Page 171 - Industrial Power Engineering and Applications Handbook

Static controls and braking of motors 6/151
Armature vottage control Field contml
(Field strengtheniq region) (Field weakening region)
- Constant toque region Constant HP region

/,
Va e, Ta 0 . = Constant

Armature voltage fixed
field current reduced la = Armature current
@ = Torque cutve
@ = output cum

SPeed(=9 - (T,, HP and V,)
the rated parameters are referred
0' N Base speed - It is normally the rated speed at which
Basa speed

Figure 6.51 Variation of toque with speed in a d.c. machine (the same as for an a.c. machine)

251-l-rl A. External: mechanical or friction braking

This type of braking is suitable for small motors and can
I I I be achieved through

1 Solenoid-operated brakes,
2 Electro-magnetically operated brakes, or
3 Magnetic particle brakes.

In the first two types a brake shoe, operated by an external
auxiliary supply, is mounted on the extended shaft at the
NDE (non-driving end) of the motor. These brakes are
normally operated after the motor is switched OW. The
heat of braking appears in the external circuit and the
motor windings &e not affected. For motors with this
braking, only the starting heat need be considered,
depending upon the frequency of starts and not the heat
of braking.
Figure 6.52 An approximate illustration of 7 and loss variation,
with change in speed in a variable-speed fluid coupling Nofe Friction braking may be employed for all sizes of drives,
either as the only braking means as noted below, or as a supplementary
safety means to keep the drive locked stationary when required.
An analogue to starting heat (equation (2.10)) gives the
braking heat Hb as 1 AC solenoid brakes These are employed for small
motors, say, up to 15-20 h.p. They are suitable for
GD;
H --. (Np - Ni ) watt-seconds (Ws) (6.9) applications such as conveyors, hoists, cranes, machine
- 730 tools, lock gates and dumb waiters (Figure 6.53). The
The wider the speed range of braking, the greater will be brakes are spring loaded and mounted on two
the heat generated. mechanically opposing brake shoes. They grip a brake
drum or disk, coupled rigidly at the NDE of the motor
6.20.1 Types of braking shaft. The brakes are applied mechanically and released
electrically. The braking action takes place by de-
There are several methods of braking, external or internal, energizing the spring. The brakes are normally applied
and they are briefly discussed below. Any of them can be in the OFF position for reasons of safety in the event
employed, depending upon the torque requirement, i.e. of a power failure. They are released only when the
size of motor, its speed, the type of load, etc. solenoid is energized.