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construction of DC motor

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DC Motors

There are four main types of DC motor, namely permanent magnet, series, shunt and seperately excited. The latter three all use field coils in the stator (the part which doesn't move) to generate a magnetic field for the rotor to spin in, and their name simply refers to the way the field coils are wired with respect to the rotor coils. All four types use a commutator to control which rotor coils are energised at any given time in order to maintain rotation, and it is enough just to apply a DC voltage across their terminals to get the motor to spin, so they are relatively easy to control.
Currently series DC are the most economical and commonly used type of motor in electric vehicle conversions. Being a mature technology, they do perform well, with efficiencies up to 90% and only needing servicing every 100,000kms or so. However using a commutator is restrictive and is a source of inefficiency. Also, with series DC motors regenerative braking is very difficult to do, so is not commonly seen. Regen can increase your range by 10-20%, so is quite valuable.
Though still popular for EV conversions, all modern mass-produced electric vehicles tend not to use DC motors, instead favouring BLDC or AC induction motors.
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Permanent Magnet DC Motor


DC Motor with wound stator

 

Brushless DC Motors

In a brushless DC motor (BLDC, also known as Permanent Magnet Synchronous Motor/PMSM or Permanent Magnet AC), the rotor has permanent magnets and the stator has an electronically-controlled rotating field, using sensors (rotor angle sensors or back-EMF) to detect rotor position. As such they have no commutator, and tend to be more efficient and more power-dense than commutated motors. They do require a more complicated motor controller, although as the technology matures and costs come down they are becoming increasingly popular, particularly for smaller motors.
The main disadvantage for EV use is the cost of the large permanent magnet(s) required for the rotor, and the added expense of the more complicated motor controller. At present BLDC drive systems are still considerable more expensive than their brushed DC counterparts.


Brushless DC Motor ("in-runner" type)

 

AC Motors

Although there are a variety of motors which will run on AC power (including BLDC above), for EV use it usually refers to AC induction.
The operation of induction motors is a somewhat difficult concept to grasp at first. Basically they use a rotating magnetic field in the stator to induce a magnetic field in the rotor and hence a current to flow in the rotor's coils. The rotor coils actually just loop around on themself - they are not explicitly powered. The induced field in the rotor tries to stay aligned with the rotating field of the stator, so it turns to chase the stator's field. Due to loads on the motor, the rotor's field is forced to rotate slightly slower than the stator's field (if it kept up exactly, there would be no difference in the fields and hence no torque).
Three phase induction motors are very common for industrial use because they are highly efficient and reliable. These same advantages apply for electric vehicle use, except for the added complication that a variable-speed inverter (sometimes called a Variable Frequency Drive or VFD) is required to control the AC motor from a DC power supply (the battery). These are a relatively expensive piece of hardware. Although they do include regenerative braking and are generally more efficient, primarily due to the expensive controller AC systems currently cost at least twice as much as series DC for equivalent power.


AC Induction Motor



 

                                                                        



Construction of DC Motor | Yoke Poles Armature Field Winding Commutator Brushes of DC Motor

A DC motor like we all know is a device that deals in the conversion of electrical energy to mechanical energy and this is essentially brought about by two major parts required for the construction of DC motor, namely.


1) Stator – The static part that houses the field windings and receives the supply and,

2)Rotor – The rotating part that brings about the mechanical rotations.

Other than that there are several subsidiary parts namely the


3)Yoke of DC motor.

4)Poles of DC motor.

5)Field winding of DC motor.

6)Armature winding of DC motor

7)Commutator of DC motor

8)Brushes of DC motor.


All these parts put together configures the total construction of a DC motor.

Now let’s do a detailed discussion about all the essential parts of DC motor.

parts of dc machine

Yoke of DC Motor


frame yokepole of dc machine

The magnetic frame or the yoke of DC motor made up of cast iron or steel and forms an integral part of the stator or the static part of the motor. Its main function is to form a protective covering over the inner sophisticated parts of the motor and provide support to the armature. It also supports the field system by housing the magnetic poles and field winding of the dc motor.

Poles of DC Motor

The magnetic poles of DC motor are structures fitted onto the inner wall of the yoke with screws. The construction of magnetic poles basically comprises of two parts namely, the pole core and the pole shoe stacked together under hydraulic pressure and then attached to the yoke. These two structures are assigned for different purposes, the pole core is of small cross sectional area and its function is to just hold the pole shoe over the yoke, whereas the pole shoe having a relatively larger cross-sectional area spreads the flux produced over the air gap between the stator and rotor to reduce the loss due to reluctance. The pole shoe also carries slots for the field windings that produce the field flux.


Field Winding of DC Motor


field winding of dc machine


The field winding of DC motor are made with field coils (copper wire) wound over the slots of the pole shoes in such a manner that when field current flows through it, then adjacent poles have opposite polarity are produced. The field winding basically form an electromagnet, that produces field flux within which the rotor armature of the dc motor rotates, and results in the effective flux cutting.


Armature Winding of DC Motor


armature winding


The armature winding of DC motor is attached to the rotor, or the rotating part of the machine, and as a result is subjected to altering magnetic field in the path of its rotation which directly results in magnetic losses. For this reason the rotor is made of armature core, that’s made with several low-hysteresis silicon steel lamination, to reduce the magnetic losses like hysteresis and eddy current loss respectively. These laminated steel sheets are stacked together to form the cylindrical structure of the armature core.

The armature core are provided with slots made of the same material as the core to which the armature winding made with several turns of copper wire distributed uniformly over the entire periphery of the core. The slot openings a shut with fibrous wedges to prevent the conductor from plying out due to the high centrifugal force produced during the rotation of the armature, in presence of supply current and field.

armature winding of dc mach

The construction of armature winding of DC motor can be of two types:-

Lap Winding

In this case the number of parallel paths between conductors A is equal to the number of poles P.



i.e A = P

***An easy way of remembering it is by remembering the word LAP-----→ L A=P

Wave Winding

Here in this case, the number of parallel paths between conductors A is always equal to 2 irrespective of the number of poles. Hence the machine designs are made accordingly.


Commutator of DC Motor


commutator brush

The commutator of DC motor is a cylindrical structure made up of copper segments stacked together, but insulated from each other by mica. Its main function as far as the DC motor is concerned is to commute or relay the supply current from the mains to the armature winding housed over a rotating structure through the brushes of dc motor.

Brushes of DC Motor


The brushes of DC motor are made with carbon or graphite structures, making sliding contact over the rotating commutator. The brushes are used to relay the current from external circuit to the rotating commutator form where it flows into the armature winding. So, the commutator and brush unit of the dc motor is concerned with transmitting the power from the static electrical circuit to the mechanically rotating region or the rotor.


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