Instructions for use and maintenance
Electric motors 50-400 V CA
Page
16
of
22
initially send an audible or visual signal to the operator, and only at a later stage, if the operator
has not intervened, can it be stopped. Naturally, intervention levels and delay times should be
chosen in such a way as to exclude possible hazardous conditions.
In the case of motors powered by converters (e.g. inverters), protection may also be provided
by limiting the current consumption; this limitation, however, is generally set at a value higher
than the rated current in order to allow the motor to withstand any peak load and have
sufficient starting torque. Therefore it must be coupled to another device able to intervene in
the case of over a certain time of overload conditions for the motor.
•
Excessive speed of machine elements.
Any over-speed of the motor may occur in the case of power supply by means of converters,
following a failure or abnormal operation of the converter itself, or in the case of dragging by
the load, in the case of absence or insufficiency of braking action. Protection, which is essential
when over-speed can cause dangerous conditions, can be obtained by preventing such
abnormal operations, for example by the use of devices sensitive to motor speed (centrifugal
switches or voltage relays connected to tachymetric dynamos), interrupting the power supply
of the motor, possibly including braking devices, in the event that the power failure is not able
to stop the controlled load (e.g.: uplift).
•
Protection against direct contact.
To avoid direct contact, the active parts of the motor (which are normally live) are placed inside
the casing (motor casing) and the opening of the terminal box, which makes them accessible,
and made possible only through the use of a tool. The removal of the terminal box must be
carried out, in case of maintenance, only by qualified experienced personnel with visibly
sectioned power supply network, including auxiliary circuits (e.g. for anti-condensation treads)
so as not to have accessible live parts.
In the case of active parts capable of maintaining the voltage, such as mains capacitors visibly
disconnected, drain the capacitors before performing any other intervention.
•
Protection against indirect contacts.
The protection against indirect contacts, that is, the contacts due to the manifestation of faults
in the insulation which thus bring the external metallic masses in contact with active parts under
tension, is mainly achieved by connecting all accessible masses to a protective conductor, which
in turn is connected to the ground, and by using protective devices which act by disconnecting
the power supply when, due to the fault in the winding, the passage of currents towards the
ground occurs.
The basic principle of such protection shall be that, in the event of failure, it shall not occur
between a part accessible to the ground or between two parts which are simultaneously
accessible, a tension sufficient to cause a risk of harmful physiological effects in a person who
comes into contact. They are distinguished:
-
Protection connection made according to the system with protection conductor
connected to the ground in an autonomous way compared to the power supply; the
protection can be performed by means of a switch of maximum current or differential
switch;
