Download SPEED CONTROL of SHUNT DC MOTOR

s.p.b. Patel engineering
college
Subject: Dc Machine & Transformer
Topic : speed control of dc machine
Branch : electrical
Prepared by,
Shah yash h.
(130390109058)
guided by,
Prof. Malay bhatt
prof. manish patel
Methods of Speed Control
Armature Voltage Control
Field Flux Control
V
Ra
m 

T
2
K e  K e 
Armature Resistance Control
Armature Voltage Control
1.Controlled Rectfier
V
Ra
m 

T
K e  K e 2
2. Chopper (DC-DC Converter)
Field Flux Control
Armature Resistance Control
Separately or shunt field
m
V
Ra  Rext
m 

T
K e K e 2
m
TR
Increasing
Re
Re
Torque
Armature Resistance Control
V
1
Ra
m 

Series field
m
Ke K f
T
Increasing
Re
Torque
Ke K f
SPEED CONTROL of SHUNT DC
MOTOR
•
•
Two common methods:
1- Adjusting the field resistance RF (and thus
the field flux)
2- Adjusting the terminal voltage applied to the
armature
Less common method:
3- Inserting a resistor in series with the
armature circuit
SPEED CONTROL of SHUNT DC
MOTOR
• Changing the Field Resistance
• If the field resistance increases, field current
decreases (IF↓ = VT/RF↑), and as the field current
decreases, flux decreases as well.
• A decrease in flux causes an instantaneous decrease
in the internal generated voltage EA↓ (=Kφ↓ω), which
causes a large increase in the machine’s armature
current since
VT  E A 
I A 
RA
SPEED CONTROL of SHUNT DC
MOTOR
• Induced torque in a motor is given by ind =KφIA
since flux in machine decreases while current IA
increases, which way does the induced torque
change?
Look at
this example: 
• armature
current flow is
IA=(250V-245V)/
0.25Ω= 20A
SPEED CONTROL of SHUNT DC
MOTOR
• What happens in this motor if there is a 1%
decrease in flux?
• If the flux decrease by 1%, then EA must
decrease by 1% too, because EA = Kφω
Therefore, EA will drop to:
EA2 = 0.99 EA1 = 0.99 (245) = 242.55 V
• armature current must then rise to
IA = (250-242.55)/0.25 = 29.8 A
• Thus, a 1% decrease in flux produced a 49%
increase in armature current
SPEED CONTROL of SHUNT DC
MOTOR
• back to original discussion, the increase in
current predominates over the decrease in flux
so, ind>load , the motor speeds up
• However, as the motor speeds up, EA rises,
causing IA to fall. Thus, induced torque ind
drops too, and finally ind equals load at a higher
steady-sate speed than originally
• Summarizing behaviour:
1- increasing RF causes IF (=VT/RF) to decrease
2- decreasing IF decrease φ
SPEED CONTROL of SHUNT DC
MOTOR
3 – Decreasing φ lowers EA(=Kφω)
4 - Decreasing EA increases IA (=VT-EA)/RA
5- increasing IA increases Tind (=KφIA), with change in IA
dominant over change in flux
6-increasing Tind makes Tind>Tload & speed ω increases
7-increase in ω, increases EA= Kφω again
8-increasing EA decreases IA
9-Decreasing IA decreases Tind until Tind=Tload at a higher
ω
• Effect of increasing RF on O/P characteristic of a shunt
motor shown in next figure
SPEED CONTROL of SHUNT DC
MOTOR
• Effect of RF speed control on a shunt motor’s
torque-speed (over motor’s normal operating
range)
SPEED CONTROL of SHUNT DC
MOTOR
• Effect of RF speed control on a shunt motor’s
torque-speed (over entire range from no-load to
stall conditions)
SPEED CONTROL of SHUNT DC
MOTOR
• According to equation of speed presented before:
(a) no-load speed is proportional to reciprocal of flux in
motor
(b) while slope of the curve is proportional to reciprocal
of flux squared
• Therefore a decrease in flux causes slope of torquespeed to become steeper
• over this range, an increase in field resistance
increases motor’s speed
• For motors operating between no-load & full-load
conditions, an increase in RF may reliably be expected
to increase operating speed
SPEED CONTROL of SHUNT DC
MOTOR
• In previous figure (b) terminal characteristic of motor over full
range from no-load to stall shown
• In figure can see at very slow speeds, an increase in field
resistance will actually decrease speed of motor
• This effect occurs because at very low speeds, the increase in
armature current caused by decrease in EA not enough to
compensate for decrease in flux in induced torque field
resistance
• Some small dc motors used for control purposes operate at
speeds close to stall conditions
• For these motors, an increase in RF might have no effect or it
might even decrease speed of motor
• Since the results are not predictable, field resistance speed
control should not be used in these types of dc motors. Instead,
the armature voltage method should be employed
SPEED CONTROL of SHUNT DC
MOTOR
• CHANGING ARMATURE VOLTAGE
• This method involves changing the voltage applied to
the armature of the motor without changing the
voltage applied to the field
• If the voltage VA is increased, then the IA must rise [ IA
= (VA ↑ -EA)/RA]. As IA increases, the induced torque
ind =KφIA↑ increases, making ind > load , and the
speed of the motor increases
• But, as the speed increases, the EA (=Kφω↑)
increases, causing the armature current to decrease
This decrease in IA decreases the induced torque,
causing ind = load at a higher rotational speed
SPEED CONTROL of SHUNT DC
MOTOR
• Effect of armature voltage speed control
SPEED CONTROL of SHUNT DC
MOTOR
• Inserting a Resistor in Series with the Armature
Circuit
• If a resistor is inserted in series with the armature
circuit, the effect is to drastically increase the slope of
the motor’s torque-speed characteristic, making it
operate more slowly if loaded. This fact can be seen
from the speed equation:
VT
RA



2 ind
K ( K )
• The insertion of a resistor is a very wasteful method of
speed control, since the losses in the inserted resistor
are very large. For this reason, it is rarely used