ABB
132
ABB/ BU Machines / HV Induction motors IEC catalogue / Motors for variable speed drives EN 12-2007
3
Frequency Converter & By-pass Drive
In some applications, the frequency converter is fed
directly from the mains, the direct-on-line (DOL) method.
In these applications, the motor should be checked to
ensure it is:
-
suitable for direct-on-line operation (rated frequency
and voltage vs. the network)
-
capable of starting the load (starting torque curve vs.
load curve, inertia).
To be suitable for DOL operation the field weakening
point choice and that the operational speed is defined
by the net frequency. The load torque must be checked
as the power requirement will change if the operating
speed changes and the motor capacity must be
checked. For example, the pump loads can be much
higher if the operating speed is increased because of
the higher frequency.
In many cases the starting torque available with the
frequency converter which may affect the ability of the
motor to start the load. During frequency converter
operation, the motor is normally capable of starting with
a higher mass of inertia and higher starting torque. In
some cases the load torque can be decreased by, for
example, closing the valves of the pump (or fan).
The starting current is normally much higher than the
rated current (often 600%), thus during by-pass starting
the voltage drop in the network might exceed given
limits. In all cases the transients in DOL-starts must be
considered, such as, forces to the foundation and the
shaft torques.
The table below shows the rating plate values for
converter supply and DOL operation. In this example,
the rated frequency is the same for both applications.
If the motor is designed for by-pass operation it is
equipped with two rating plates: one for converter
supply and one for by-pass (DOL).
CONVERTER SUPPLY
S1
0
-
680
kW
0
-
660
V
0 - 50
Hz
0 - 728
A
0 - 995
rpm
0 - 0.84
PF
Table above: Rating plate values for a by-pass drive.
BY-PASS
800
kW
660
V
50
Hz
849
A
994
rpm
0.85
PF
Rating plate