VMX
2
Series Digital Solid State Soft Starter User Manual
73 |
P a g e
Accel Ramp Time.
This Function sets the maximum allowable time for
ramping from the Initial Torque setting to either of the following:
1) Current limit setting when the motor is still accelerating, or
2) Full output voltage if the Current Limit is set to maximum.
Increasing the Ramp Time softens the start process by lowering the
slope of increase in voltage or current. This should be set to provide the
softest possible start without stalling unless it is determined that the
application has other considerations. Applications where this setting
should be shorter include Centrifugal Pumps, because pump problems
may occur as a result of insufficient torque during acceleration through
the pump curve.
▪
Note:
Ramp Time is affected by the following conditions:
1. Current Limit
will automatically override the Ramp Time if set
to Voltage Ramp (F010=1) and if the motor does not reach full
speed while in current limit mode.
2.
An “Anti-Oscillation” circuit built-in to the
VMX2 Series
will
shorten the Ramp Time if the motor reaches full speed before
end of ramp.
Current Limit.
This Function sets the maximum motor current that the
starter allows during Ramping. It is active in both the Voltage Ramp and
Current Ramp modes. As the motor begins to ramp, this feature will set
a maximum at which the current draw will be held. Current Limit will
remain in effect until one of the following two events occur:
1. The motor reaches full speed as detected by the At-Speed
detection circuit.
2. The Overload Protection trips on Motor Thermal Overload (see
Ch.3).
Once the motor has reached full speed, the Current Limit feature
becomes inactive.
▪
In the Voltage Ramp Profile
, the Voltage output is increased until
the Current Limit is reached. The Ramp Time is the maximum time
it will take for the voltage to increase until the Current Limit setting
takes over.
▪
In the Current Ramp Profile
, output voltage varies to provide a
linear increase in current up to the Current Limit setting, and Ramp
Time is the time that it will take to get there. A closed loop feedback
of motor current allows continuous updating of the output to
maintain this ramp profile.
Because most AC induction motors will not start below 200% current,
the current limit set point is adjustable down to only 200%.
Caution
:
While in the Start mode there is no maximum Current Limit time.
Excessive start time may lead to motor stalling, causing an
Overload Trip. If this happens, try raising the Current Limit setting
to accommodate your load conditions. If the Current Limit setting
cannot be increased, try using Current Limit without ramping
features (“Current Limit Only” as described previously).
