Reduced Voltage Starting - Emotron MSF 2.0 Instruction Manual

St Conel Street Nudgee SPS SP043 Electrical Switchboard Operation and Maintenance Manual (Whelan)
Some applications like conveyors or screws may need an ini-
tial torque boost. However, for many applications it can be
seen that the torque needed is much lower than the torque
delivered by the induction motor in a DOL start.
A common method to reduce both starting torque and cur-
rent is to decrease the motor voltage during starting. The
following figure shows how the motor's torque and current
characteristics are changed when the supply voltage is
reduced.
T/T
n
2.5
2
1.5
1
0.5
0
0
I/I
n
8
U
n
7
6
U <U
2
5
4
U <U
3
3
2
1
0
0
Fig. 6 Reduced voltage start
A general rule of thumb is that the torque at each operating
point is roughly proportional to the square of the current.
This means when the motor current is decreased by a factor
of two by means of reducing the supply voltage, the torque
delivered by the motor will be decreased by a factor of four
(approximately).
2
T ~ I
I = 1/2 I
LV
I = 1/3 I
LV
LV=low voltage
DOL=Direct on line
10 Description
Q-Pulse Id TMS484
Torque
0.5
Current
n
2
0.5
≈ 1/4 T
-> T
DOL LV DOL
≈ 1/9 T
-> T
DOL LV DOL
This relationship is the base for any starting method using
reduced voltage. It can be seen that the possibility of reduc-
ing the starting current depends on the correlation between
the motor's and the load's torque characteristic. For the com-
bination of an application with very low starting load and a
motor with very high starting torque, the starting current
may be reduced significantly by means of decreasing the
voltage during start. However, for applications with high
starting load it may – depending on the actual motor – not
be possible to reduce the starting current at all.
2.2

Reduced voltage starting

This section describes different starting methods which are
based on the reduced-voltage principle explained above. A
pump and its quadratic torque characteristic are used as an
example.
The star-delta starter is the simplest example of a reduced
U
n
voltage starter. The motor phases are first star connected; at
about 75% of nominal speed the phase connection is then
changed to delta. To enable star-delta start, both ends of all
U <U
2 n
three motor windings have to be available for connection.
Moreover, the motor has to be dimensioned for the (higher)
U <U
voltage in the delta connection. The following figure shows
3 2
the resulting torque and current characteristics.
n/n
n
1
T/T
2.. 5
n
2
1.5
1
0.5
0
n/n
n
I/I
8
1 n
7
6
5
4
3
2
1
0
0
Fig. 7 Star-delta start
Active 03/12/2013
Torque
0 0.5
Current
0.5
Emotron AB 01-4135-01r1
n/n
n
1
n/n
n
1
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