VMX
2
Series Digital Solid State Soft Starter User Manual
72 |
P a g e
3. Closed Loop Current (Torque) Ramp
provides a smooth increase
of output torque. Ramp Time becomes the time from Initial Torque to
the Current Limit setting. Output current is constantly updated using an
internal PID feedback loop to provide a linear current ramp, therefore
the available torque is maximized at any given speed. The best use of
this feature is for applications where rapid changes in torque may result
in load damage or equipment changes. Examples would be:
▪
Long overland conveyors where belt stretching may occur.
▪
Low pressure pumps where slight surges at the end-of-ramp may
pose mechanical problems.
▪
Fans and mixers where blade warping is a problem.
▪
Material handling systems where stacked products may fall over, or
break.
This feature can be used with or without the Maximum Current Limit
setting.
To achieve Current Ramping with the
VMX2 Series
, set the Ramp
Profile (F010) to 0002 or 0004 (Current Ramp), and the Maximum
Current Limit setting (F014) to the desired level. The initial torque value
comes from the initial current setting in F012.
4. Current Limit Only (Current Step)
uses the Current Limit feature
exclusively without the benefit of soft starting by ramping the voltage or
current first (also known as
Pedestal Starting
). This will maximize the
effective application of motor torque within the limits of the motor. In this
mode, Initial Voltage is set to maximum and Ramp Time is set to
minimum, so that the output current steps to the Current Limit setting
immediately. Examples of when to use this mode are:
▪
Applications with a severely limited power supply.
▪
When starting a difficult load such as a centrifuge or deep well
pump.
▪
When the motor capacity is barely adequate without stalling or
overloading.
▪
It is a good choice when other starting modes fail.
Since ramp times are set to minimum, this mode functions in either
Voltage Ramp or Current Ramp setting.
Ramp Parameter Descriptions
Following are detailed descriptions of the individual settings that make
up each ramp profile.
Initial Voltage or Initial Current
These functions set the initial start point of either the Voltage Ramp or
Current Ramp as programmed in F010. Every load requires at least
some amount of torque to start from a standstill. It is not efficient to
begin ramping the motor from zero every time, since between zero and
the (WK
2
) breakaway torque level, no useful work is being performed.
The initial torque level should be set to provide just enough torque to
make the motor shaft begin to rotate, enabling the softest possible start
and preventing torque shock damage to the mechanical components.
Setting this function too high will not damage the starter, but may
reduce or eliminate the soft start advantages.
Initial Torque
Current Limit
C
u
rr
e
n
t
Time
Figure APP 1.3:
Curre nt / Torque Ramp
Ramp Time
At
Speed
Initial Torque
Current Limit
C
u
rr
e
n
t
Time
Figure APP 1.4:
Curre nt Ste p Starting
At
Speed
