1. DC motors
2. Induction motors
3. Synchronous motors.

Plugging Braking of DC Motor

In order to under- stand the concept of plugging, consider a D.C shunt motor running with a constant speed, N as shown in figure (1). The polarities of the armature and the field can be observed under running condition as shown in figure (1). Under running operation, the back e.m.f of the motor E is nearly equal to the supply voltage V and acts in the opposite direction. Now, to perform the braking operation by plugging, consider the connection diagram of the same shunt motor as shown in figure (2). It can be observed in figure (2) that during the braking operation the polarities of the armature are reversed with the polarities of field unchanged. Thus, a reverse voltage appears across the armature. This causes the induced armature voltage (back emf) E to act along the direction of the supply voltage V. Hence. E no longer opposes V and as a result voltage aiding a large reverse current flows through the armature. This reverse current would be of twice the magnitude that of normal current, as twice the supply voltage (i.e., V + E ≅ 2V) is impressed on the armature having low resistance.

A large reverse current leads to a large negative torque (braking torque). In case of starting the motor during plugging operation also, it is necessary to limit the reverse current. The limitation of reverse current limits the reverse torque and thus reduces the stress on the motor. This is achieved by inserting an additional resistor called the braking resistor R_B in series with the armature during plugging. Current at the instant of braking,

\[{{I}_{a}}=\frac{V+E}{{{R}_{B}}}….(1)\]

Electric braking torque,

\[{{T}_{b}}=\frac{\text{Braking Power}}{n}\]

\[{{T}_{b}}=\frac{{{P}_{B}}}{n}\]

\[=\frac{\left[ E_{a}^{2}/{{R}_{B}} \right]}{n}\]

\[{{T}_{b}}={{k}_{1}}\phi \left[ \frac{V+{{k}_{2}}N\phi }{{{R}_{B}}} \right]\text{      }\left[ E={{k}_{2}}N\phi  \right]\]

\[=\frac{{{(n{{k}_{a}})}^{2}}}{{{R}_{B}}n}\]

\[=\frac{nk_{a}^{2}}{{{R}_{B}}}\]

The instant when the motor attains zero speed after the braking operation, the supply must be disconnected. This is done because the motor starts rotating in the reverse direction.

Thus, plugging has two modes. They are,

1. Plug to stop
2. Plug to reverse.

If plugging is required only for stopping the motor, then the supply has to be switched OFF at zero speed. This mode represents the ‘plug to stop’ mode. If the supply is continued, then the motor operates in the reverse direction and the mode is called plug to reverse mode.

The speed-torque characteristics of the shunt motor during plugging are shown in figure (3).

The post What is Plugging Braking? Connection Diagram & Working appeared first on Electrical and Electronics Blog.

Figure 1: Arrangement of Swinburne’s test.

In Swinburne’s test, R and are measured separately so that losses can be calculated easily on various loads. The machine whether motor or generator is run as a shunt motor on no-load, the field rheostat is adjusted to give rated speed and rated voltage when shunt is applied across the terminals of motor.

The circuit diagram for determining the no-load losses of a D.C shunt machine is shown in figure.

Let,

V is the voltage measured by voltmeter

I₀ is the input motor current which is measured by ammeter A₁.

I_sh is the shunt field current measured by ammeter A₂.

We know that,

Input power Output + Losses

Since machine is on no-load,

Input power = Losses                     [Since, Output = 0]

From the above equation, it is clear that no-load input power is used to supply internal losses in the machine that is shunt field copper loss, armature copper loss, stray losses in shunt machine.

Power input = VI₀

Shunt field copper loss = VI_sh

Armature copper loss = I²_a0 = (I₀ – I_sh)²R_a

Here, R_a  is the Armature resistance. Since,

Power input = Total losses

\[V{{I}_{0}}=V{{I}_{sh}}+I_{{{a}_{0}}}^{2}{{R}_{a}}+\text{ Strey losses}\]

\[\text{Strey losses}=V{{I}_{0}}-V{{I}_{sh}}-{{({{I}_{0}}-{{I}_{sh}})}^{2}}{{R}_{a}}\]

If the shunt field copper loss is added to stray loss then the constant loss can be obtained which remains constant irrespective of the load on the machine.

Constant losses = Stray losses + Shunt copper losses

\[=V{{I}_{0}}-{{({{I}_{0}}-{{I}_{sh}})}^{2}}{{R}_{a}}\]

Since constant losses are known, the efficiency can be determined at any load. Let ‘I’ be the load current where the efficiency of machine is to be determined.

Efficiency when Running as Generator

Generator output = VI watts

Total losses = Armature copper loss + Field copper loss + Stray losses

\[=I_{a}^{2}{{R}_{a}}+{{W}_{c}}\]

\[={{(I+{{I}_{sh}})}^{2}}{{R}_{a}}+{{W}_{c}}\]

Input = Output + Losses

\[=VI+{{(I+{{I}_{sh}})}^{2}}{{R}_{a}}+{{W}_{c}}\]

\[\text{Efficiency = }\frac{\text{Output}}{\text{Input}}=\frac{VI}{VI+{{(I+{{I}_{sh}})}^{2}}+{{W}_{C}}}\]

Efficiency when Running as a Motor

Motor input = VI

Total losses = Armature copper loss + W_c

\[=I_{a}^{2}{{R}_{a}}+{{W}_{c}}\]

\[={{(I-{{I}_{sh}})}^{2}}{{R}_{a}}+{{W}_{c}}\]

Motor output = Input – Losses

\[=VI-{{(I-{{I}_{sh}})}^{2}}{{R}_{a}}-{{W}_{c}}\]

\[\text{Efficiency of motor = }\frac{\text{Output}}{\text{Input}}\]

\[=\frac{VI-{{(I-{{I}_{sh}})}^{2}}{{R}_{a}}-{{W}_{c}}}{VI}\]

The post What is Swinburne’s Test of DC Motor? Theory & Diagram appeared first on Electrical and Electronics Blog.

Figure 1: Arrangement of Retardation Test.

In this method of testing DC machines, the machine runs slightly above its normal speed and supply to the armature is cut-off while the field remains excited. As a result, the armature slows down and its kinetic energy is utilized to overcome rotational losses.

\[\text{K}\text{.E}\text{. of armature = }\frac{1}{2}I{{\omega }^{2}}\]

Where,

I = M.I (i.e., moment of inertia) of armature

ω = Angular speed of armature

P_R = Rate of change of K.E.

\[=\frac{d}{dt}\left( \frac{1}{2}I{{\omega }^{2}} \right)=I\omega \frac{d\omega }{dt}\]

\[=I\left( \frac{2\pi N}{60} \right)\frac{d}{dt}\left( \frac{2\pi N}{60} \right)\]

\[={{\left( \frac{2\pi }{60} \right)}^{2}}IN\frac{dN}{dt}….(1)\]

Hence to determine stray losses, the value of M.I of armature i.e., I and rate of change of speed i.e., \(\frac{dN}{dt}\) must be known. Determination of \(\frac{dN}{dt}\).

The connections for conducting retardation test are shown in figure. The voltmeter V across armature shows the instantaneous back e.m.f. Since E_b ∝ N, the voltmeter can be suitably calibrated to indicate speed. When the supply to armature is cut off the speed of machine decreases with time. The speed or readings of voltmeter are noted at different intervals of time and a curve is drawn between speed and time.

From any point, ‘P’ lying on speed time curve, tangent is drawn meeting the X-axis and Y-axis at A and B respectively. Then,

\[\frac{dN}{dt}=\frac{\text{OB in r}\text{.p}\text{.m}}{\text{OA }\text{in sec}}\]

Determination of M.I (Moment of Inertia) :

First speed time curve is plotted with armature alone. Then a flywheel of known M.I i.e., I₁ is keyed to the shaft and speed time curve is plotted again.

Due to the addition of flywheel, the time taken to lower the speed will be longer. \(\frac{dN}{d{{t}_{1}}}\) and \(\frac{dN}{d{{t}_{2}}}\) will be determined as before, losses will remain the same in both cases since the addition of flywheel will not make much difference.

Rotational losses before the addition of flywheel,

\[{{\left( \frac{2\pi }{60} \right)}^{2}}IN\frac{dN}{d{{t}_{1}}}….(2)\]

Rotational losses after the addition of flywheel,

\[{{\left( \frac{2\pi }{60} \right)}^{2}}(I+{{I}_{1}})N\frac{dN}{d{{t}_{2}}}….(3)\]

Equating equation (2) and (3), we get,

\[{{\left( \frac{2\pi }{60} \right)}^{2}}(I+{{I}_{1}})N\frac{dN}{d{{t}_{2}}}={{\left( \frac{2\pi }{60} \right)}^{2}}IN\frac{dN}{d{{t}_{1}}}\]

\[=I\left( \frac{dN}{d{{t}_{1}}}-\frac{dN}{d{{t}_{2}}} \right)={{I}_{1}}\frac{dN}{d{{t}_{2}}}\]

\[I=\frac{{{I}_{1}}\left( \frac{dN}{d{{t}_{2}}} \right)}{\frac{dN}{d{{t}_{1}}}-\frac{dN}{d{{t}_{2}}}}\]

Substituting the values of I₁,

\(\frac{dN}{d{{t}_{1}}}\) and \(\frac{dN}{d{{t}_{2}}}\) in above equation the value of I can be determined using retardation test.

The post What is Retardation Test (or Running Down Test) of DC Motor? Theory & Circuit Diagram appeared first on Electrical and Electronics Blog.

Figure 1: Arrangement of Brake Test.

Brake test is a direct testing method used to find the efficiency of a DC motor. The arrangement for performing brake test on a DC motor is shown in figure (1). In this test, brakes are applied on a water cooled pulley mounted on the motor shaft depending upon the rating of the machine. The brake rope is fixed with the help of wooden blocks gripping the pulley. One end of the rope is fixed to the earth via spring balance ‘B’ and the other end is connected to a suspended weight ‘W₁‘. Now, the motor is allowed to run under this condition and the load on the motor is so adjusted that it carries full-load current by adding or removing the weights to suspended weight ‘W₁’ Let reading on spring balance ‘B’ be ‘W₂‘.

While increasing the load, the load is increased in step by step manner until full-load condition is achieved and for each step various values of voltmeter, ammeter, the suspension weights i.e., W₁ and W₂ and speed of the motor are noted and the efficiency of the rotor can be calculated as follows,

The net pull on the rope due to friction at the pulley is given as,

\[=({{W}_{1}}-{{W}_{2}})\text{ kg wt}\]

\[\text{= 9}\text{.81 (}{{W}_{1}}-{{W}_{2}}\text{) Newton}\]

The shaft torque T_sh developed by the motor is given by,

\[{{\text{T}}_{sh}}=({{W}_{1}}-{{W}_{2}})\text{ R kg-mt}\]

\[\text{= 9}\text{.81 }({{W}_{1}}-{{W}_{2}})\text{ R N-m}\]

Where,

R = Radius of pulley (mts)

Motor power output is given by,

\[{{P}_{out}}={{T}_{sh}}\times \omega \text{ watts}\]

\[\text{=}\frac{2\pi \times 9.81}{60}({{W}_{1}}-{{W}_{2}})\text{ RN watts}\]

\[\text{= 1}\text{.027 N }({{W}_{1}}-{{W}_{2}})\text{ R watts}\]

Where,

Let,

V = Supply voltage

I = Full-load current taken by the motor.

Input power is given is,

\[{{\text{P}}_{in}}=VI\text{ watts}\]

The efficiency is given by,

\[%\eta =\frac{\text{Output power}}{\text{Input power}}\times 100\]

\[=\frac{\frac{2\pi \times 9.81}{60}({{W}_{1}}-{{W}_{2}})\text{ RN}}{VI}\times 100\]

\[=\frac{1.027({{W}_{1}}-{{W}_{2}})\text{ RN}}{VI}\times 100\]

Advantages of Brake Test

1. Test requires no other machines. Hence, it reduces the cost and energy.
2. This method is very simple.
3. Very much convenient for small motors.

Disadvantages of Brake Test

1. This test is performed with only small motors. In case of large motors it is difficult to dissipate the large amount of heat generated at the brake.
2. The output of the motor cannot be determined accurately because of the unavoidable errors occurring in spring balance which leads to incorrect determination of the internal losses or efficiency of a DC machine.
3. In case of series wound machines care should be taken so that the brake applied is tight otherwise the motor will attain dangerously high speed, which leads to severe damage.

The post What is Brake Test of DC Machine? Theory, Circuit Diagram & Advantages appeared first on Electrical and Electronics Blog.

1. Armature control method
2. Field control method
3. Ward-Leonard method.

Armature Control Method

In this method of speed control, an external variable resistance is connected between the armature terminals and the line.

When the resistance added is maximum due to higher resistance, the current flowing through the armature would be reduced proportionally. The field flux remains unchanged thus, with the reduction in the armature current, the torque is also reduced.

\[T={{k}_{a}}\oint{{{I}_{a}}}\]

As the torque of the machine is reduced, the speed also reduces. If the armature resistance is kept minimum, the rated current flows through the armature keeping the speed at rated value. Thus by analysis, it could be noted that the speed variation is possible only below the rated speed but not above the rated speed. The major advantage is that due to higher resistance in the armature the power losses considered would be large. So, it is used for intermittent periods only. The circuit and the characteristics are shown in figure (1).

Advantages of Armature control method

1. This method of speed control is simple.
2. Speed can be easily varied.
3. Speeds below base speed or normal speed can be achieved easily.
4. Initial cost required is low.
5. Economical for short time speed control applications.

Disadvantages of Armature control method

1. The operating cost increases for low speeds.
2. There occurs power loss in controller resistance.
3. Output and efficiency is reduced due to power loss.
4. Speed regulation is poor.

Field Control Method

In field control method, a varying external resistance is connected in the field circuit. When the resistance added to the field is zero then the current through the armature would be the rated value. As the resistance increases the current through the field reduces thereby, reducing the flux of the machine. But this reduction of current in the field increases the current through the armature above the rated value and thus, the operating torque also increases which in turn increases the speed above the rated value. While starting, the field resistance should be kept minimum else huge current flows though the armature which damages the armature coils. So, from this it can be said that the variation of speed above the rated speed can also be obtained by increasing the field resistance.

Advantages of Field control method

The advantages of speed control of D.C motor using flux control method are as follows.

1. Speed can be increased upto twice the rated speed.
2. It is a very simple.
3. Less amount of power is wasted as the field rheostat is connected across the field.
4. Independent of load.
5. It is the most efficient and economical method.

Disadvantages of Field control method

1. Armature reaction is greater and results in instability at high speeds.
2. Speeds below the normal speed cannot be controlled.
3. Poor commutation.

Ward-Leonard Method

Ward Leonard system is used to control the speed of DC shunt motors. It comes under the category of voltage control method. In this method, the basic adjustable voltage to the armature is achieved by means of an adjustable voltage generator. The arrangement of Ward Leonard system is shown in figure (3).

As shown in figure, Ward-Leonard system consists of two motors M₁ and M₂. M₁ is the motor whose speed is to be controlled. A motor-generator set consists of either DC or an A.C motor and is directly coupled to the generator G.

The input to the generator ‘G’ is fed from motor ‘M₂‘ and the output of generator ‘G’ is applied to motor ‘M₁ Motor ‘M2’ operates at constant speed. The voltage applied to motor ‘M₁‘ is varied by connecting the field of generator to a regulator and hence variable voltage is applied to the armature of the main motor ‘M₁‘ . Thus, speed control is achieved. The switch RS is used to reverse the direction of field current of generator ‘G’ i.e., when switch RS is in connected position, reverse voltage is generated in generator ‘G’ and is applied to motor ‘M₁‘ due to which the direction of rotation of motor ‘M₁‘ reverses.

Advantages of Ward Leonard method

1. Speed can be controlled over the range from zero to normal speed in both the directions.
2. Speed regulation is smooth.
3. For large motors when a D.C supply is unavailable, this method is very much suitable.

Disadvantages of Ward Leonard method

1. This method is expensive as it requires two extra machines.
2. The overall efficiency of the system for light loads is low.
3. There is a possibility for the armature winding to get damage when the applied voltage across the armature increases more than the rated voltage i.e., for a very long duration.

Comparison between different Methods of speed control of DC Shunt Motor

The post Speed Control for DC Shunt Motor – Methods & Diagram appeared first on Electrical and Electronics Blog.

A four point starter is a protective device used in shunt and compound DC motors to limit the high starting current in the absence of back e.m.f. The construction of four point starter is similar to three point starter except the connection of NVC (No-volt coil). The N-V coil connected in series with field winding in three point starter is replaced in four point starter by connecting it across the supply through an additional terminal, N as shown in figure. In this type of arrangement, when the armature touches stud number 1, the line current divides into three parts. In which, first part passes through starting resistance, series field and armature. The second part passes through shunt field winding and the third part passes through N-V coil and protective resistance.

In four-point starter since N-V coil current is independent of shunt field circuit current, any change of current in shunt field does not affect the current passing through the N-V coil. Thus, the electromagnetic pull exerted by the N-V coil will always be sufficient to hold the handle in RUN position under all the operating conditions and prevents the spiral spring from resisting the starting arm to OFF position. The function of No-volt release and overload release in four point starter is similar to three point starter.

Difference between 3 -point and 4-point starters.

The post What is a 4 Point Starter? Working Principle, Construction & Diagram appeared first on Electrical and Electronics Blog.

The speed-torque characteristics of DC shunt motor are also known as mechanical characteristics. They can be obtained from torque-current and speed-current characteristics of DC shunt motor.

The expression for back e.m.f in a DC motor is given by,

\[{{E}_{b}}={{k}_{a}}\phi N\text{     }\left[ {{k}_{a}}=\frac{ZP}{60A} \right]…(1)\]

Also, we have,

\[{{E}_{b}}=V-{{I}_{a}}{{R}_{a}}…(2)\]

On equating equations (1) and (2), we get,

\[{{k}_{a}}\phi N=V-{{I}_{a}}{{R}_{a}}\]

\[N=\frac{1}{{{k}_{a}}\phi }\left[ V-{{I}_{a}}{{R}_{a}} \right]…(3)\]

The expression for torque of a DC motor is given by,

\[T={{k}_{a}}\phi {{I}_{a}}\]

\[{{I}_{a}}=\frac{T}{{{k}_{a}}\phi }\]

Substituting the above value of I_a in equation (3), we get,

\[N=\frac{1}{{{k}_{a}}\phi }\left[ V-\left( \frac{T}{{{k}_{a}}\phi } \right){{R}_{a}} \right]\]

\[N=\frac{V}{{{k}_{a}}\phi }-\frac{T{{R}_{a}}}{{{({{k}_{a}}\phi )}^{2}}}…(4)\]

As the torque in a DC motor increases, flux (ϕ) decreases. This is because when torque is increased, armature current increases resulting in reduction of air gap flux ϕ i.e., due to armature reaction and saturation. Therefore, for increased torque there is an increase in the value \(\frac{T}{{{\phi }^{2}}}\) of equation (4) causing a drop in the speed of the motor. However, when the effect of armature reaction is neglected, the value of flux (ϕ) remains constant due to which for increase in torque there will not be much drop in speed of the DC shunt motor. The speed-torque characteristics of a DC shunt motor are drawn as shown below. Dotted line in above figure represents the ideal speed- torque characteristics of a DC shunt motor. But, due to the reduction in flux caused by the armature reaction, the curve is obtained as shown by the thick line.

Speed-Torque Characteristics of DC Series Motor

The speed-torque characteristics of DC series motor can be obtained or derived from the torque-current and speed-current characteristics of DC series motor. Therefore, armature torque and speed has inverse relationship as shown in figure (3).

We know that,

\[N\propto \frac{{{E}_{b}}}{\phi }\propto \frac{{{E}_{b}}}{{{I}_{a}}}\]

Since on no-load the speed is dangerously high, the series motors are never started on no-load. When the motor is connected across supply mains without load, the current drawn is small and hence ϕ is small and the speed tends to increase \(\left[ N\propto \frac{{{E}_{b}}}{\phi } \right]\). With increase in speed, E_b increases \(\left[ {{E}_{b}}=\frac{\phi ZN}{60}\times \frac{P}{A} \right]\) thus the field current decreases \(\left[ {{I}_{a}}=\frac{V-{{E}_{b}}}{{{R}_{a}}} \right]\) which in tum leads to decrease in flux and hence speed increases gradually. This process continues until the armature gets damaged. Hence, series motors are not suitable for the services where the load may be entirely removed.

The post Speed Torque Characteristics of DC Motor (Shunt & Series) appeared first on Electrical and Electronics Blog.

Consider a DC motor with stator and rotor. When the stator coils are energized, a stator magnetic flux is set up and follows the path as shown in figure (a) with no current in the rotor conductor.

When rotor conductor carries current. magnetic flux is set up and flows in the direction as shown in figure (b) with no current in the stator coil.

Now when both the stator coil and the rotor conductor carries current, then an interaction between the stator flux and the flux produced by the rotor takes place and the resultant magnetic field is set up as shown in figure (c). The rotor conductors experience a force in the upward direction and develop a torque. Thus, the torque is produced in a DC motor when the stator flux and flux produced by the rotor conductor interacts with each other.

Derivation for Torque Equation of a DC Motor

Let,

F — Force in Newton

r — Radius of armature in meter

T_a — Armature torque in N-m

S — Circumferential distance

f — Flux/pole in Wb

P — Number of poles

Z — Number of armature conductors

A — Number of armature paths

I_a — Armature current

N — Speed of armature in r.p.m. Since, torque is the twisting movement produced across the armature.

\(\frac{N}{60}\) = Speed in r.p.m

dt = \(\frac{60}{N}\) = Time taken for one revolution.

Mechanical work done per second is given as,

\[\text{Mechanical W}\text{.D / sec = }\frac{F\times \text{ Circumferential distance}}{\text{Time}}\]

\[=\frac{F\times S}{{}^{60}/{}_{N}}\]

\[=F\times 2\pi r\times \frac{N}{60}\text{       }\left[ S=2\pi r \right]\]

\[=F\times r\times \frac{2\pi N}{60}\]

\[=\frac{{{T}_{a}}\times 2\pi N}{60}\text{ N-m / sec  }\left[ {{T}_{a}}=F\times r \right]\]

The electrical work done per second,

\[\text{Electrical W}\text{.D / sec = }{{E}_{b}}\times {{I}_{a}}\]

\[=\frac{\phi PN}{60}\times \frac{Z}{A}{{I}_{a}}\text{     }\left[ {{E}_{b}}=\frac{\phi PN}{60}\times \frac{Z}{A} \right]\]

As 1 N-m/sec= 1 watt, equating mechanical and electrical power, we get,

\[\frac{{{T}_{a}}\times 2\pi N}{60}=\frac{\phi PN}{60}\times \frac{Z}{A}{{I}_{a}}\]

\[{{T}_{a}}\times 2\pi =\frac{\phi PZ{{I}_{a}}}{A}\]

\[{{T}_{a}}=\frac{1}{2\pi }\times \frac{\phi PZ{{I}_{a}}}{A}\]

\[{{T}_{a}}=\frac{0.159}{9.81}\frac{\phi PZ{{I}_{a}}}{A}\text{ kg-m}\]

\[{{T}_{a}}=\frac{0.0162\phi PZ{{I}_{a}}}{A}\text{ kg-m (MKS}\text{ unit)}\]

Shaft Torque

Shaft torque is the torque which is available for doing useful work. It is usually denoted by T and is always available at the shaft.

\[\text{Motor output = }\omega {{\text{T}}_{sh}}\text{ watts = }\frac{2\pi N}{60}.{{T}_{sh}}\text{ watts}\]

Shaft torque,

\[{{\text{T}}_{sh}}=\frac{\text{Output in watts}}{\frac{2\pi N}{60}}\]

\[=\frac{60}{2\pi }\times \frac{\text{Output}}{N}\text{ N-m = 9}\text{.55 }\times \text{ }\frac{\text{Output}}{N}\text{ N-m}\]

Lost Torque

When the mechanical power developed is transmitted to the shaft of the motor, there occurs a power loss due to friction, windage and iron loss. Therefore the torque required to overcome these losses is known as lost torque or lost torque. The total armature torque is the combination of shaft torque and lost torque.

\[{{T}_{a}}={{T}_{sh}}+{{T}_{f}}\]

Lost torque,

\[{{T}_{f}}={{T}_{a}}-{{T}_{sh}}\]

The post DC Motor Torque Equation – Theory, Diagram & Derivation appeared first on Electrical and Electronics Blog.

Fig. 1: Armature Control Method of DC Motor.

Working and Diagram of Armature Control Method

As the back emf in DC motor, E_b = V – I_a R_a, by varying the armature resistance we can obtain various speeds. A variable resistance (R) called controller is connected in series with the armature as shown in Fig 1. The controller should be selected to carry the armature current for a longer period. Let the initial and final speeds of the motor be N₁ and N₂, and the back emf be E_b1 and E_b2 respectively,

\[{{N}_{2}}=\frac{{{E}_{b2}}{{N}_{1}}}{{{E}_{b1}}}\]

 By varying the controller resistance (R) value in the armature circuit, the back emf can be varied from E_b1 to E_b2, thereby, the speed can be varied from N₁ to N₂.

Advantages of Armature Control Method

This method is suitable for constant load drives where speed variations from low speed up to normal speed are only required.

Disadvantages of Armature Control Method

1. Speeds below normal can only be obtained.
2. A stable speed cannot be maintained when the load changes.
3. Power loss in the control resistance is high due to the higher current rating, leading to the low efficiency of the motor.
4. The cost of control resistance is high due to the fact it has to be designed to carry the armature current.
5. Requires expensive arrangements to dissipate the heat developed in the control resistance.

Application of Armature Control Method

Suitable for DC shunt and compound motors used in printing machines, cranes, and hoists where the duration of low-speed operation is minimum.

The post What is Armature Control Method of DC Motor? Working Principle, Diagram & Applications appeared first on Electrical and Electronics Blog.

Fig. 1: Ward Leonard Method of Speed Control.

In all the Speed Control methods, it is clear that the speed cannot be varied from zero to above normal by any one method and atleast two methods are required to be combined to do so. Further, the efficiency of the above mentioned controls is much less, due to power loss and instability due to load variation.

Working of Ward Leonard Method of Speed Control

In this method, a DC generator is mechanically coupled to a constant speed DC motor or an AC 3-phase induction motor as shown in Fig 1. The generated supply from the DC generator is fed directly to the armature of the controlled DC motor. The fields of both the DC generator and the controlled DC motor are separately excited from a suitable DC supply. The field of the DC generator is controlled through a field rheostat and a change-over switch to vary the generated voltage and to change the polarity respectively. This enables the supply to the controlled DC motor to vary at a wide range and also makes it possible to reverse the supply voltage polarity. This, in turn, changes the speed of the controlled DC motor to vary from zero to above normal speed as well as change the direction of rotation, if necessary. The controlled DC motor speed can be brought down to zero by reducing the supply voltage of the generator to a suitable level.

Advantages of Ward Leonard Method of Speed Control

1. By this system, a speed as low as zero and as high as two times the normal speed could be achieved.
2. The direction of rotation of the controlled DC motor can be changed simply by reversing the controller in the field circuit of the generator.
3. As there is not much power loss in the field rheostat, the speed variations are achieved at higher efficiency.
4. The speed of the controlled DC motor is independent of the load.

Disadvantages of Ward Leonard Method of Speed Control

This method requires high initial cost and low overall efficiency due to three machines in operation.

Applications of Ward Leonard Method of Speed Control

This system is used in steel rolling mills and paper mill drives, hoists, elevators etc, where precise control of speed over a wide range is required.

The post What is Ward Leonard Method of Speed Control? Working Principle, Diagram & Applications appeared first on Electrical and Electronics Blog.