reminder of the various types of AC motors
This device is practically indispensable
when the synchronous motor accounts
for a large part of total installed power.
c or apply the exciting current in two
stages, automatically or manually.
The exciting sources can be either
separate:
c motor set, exciter;
c thyristor rectifier;
or placed at the end of the motor shaft:
c reversed generator;
c rotating "diode rectifier and armature"
reversed AC generator.
The techniques most frequently used
are the thyristor rectifier and the
"rotating diodes".
The latter does away with brushes,
removes the exciting cubicle and, in
addition, is often fitted with a
synchronisation and recoupling
mechanism should synchronism fail.
These motors can supply reactive
energy by increasing the exciting
current. This characteristic, which
enables compensation of network
reactive loads, is one of the main
reasons these motors are chosen.
The curves in figure 5 show stator
current variation as a function of
exciting current for a given constant
load (Mordey curves). Use of this type
of motor for small powers is fairly rare.
On the other hand, it is frequently used
above 2,000 kW for its excellent
efficiency and control of its power
factor. In the case of highly regular
movements, synchronous motors are a
necessity. However, the machines
being driven must have a relatively low
load torque during starting, and sizing
of the dampening cage limits starting
rate.
sizing tolerances
The electromechanical characteristics
of motors are defined by standard
IEC 34-1. In the case of certain rated
characteristic values, the standard
defines the tolerances to be complied
with by the manufacturer. It is useful to
be familiar with these tolerances since,
for certain characteristics, they directly
affect the choice of motor and
equipment power and the setting of the
protection devices.
The table in figure 6 gives the
tolerances
dielectric withstand and
tests
Motors, just like all electrical network
components, are subjected to a variety
of surge voltages. They are particularly
sensitive to steep front surge voltages
or high frequency since they are
"jammed" by the first turns of the stator
windings.
Switching surge voltages
These are the result of transient
phenomena occurring during changes
in status in the supply network.
The following phenomena, specific to
inductive circuits and thus to motors,
must be taken into consideration:
c current pinch-off on current breaking;
c multiple re-ignitions on breaking and
prearcing on current making if the
breaking device is capable of breaking
the high frequency currents
corresponding to these phenomena.
Steep front surge voltages
These are the result of direct or indirect
lightning strokes. They spread onto the
network, creating a dielectric stress
which, even when limited by the use of
surge arresters, can be considerable.
Surge voltage is studied in detail in
"Cahier Technique" n° 151
"Overvoltages and insulation
coordination in MV and HV" and
special motor sensitivity in
"Cahier Technique" n° 143 "Behaviour
of the SF6 MV circuit-breakers Fluarc
for switching motor starting currents".
To check the motor’s capacity to
withstand to these various surge
voltages, the motors undergo
standardized tests performed as
defined by IEC 34-1 standard.
The test voltage is applied between the
winding being tested and the body of
the machine to which the magnetic
circuits and all the other stator and rotor
windings are connected.
Two types of tests are stipulated in the
standards: standard frequency tests
and impulse withstand tests.
Standard frequency test
Withstand to switching surge voltages
is checked in compliance with standard
IEC 71, by the standard frequency
withstand test. Testing commences
with a voltage of less than U/2 which is
gradually increased up to 2 U + 1,000V,
at which level it is applied for one
minute.
For the stator, U is the specified supply
voltage. For the rotor, U is the voltage
which appears, with the rotor circuit
open, when the specified stator
supply voltage is applied with the
rotor locked in rotation. If the motor
is reversible (change of rotation
direction of motor already started), the
test voltage applied to the rotor will be
4 U + 1,000 V.
Impulse withstand test
This test consists in applying an
impulse voltage representative of
lightning:
c buildup time: 1.2 ms;
c dropdown time at Upeak/2: 50 ms;
c test voltage:
Upeak = 4 U + 5,000 V.
The windings are subjected to a
number of positive and negative waves.
Impulse withstand tests are not
currently mandatory in standards, as
they can lead to early ageing of
armature and winding insulation.
More generally, the dielectric tests
must not be repeated; if a second test
is performed, it will be carried out at
80 % of the voltages indicated above.
control monitoring and protection of HV motors