Download Explain the two methods off speed control for a DC motor

Explain the two methods off speed control for a DC motor and calculate the speeds for
the following motor.
A 480v shunt wound motor has a base speed of 1000rpm (16.67 rev/s) with an armature
current of 150A and resistance of 0.2ohms. calculate the speed if the armature current is
75A and additional resistance of 0.6ohms is connected in series with the armature. (field
current remains constant) calculate the speed when the armature is 75A and field current
is reduced by 80% .
There are mainly two methods of speed control of DC motors, that is, armature control
and field control. Speed of a DC motor is given by

V  I a Ra
k
Where ω is speed(rad/sec), V is supply voltage across armature,  is field flux, Ra is
armature resistance and k is constant. We note that we can control speed by varying
numerator or denominator. When we vary numerators, we basically vary voltage across
the armature; hence, it is called armature control of speed of DC motors.
When we control denominator, we basically control field flux. Armature control method
is used for controlling speeds up to rated speed. Filed control method is used when speed
is to be controlled beyond rated speed.
Speeds up to rated speed can be achieved by controlling armature voltage. Speeds beyond
rated speed can be achieved by increasing armature voltage but it will reduce the motor
life unnecessarily. Thus, speeds beyond rated speed are never achieved by armature
voltage control. For this purpose, field current is reduced. Reduction of field flux results
in less produced torque. However, power remains same. Hence, this is also called
constant power control. Armature voltage results in constant torque. Hence, armature
voltage control is also called constant torque control method of controlling speed.
460V DC shunt motor, 1000 rpm rated speed, 0.2 ohms armature resistance.
From the figure, we have back EMF
Eb = 460 – 150*0.2 = 430 volts
We know that Eb 
 ZNp
60a
Hence,

kI f ZNp
60a
430 
if field flux is proportional to field current I f .
kI f Zp 1000
60a
--- (1)
Ia = 75 amps, armature resistance = 0.2 +0.6 = 0.8ohms.
Hence, back EMF Eb = 460 -75*0.8 = 400 volts. If speed is N rpm, we have
400 
kI f Zp  N
60a
--- (2)
Dividing (1) by (2), we get
430 1000

400
N
Or,
N = (400/430)*1000
= 930.232 rpm
Ans.
If field current is reduced by 80%, we have
400 
Dividing (1) by (3), we get
k  0.2 I f Zp  N
60a
--- (3)
430 1000

400 0.2N
Or,
N = (400/430*0.2)*1000
= 4651.162 rpm
Ans.
Explain using a minimum of 200 words the concepts of speed control using. a)a chopper
circuit from a DC source (b) a full bridge circuit from a DC source (c) compare a) and B)
with reguards a controlled AC to DC rectifier circuit and how forward and reverse
operation is possible. (d) the comparisons between speed control and torque control using
closed loop feedback.
(a)CHOPPER FED DC DRIVES
Choppers basically reduce the voltage to suit the requirements. Voltage is applied to the
motor terminals in pulses. The motor responds to the average voltage and not the
maximum voltage. A chopper basically acts as a fast acting switch. When switch is on,
the supply voltage is applied to the motor terminals. The motor accelerates during on
period. When chopper is off, the motor decelerates. The current during coasting period is
circulated through a freewheeling diode. As armature has generally quite high
inductance, the operation, in most of the cases, results in continuous current mode.
Four quadrant operation is possible through choppers. There are five types of choppers,
that is, class A,B,C,D and E. Class A chopper operates in quadrant I only of torque-speed
diagram. Type B chopper operates in 1st and 2nd quadrants. Four quadrant choppers are
used for very high loads.
Choppers are generally voltage impulse controlled or current pulse controlled. Fast
acting, that is, inverter grade thyristors were initially used in choppers but, at present,
MOSFETS or IGBTs are used in choppers. This became essential as switching off of
thyristors is quite difficult. When duty ration is high, thyristors can be used. Design of
commutating circuit for choppers is challenging job.
A chopper fed motor has to dissipate more losses because of the presence of ripple in the
output of the choppers. This requires special design of the motors. Choppers find
extensive use in railway traction.
(b) and (c)FULL BRIDGE CIRCUIT
AC power, 1-phase or 3-phase, is generally available. We convert AC power into DC
power for operating DC drives. When DC is available, it may directly be used for
operating DC drives. When the available voltage level is higher than the drive can use,
we reduce the voltage available voltage level by using choppers or any other
means. DC series motors are frequently used in railway traction where available voltage
is quite higher than what motors require. Separately excited motors are extensively used
in industry as we can control precisely control speed as well as torque. Induction motors
can not be used where precise control of speed and torque is required. This property of
DC motors has made them to survive them in the era of AC transmission and distribution.
Vector control drives are coming in the market but time will tell whether they will be able
to replace DC drives or not.
As the name indicates, a full bridge may be single phase or three phase bridge. When
load is high, three phase bridge circuit is used. A three phase bridge circuit may be full
controlled or half controlled. A three phase bridge, fully controlled or half controlled, is 6
pulse converter. Many times 12 pulse converters are also used to simplify the design of
filter circuit and control ripple.
Bridge circuits also suffer from the disadvantage of ripple. Harmonics are introduced in
supply as well as load side. Power factor control is also a problem in these drives as these
suffer from poor power factor. Supply voltage goes to zero in every half cycle; hence,
line commutation is used. As a result, control circuit design becomes simple.
Thyrsiors were the devices used earlier but MOSFETS or IGBTS are used as controlling
devices.
Four quadrant operation is possible with back to back connected bridge circuits. In case
of 4 quadrant operation, torque as well as speed direction can be controlled. Four
quadrant operation can be achieved by reversing the field current or dual converters (1phase as well as 3-phase) are used. Single-phase dual converters are used up to 15kW and
3-phase dual converters are used beyond 15 kW. Both circulating and non-circulating
type dual converters are used as each has advantages and disadvantages. Circulating
type dual converters require inter phase reactance. This increases the cost and incurs
losses but provides faster response. Regenerative braking is the problem with these
drives.
(d) Open loop and closed loop control
These are basically speed control methods. As mentioned above, constant torque or
constant power control is possible through closed loop control. With open loop control, it
is not possible. Separately excited motors are generally used for this purpose in industry.
In traction, DC series motors are used. For position control, DC servo motors are used.
These find extensive used in robots. Size of DC servo motor is generally less compared to
AC servo motors. Also AC servo motors incur more losses. In DC servo motors, design
of commutator is quite challenging task.
Time constants of field as well as armature circuit play important role in closed and open
loop control. Time constant of field circuit is generally quite high. Large time constant of
field circuit does not allow speed to change fast when field current is varied. Hence,
where fast response is desired, open loop control is preferred.
Armature control is achieved by varying the supply voltage generally and not by inserting
resistance in the armature circuit as it leads to in-efficient operation.
Tachometer is required for open as well as closed loop control. It generates voltage
proportional to the speed.