classical HV starting processes
The main HV motor starting processes
are as follows:
c direct stator starting on full voltage;
c stator starting on reduced voltage by
star-delta connection, by reactance or
by autotransformer;
c stator starting by capacitors;
c rotor starting.
direct stator starting on full
voltage
This starting mode is used for
asynchronous motors with cage rotor
and for synchronous motors.
Current peak on starting is around 4
to 7 In, according to motor
characteristics, and can last for roughly
1 to 10 seconds depending on the
moment of total inertia (motor +
machine), motor torque and load
torque.
If this starting mode is used, the
network must be able to withstand the
above current overload without
disturbing the other loads, and the
machine being driven must be able to
withstand the mechanical impact due to
the motor torque. The simplicity of both
equipment and motor and the resulting
savings mean that this mode is very
popular and even recommended
provided that voltage drop on the
network on start up is acceptable. The
decisive factor lies in the motor power/
short-circuit power ratio.
stator starting on reduced
voltage
Star-delta starting
This starting mode is used to reduce:
c current in a ratio of e;
c starting torque by a third.
It is used in LV and for low powers, but
rarely in HV due to the high current
peaks when moving to delta. In this
case it is replaced by reactance
starting.
Voltage reduced by resistance
Commonly used in LV, it is rarely used
in HV due to the Joules to be dissipated
and resistance insulation problems.
Voltage reduced by reactance
This starting mode (refer to power
diagram, figure n° 8) is the one which
reduces current inrush on the network
in the simplest manner. Since motor
starting torque is low, the machines
being driven must have a relatively low
load torque during start up:
compressors, centrifugal pumps,
converter sets, etc.
In point of fact, asynchronous motor
torque varies according to square of the
supply voltage , whereas the absorbed
current remains proportional to this
voltage.
C'd = Cd
Ud
Un
æ
è ç
ö
ø ÷
2
where :
C'd: starting torque with reduced
voltage,
Cd: starting torque with full voltage,
Ud: starting voltage,
Un: rated operating voltage.
I'd = Id
Ud
Un
where:
I'd: starting current with reduced
voltage,
Id: starting current with full voltage.
These relationships can also be
expressed using the rated
characteristics as follows:
I'd
In
=
Id
In
Ud
Un
.
The curves in figure 7 give the ratio
variations as a function of
the ratio
Ud
Un
.
Voltage at the motor terminals
increases gradually during starting. The
resulting start up is smooth.
c operation and schematic diagram:
v first stage:
operation on reduced voltage by
closing the line contactor CL,
v second stage:
normal operation by closing the shortcircuit
contactor CC;
c determining a starting reactance
(see fig. 9)
Starting voltage is determined by the
maximum current inrush l'd authorised
on the network:
Ud = Un
I'd
Id
.
The phase-to-phase voltage drop in the
reactance has the following value:
Un
- Ud
= j 3 L w I'd
The diagram in figure 9 shows that this
relationship can be expressed in an
arithmetic form for asynchronous
motors, since the power factor at the
initial moment start up moment is
initiated is virtually the same as the
power factor of the starting inductance.
Therefore:
L w =
Un - Ud
3 I'd
.
Knowledge of start up time and
operation rate is required to determine
reactive power.
Voltage reduced by autotransformer
This starting mode sometimes makes it
possible to reconcile reduction of
current inrush on the network and
motor torque value. In point of fact, it
has the advantage of reducing current
inrush according to the square of the
winding ratio:
control monitoring and protection of HV motors