programming range of the adjustable frequency drive. Consult the Instruction Manual for more
on this function.
Parameters 1-30 and 1-35, as stated, should be set by the AMA function, values supplied by the
motor manufacturer, or left at the factory default values. Never adjust these parameters to random
values even though it may seem to improve operation. Such adjustments can result in unpredict-
able operation under changing conditions.
Par. 4-17
Torque Limit Generator Mode
sets the limit for adjustable frequency drive torque. The
factory setting is 160% for FC 302 series and 110% for FC 102/202 series and will vary with motor
power setting. For example, an adjustable frequency drive programmed to operate a smaller rated
motor will yield a higher torque limit value than the same adjustable frequency drive programmed
to operate a larger size motor. It is important that this value not be set too low for the requirements
of the application. In some cases, it may be desirable to have a torque limit set at a lesser value.
This offers protection for the application in that the adjustable frequency drive will limit the torque.
It may, however, require higher torque at initial start up. Under these circumstances, nuisance
tripping may occur.
Par. 14-25
Trip Delay at Torque Limit
works in conjunction with torque limit. This parameter selects
the length of time the adjustable frequency drive operates in torque limit prior to a trip. The factory
default value is off. This means that the adjustable frequency drive will not trip on torque limit,
but it does not mean it will never trip from an overload condition. Built into the adjustable fre-
quency drive is an internal inverter thermal protection circuit. This circuit monitors the output load
on the inverter. If the load exceeds 100% of the continuous rating of the adjustable frequency
drive, a timer is activated. If the load remains excessive long enough, the adjustable frequency
drive will trip on inverter time. Adjustments cannot be made to alter this circuit. Improper pa-
rameter settings effecting load current can result in premature trips of this type. The timer can
be displayed.
5.1.1 Overvoltage Trips
This trip occurs when the DC bus voltage reaches its DC bus alarm voltage high (see ratings tables
in introductory section). Prior to the trip, the adjustable frequency drive will display a high voltage
warning. Most times an overvoltage condition is due to fast deceleration ramps with respect to
the inertia of the load. During deceleration of the load, inertia of the system acts to sustain the
running speed. Once the motor frequency drops below the running speed, the load begins over-
hauling the motor. At this point the motor becomes a generator and starts returning energy to
the adjustable frequency drive. This is called regenerative energy. Regeneration occurs when the
speed of the load is greater than the commanded speed. This return voltage is rectified by the
diodes in the IGBT modules and raises the DC bus. If the amount of returned voltage is too high,
the adjustable frequency drive will trip.
There are a few ways to overcome this situation. One method is to reduce the deceleration rate
so it takes longer for the adjustable frequency drive to decelerate. A general rule of thumb is that
the adjustable frequency drive can only decelerate the load slightly faster than it would take for
the load to naturally coast to a stop. A second method is to allow the overvoltage control circuit
to take care of the deceleration ramp. When enabled the overvoltage control circuit regulates
deceleration at a rate that maintains the DC bus voltage at an acceptable level. One caution with
overvoltage control is that it will not make corrections to unrealistic ramp rates. For example, if
the deceleration ramp needs to be 100 seconds due to the inertia, and the ramp rate is set at 3
5 Adjustable Frequency Drive and Motor Ap-
plications
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
5-2
MG.90.L1.22 - VLT
®
is a registered Danfoss trademark
5
Summary of Contents for VLT HVAC FC 100
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