control monitoring and protection of HV motors
reminder of the various types of AC motors
Both high and low voltage AC motors,
offer a large variety of electrical,
dynamic and technological
characteristics. However, except for a
small number of motors used for highly
specific applications, they can be
divided into three families, namely:
c asynchronous cage motors;
c asynchronous slipring rotor motors;
c synchronous motors.
They differ from each other in:
c starting current and torque values;
c speed variation in normal operation;
c power factor and efficiency values as
a function of load.
HV motors are supplied with a voltage
rarely exceeding 7.2 kV, their power
ranges from 100 kW to over 10 MW,
with an average of 800 kW.
asynchronous cage motors
These HV motors fall into two main
categories according to their rotor
composition which can be single or
double cage.
This enables choice of starting current
and torque characteristics:
c single cage rotors have:
v a relatively low starting torque (0.6
to 1 Cn),
v a maximum torque of around 2
to 2.2 Cn,
v a starting current ranging from 4.5
to 5.5 In,
(Cn: rated torque, In: rated current);
c double cage or deep slot rotors have:
v a slightly higher starting torque (0.8
to 1.2 Cn),
v a maximum torque of around 2
to 2.2 Cn (slightly higher for deep slots),
v a starting current ranging from 5
to 6.5 In.
Figures 1 and 2 show the form of these
curves as a function of speed (N/Ns).
Note that:
c single cage motors have a minimum
torque (0.5 to 0.6 Cn), whereas the
torque curve, varying according to the
speed of the double cage or deep slot
motors, continues to increase up to
maximum torque.
c these motors are ideal for intensive use
and dangerous environments, due to
v the simplicity of rotor design in
short-circuit providing them with an
excellent mechanical and electrical
robustness,
v absence of brushes.
These two features allow maintenance
to be reduced to a minimum.
The torque characteristics of
asynchronous cage motors are
especially suitable for machines such
as centrifugal pumps, compressors,
converter sets, machine-tools and fans.
However, all these motors have the
drawback of a relatively low power
factor, around 0.8 to 0.9 on full load,
and even less when they are running
on low load (see fig. 3).
If asynchronous motor installed power
is high, reactive power compensation is
required. This may either be global, for
each set of motors or for each motor
(large units).
asynchronous slipring
rotor motors
The rotor winding of these motors
connected to sliprings means the
resistance of this circuit can be
modified by introducing external
resistances.
In the motor stability zone,
corresponding to the positive slope of
curve C = f (g) (see fig. 4), the slippage
"g" is proportional to the rotor
resistance:
g =
1
A
Rr C
where g% =
Ns - N
Ns
100
where:
Ns : synchronous speed,
N : operating speed.
A = 3V2
p
w
M
L1
= constant
where:
V: phase to neutral supply voltage,
p: number of pole pairs,
w: pulsation of supply currents,
M: reciprocal stator-rotor inductance,
L1: total stator choke,
(L1 = M + Ls)
Rr: rotor resistance = rotor inherent
resistance + external resistances,
C: motor torque.
Cahier Technique Merlin Gerin n° 165 / p.6
c the power factor which may be set by
the exciting current.
Technologically, they are identical to
AC generators.
In order to obtain asynchronous torque
and avoid oscillations, the rotors are
equipped with a damping cage. This
cage means synchronous motors can
be started with a low load torque in
similar fashion to asynchronous single
cage motors (they have practically the
same characteristic torque and current
curves). To avoid surge voltages in the
exciting circuit, this circuit is shunted
during starting and on tripping by a
resistance with a value chosen
between 5 and 10 times the resistance
of the exciting circuit.
In view of the fact that the
asynchronous torque tends to zero on
approaching synchronous speed,
coupling to the network when motor
starting is complete cannot take place
at synchronous speed as is the case for
AC generators. The result is invariably
a transient state varying according to
the speed acquired at the end of
starting and motor power. To limit this
transient state:
c either use a relay to monitor slippage
by measuring frequency of the rotor
current passing through the starting
resistance. This relay controls exciting
circuit supply when slippage is at its
lowest.