Design Guide | VLT® AutomationDrive FC 360
3.2.3 Overvoltage Protection
Motor-generated overvoltage
The voltage in the DC link is increased when the motor acts as a generator. This situation occurs in the following cases:
l The load rotates the motor at constant output frequency from the drive, that is, the load generates energy.
l During deceleration (ramp-down) if the inertia moment is high, the friction is low, and the ramp-down time is too short for the
energy to be dissipated as a loss throughout the drive system.
l Incorrect slip compensation setting causing higher DC-link voltage.
l Back EMF from PM motor operation. If coasted at a speed which is higher than nominal speed, the PM motor back EMF can
potentially exceed the maximum voltage tolerance of the drive and cause damage. To help prevent this situation, the value of
parameter 4-19 Max Output Frequency is automatically limited based on an internal calculation based on the value of parameter
1-40 Back EMF at 1000 RPM, parameter 1-25 Motor Nominal Speed, and parameter 1-39 Motor Poles.
l The overvoltage can be handled either using a brake function (parameter 2-10 Brake Function) and/or using overvoltage control
(parameter 2-17 Over-voltage Control).
Brake functions
AC brake is an alternative to improving braking without using a brake resistor. This function controls an over-magnetization of the motor
when the motor is acting as a generator. Increasing the electrical losses in the motor allows the OVC function to increase the braking
torque without exceeding the overvoltage limit.
NOTICE
AC brake is not as effective as dynamic braking with a resistor.
Overvoltage control (OVC)
By automatically extending the ramp-down time, OVC reduces the risk of the drive tripping due to an overvoltage on the DC link.
NOTICE
Do not enable OVC in hoisting applications.
Ground fault
The drive provides low-impedance ground fault protection and high-impedance ground fault protection due to configuration of current
transducer on the output phases. Depending on whether the LCP parameter 0-06 GridType is set to IT grid or not, different protection
algorithms are executed.
Ground fault is detected during start-up stage and on-the-fly stage. After receiving the protection signals, the drive is coasted.
3.2.4 Missing Motor Phase Detection
The missing motor phase function (parameter 4-58 Missing Motor Phase Function) is enabled by default to avoid motor damage if a
motor phase is missing. The default setting is 1000 ms, but it can be adjusted for faster detection.
3.2.5 Supply Voltage Imbalance Detection
Operation under severe supply voltage imbalance reduces the lifetime of the motor and drive. If the motor is operated continuously near
nominal load, conditions are considered severe. The default setting trips the drive if there is supply voltage imbalance.
A fast trip/warning option is available by utilizing LCP parameters (parameter 14-17 Fast Mains Phase Loss Level and parameter 14-18
Fast Mains Phase Loss Min Power) when a quick response to mains imbalance is expected.
Danfoss A/S © 2024.01 AJ435824192086en-000101 / 130R1295 | 15
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