ABB
ABB/ BU Machines / HV Induction motors IEC catalogue / Motors for variable speed drives EN 12-2007
131
3
Constant Torque Load
Certain applications can be regarded as constant
torque loads. These include paper machines, and
extruders and other gear pumps. One of the advantages
of AC drives is that, because there is no commutator,
the AC motor can produce torque at zero speed without
harmful effects, which is needed by some applications.
The amount of continuous torque available at low
speeds is dependent on the cooling method and often a
separately driven cooling fan will be the optimal choice.
The table below shows the rated values for a constant
torque drive. Because it is a self-cooled machine,
the rated torque cannot be utilized at zero speed
continuously. The continuous operational speed is from
340 rpm to 1750 rpm. In this example, the motor is also
capable of delivering an overload of up to 130% of the
rated torque.
CONVERTER SUPPLY
S1
183
-
750
-
940
kW
162
-
660
-
660
V
17.2 - 70.0
-
87.8 Hz
828 - 828
-
992 A
340 - 1396
-
1750 rpm
0.82 - 0.82
-
0.85 PF
OVERLOAD 130% 60 s / 10 min
1043 -
1043
-
1314
A
0
-
1395
-
1750
rpm
INVERTER PARAMETER SETTING:
940 kW / 660 V / 70.0 Hz /1009 A / 1395 rpm /
0.84 PF / TRN = 1785 ms / Tmax/Tn = 2.2
Table above: Rating plate values for a constant torque
drive.
Figure above: Load torque curve for a constant torque
drive.
0
0.5
1
1.5
0
1
speed
Constant Torque and Constant Power
For some specialist applications such as paper
unwinders, reelers and metal industry coilers, the
operational speed range is divided into two sections.
When operating between zero speed and base speed
the load has a constant torque and from base speed to
maximum speed it operates as a constant power. The
application requires constant tension (pull) in the web,
that requires constant power on the motor shaft if the
roll is centre driven. When the web speed is kept
constant the motor speed increases or decreases
as the roll diameter increases or decreases. Over a
specific speed range this results in a constant power
load. To overcome these challenges a relatively high
overload is often required at base speed. The field
weakening frequency is often slightly higher than the
base speed, giving an ideal combination for the inverter
and motor.
The table below shows the rating plate values for a
winder/unwind drive. In this case the normal operation
speed range is from 540 rpm to 2130 rpm. The field
weakening point is at 36 Hz, since that is the frequency,
when the nominal voltage (380 V) is reached. The drive
requirement is to deliver a varying overload from
707 kW at 540 rpm to 386 kW at 2130 rpm.
CONVERTER SUPPLY
S1
0
-
284
-
284 -
284
kW
0
-
287
-
380 -
380
V
0
- 27.2
-
36.0 -
107.0 Hz
770 - 770
-
645 -
520
A
0
- 540
-
717 -
2130 rpm
- 0.77
-
0.69 -
0.87 PF
OVERLOAD 60 s / 10 min
1043 -
1043
-
1314
A
0
-
1395
-
1750
rpm
INVERTER PARAMETER SETTING:
407 kW / 380 V / 36.0 Hz / 811 A / 715 rpm /
0.79 PF / TRN = 1160 ms / Tmax/Tn = 3.2
Table above: Rating plate values for a constant torque
drive.
Figure above: Load torque curve for a constant torque
drive.
0
1
2
3
0
1
2
3
speed
torque