Hitachi H20-T User Manual

31
3-3-4 Inching and Plugging Operation
Since frequent inching and plugging operations actuate the thermal overload relay, continuous operation becomes
impossible .
In order to avoid this, the setting value of current of the thermal overload relay may be increased over the rated
current of motor; however, it is not advisable to protect the motor.
In the case of inching and plugging operation, the starting current flows intermittently and the self-cooling effect
of the cooling fan decreases. In this state the temperature rise is different from that of normal operation.
Taking into consideration the above, in order to protect the motor when carrying out inching and plugging operations,
it is most suitable to protect the motor directly detecting the temperature by incorporating the thermistor in the
motor winding.
In the case of slip-ring motor, the rotor must be protected from burnt-out. In addition to the thermistor incorporated
in the stator winding, the thermal overload relay should be concurrently used for restraint protection, thus ensuring
perfect protection.
3-3-5 Where Starting Time is Long
For the fan, press machine, etc. whose mechanical inertia (GD
2
) of load is great, the starting time is long.
If the starting time is long, it is very difficult to achieve compatibility of prevention from malfunction with protection
of motor, so that for practical purposes, such a method is adopted that the protective performance at restraint is
somewhat lowered (since the probability of restraint troubles is low).
The following table classifies the general countermeasures.
Table 23. General countermeasure
Starting time Applicable thermal
overload relay
Connection Merit Demerit
Approx. 5 to
15 sec.
Thermal overload relay
with saturation reactor
(Please consult with us)
Same as standard Simple circuit In both starting and
running times, the
protective perform-
ance at restraint is
insufficient .
15 sec. or
longer
Standard thermal
overload relay
Timer and contactor
are additionally used. Refer to Fig. 28.
The protection
during operation
is certain.
The restraint
protection at
starting can be
carried out to
some extent by
changing the
setting of the
time .
The circuit is
complicated.

Hitachi H20-T Specifications

General

Brand	Hitachi
Model	H20-T
Category	Controller
Language	English