DC DRIVES
INTRODUCTION
✓ The speed of a given machine (DC) has to be controlled for the required speed
variations of an operation.
✓ Either armature voltage or field current can be varied or controlled. A separately
excited motor is a versatile variable speed motor. The speed control using the
variation of the armature voltage can be used for constant torque application in the
speed range from zero to base or rated speed.
✓ The speed control using the field weakening can be used for constant power
application in the speed range from zero to above base or rated speed.
Review of DC Motor Theory
DC motor construction:
Dc motor is mainly constructed from two main parts, the stator and the rotor as shown in Fig.1.
Fig.1 DC machine construction
Types of DC motors: There are several types of dc motors:
•
•
•
•
Separately Excited motor
Shunt DC Motor
Series motor
Compound motor
The circuit connection diagrams of these four types of dc motors areshown in Fig.2.
Fig.2.Connection of the field circuit and armature circuit for (a) separately exited
motor,(b) series motor (c ) shunt motor and (d)compound motor.
In this lecture we shall consider mainly the separately excited and the shunt DC motors.
Separately Excited and Shunt DC Motors
A separately excited dc motor is a motor whose field circuit is supplied from a separate
constant-voltage power supply, while a shunt dc motoris a motor whose field circuit gets its
power directly across the armature terminals of the motor.
When the supply voltage to a motor is assumed constant, there is no practical difference in
behavior between these two machines. Unless otherwise specified, whenever the behavior of
a shunt motor is described, the separately excited motor is included too.
The equivalent circuits of these two DC motors are shown in Fig.3. The KVL equation for the
armature circuit is:
VT = EA + IARA
Fig.3. The equivalent circuit of (a) separately excited dc motor and (b)shunt dc motor.
The internal generated voltage (back emf) is given by:
E A = Ke φ n
Where,
n= speed of the motor in revolution per minute (rpm),
Φ = flux per pole in Weber (Wb),
Ke = Machine constant
Developed motor torque is,
Td = KT IA φ
KT = Torque constant = 9.55 Ke
IA = armature current (A)
The speed equation and the terminal characteristics of a DC Motor
A terminal characteristic of a dc machine is a plot of its output torque versus speed.
The output characteristic of a separately excited and shunt dc motors is approximately the
same and can be derived from the induced voltage and torque equations of the motor plus
the KVL as follows:
KVL : VT = EA + IARA
The induced voltage
EA = Ke φ n
VT = Ke φ n + IARA
Since Td = KT IA φ
current IA can be expressed as:
Combining the VT and IA equations:
Finally, solving for the motor’s speed:
Where n= speed in rpm.
This equation is called the dc motor speed equation and is just a straight line with a negative
slope. The resulting torque-speed characteristic of ashunt dc motor is shown in fig.4:
Speed
no
Separately exited
Shunt
Torque
Fig.4. Speed - torque characteristic of a shunt or separately excited dcmotor.
Where : no = no load speed (i.e. when Td =0) or
Four Quadrant operation of a DC Motor
Braking operation
Assume that flux is kept constant, and motor is initially driving a load at a speed of m. To reduce
the motor speed if Vt is reduced below Ea, then the current Ia will reverse in direction. The
electromagnetic torque Tem now reverses in direction and the kinetic energy is converted to
electrical energy by the DC machine which now act as a generator.
During the braking operation, polarity of Ea does not change since the direction of rotation has not
changed. If the terminal voltage polarity is reversed, the direction of rotation of the motor will
reverse. Therefore, a DC motor can be operated in either direction and its electromagnetic torque
can be reversed for braking as shown by the four quadrants of the torque-speed plane.
Fig.5 Four quadrant operation of a DC motor
DC Motor Drive Transfer Function Model
Fig.6 shows a DC motor operating in a closed loop to deliver controlled speed or controlled
position. To design a proper controller that will result in high performance it is important to know
the transfer function of the motor.
Fig.6. DC Motor Drive
For analyzing small signal dynamic performance of the motor load combination around a steady
state operating point, the following equations can be written in terms of small deviations around
their steady state values.
By taking the Laplace transformation of these quantities,
Using above equations input (Vt) output (m) relationship can be obtained as,
This equation results in two closed loop transfer functions,
Fig.7. Transfer Function Bock Diagram
Neglecting friction term by setting B=0 and considering just the motor without the load (J = Jm),
Electrical time constant e determines how quickly the armature current builds up as shown in Fig.8
in response to a step change in the terminal voltage, where the rotor speed is assumed to be
constant.
Fig.8
Mechanical time constant m determines how quickly the speed builds up in response to a step
change in terminal voltage provided that e is assumed to be negligible and hence the armature
current can change instantaneously.
Conventional Methods of Speed Control
Speed control of DC Shunt Motors:
✓ By varying the resistance in the armature circuit
✓ By varying the flux (field)
✓ By varying the applied Voltage
Armature Resistance Control
✓ Speed of the motor is directly proportional to the back emf Ea
Ea = V- IaRa.
✓ That is when supply voltage V and armature resistance Ra are kept constant, speed is directly
proportional to armature current Ia. Thus if we add resistance in series with armature, Ia
decreases and hence speed decreases.
✓ Greater the resistance in series with armature, greater the decrease in speed.
Advantages:
✓ Simple method of speed control
Disadvantages:
✓ The change in speed with the change in load becomes large.
✓ More power is wasted in this controller resistance.
Field flux control:
✓ Speed of the motor is inversely proportional to flux. Thus, by decreasing flux speed can
be increased and vice versa.
✓ To control the flux, a rheostat is added in series with the field winding, as shown in the circuit
diagram.
✓ Adding more resistance in series with field winding will increase the speed, as it will decrease
the flux.
✓ Field current is relatively small and hence I2R loss is small, hence this method is quite
efficient.
✓ Though speed can be increased by reducing flux with this method, it puts a limit to maximum
speed as weakening of flux beyond the limit will adversely affect the commutation.
Solid state Speed Control of DC Motor
✓ The DC Motor speed can be controlled through power semiconductor switches. In this way,
terminal voltage is adjusted. The power semiconductor switches are SCR, MOSFET, IGBT
etc..
✓ Types of DC Drives:
➢ Phase controlled rectifier fed DC drives
➢ Single phase rectifier fed DC drives
➢ Three phase rectifier fed DC drives
➢ One quadrant converter
➢ Two quadrant converter
➢ Four quadrant converter
➢ Chopper fed DC drives
➢ One quadrant Chopper drives
➢ Two quadrant Chopper drives
➢ Four quadrant Chopper drives
Single phase Controlled rectifier fed DC drives:
AC
Source
Rectifier
DC
Motor
Load
Half wave controlled rectifier
The average output voltage (VL) from a half-wave controlled rectifier for the given input ac voltage
V=Vmax sin  t,
Average output current,
Thus, the desired value of load current IL can be obtained by varying firing angle α.
Hence, load current decreases with the increase in value of firing angle α. So, the terminal voltage
decreases the motor run slowly and vice versa.
Full controlled rectifier
The average output voltage (VL) for the given input ac voltage V=Vmax sin  t,
Average output current,
Advantages:
✓
✓
✓
✓
Basic operation is simple and reliable
Time response is faster
Small size
Less weight
Disadvantages:
✓ Introduce current and voltage harmonics into supply systems
✓ The overload capacity is lower
✓ Due to switching of SCR distortion of the AC supply voltage and telephone
interference may be produced.
Chopper Fed DC Drives
Fixed DC
DC Chopper Variable DC
DC
Load
Moto
✓
✓
✓
✓
✓
Fixed DC voltage is fed to the DC chopper circuit.
DC chopper converts fixed DC into variable DC voltage.
This variable DC Voltage is fed to the motor.
By varying the DC voltage, the motor speed can be controlled.
Self-commutated devices such as MOSFET‟s, Power transistors, IGBT‟s and IGCT‟s
are used for building choppers because they can be commutated by a low power control
signal and do not need commutation circuit and can be operated at a higher frequency
for the same rating.
Advantages:
✓ High efficiency
✓ Light weight
✓ Flexibility in controls
✓ Small size
✓ Quick response
Applications:
✓ Battery operated vehicles
✓ Traction motors
✓ Hoists
✓ Electric braking
✓ Trolley car
Example: A separately excited DC motor has the following parameters:
Ra = 3 Ω
,
Ke = 0.52 V/rpm.Wb
, Φ (flux per pole) = 150 mWb.
The motor speed is controlled by a semiconverter (2 pulse). The firing angle α is set at 60°,
and the average speed is 1250 rpm. The applied a.c. voltage to the bridge is 230 V at 50 Hz.
Assuming the motor current is continuous; calculate the armature current drawn by the motor and
the steady-state torque.
Note: Semi converter output voltage is given by,