What is a DC MOTOR : Basics, Types & Its Working

Almost every mechanical development that we see around us is accomplished by an electric motor. Electric machines are a method of converting energy. Motors take electrical energy and produce mechanical energy. Electric motors are utilized to power hundreds of devices we use in everyday life. Electric motors are broadly classified into two different categories: Direct Current (DC) motor and Alternating Current (AC) motor. In this article, we are going to discuss the DC motor and its working. And also how a gear DC motors work.

What is DC Motor?

A DC motor is an electric motor that runs on direct current power. In an electric motor, the operation is dependent upon simple electromagnetism. A current-carrying conductor generates a magnetic field, when this is then placed in an external magnetic field, it will encounter a force proportional to the current in the conductor and to the strength of the external magnetic field. It is a device that converts electrical energy to mechanical energy. It works on the fact that a current-carrying conductor placed in a magnetic field experiences a force that causes it to rotate with respect to its original position. Practical DC Motor consists of field windings to provide the magnetic flux and armature which acts as the conductor.

The input of a brushless DC motor is current/voltage and its output is torque. Understanding the operation of the DC motor is very simple from a basic diagram is shown below. DC motor basically consists of two main parts. The rotating part is called the rotor and the stationary part is also called the stator. The rotor rotates with respect to the stator.

The rotor consists of windings, the windings being electrically associated with the commutator. The geometry of the brushes, commutator contacts, and rotor windings are such that when power is applied, the polarities of the energized winding and the stator magnets are misaligned and the rotor will turn until it is very nearly straightened with the stator’s field magnets.

As the rotor reaches alignment, the brushes move to the next commutator contacts and energize the next winding. The rotation reverses the direction of current through the rotor winding, prompting a flip of the rotor’s magnetic field, driving it to keep rotating.

Construction of DC Motor

The construction of the DC motor is shown below. It is very important to know its design before knowing it’s working. The essential parts of this motor include armature as well as stator.

The armature coil is the rotating part whereas the stationary part is the stator. In this, the armature coil is connected toward the DC supply which includes the brushes as well as the commutators. The main function of the commutator is to convert the AC to DC which is induced in the armature. The flow of current can be supplied by using the brush from the motor’s rotary part toward the inactive outside load. The arrangement of the armature can be done in between the two poles of the electromagnet or permanent.

DC Motor Parts

In DC motors, there are different popular designs of motors that are available like a brushless, permanent magnet, series, compound wound, shunt, otherwise stabilized shunt. In general, the parts of dc motor are the same in these popular designs but the whole operation of this is the same. The main parts of dc motor include the following.

Stator

A stationary part like a stator is one of the parts in DC motor parts which includes the field windings. The main function of this is to get the supply.

Rotor

The rotor is the dynamic part of the motor that is used to create the mechanical revolutions of the unit.

Brushes

Brushes using a commutator mainly work as a bridge to fix the stationary electrical circuit toward the rotor.

Commutator

It is a split ring that is designed with copper segments. It is also one of the most essential parts of dc motor.

Field Windings

These windings are made with field coils which are known as copper wires. These windings round approximately the slots carried through the pole shoes.

Armature Windings

The construction of these windings in the DC motor is two types like Lap & Wave.

Yoke

A magnetic frame like a yoke is designed with cast iron or steel sometimes. It works like a guard.

Poles

Poles in the motor include two main parts like the pole core as well as pole shoes. These essential parts are connected together through hydraulic force & are connected to the yoke.

Teeth/Slot

The non-conducting slot liners are frequently jammed among the slot walls as well as coils for safety from scratch, mechanical support & additional electrical insulation. The magnetic material between the slots is called teeth.

Motor Housing

The housing of the motor gives support to the brushes, the bearings & the iron core.

Working Principle

An electrical machine that is used to convert the energy from electrical to mechanical is known as a DC motor. The DC motor working principle is that when a current-carrying conductor is located within the magnetic field, then it experiences a mechanical force. This force direction can be decided through Flemming’s left-hand rule as well as its magnitude.

If the first finger is extended, the second finger, as well as the left hand’s thumb, will be vertical to each other & primary finger signifies the magnetic field’s direction, the next finger signifies the current direction & the third finger-like thumb signifies the force direction which is experienced through the conductor.

F = BIL Newtons

Where,

‘B’ is the magnetic flux density,

‘I’ is current

‘L’ is the conductor’s length in the magnetic field.

Whenever an armature winding is given toward a DC supply, then the flow of current will be set up within the winding. Field winding or permanent magnets will provide the magnetic field. So, armature conductors will experience a force because of the magnetic field based on the above-stated principle.
The Commutator is designed like sections to attain uni-directional torque or the path of force would have overturned each time once the way of the conductor’s movement is upturned within the magnetic field. So, this is the working principle of the DC motor.

Types of DC Motors

The different types of dc motors are discussed below.

Geared DC Motors

Geared motors tend to reduce the speed of the motor but with a corresponding increase in torque. This property comes in handy, as DC motors can rotate at speeds much too fast for an electronic device to makes use of. Geared motors commonly consist of a DC brush motor and a gearbox attached to the shaft. Motors are distinguished as geared by two connected units. It has many applications due to its cost of designing, reduces the complexity, and constructing applications such as industrial equipment, actuators, medical tools, and robotics.

• No good robot can ever be built without gears. All things considered, a good understanding of how gears affect parameters such as torque and velocity is very important.
• Gears work on the principle of mechanical advantage. This implies that by using distinctive gear diameters, we can exchange between rotational velocity and torque. Robots do not have a desirable speed to torque ratio.
• In robotics, torque is better than speed. With gears, it is possible to exchange the high velocity with better torque. The increase in torque is inversely proportional to the reduction in speed.

Speed Reduction in Geared DC Motor

Speed reduction in gears comprises of a little gear driving a larger gear. There may be few sets of these reduction gear sets in a reduction gearbox.

Sometimes the objective of using a gear motor is to reduce the rotating shaft speed of a motor in the device being driven, for example in a small electric clock where the tiny synchronous motor may be turning at 1,200 rpm however is decreased to one rpm to drive the second hand and further reduced in the clock mechanism to drive the minute and hour hands. Here the amount of driving force is irrelevant as long as it is sufficient to overcome the frictional impacts of the clock mechanism.

Series DC Motor

A Series motor is a DC series motor where field winding is connected internally in series to the armature winding. The series motor provides high starting torque but must never be run without a load and is able to move very large shaft loads when it is first energized. Series motors are also known as a series-wound motor.

In series motors, the field windings are associated in series with the armature. The field strength varies with progressions in armature current. At the time its speed is reduced by a load, the series motor advances more excellent torque. Its starting torque is more than different sorts of DC motor.

It can also radiate more easily the heat that has built up in the winding due to a large amount of current being carried. Its speed shifts considerably between full-load and no-load. When the load is removed, motor speed increases, and current through the armature and field coils decreases. The unloaded operation of large machines is hazardous.

Current through the armature and field coils decreases, the strength of the flux lines around them weakens. If the strength of the flux lines around the coils was reduced at the same rate as the current flowing through them, both would decrease at the same rate at

which the motor speed increases.

Advantages 

The advantages of a series motor include the following.

• Huge starting torque
• Simple Construction
• Designing is easy
• Maintenance is easy
• Cost-effective

Applications 

Series Motors can produce enormous turning power, the torque from its idle state. This characteristic makes series motors suitable for small electrical appliances, versatile electric equipment and etc. Series motors are not suitable when constant speed is needed. The reason is that the velocity of series motors varies greatly with varying loads.

Shunt Motor

Shunt motors are shunt DC motors, where the field windings shunted to or are connected in parallel to the armature winding of the motor. The shunt DC motor is commonly used because of its best speed regulation. Also hence both the armature winding and the field windings are presented to the same supply voltage, however, there are discrete branches for the stream of armature current and the field current.

A shunt motor has somewhat distinctive working characteristics than a series motor. Since the shunt field coil is made of fine wire, it cannot produce a large current for starting like the series field. This implies that the shunt motor has extremely low starting torque, which requires that the shaft load be quite little.

When voltage is applied to the shunt motor, a very low amount of current flows through the shunt coil. The armature for the shunt motor is similar to the series motor and it will draw current to produce a strong magnetic field. Due to the interaction of the magnetic field around the armature and the field produced around the shunt field, the motor starts to rotate.

Like the series motor, when the armature begins to turn, it will produce back EMF. The back EMF will cause the current in the armature to begin to diminish to a very small level. The amount of current the armature will draw is directly related to the size of the load when the motor reaches full speed. Since the load is generally small, the armature current will be small.

Advantages

The advantages of shunt motor include the following.

• Simple control performance, resulting in a high level of flexibility for solving complex drive problems
• High availability, therefore minimal service effort needed
• High level of electromagnetic compatibility
• Very smooth running, therefore low mechanical stress of the overall system and high dynamic control processes
• Wide control range and low speeds, therefore universally usable

Applications

Shunt DC motors are very suitable for belt-driven applications. This constant speed motor is used in industrial and automotive applications such as machine tools and winding/unwinding machines where a great amount of torque precision is required.

DC Compound Motors

DC compound motors include a separately excited shunt field which has an excellent starting torque however it faces troubles within the variable speed applications. The field in these motors can be connected in series through the armature as well as a shunt field which is separately excited. The series field gives a superior starting torque whereas the shunt field gives enhanced speed regulation. But, the series field causes control issues within the applications of variable speed drive & is normally not utilized in 4-quadrant drives.

Separately Excited

As the name suggests, the field windings otherwise coils are energized through a separate DC source. The unique fact of these motors is that the armature current does not supply throughout the field windings, because the field winding is strengthened from a separate exterior DC current source. The torque equation of DC motor is Tg = Ka φ Ia, In this case, the torque is changed through changing filed flux ‘φ’ & independent of the ‘Ia’ armature current.

Self Excited

As the name suggests, in this type of motor, the current within the windings can be supplied through the motor otherwise machine itself. Further, this motor is separated into the series wound and shunt-wound motor.

Permanent Magnet DC Motor

The PMDC or permanent magnet DC motor includes an armature winding. These motors are designed with permanent magnets by placing them on the inside margin of the stator core for generating the field flux. On the other hand, the rotor includes a conventional DC armature including brushes & commutator segments.

In a permanent magnet DC motor, the magnetic field can be formed through a permanent magnet. So, the input current is not used for excitation which is used in air conditioners, wipers, automobile starters, etc.

Connecting DC Motor with Microcontroller

Microcontrollers can’t drive the motors directly. So we need some kind of driver to control the speed and direction of motors. The motor drivers will act as interfacing devices between microcontrollers and motors. Motor drivers will act as current amplifiers since they take a low current control signal and provide a high current signal. This high current signal is used to drive the motors. Using L293D chip is an easy way for controlling the motor using a microcontroller. It contains two H-bridge driver circuits internally.

This chip is designed to control two motors. L293D has two sets of arrangements where 1 set has input 1, input 2, output1, output 2, with enable pin while another set has input 3, input 4, output 3, output 4 with other enable pin. Here is a video related to L293D

Here is an example of a DC motor that is interfaced with the L293D microcontroller.

L293D has two sets of arrangements where one set has input 1, input 2, output 1, and output 2 and another set has input 3, input 4, output 3, and output 4, according to the above diagram,

• If pin no 2 and 7 are high then pin no 3 and 6 are also high. If enable 1 and pin number 2 are high leaving pin number 7 as low then the motor rotates in the forward direction.
• If enable 1 and pin number 7 are high leaving pin number 2 as low then the motor rotates in the reverse direction.

Today dc motors are still found in many applications as small as toys and disk drives or in large sizes to operate steel rolling mills and paper machines.

DC Motor Equations

The magnitude of flux experienced is

F=BlI

Where, B-   Flux density due to flux produced by field windings

l- Active length of the conductor

I-Current passing through the conductor

As the conductor rotates, an EMF is induced which acts in a direction opposite to the supplied voltage. It is given as

Where,   Ø- Fluz due to the field windings

P- Number of poles

A-A constant

N – Speed of the motor

Z- Number of conductors

The supply voltage, V = E_b + I_aR_a

The torque developed is

Thus the torque is directly proportional to the armature current.

Also, speed varies with armature current, hence indirectly torque and speed of a motor are dependants on each other.

For a DC shunt motor, speed remains almost constant even if torque increases from no load to full load.

For a DC series motor, speed decreases as torque increases from no load to full load.

Thus torque can be controlled by varying the speed. Speed control is achieved either by

• Changing flux by controlling the current through field winding- Flux Control method. By this method, speed is controlled above its rated speed.
• Armature Voltage Control –  Provides speed control below its normal speed.
• Supply Voltage Control – Provides speed control in both directions.

4 Quadrant Operation

Generally, a motor can operate in 4 different regions. The four-quadrant operation of dc motor includes the following.

• As a motor in a forward or clockwise direction.
• As a generator in the forward direction.
• As a motor in a reverse or anticlockwise direction.
• As a generator in the reverse direction.

• In the first quadrant, the motor is driving the load with both the speed and torque in a positive direction.
• In the second quadrant, torque direction reverses and the motor acts as a generator
• In the third quadrant, the motor drives the load with speed and torque in a negative direction.
• In the 4^th quadrant, the motor acts as a generator in reverse mode.
• In the first and third quadrant, the motor acts in both forward and reverse directions. For example, motors in cranes to lift the load and also put it down.

In the second and fourth quadrant, the motor acts as a generator in forward and reverse directions respectively, and provides energy back to the power source. Thus the way to control a motor operation, to make it operate in any of the 4 quadrants is by controlling its speed and direction of rotation.

The speed is controlled either by varying the armature voltage or weakening the field. The torque direction or direction of rotation is controlled by varying the extent to which applied voltage is greater than or less than the back emf.

Common Faults in DC Motors

It is significant to know as well as to understand the failures & faults of the motor to describe the most appropriate safety devices for every case. There are three types of motor failures like mechanical, electrical & mechanical that grow into electrical. The most frequently occurred failures include the following,

• Breakdown of insulation
• Overheating
• Overloads
• Failure of bearing
• Vibration
• Locked Rotor
• Misalignment of Shaft
• Reverse Running
• Imbalance of phase

The most common faults that occur within AC motors, as well as DC motors, include the following.

• When the motor is not correctly mounted
• When the motor is blocked through dirt
• When the motor contains water
• When the motor is overheating

12 V DC Motor

A 12v DC motor is inexpensive, small as well as powerful which is used in several applications. Selecting the suitable DC motor for a particular application is a challenging task, so it is very essential to work through the exact company. The best example of these motors is METMotors, as they making PMDC (permanent magnet DC) motors with high-quality for over 45 years.

How to Select the Right Motor?

The selection of a 12v dc motor can be done very easily through METmotors because the professionals of this company will first study your correct application and after that they consider numerous characteristics as well as specifications to guarantee you finish up with the finest product possible.
The operating voltage is one of the characteristics of this motor.

Once a motor is power-driven through batteries, then low operating voltages are normally chosen as fewer cells are necessary to get the particular voltage. But, at high voltages, drive a dc motor is normally more efficient. Even though, its operation is achievable with 1.5 volts that goes up to 100V. The most frequently used motors are the 6v, 12v & 24v. Other main specifications of this motor are speed, operating current, power & torque.

The 12V DC motors are perfect for different applications through a DC supply requiring running torque as well as high starting. These motors operate at fewer speeds as compared with other motor voltages.
The features of this motor mainly vary based on the manufacturing company as well as application.

• Motor speed is 350rpm to 5000 rpm
• Rated torque of this motor ranges from 1.1 to 12.0 in-lbs
• The output power of this motor ranges from 01hp to.21 hp
• Frame sizes are 60mm, 80mm, 108 mm
• Replaceable brushes
• The typical life of brush is 2000+ hours

Back EMF in DC Motor

Once the current-carrying conductor is arranged in a magnetic field, then the torque will induce over the conductor and the torque will rotate the conductor which slices the magnetic field’s flux. Based on the phenomenon of Electromagnetic induction once the conductor slices the magnetic field, and then an EMF will induce within the conductor.

The induced EMF direction can be determined through Flemming’s right-hand rule. According to this rule, if we grip our thumbnail, index, and center finger with 90° of an angle, after that the index finger will signify the way of the magnetic field. Here, the thumb finger represents the conductor’s way of motion & the middle finger denotes the induced EMF over the conductor.

By applying Flemming’s right-hand rule, we can notice that the induced emf direction is reverse to the voltage applied. So the emf is called the back emf or counter emf. The development of back emf can be done in series through the voltage applied, however, reverse in direction, that is the back emf resists the flow of current which causes it.

The back emf magnitude can be given through a similar expression like the following.

Eb = NP ϕZ/60A

Where

‘Eb’ is the motor’s induced EMF called Back EMF

‘A’ is the no. of parallel lanes throughout the armature among the reverse polarity brushes

‘P’ is the no. of poles

‘N’ is the speed

‘Z’ is the whole number of conductors within the armature

‘ϕ’ is a helpful flux for each pole.

In the above circuit, the back emf magnitude is always low as compared with the voltage applied. The disparity among the two is almost equivalent once the dc motor works beneath usual conditions. The current will induce on the dc motor due to the main supply. The relation among the main supply, back EMF & armature current can be expressed as Eb = V – IaRa.

Application to Control DC Motor Operation in 4 Quadrants

Control of DC motor operation in 4 quadrants can be achieved using a Microcontroller interfaced with 7 switches.

Case1:  When the start and clockwise switch is pressed, the logic in the Microcontroller gives an output of logic low to pin 7 and logic high to pin2, making the motor rotate in a clockwise direction and operate in the 1^st quadrant. The speed of the motor can be varied by pressing the PWM switch, causing an application of pulses of varying duration to the enable pin of the driver IC, thus varying the applied voltage.

Case 2: When the forward brake is pressed, Microcontroller logic applies logic low to pin7 and logic high to pin 2 and the motor tends to operate in its reverse direction, causing it to stop instantly.

In a similar way, pressing the anti-clockwise switch causes the motor to move in the reverse direction, i.e. operate in the 3^rd quadrant, and pressing the reverse brake switch causes the motor to stop instantly.

Thus through proper programming of the microcontroller and through switches, the motor operation can be controlled in each direction.

Thus, this is all about an overview of the DC motor. The advantages of dc motor are they provide excellent speed control for acceleration and deceleration, easy to understand design, and a simple, cheap drive design. Here is a question for you, what are the drawbacks of DC motor?

Photo Credits:

• Brushless DC Motors Work by news.softpedia
• 4 Quadrant Operation of DC Motor by lh5.ggpht
• Geared DC Motor by wikimedia
• Shunt Motor by zone