As we already know, a DC motor transforms electrical energy into mechanical energy, and it does … [Read more...] about Separately Excited DC Motor
These motors work with direct current (DC) and produce their rotation force (torque) by switching the current mechanically or electronically.
In other words, they can change the polarity of the coils, either mechanically or electronically.
It is easy to control their speed since only the current of the induced coil needs to be adjusted.
Types of DC Motors
For their classification, DC motors are first divided into two:
Types of Brushed DC Motors
-Self-excited DC Motors
-Permanent Magnet Motors
Types of Brushless DC Motors (BLDC)
DC motor, and DC generators both descended from the same family of DC machines. In other words, this means that there isn’t a big difference between the DC motor and DC generators.
Contrary we will find lots of similarities between them so let’s go.
In a simple definition, DC motor is also an electrical machine, but it converts electrical power into mechanical power.
The direction of the mechanical force produced when we place a current-carrying conductor in a magnetic field is determined by Fleming’s left-hand rule. The magnitude of this force is:
B= magnetic flux density.
L= the length of the conductor.
Fleming’s left-hand rule
Applications of DC motor
DC motor is the latest electrical machine used to produce mechanical force. So we can find it in many applications where constant or low-speed torque is required, those applications include:
Fractional HP applications.
Textile, and so on everywhere around us.
Advantages of DC motor
DC motor has many advantages like:
Simple and efficient design.
Easy to maintain and service.
Small converters and drives.
High power density.
Full torque at zero speed.
Varying speed easily.
And less inertia.
Working principle of DC motor
The stator of the DC motor is a permanent magnet and it provides us with a constant magnetic field, and the rotating part is the armature and it’s a simple coil.
We use a pair of commutator rings to connect the armature to a DC power source. When the current flow through the armature coil an electromagnetic force is induced in the coil so the coil starts to rotate.
When the coil rotates, the commutator rings will connect with the opposite polarity of the power source.
So the electricity on the left side of the coil will always flow away and it will flow towards the right side.
So the torque action will also be in the direction of motion so the coil will continue rotating.
But if the coil becomes perpendicular to the magnetic flux, the torque action will be near zero. If we run this motor there may be an irregular motion of the rotor.
To overcome this problem, we add one more loop with a separate commutator to the rotor and by that, if there is a loop in the vertical position the other will be connected to the power source.
Thus, to ensure continuity of the produce of motive force in the system. In the same way, the more loops we add, the smoother rotation of the motor we get.
Here, we care to fit the armature loops inside the highly permeable steel layers slots to enhance the interaction of the magnetic flux.
Types of DC motor
As DC motor is much similar to the DC generator, we would find the same types as DC motor. They are;
1. Separately excited Dc motor:
From the name, we deduce that we use a separate DC source to energize the field coils (field windings).
Permanent magnet DC motor (Full details about it here >>> Permanent magnet DC motor)
2. Self-excited DC motor:
Self-excited means that the motor depends on itself to supply the current flows in the windings.
3. Series wound DC motor:
In this motor, we connect the field winding in series with the armature winding, and so, it has a good starting torque but the speed drops drastically with the load.
We can find series wound DC motors in:
Sewing machines and so on, where high starting torque is required and speed variation are possible.
4. Shunt-wound DC motor:
It is the most common type of DC motor, and the field winding is connected parallel with the armature. This motor has the addition of a low starting torque but at the same time, it runs at a constant speed.
The shunt-wound motor can be used with:
Spinning machines, and everywhere where constant speed is required.
5. Compound wound DC motor:
In the compound motor, there are a shunt and series field windings. It can be subdivided into a cumulative compound where the flux is produced by both the windings.
Or a differential compound where the flux produced by the series winding opposite the flux produced by the shunt winding.
cumulative compound excited Dc motor
We can use this type at:
Heavy planners, and in places where high starting torque and constant speed is required.
Finally; we can find DC motors in many portable home appliances, automobiles, and types of industrial equipment.
Types of DC motor
We said that DC motors are the latest machines used to convert electrical energy into mechanical energy. It is axiomatic in that it powers hundreds of devices we use every day.
For instance, robotics, electric shaver, automobiles, small, and also medium motoring applications. Of course, this great number of devices can’t be powered by only one type of DC motors, so there is the multiplicity of types of DC motor as follows:
1. Separately excited DC motor:
In the separate motor, we supply the field winding from a separate DC power source.
The torque of this motor depends upon the field flux and we discovered that from the torque equation of DC motor
Tg =Ka Q Ia.
and the torque is independent o the armature current.
The most popular of the separately excited motor is:
Permanent magnet DC motor:
In permanent magnet DC motor, we use a permanent magnet to create the field flux so it’s called permanent. we can get a great starting torque and a good speed regulation from this motor.
But this torque is limited so it’s better with low horsepower applications.
The torques of this motor and according to the equation, T= Ka Q Ia will change only by controlling the armature supply because the flux is constant as we chose the permanent magnet of required flux density with construction and it can’t be changed.
2. Self-excited DC motor:
In the self-excited motor, the field winding is energized by the current produced by the motor itself with the help of residual magnetism.
We connect the field and armature windings in a way to help to achieve the performance characteristics. That means field and armature windings can connect in parallel or series. So self-excited motor is classified into:
Series wound DC motor:
Here we can get a large amount of starting torque, but we can’t regulate the speed. Unfortunately, running with no load can be damaged so it’s suitable for small electrical appliances and versatile electric equipment.
The advantages of the series motor are high starting torque, simple construction, easy in design, easy maintenance, and effective cost.
The speed in series motor varies with load and we discover that by applying KCL:
I= Ise =I a
While Ise is the series current
And from KVL:
V= E+ I(Ra + Rse )
To get the power we multiply the equations so;
VI= EI+ I^2 (Ra +Rse ).
And as we know: input power= mechanical power+ armature losses + field losses
VI= P + I^2 Ra +I^2 Ra
Shunt-wound DC motor:
Shunt motor is the most common type of DC motor where we connect the field winding parallel with the armature windings. So this motor can be self-excited from the armature windings.
This gives it the feature of greatest speed regulation, simplified reversing control, and low starting torque. So shunt motor is suitable for belt-driven applications in industrial and automotive applications.
It has the advantages of simple control performance, high availability, smooth running, wide control range, and low speeds.
The shunt motor is a constant speed motor as the speed doesn’t change with varying the mechanical load. This is explained in the equation below;
By applying KCL;
I= Ia +Ish.
While Ish is the shunt field current.
And by applying KVL the voltage for field winding will be:
V= Ish Rsh.
And for armature winding:
V= E + Ia Ra
So, the power will be:
Input power= mechanical power+ armature losses+ field losses.
VI= Pm +Ia^2 Ra + Ish^2 Rsh
VI = Pm + Ia^2 Ra +V Ish
Pm = VIa- Ia^2 Ra =( V- IaRa) Ia.
To get the electrical power supplied to the armature (VIa)
V Ia= E Ia+ Ia^2 Ra .
V Ia= Pm+ Ia^2 Ra.
Compound wound DC motor:
In the compound motor, we have both series and shunt field winding. This motor has a good starting torque.
This motor is also subdivided into a cumulative compound motor where the produced flux by both windings is in the same direction ( Φ= Φsh + Φse).
And differential compound motor where the flux produced by the series winding opposes the flux produced by the shunt winding( Φ= Φsh- Φse).
Also cumulative and differential motors can be long or short type depending on the arrangement nature:
Long shunt DC motor:
In the long shunt motor, the shunt field winding parallels both the armature and the series field windings.
Short shunt Dc motor:
In short shunt motor, the shunt field winding is parallel to the armature winding and both shunt and armature windings series the series field winding.
And to be fair there is the abandoned type of DC motor which is Brushless Dc motor.
3. Brushless DC motor:
It is a special type of motors as it doesn’t contain brushes. It has a high efficiency typically around 85-90% in producing large amounts of torque over a vast speed range.
And as it has a high power to weight ratio, high speed and electronic control we use it in many applications as:
In computer peripherals (disk drives, printers).
Hand-held power tools.
Vehicles ranging from aircraft to automobiles.
In Small cooling fans.
And for gramophone records in direct-drive turntables.
It also has many advantages like;
It’s more efficient as the velocity is determined by the frequency which depends on the current, not the voltage.
It has less mechanical energy loss as the friction is less and that enhances the efficiency.
It can operate at high-speed under any condition.
There is no sparking so we have less noise during operation.
And unfortunately, this motor has some disadvantages as:
The cost is higher than in a brushed motor.
Insulation of winding may get damaged as heat weakens the magnets because we can supply limited high power to the motor.
Construction of DC motor
Construction of Dc motor nearly looks like the construction of DC generators. In other words, when the DC machine starts working we can’t identify whether it’s a DC generator or motor.
Briefly, DC motor consists of:
Stator: it’s the stationary part containing the field winding and receives the supply.
Rotor: This is the rotating part.
The details of the construction of DC motor involves a yoke (Magnetic frame), pole core and pole shoes, the armature core, armature winding (conductor), commutator, and brushes and bearings.
Each part of those has its identity and great importance, advantage, and sometimes problems, now we will separately explain each part.
The yoke of DC motor:
The yoke is the outer cover (frame) of the machine, it’s made of cast iron in small machines where cheapness is an important consideration.
Whiles in large machines, it’s made of cast, silicon, or rolled steel to provide high permeability.
It has the importance that it:
Provides mechanical support to poles.
Provide protection from dust, moisture, and various gases to the whole machine.
Carries the magnetic flux produced by the poles.
Pole shoes and pole core of DC Motor:
Pole core is a solid piece made of cast iron or cast steel build of thin laminations which are perfect to reduce power losses due to eddy current,
the function of these poles is:
Provide magnetic flux when the field winding in the excitation case.
Directs the flux produced through the air gap to armature core to the next pole.
Pole shoe is a lamination of annealed steel made of cast iron or cast steel but it’s used to:
Enlarge the pole area and reduce the reluctance of the magnetic path.
Provide more spread out of flux in the air gap.
Field Winding of DC Motor:
It’s usually a copper conductor wound around the pole core with a definite direction. We usually connect it in series and when the current passes through it, the electro poles magnetizes and produces the necessary flux.
We must make the field winding of conducting materials like aluminum or copper because it carries the produced current.
Armature core of DC Motor:
Armature core is a cylindrical shape mounted to the shaft and rotates between the field poles and keyed to the machine shaft. It uses high permeability low reluctance materials like cast iron or cast steel.
Similarly, there are also laminations coated with a thin film to reduce losses due to eddy currents. Furthermore, laminations also provide mechanical security to the armature winding.
There is a large number of slots in the armature core-periphery and we place the armature conductor in those slots.
The armature core is important as:
It provides a path of low reluctance to the flux produced by the field winding.
It’s the house of the armature conductors.
The armature winding of DC motor:
Armature winding is the interconnection of armature conductors. And we can make those windings of conducting materials like copper.
When we use a prime mover to rotate the armature, magnetic flux, and voltage is induced in it.
It’s important because it carries the current supplied to the machine.
We can connect the armature conductors in series to increase the voltage. Similarly, in parallel to increase the current. There are two types of windings:
The commutator of DC Motor:
The commutator is a cylindrical mechanical rectifier made of edge shaped copper segments insulated from each other by a thin layer of mica. These segments are made of hard drawn copper.
The function of the commutator is:
Converts the alternating current or voltage produced in the armature winding to direct current and voltage with the help of brushes.
Provide unidirectional torque for DC motor.
Facilitate the collection of current from armature conductors.
Brushes of DC Motor:
Brushes are carbon or graphite rectangular shapes inspected regularly because they wear with time. Also, they rest on the surface of the commutator.
We use brushes to collect the current from the commutator and apply it to the external load. In other words, they ensure the electrical connection between the rotating commutator and the stationary external load circuit.
Nonetheless, there are brushless DC motors that we can use in high-speed applications. Similarly, multiple machines have the number of brushes as the same as the number of poles.
Bearings of DC motor:
Ball bearings are the most popular because they are more reliable. So they are perfect on heavy-duty machines.
Torque equation of DC motor
Dr. Hugh d Young explains torque is the propensity of force to cause a rotational motion. In DC motor, the torque develops between the armature and the stator.
It is given by the product of the force and the radius at which the force acts.
DC Motor Torque equation:
It will be easy if we mathematically explain the torque. We can start from:
B: is the flux density (in Wb/m^2).
I: the current flows through armature conductor (in Amp).
L: the length of the armature conductor.
R: the radius of the armature drum.
Φ: The total flux cut per pole (in Wb).
A: the area.
Ia: is the total armature current.
A: the parallel path.
By substituting in torque equation, So we will have:
And as the total number of conductors is Z, so
So the torque will equal;
And as we know P, Z and A are constant so
Thus, this means that the torque produced by a DC motor is directly proportional to the main flux and the armature current.
The torque of permanent magnet DC motor
We said before that permanent magnet DC motor uses a magnet to supply field flux. Hence, this makes the motor have excellent starting torque capability with good speed regulation.
However, the torque is usually limited to 150% of rated torque which is a big disadvantage. As such, this makes the use of permanent motor for low horsepower applications.
Torque equation of Dc series motor
In series DC motor, we connect the field winding in series with the armature winding so we have a large amount of starting torque.
Also, the speed varies widely between no-load and full-load. Likewise, this high starting torque doesn’t allow us to use a series motor where we require a constant speed under varying loads.
Torque equation of shunt Dc motor
In shunt Dc motor we connect the field winding in parallel with the armature winding. In addition, we have a good speed regulation.
Also, a low starting torque which makes this motor suitable for belt-driven applications in industrial and automotive applications.
Torque equation of compound DC motor:
In compound motor, we have the benefits of both shunt and series motors. Likewise, we have a better starting torque and a better speed regulation which makes it the best for many applications.
Starting of DC motor
Firstly, DC motor has a very high starting current which makes it unlike other types of motors.
Secondly, it also makes the starting of DC motor different from the starting of all other types.
It’s the main reason that leads to thinking of starting methods of DC motor to limit the starting current. This happens by a starter or other devices containing variable resistance connected in series to the armature winding.
However, there is a need to find why the DC motor has this high current. Thus, the need for the basic operational voltage equation:
We know that:
E: the supply voltage.
Ia: the armature current.
Ra: the armature resistance.
Eb: the back emf.
And the back emf will be:
We will find that Eb is directly proportional to the speed(N) and as we know at starting N=zero, Eb=zero, so the voltage equation at start will be:
So the starting current will be:
Usually, we keep the armature resistance very small and a constant voltage supply. Thus, we will have a starting current equals 440 A.
Nonetheless, this high current creates two problems:
Firstly, this high current has the potential of damaging the internal circuit of the armature winding.
Secondly, we will have a high electromagnetic starting torque because the torque is directly proportional to the current. Thus, it helps to produce a huge force capable of flying off the rotor winding.
Types of starting DC motor
Starting of Dc motor:
To avoid this high starting current, we add external electrical resistance to the armature winding to increase the effective resistance. This is to have a rated armature current described by:
And after a period of working, a back EMF develops and increases. So the current decreases to be:
To reach the armature current rated value, we decrease the external resistance and that happens by a starter. The following types of starting DC motors are used.
3 point starter:
We use the 3 point starter for the starting of the shunt-wound DC motor or compound wound DC motor. Hence, it’s important to search in the construction of 3 point starter. This is how it works, let’s see:
Construction of 3 point starter:
Firstly, it consists of sections called studs marked as off, 1, 2, 3, 4, 5, and, run. Subsequently, there are also three main parts which are
A line terminal connected to the positive of the supply(L).
Armature terminal connected to the armature winding(A).
And field terminal connected to the field winding(F).
There are also overload releases and no-volt coil which protects devices of the starter.
Working of 3 point starter:
First of all, when we switch on the supply to the DC motor, the spring-loaded starter handle moves from off to the first stud position.
Additionally, in this position, we have a high starting resistance, and as we slowly move the handle towards the run position, the series resistance decreases. Hence, the motor gains speed and by that way, the EMF increases.
When the current flows through the starter, the no voltage coil is magnetized. Subsequently, the handle remains in the run position. During motor operation, this no voltage coil acts as a safeguard.
Thus, when any kind of supply failure happens, the no voltage coil demagnetized. Hence, the handle spring returns to off position to effectively cut the motor.
But unfortunately, this 3 point starter suffers from a serious drawback with a large variation of speed. This results in the 4 pointer starter.
4 point starter:
The 4 point starter doesn’t have a big different than 3 point start as we also use it with series wound DC motor, shunt-wound DC motor, and compound wound DC motor.
Also, it mainly has the same construction except for an additional terminal(N) which links the supply to the no voltage coil.
Operation of 4 point starter:
Firstly, the motor is supplied and a current flow through the starter. However, this current will divide into 3 parts and flows through 3 points:
Through the resistance(R1+R2+R3+…..) then to the armature.
Through the field winding.
And through the no voltage coil in series with the protective resistance(R).
And it’s different than the 3 point starter.
If there is any change in the shunt field circuit, there won’t be any change in the no voltage coil. Because the two circuits are independent of each other.
In other words, this means that the electromagnet pull subject upon the soft iron bar of the handle should be high enough to keep the handle at the run position.
Likewise, prevent the spring force to restore the handle at the original position off.
Series motor starter:
From the name, this starter is used with a series DC motor. However, we can call this starter two-point starter because it has only two terminals:
The line connecting the positive supply and the starting handle (L).
The field/armature connection to the motor itself.
Operation of series starter:
Firstly, the series starter is identical in operation to the 3 point and 4 point starter. For instance, the handle moves from off to run a position to start the motor.
Additionally, the no-load release coil holds the starter arm to run position, and the voltage supply lost leaves the arm.
If we focused on the above starters; we will note that they are manually controlled. Also, the starter handle is moved by an operator. Therefore, this is an electronically controlled soft starter, which doesn’t contain moving parts.
As such, it improves the time taken to ignite the motor. In this starter, we use a microcontroller and thyristors to control the current flows through the motor and manage the speed.
Speed control of DC motor
Speed control of DC motor is the most important feature. Certainly, when we control the speed, we vary it according to the requirements and the operation needed.
The effect of speed control of DC motor is evident in the movement of robotic vehicles, movement in elevators, movement in paper mils, and so on.
The principle of speed control of DC motor:
The working principle of the DC motor is that:
Eb=(PΦ N Z)/60A
P: number of poles.
A: just a constant.
Z: number of conductors.
N: speed of the motor.
And by substituting in the voltage equation:
V=(PΦ N Z)/60A + Ia.Ra
And the speed will be;
N= (P.Z /60A)(V-Ia.Ra) /Φ.
And we know that P, Z, and A are constants which we can’t change in working so the speed equation can be:
N= K.(V-Ia.Ra) /Φ.
We deduce that:
The motor speed is directly proportional to the supply voltage.
The speed is inversely proportional to the armature voltage drop.
Also, the motor speed is inversely proportional to the flux.
This means that we can control the speed by:
Varying the supply voltage.
We can Vary the flux.
Also, Varying the armature voltage.
And as there are many types of DC motors, we should have many types of speed control of DC motor like:
Types of speed control of DC Motors
Speed control of shunt motor:
We can control the speed of a shunt motor by:
Flux control method.
Armature and Rheostatic control method.
Voltage control method (Multiple voltage control & Ward Leonard system).
Speed control of series motor:
We can also control the speed of a series motor in many ways such as:
Variable resistance in series with the motor (Armature control of DC series motor).
Flux (Field) control method: (Field diverter – Armature diverter – Tapped field control – Paralleling field coils)
Series-parallel control method.
Let’s start with a speed control of DC series motor:
1- Armature control of DC series motor:
We can use more than one technique in armature control like:
Armature resistance control method:
It’s the most common method where we connect a controlling resistance in series with the supply.
And it’s the most economical for constant torque, especially for DC series motor driving cranes, hoists, trains, and so on.
Shunted armature control:
In this method, we use a combination of a rheostat shunting, the armature, and a rheostat in series with the armature. Here, the voltage applied to the armature varies with varying the series rheostat.
Also, the exciting current varies with varying the armature shunting resistance.
Unfortunately, this method isn’t economical due to power losses in speed controlling resistance. Additionally, the speed control is obtained below normal speed.
Armature terminal voltage control:
We rarely use this method because it involves high cost as we supply the power to the motor from a separate variable voltage supply.
2- Field control of DC series motor:
We can also control the speed by:
Field diverter method:
In this method, we use a diverter and we vary the field flux by varying the current and in turn vary the speed.
Likewise, this method is most common in electrical drives because we have a speed above the normal.
Armature Diverter method:
If we have constant load torque, we reduce Armature current and the flux will increase.
We know that the torque is directly proportional to flux and current. So, if the current increases, the flux will also increase and the speed will decrease.
Tapped field control:
Here we also increase the speed by reducing the flux, but with lowering the number of turns of field winding where the current flows.
Paralleling field coils:
In this method, we can control the speed of the motor by adding groups of coils as shown in fig.
3- Series-parallel control method.
In this method, we use two or more mechanisms coupled with series motors.
Firstly, if we require lower speed, we must join motors in series because in this case, the motors have the same current passing through them.
Secondly, if we require higher speed, we must join motors in parallel because in this case, the motors have the same voltage across them.
And this method is usually used in electric traction.
We can control the speed by:
Field rheostat control method:
We control the speed by adding a variable resistance in series with the shunt field. Thus, when we increase the resistance, the field current reduces. In that case, the flux reduces, and the speed increases.
Firstly, this method is independent of the load. Secondly, the power wasted in controlling the resistance is very less. Lastly, we also use this method of control in the Dc compound motor.
Field voltage control:
In this method, we need a variable voltage supply for the field circuit. We separate it from the main power supply and that can be obtained by an electronic rectifier.
Armature control of Dc shunt motor:
We have two ways:
Armature resistance control:
We add variable resistance to the armature circuit and the field connected directly to the supply. Thus, it doesn’t change with the variation of series resistance.
We usually use this method in the printing press, cranes, hoists, and so on where we use a speed slower than the normal.
Armature voltage control:
In this method, we use a variable source of voltage separated from the source supplying the field current. Therefore, this method has a poor speed regulation and low efficiency of armature resistance control methods.
Thus, to make the armature voltage control method more efficient, we use an adjustable voltage generator called the Ward Leonard system. Hence, we use a motor-generator set, which makes it suitable for steel rolling mills, paper machines, elevators.
Flux control method:
In shunt or series type, we vary the flux produced by the field winding in order to vary the speed of the motor.
The magnetic flux depends on the current flowing through the field winding. So we can vary the flux by varying this current and that can happen by using variable resistance in series with the field winding resistor.
Also, when we keep the resistance in its minimum position, we have a rated current due to a rated voltage. So the speed will be in its normal value.
Likewise, when we gradually increase the resistance the current decreases and in turn the flux produced decreases, so the speed of the motor increases its normal value.
Armature control method:
In the armature control method, we can control the speed by controlling the armature resistance. Similarly, by controlling the voltage drop across the armature. Again, we also use a variable resistor in series with the armature.
When we keep the resistance to its minimum value we have a normal armature voltage drop. And when we gradually increase the resistance the voltage across the armature decreases.
As a result, the speed of the motor decreases, and here we can reach a speed below the normal range.
Voltage control method:
Both the flux and armature control methods can’t provide speed control in the desirable range. Hence, the voltage control method that controls the speed by controlling the supply voltage.
In the voltage control method, we fix the field winding voltage and vary the armature voltage and that can happen by:
Using a switchgear mechanism to provide the variable voltage to the armature.
Or using an AC motor-driven generator to provide a variable voltage to the armature (Ward-Leonard system)
And the most widespread technique used in those two ways is the use of pulse width modulation. It involves the application of varying width pulses to the motor driver to control the supply voltage.
As such, it’s the most efficiently used method because of the power losses kept at a minimum value.
Ward Leonard method of speed control
Speed regulation of DC motor
We know that the speed of any motor decreases automatically when we apply a load to it. Thus, to keep the constant speed we should maintain the difference between no-load and full-load speed (speed regulation) very less.
And the speed regulation of the different types of DC motor is:
10 to 15% for the permanent magnet DC motor.
10% of the DC shunt motor
And for compound Dc motor it’s around 25% for the cumulative and 5% for the differential.
And to understand the speed regulation, we should start with the speed of the Dc motor.
The speed of a DC motor:
From the EMF equation:
N: speed of rotation.
P: number of poles.
A: number of parallel paths.
Z: total number conductors in the armature.
So, the speed will be:
while A, P, and Z are constant the speed will be:
From this equation, we deduce that:
The speed is directly proportional to the EMF.
While we add a load to the motor it produces more torque to overcome the added load.
As the torque is proportional to the current so the armature current increases.
Also to produce more torque we will increase the pole magnetic field and that can be achieved when the armature speed decreases.
When the back EMF decreased a more current flow through the armature and causes an increase in the magnetic field strength. This means that the speed is proportional to the armature resistance or the speed is inversely proportional to the flux (Φ).
The speed regulation of the DC motor is defined as the change in the speed from no-load to full load expressed as a fraction of the full load speed.
Ward Leonard method of speed control
Ward Leonard's method of speed control is based on varying the applied voltage to the armature. Thus, in this system, we have a DC motor (M) powered by a DC generator (G).
Likewise, we use a motor-generator set ( which consists of either AC or DC motor) to have the variable voltage we need.
The principle of Ward Leonard method:
Firstly, we use a driving or prime motor which can be:
A Dc shunt motor.
A 3 phase induction motor.
A diesel engine or any other constant speed drove.
Secondly, the Ward Leonard system is made up of a constant speed driven motor and powers a DC generator. However, the output of the generator is fed to a DC motor.
Similarly, when we vary the generator’s field current, its output voltage will change so the speed of the controlled motor can vary smoothly zero to full speed.
As the generator shunt field current is the main base of control, we require a control equipment for small current values. We use a potentiometer or rheostat in the generator field circuit to vary the output voltage from zero to the full value.
Also, in either direction, the speed and direction of the controlled motor are determined by the generator output. As a result, the controlled motor has a constant excitation.
Likewise, we may use additional switching to reverse the controlled motor depending upon the compounding arrangements between the generator and the motor.
Advantages of Ward Leonard system:
We have many advantages of Ward Leonard method of speed control as:
The availability of a very large range of speed variation.
It’s very easy to reverse the direction of rotation by reversing the generator field current.
The motor can run with a uniform acceleration.
There is an inherent braking capacity.
We have a good speed regulation.
And the high overall efficiency.
Disadvantages of Ward Leonard system:
Unfortunately, we can find drawbacks in all systems and in Ward Leonard system we would find that:
We use two additional machines (motor-generator set ) of the same rating of the main Dc motor so it has a higher initial cost.
It has a large size and weight.
This system requires more floor area and by the way costly foundation.
There are higher losses so it has a lower efficiency.
It also produces more noise.
It needs frequent maintenance.
Application of Ward Leonard system:
We usually use the Ward Leonard method of speed control in:
Steel rolling mills.
Static Ward Leonard system:
Static Ward Leonard system or solid-state control is the most used nowadays. As we replace the motor-generator set with a solid-state converter to control the speed of the DC motor, we use controlled rectifiers and choppers as a converter.
If we use an AC supply, we use the controlled rectifiers to convert fixed AC supply voltage to a variable AC supply voltage.
And if we use a DC supply, we use choppers to convert fixed DC voltage to variable Dc voltage.
Applications of DC motor
We perfectly know that DC motor is an electrical device that converts electrical power into mechanical power. As such, we can find many applications of DC motor.
Also, from the name, these types of motors run on DC power which is an easily available source. So it can be used in toys as it can be designed in small size.
Likewise, we said DC motor has a good speed regulation, so we use it in lifts, suburban trains, electric traction, and so on.
And as it also has a constant or low-speed torque, we use it in dynamic braking and reversing applications.
Lastly, we know there is more than one type of DC motor, thus, DC motor is everywhere depending on the type.
Applications of DC series motor:
DC series motor is the best of DC motors because it is suitable for both high and low power drives, for fixed and variable speed electric drives.
Likewise, it has a simple construction, it’s easy to design and maintenance. Also, it has a high starting torque that can be found toys and automotive applications like:
Versatile electric equipment.
Vacuum cleaner, and so on where we also need a variation in speed.
Sewing machines and power tools.
Applications of DC shunt motor:
We know that Dc shunt motor is a constant speed motor so we use it where we need almost constant speed from no load to full load like in:
Spinning and weaving machines.
Applications of separately excited Dc motor:
Separately excited Dc motor or permanent magnet Dc motor is a special type where we use a permanent magnet to create the required magnetic field.
Similarly, we also know that it doesn’t need to control the speed so it’s usually used in:
Automobiles as a starter motor.
Blowers in heaters and air conditioners.
Personal computer disc drives.
And also in small fractional and sub-fractional KW motors.
Applications of compound Dc motor:
We have two types of compound motor. Firstly, the differential compound and we rarely use it because it has poor torque characteristics. Secondly, the other is the cumulative compound which has a high starting torque and good speed regulation at high speed so it’s the most used in:
Heavy planners, and so on.
Applications of brushless DC motor:
Previously, we said brushless DC motor is a special motor because it doesn’t contain brushes. Also, it has high efficiency, high speed, and electronic control so we use it in many applications like:
In computer peripherals (disk drives, printers).
Hand-held power tools.
Heating and ventilation.
Vehicles ranging from aircraft to automobiles.
For Small cooling fans.
And for gramophone records in direct-drive turntables.
Finally, it's clear that applications of DC motor are everywhere around us in homes, workshops, factories and so on which make it very important to us.
The Function of a Commutator in a DC Generator Even though we've already mentioned the commutators … [Read more...] about The Commutator And It’s Function in a DC Motor
Shunt-wound DC motor or DC shunt motor is a self-excited DC motor that operates on direct current, … [Read more...] about DC Shunt Motor
Series wound DC motor is a self-excited DC motor that converts electrical power into mechanical … [Read more...] about Series Wound DC Motor
A DC compound motor or Compound wound DC motor is also a self-excited motor, but it is a compound of … [Read more...] about DC Compound Motor
Permanent magnet DC motor or PMDC motor is a separately excited motor. This motor is different from … [Read more...] about Permanent Magnet DC Motor [PMDC Motor]
Brushless DC motors or BLDC motors are special motors as they don’t contain brushes. Also, it has … [Read more...] about Brushless DC Motor