User Manual Instruction Manual

Application Notes 13-15
13.7 Overspeed
Overspeed Limit is a user programmable value that allows operation at maximum
speed, but also provides an “overspeed band” that will allow a speed regulator such as
encoder feedback or slip compensation to increase the output frequency above
maximum speed in order to maintain maximum motor speed.
The figure below illustrates a typical Custom V/Hz profile. Minimum Speed is entered
in Hertz and determines the lower speed reference limit during normal operation.
Maximum Speed is entered in Hertz and determines the upper speed reference limit.
The two “Speed” parameters only limit the speed reference and not the output
frequency.
The actual output frequency at maximum speed reference is the sum of the speed
reference plus “speed adder” components from functions such as slip compensation.
The Overspeed Limit is entered in Hertz and added to Maximum Speed and the sum
of the two (Speed Limit) limit the output frequency. This sum (Speed Limit) is
compared to Maximum Frequency and an alarm is initiated which prevents operation if
the Speed Limit exceeds Maximum Frequency.
13.8 Power Loss Ride Through
When AC input power is lost, energy is being supplied to the motor from the DC bus
capacitors. The energy from the capacitors is not being replaced (via the AC line),
thus, the DC bus voltage will fall rapidly. The drive must detect this fall and react
according to the way it is programmed. Two parameters display DC bus voltage:
• DC Bus Voltage (12) - displays the instantaneous value
Figure 13.4 – Overspeed
Frequency
Overspeed
Limit
(83)
Frequency Trim
due to Speed
Control Mode
Max Volts
(54)
Motor Volts
(41)
Run
Boost
(70)
Break Volts
(71)
Start Boost
(69)
Allowable Output Frequency Range - Normal Operation
Allowable Speed Reference Range
Allowable Output Frequency Range -
Bus Regulation or Current Limit
Maximum
Freq
(55)
Motor NP Hz
(43)
0 Break
Frequency
(72)
Max
Speed
(82)
Min
Speed
(81)
Output
Freq Limit
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