dc motors ee153.pdf
TRANSCRIPT
-
7/27/2019 dc motors ee153.pdf
1/19
MOTOR converts electrical energy to mechanical
energy Its action is based on a principle that when acurrent-carrying conductor is placed in amagnetic field, it experiences a mechanicalforce, whose direction is given by FlemingsLeft Hand Rule
-
7/27/2019 dc motors ee153.pdf
2/19
-
7/27/2019 dc motors ee153.pdf
3/19
Significant Features of DC MotorsDC motors convert electrical energy into mechanical energy
Constant mechanical power output or constant torqueTypes Shunt motors, series motors and compound motorsRapid acceleration or deceleration
Extensively used as a positioning device because its speed as
well as torque can be controlled precisely over a wide range1W to 10,000 hp
Applications in automobiles, robots, VCRs, movie camera,
electric vehicles, in steel and aluminum rolling mills, electrictrains, overhead cranes, control devices, etc.
Biggest advantage over other motors Torque-speed characteristics of dc motors can be varied over a wide
range while retaining high efficiency
-
7/27/2019 dc motors ee153.pdf
4/19
+
VT
_
Rf
Ra
+Eb
_
Iin
IaIf
EQUIVALENT CIRCUIT OF A SHUNTMOTOR
fain III +=
aab RIVE !=
Eb back or counter
emfV input voltageIin input current
-
7/27/2019 dc motors ee153.pdf
5/19
Back emf depends, among other factors, upon the armature speed. High speed!high Eb!small IaLow speed!low Eb!high Ia
Eb acts like a governor, i.e., it makes a motor self-regulating so that itdraws as much current as is just necessary.
CONDITION for MAXIMUM
EFFICIENCY
Thus, gross mechanical power developed by the motor is
maximum when back emf is equal to half the applied
-
7/27/2019 dc motors ee153.pdf
6/19
F
N rpsPulley
r
TORQUE
T=F x r (N-m)Work done by this force in onerevolution= force x distance
= F x 2"r JoulePower developed = F x 2"r x N
= F x r x 2"N
Watt Thus, P=2"NT
Where;P power developed in W N speed in rps T torque in N-m
-
7/27/2019 dc motors ee153.pdf
7/19
Armature Torque
EbIa = 2"NTa where aPZN
Eb
!=
!
"
#$
%
&=
A
PZIT
aa '(2
1
Thus, For shunt motor; Ta#IaFor series motor; Ta#Ia2
aa IT !"
-
7/27/2019 dc motors ee153.pdf
8/19
Shaft Torque
torque available for doing useful work
N2
watts
!
inoutputTsh =
where N in rps Lost Torque = Ta-Tsh
- due to iron and friction losses of the motor
-
7/27/2019 dc motors ee153.pdf
9/19
SPEED OF A DC MOTOR From the equation We get
and we assume $ # field currentFor Shunt motor;
For series motor;
a
PZN
Eb!
=
!
bE
kN =
1
2
1
2
b
b
E
E
N
N=
2
1
1
2
1
2
a
a
b
b
I
I
E
E
N
N!=
-
7/27/2019 dc motors ee153.pdf
10/19
Speed Regulation
Torque and Speed of a DC Motor
-
7/27/2019 dc motors ee153.pdf
11/19
-
7/27/2019 dc motors ee153.pdf
12/19
While studying motor characteristics, keep in mind the following relations:
-
7/27/2019 dc motors ee153.pdf
13/19
-
7/27/2019 dc motors ee153.pdf
14/19
Three possible methods of speed control: By Variation of Flux or Flux-control Method
-
7/27/2019 dc motors ee153.pdf
15/19
Torque
Speed
MaximumTorque
Flux Decreasing
Trated
Slow transient response Does not maintain maximum torque capability
By Decreasing Flux
-
7/27/2019 dc motors ee153.pdf
16/19
Armature or Rheostatic Control Method(Variation of Ra)
-
7/27/2019 dc motors ee153.pdf
17/19
Torque
Speed
MaximumTorque
By Changing Ra
Ra increasing
Power loss in Ra Does not maintain maximum torque capability Poor speed regulation
-
7/27/2019 dc motors ee153.pdf
18/19
dcmotor 18
Voltage Control Method
(By Controlling Terminal Voltage Vt )This method of speed control is applicable for speeds below
rated or base speed.
%m
VT
T1 T2 T3
T1
-
7/27/2019 dc motors ee153.pdf
19/19