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One classification of DC motor separate them into Brushed and Brushless, and then the brushed are further broke down into series wound, shunt wound or externally wound. Brushless motors, on the other hand, are permanent magnet.

I dont understand why we cant have brushless shunt wound motor, for instance. Why brushed motors always dont have permanent magnet.

It seems that certain combinations simply dont exist.

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  • \$\begingroup\$ How do you connect the armature windings to the field windings without brushes? \$\endgroup\$ – user253751 Jul 30 '18 at 0:45
  • \$\begingroup\$ The reason why certain combinations don't exist is because they don't work. The BLDC motor is relatively new to the electro-mechanical conversion process (1840's). As BLDC technology evolves it should replace many brushed motors. \$\endgroup\$ – StainlessSteelRat Jul 30 '18 at 0:45
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this is mainly because brushless motors have no windings on the rotor.

If you put elecromagnets on the rotor you need some way to get power to them.

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First, not all brushless motors use permanent magnets (PM). Some are switched-reluctance, e.g stepper motors, or induction motors, particularly large polyphase motors. BTW, one characteristic of PM steppers is that they can be self-locking or cogging, acting as if a detent keeps the armature in position.

Second, it's a matter of definition. A shunt-wound motor uses the same power source in parallel for the field and armature, using a mechanical commutator. Replace the commutator with a Hall-effect sensor and electronic switch and it's a brushless motor, but the term "shunt-wound" is not applied. They're still the same basic design, though.

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It is part economics and part technical conflicts. If you have a huge 5,000 HP motor to pump oil from deep in the ground then 4,160 VAC windings are far cheaper than permanent magnets. I have not heard of permanent magnets being used on motors over 200 HP, but they may exist someday.

Small motors give you lots of options. A universal motor can be AC or DC and reversible. Brushless ball-bearing motors are used when long life is important.

To a large degree the type of power available to use plays a part as well. AC motors with no electronics tend to have low cost and can be brushless if high torque is not needed. If DC power is available then powerful brushless motors are an option if cost is accounted for, else go cheap and use brush type motors.

For cost, which often determines the type of motor used, powerful and expensive permanent magnets are used with a VFD for electric or hybrid cars, as the cost is built into the car. For a disc player it is hard to justify anything but a brush type motor.

An electric hand drill uses a universal motor as it is reversible, runs on AC or DC, and the brushes are made easy to replace. Any other type of motor would cost more and weigh much more.

This answer may not cover all scenarios but should point out the more obvious reasons for choosing one motor type over another, and why some combinations do not make cost or engineering sense.

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  • \$\begingroup\$ Actually Milwaukee has switched to brushless DC for their top line of drills. Mine are too new to take apart just to see what's inside, but they're advertised as having 10x the motor life, and the improvement in torque over their previous models is quite noticeable. The last high end drill I've taken apart is about 6 years old now, but very high end for a brushed drill. Solidly built through and through, but you can see where they left room for improvement here and there. \$\endgroup\$ – K H Jul 29 '18 at 3:15
  • \$\begingroup\$ @KH so this is directly related to cost - Milwaukee are a brand known for a high quality durable product - not a chinesium design that works for 5 minutes (if you are lucky) \$\endgroup\$ – Solar Mike Jul 29 '18 at 4:26
  • \$\begingroup\$ This is true. Just wanted to point it out because I think it's brand new tech for small motors but relevant. \$\endgroup\$ – K H Jul 29 '18 at 4:35
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    \$\begingroup\$ Brushes made easy to replace? You must be still living in the 80s \$\endgroup\$ – PlasmaHH Jul 29 '18 at 8:52
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A motor with a commutator has a stationary magnetic field (stator field) that can be provided with either permanent or wound-field magnets. The magnetic field of the rotor (armature field) remains in a relatively fixed position with respect to the rotor. For the rotor to turn, while the rotor magnetic field remains relatively stationary with resect to the stator, the commutator must switch the rotor current to a different combination of rotor conductors every few degrees.

AC motors of all types and brushless DC motors have a rotating magnetic field that resides in the motor stator. Most have a magnetic field in the rotor that rotates synchronously with the stator field. The rotor field can be supplied with permanent magnets, with a wound or cage rotor by induction, with a wound rotor energized through slip-rings, or with a wound rotor energized by a rotating exciter. There are also AC motors that have only a rotating stator field that produces torque in the rotor by acting of the magnetic reluctance of a rotor with salient poles.

The above description divides motors into two general classes, those with commutators and those without. The concept of series, shunt, separately excited and stationary permanent-magnet stator fields pertains only to commutator motors because of their fundamental design concept. Motor without commutators are fundamentally different as a class and therefore have different sub-classifications.

Universal motors are fundamentally DC commutator motors with series fields.

The many types of motors were invented over many years as improvements from the standpoint of performance, reliability, manufacturing-cost etc. As new designs prove their performance and reliability and the cost of manufacturing them has become competitive, they have gained market share. Some designs have proven to be more advantageous than others in different ranges of torque and power ratings. The value of various performance aspects, physical size and weight and other factors weighed against cost also keep a wide variety of designs in use.

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The point of having a brushed DC motor is mainly to provide electrical connection to the coils on the rotor on the motor. It can also be used to provide commutation (reversal of poles at intervals of rotation). If you are unable or don't want to use brushes in your motor design, you still need to have a magnetic field attached to the rotor for the stator field to interact with. This necessitates permanent magnets or a second exciter winding on the rotor.

In the case of an exciter winding, a winding in the stator energizes a second winding on the rotor, which causes current to flow in the primary rotor winding. It is this field that the field of the stator interacts with to cause the motor to rotate.

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