|
CHARACTERISTICS
AND APPLICATIONS OF D.C. MOTORS |
|
Type
of motor |
Characteristics
|
Applications
|
|
Shunt |
Approximately constant speed |
For driving constant speed
line |
| . |
. |
shafting |
| . |
Adjustable speed
|
Lathes |
| . |
Medium starting torque
|
Centrifugal pumps
|
| . |
(upto 1.5 F.L torque)
|
Machine tools
|
| . |
. |
Blowers and fans
|
| . |
. |
Reciprocating pumps
|
| . |
. |
. |
|
Series |
Variable speed
|
For traction work,i.e electric
|
| . |
Adjustable varying speed
|
Locomotives, rapid transit
systems |
| . |
High starting torque
|
Trolley cars, cranes and hoists
|
|
.
|
. |
Conveyors |
| . |
. |
. |
|
Cumulative compound
|
Variable speed
|
For intermittent high torque
loads |
| . |
Adjustable Varying speed |
For shears and punches |
| . |
High starting torque |
Elevators |
| . |
. |
Conveyors |
| . |
. |
Heavy planners
|
| . |
. |
Rolling mills
|
SPEED
CONTROL OF DC MOTORS |
A.Merits
and Demerits of Rheostatic Control Method |
| 1. Speed
changes with every change in load, because speed variations depend
not only on controlling resistance, but on load current also.This
double dependence makes it impossible to keep the speed sensibly
constant on rapidly changing loads. |
| 2. A
large amount of power is wasted in the contoller resistance. Loss
of power is directly proportional to the reduction in speed.Hence
efficiency is decreased |
| 3. Maximum
H.P is diminished on the same ratio as speed. |
| 4.It
needs expensive arrangement for dissipation of heat produced in
the controller resistance. |
| 5.It
gives speeds below the normal, not above it because armature voltagecan
be decreased by the controller resistance.This method is, therefore,
employed when low speeds are required for a short period only and
occasionally as in printing machines and for cranes and hoists where
the motor is continually started and stopped. |
|
B
. Advantages of Field Control Method |
This method is economical ,
more efficient and convenient though it can give speeds above the
normal speed . The only limitation of this method is that commutation
becomes unsatisfactory because of armature reaction is greater than
a weaker field.
t
should however be noted that by combining the two methods,speed
above and below the normal may be obtained. |
CURRENT
RATINGS FOR COPPER CONDUCTOR RUBBER INSULATED AND
SHEATHED SINGLE CORE WELDING CABLES |
| Nominal
cross-sectional area sq.mm |
No.and
diameter of wires mm. |
Current
rating amp |
|
16
|
509/.200
|
100
|
|
25
|
796/.200
|
150
|
|
35
|
1114/.200
|
230
|
|
50
|
1591/.200
|
400
|
|
70
|
2228/.200
|
600
|
|
95
|
1344/.300
|
|
|
