In this method, the field coil is divided into various parts.These parts can be connected in series or parallel as per the requirement.
The Figure shows the two parts of field coil connected in series and parallel.
Two Parts Of Field Coil Connected In Series And Parallel
For the same torque, if the field coil is arranged in series or parallel, mmf produced by the coil changes.Hence the flux produced also charges.
In parallel grouping, the m.m.f produced decreases; hence higher speed can be obtained by parallel grouping.Used in case of fan motors.
2. Armature control of DC series motor
Speed adjustment of dc series motor by armature control may be done by any one of the methods that follow,
1. Armature resistance control method:
This is the most common method employed. Here, the controlling resistance is connected directly in series with the supply to the motor as shown in the figure.
The power loss in the control resistance of dc series motor can be neglected because this control method is utilized for a large portion of time for reducing the speed under light load condition.
This method of speed control is most economical for constant torque. and speed control is employed for dc series motor driving cranes, hoists, trains etc.
2. Shunted armature control:
The combination of a rheostat shunting the armature and a rheostat in series with the armature is involved in this method of speed control.
The voltage applied to the armature is varies by varying series rheostat R. The exciting current can be varied by varying the armature shunting resistance R2.
This method of speed control is not economical due to considerable power losses in speed controlling resistance.Here speed control is obtained over wide range but below normal speed.
3. Armature terminal voltage control:
The speed control of dc series motor can be accomplished by supplying the power to the motor from a separate variable voltage supply. This method involves high cost so it rarely used.
Rheostat control and its characteristics curve
4.2 Armature and field control
Here we see an example for a 200 V, 10.5 A 2000 RPM shunt motor has the armature and field resistances of 0.5 and 400Ω respectively. It drives a load whose torque is constant at rated motor torque. Let us calculate the motor speed if the source voltage drops to 175 V.
Load torque is constant
Field control is employed for getting speeds higher than rated speed and armature voltage control is used for getting speed less than rated.
It is seen that speed of the motor is inversely proportional to flux. Thus by decreasing flux speed can be increased and vice versa.
In field control method to control the flux, a rheostat is added in series with the field winding. Adding more resistance in series with field winding will increase the speed, as it will decrease the flux.
Hence this method can be used for speeds greater than the rated value.W.k.t speed of the motor is directly proportional to the back emf Eb and Eb = V- IaRa.
That is when supply voltage V and armature resistance Ra are kept constant, speed is directly proportional to armature current Ia.Thus in case of armature control method if we add resistance in series with armature, Ia decreases and hence speed decreases.
In Greater the resistance in series with armature, greater the decrease in speed.Hence this method is used for speeds less than the rated value.
Eb → back emf in volts.
ϕ → flux in webers.
4.3 Ward-Leonard control system
Ward Leonard method of speed control is used for controlling the speed of a DC motor.
It is a basic armature control method.
This control system is consisting of a dc motor M1 and powered by a DC generator G.In this method the speed of the dc motor (M1) is controlled by applying variable voltage across its armature.
This variable voltage is obtained using a motor-generator set which consists of a motor M2(either ac or dc motor) directly coupled with the generator G.It is a very widely used method of speed control of DC motor.
Ward Leonard Speed Control System
Ward – Leonard speed control method
The ward Leonard speed control is shown in above figure. This system was introduced in 1890’s was a significant step in the evolution of the dc drives.
This system mainly used where very sensitive speed control is required like.
4Main drives in steel mills and paper mills.
The speed of motor M1 is to be controlled which is powered by the generator G.
The shunt field of the motor M1 is connected across the dc supply lines. Now, generator G is driven by the motor M2. The speed of the motor M2 is constant.
When the output voltage of the generator is fed to the motor M1 then the motor starts to rotate.When the output voltage of the generator varies then the speed of the motor also varies.Now controlling the output voltage of the generator the speed of motor can also be controlled.
This purpose of controlling the output voltage, a field regulator is connected across the generator with the dc supply lines to control the field excitation.
The direction of rotation of the motor M1 can be reversed by excitation current of the generator and it can be done with the help of the reversing switch R.S. But the motor generator set must run in the same direction.
1. It is a very smooth speed control system over a very wide range (from zero to normal speed of the motor).
2. The speed can be controlled in both the direction of rotation of the motor easily.
3. The motor can run with a uniform acceleration.
4. Speed regulation of DC motor in this ward Leonard system is very good.
1.High initial cost.
2.The overall efficiency is low, less than 80% because of an additional motor generator set.
4.4 Using controlled rectifiers and DC choppers
A controlled rectifier converts ac voltage to a dc voltage. This output voltage can be controlled by varying the firing angle of SCR’s.
Types of thyristor drives:
DC motors for different applications require speed control in a forward direction, reverse direction and regenerative braking. Closed loop control is invariably employed in all thyristor drives.
The various types of thyristor drives employed are:
1.Single phase half-wave controlled rectifier circuits for dc motors upto 1 kW rating.
2.Single phase half bridge circuits for dc motors upto 5 kW rating.
3.Three phase half bridge circuits for dc motors of 5 to 75 kw rating.
4.Three phase full bridge circuits for dc motors of 75 to 400 kw rating.
5.Twelve pulse converters for dc motors of rating exceeding 400 kW.
Single-phase half-wave thyristor drive
This type of drive is used for dc motors, which are of permanent magnetic fields and rated for 1 kW or less.
The circuit diagram is shown in Figure.
Single-phase half-wave thyristor drive
The dc voltage across the armature can be controlled by varying the triggering or firing angle of the thyristor. With the increase in firing angle a, the armature voltage and current are reduced and the motor runs slower.
With the decrease in firing angle the armature voltage and current are increased and motor runs faster.
An alternative arrangement for speed control of a separately excited dc motor is illustrated in figure in which alternating voltage is converted into dc voltage and supplied to the field winding and SCR provides half wave rectification and control to the armature winding.
The dc voltage applied to the armature is controlled by varying firing angle. The gate trigger control circuit can be either an open loop circuit or closed loop, automatically connecting circuit;
In closed-loop control, a tacho-generator is used which provides voltage to the comparator depending on the speed of the motor shaft.
Circuit for speed control of a separately excited dc motor
Single phase half-bridge thyristor drive
Bridge rectifier converts ac voltage into dc voltage.
The dc shunt field winding is directly fed from the bridge rectifier. However, the armature of the dc motor is fed from the bridge rectifier through the SCR.
The dc voltage applied to the armature is controlled by varying the triggering angle.
The free-wheeling diode D is connected across the motor armature to provide a circulating current-path (shown dotted) for the energy stored in the inductance of the armature winding at the time SCR turns-off.
In absence of free-wheeling diode, current will flow through SCR and bridge rectifier and thus prohibit SCR from turning-off.
Circuit for speed control of a DC shunt motor
Single phase full wave thyristor drive
The circuit diagram for speed control of a dc shunt motor is shown in Figure.
In Diodes D1, D2, D3 and D4 form the bridge which converts ac into dc to supply the field winding of the motor.
During the positive half cycles of input ac supply, SCR1 conducts and supplies the armature winding. During negative half cycles, SCR2 is made to conduct and supply the armature winding. Thus voltage applied to the armature controls the speed of the motor.
The angle of conduction of SCRs can be changed by varying the gate current.
Single phase full wave controlled rectifier circuit for Armature voltage control of a DC shunt motor
Armature and field control of Dc motor drive using controlled rectifiers
By varying the firing angle α1 the armature voltage applied to the motor will be varied and hence the speed can be controlled below the rated speed.when α2 is varied, then the speed can be controlled above the rated speed.
A smoothing inductor L is inserted in series with the armature circuit to reduce the ripple in armature current.
Armature and field control of DC motor drive using controlled rectifiers
Single Phase Converter Drive:
To reduce the ripples in the field circuit current, field circuit of motor is fed through a single phase semi converter drive.The load is highly inductive, hence the output current is considered to be continuous current.
During the positive half cycle of the input supply, T will be forward biased and it conducts from ωt = α to π. At ωt = π, the thyristor T will be reversed biased, hence output voltage will be zero driving ωt from π to 2π.
The inductor La in the motor will reverse its polarity due to which freewheeling diode will be forward biased.
Single Phase Fully Controlled Rectifier Drives
Single Phase Full Converter Drive
Figure shows single phase full converter drive. Assume armature current Ia is constant. Here, the load is DC motor. Full converter consists of 4 SCRs and load.During the positive half cycle (0 to π) SCR T1 and T2 are forward biased.
At ωt = α, SCR T1 and T2 are triggered and comes to the on state. These two SCRs conducts up to π + α during the period (α to π + α), SCR T1 and T2 are ON state.
At ωt = π + α, SCR T3, T4 are triggered and SCR T1 and T2 comes to the off state. Now SCR T3 and T4 conducts up to 2π + α. Figure shows input and output voltage wave forms.
Average output voltage V = V0
By varying the firing angle, the output voltage can be changed.
Input And Output Voltage Wave Forms
For firing angle α = 0°;
α = 90°; Va = 0
From the above equation, by changing the firing from 0 to 90°, we can get positive output voltage and from 90° to 180°, we can get negative output voltage.From the above discussion, the full converter is also called two quadrant converter.
It means, the average output voltage is either positive or negative but output current is always positive. It is shown in Figure.
This converter is used up to 15 kW DC motors.
A d.c. chopper is a static device used to obtain variable d.c. voltage from a source of constant d.c. voltage.
The control modes of a dc chopper drives are:
i)Power control or motoring control.
ii)Regenerative brake control.
iii)Rheostatic brake control.
iv)Combined regenerative and rheostatic brake control.
i) Motoring Control:
The control circuit consists of a forced commutated drive which offers one-quadrant drive. During t =0 to t1, the chopper will be forward biased and switched ON, so is = i0.
During t = t1 to t2, the chopper will be switched OFF by forced commutation, is = 0. But maintains load current io = ia through freewheeling diode FD.
ii)Regenerative Braking Control:
The motor will be allowed to act as a generator and the kinetic energy of motor and load is converted to electrical energy.This will be fed back to the supply.
In During t = 0 to t1, chopper is switched ON and Ia through the inductance La will increase and flows through ra, La, CH and Ea.
In During t = t1 to t2, chopper is OFF, Ea will be more than the source voltage by which D get forward biased. Hence energy stored in the armature inductance will be transferred to the source.
iii)Rheostatic Brake Control:
Here generated power is dissipated in the braking rheostat, Rb.The average current in the braking resistor is given by Ib = Ia (1 – α).
Rheostatic Brake Control
Merits and demerits of rheostatic control method
1.Simple method of speed control.
1.The input power remains constant.
2.Only lower speed operation is possible. At the same time more power is wasted in this controller resistance.
3.The speed control is highly inefficient.
iv) Combined Regenerative and Rheostatic Brake Control:
This system proves to be more energy efficient.The line voltage is continuously sensed during regenerative braking.If the voltage generated by the drive is more than a normal value, T1 will be switched ON.
Due to this the generated voltage will be diverted to the braking resistance Rb.
Combined Regenerative and Rheostatic Brake Control
Duty cycle is defined as the ratio between on time of chopper and total time of chopper
Single Quadrant Choppers:
Class A chopper drive and its characteristics
Class B chopper drive and its characteristics
Two Quadrant Choppers:
Class C chopper drive and its characteristics
Class D chopper drive and its characteristics
Class E chopper drive and its characteristics
The types of chopper are selected depending upon the applications.
In this ripples are present in the output.
To remove the ripples, the filters are used at the output of the choppers.
Muti-quadrant operation of chopper
Figure shows the power circuit diagram for a far quadrant chopper or type-E chopper.
It consists of four power semiconductor switches CH1 to CH4 and four power diodes D1 to D4. For first quadrant operation , CH4 is kept ON, CH3 is kept OFF and CH1 is operated with CH4 ON, load voltage is equal to supply voltage. i.e., Va = Vs and load current Ia begins to flow.
Here both output voltage Va and load current Ia are positive giving first quadrant operation.
When CH1 is turned OFF, positive current freewheels through CH4 & D2 in this way both output voltage Va, load current Ia can be controlled in the first quadrant. First quadrant operation gives forward motoring mode.
Power Circuit Diagram For a Far Quadrant Chopper
Forward breaking Mode:
Here CH2 is operated and CH1, CH3 and CH4 are kept OFF.
When CH2 is ON, reverse current flows through L, D4 and E during the ON time the inductor L stores energy.
When CH2 is turned OFF, current is fedback to source through diode D1.As the load voltage Va is positive and load current is negative,It is second quadrant operation of chopper.
As power flows from load to source Second quadrant operation gives forward barking mode.
Reverse motoring Mode:
For third quadrant operation CH1 is kept OFF, CH2 is kept ON and CH3 is operated.Polarity of load emf E must be reversed for this quadrant operation.With CH3 ON, load gets connected to source Vs.As both output voltage Va, load current Ia are negative It gives third quadrant operation.
It is also known as reverse motoring mode. When at 3 is turned off, negative current freewheels through CH2, D4.In this way, output voltage Va and current la can be controlled in the third quadrant.
Reverse braking mode:
Here CH4 is operated and other devices are kept OFF.Load emf E must have its polarity reversed for operation in the fourth quadrant.
With CH4 ON, positive current flows through CH4, D2 L and E.When CH4 is turned off, current is feedback to source through diodes D2, D3.
Here load voltage is negative, but load current is positive leading to the choppers operation in the fourth quadrant.Also power is flows from load to source. The fourth quadrant operation gives reverse braking mode.
The devices conducting in the fourth quadrant are indicated in Figure.
Comparison of chopper and phase control :
Input is DC.
1) Input is AC.
Switching frequency decides output ripple.
2) Input supply and rectifier decides output ripple.
Small filters are sufficient at high ripple frequencies.
3) Large filters are required.
High efficiency and better power factor.
4) Poor efficiency and poor power factor for inductive loads.
Effective regenerative braking.
5) Slow regenerative braking.
Methods to vary the duty ratio in DC choppers
The following control strategies are used in D.C chopper:
1.Pulse width Modulation method (fixed frequency method).
2.Variable frequency method.
In pulse Width Modulation method the total time period is kept constant but either duration of on period or off period is varied.So there by varying the pulse width applied to the transistor base the output level voltage can be varied.
In Variable frequency method, the pulse width is kept constant but the switching frequency is varied thereby the output load voltage can be varied.
This method is not preferred because it introduces unknown harmonics to filter.Duty cycle is defined as the ratio of on period of the switch to the sum of on and off period of the switch.
The various control strategies for varying duty cycle a or α are as follows:
1.Time ratio control method (TRC).
2.Current limit control method (CLC).
By changing the duty cycle, the motor speed also changes:
Principle of pulse – width modulation (Constant T)
This drive normally adopted in very sensitive speed control like.
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