attractive motor selection is determined by many factors. The determination can be quite complex.

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Using permanent magnet BLDC motors makes it far easier to get regenerative braking. As assuming the controller uses MOSFETS to switch the motor windings. When the motor is rotated by an external mechanical energy source. The body diodes in the MOSFETS conduct and supply power to the DC Bus in the controller. This power can then be stepped up via a switchmode converter and used to recharge batteries. Or sent to a dump resistor as a means of implementing an electronic brake.

Regeneration or electronic braking is much harder to implement with an induction motor. I have seen washing machines that have a BLDC motor. And the controller just shorts all of the motor power wires together (after disconnecting the power) if someone opens the lid during the spin cycle. As a simple means of quickly stopping the drum from spinning.

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