Table of Contents
Advertisement
The frequency, and therefore the number of motor revolutions,
is maintained constant by the motor's speed regulation system.
Generally, this regulator is of a mechanical type and presents
a droop from no-load to nominal load which is less than 5
% (static or droop), while under static conditions precision is
maintained within ±1%.Therefore, for generators at 50Hz the
no-load frequency can be 52–52.5 Hz, while for generators at
60Hz the no-load frequency can be 62.5-63Hz.
In some motors or for special requirements the speed regulator
is electronic; in these cases, precision under static operating
conditions attains ±0.25%, and the frequency is maintained
constant in operation from no-load to load (isochronal operation).
POWER FACTOR - COS ϕ
The power factor is a value which depends on the load's
electrical specifications; it indicates the ratio between the Active
Power (kW) and Apparent Power (kVA). The apparent power is
the total power necessary for the load, achieved from the sum
of the active power supplied by the motor (after the alternator
has transformed the mechanical power into electrical power),
and the Reactive Power (kVAR) supplied by the alternator. The
nominal value for the power factor is cos ϕ = 0,8; for different
values comprised between 0.8 and 1 it is important during
usage not to exceed the declared active power (kW), so as to
not overload the electricity-generating group motor; the apparent
power (kVA) will diminish proportionally to the increase of cos ϕ.
For cos ϕ values of less than 0.8 the alternator must be
downgraded, since at equal apparent power the alternator
should supply a greater reactive power. For reduction
coefficients, contact the Technical Service Department.
START-UP OF ASYNCHRONOUS MOTORS
The start-up of asynchronous motors from an electricity-
generating group can prove critical because of high start-up
currents the asynchronous motor requires (I start-up = up to
8 times the nominal current In.). The start-up current must not
exceed the alternator's admissible overload current for brief
periods, generally in the order of 250–300% for 10–15 seconds.
To avoid a group oversize, we recommend following these
precautionary measures:
- in the case of a start-up of several motors, subdivide the
motors into groups and set up their start-up at intervals of
30–60 seconds.
- when the operating machine coupled to the motor allows
it, see to a start-up with reduced voltage, star point/triangle
start-up or with autotransformer, or use a soft-start system.
In all cases, when the user circuit requires the start-up of an
asynchronous motor, it is necessary to check that there are
no utilities inserted into the installation, which in the case of
a voltage droop can cause more or less serious disservices
(opening of contact points, temporary lack of power to control
and command systems, etc.).
SINGLE-PHASE LOADS
Power to monophase utilities by means of three-phase
generators requires some operating limitations.
- In single-phase operation, the declared voltage tolerance
can no longer be maintained by the regulator (compound
or electronic regulator), since the system becomes highly
unbalanced. The voltage variation on the phases
not affected by the power can prove dangerous; we
recommend sectioning the other loads eventually
connected.
- The maximum power which can be drawn between Neutral
and Phase (start connection) is generally 1/3 of the nominal
three-phase power; some types of alternators even allow for
40%. Between two Phases (triangle connection) the maximum
power cannot exceed 2/3 of the declared three-phase power.
- In electricity-generating groups equipped with monophase
sockets, use these sockets for connecting the loads. In other
cases, always use the "R" phase and Neutral.
ELECTRIC PROTECTIONS
THERMAL-MAGNETIC SWITCH
The electricity-generating group is protected against short-
circuits and against overloads by a thermal-magnetic switch
(Z2) situated upstream from the installation. Operating currents,
both thermic and magnetic, can be fixed or adjustable in relation
to the switch model.
+ In models with adjustable operating current do not modify
the settings, since doing so can compromise the installation's
protection or the electricity-generating group's output characte-
ristics. For eventual variations, contact
our Technical Service Department.
The intervention of the protection feature
against overloads is not instantaneous,
but follows a current overload/time
outline; the greater the overload the
less the intervention. Furthermore,
keep in mind that the nominal operating
current refers to an operating temperature of 30°C, so that
protection device, check that the total absorption does not
exceed the electricity-generating group's nominal current.
26
USING THE GENERATOR
37.1
each variation of 10°C roughly
corresponds to a variation of
5% on the value of nominal
current.
In case of an intervention on
the part of the thermal magnetic
M
Advertisement
Table of Contents
