•
•
•
•
6.1.3 Overvoltage Protection
Motor-generated overvoltage
The DC-link voltage is increased when the motor acts as a generator. This occurs in the following situations:
The load drives the motor (at constant output frequency from the drive), that is, the load generates energy.
During deceleration (ramp-down) if the moment inertia is high, the friction is low, and the ramp-down time is too short for the
energy to be dissipated as a loss in the drive, the motor, and the installation.
Incorrect slip compensation setting may cause higher DC-link voltage.
Back EMF from PM motor operation. If coasted at high RPM, the PM motor back EMF may potentially exceed the maximum
voltage tolerance of the drive and cause damage. To help prevent this, the value of
parameter 4-19 Max Output Frequency
is
automatically limited based on an internal calculation. This calculation is 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
.
N O T I C E
To avoid that the motor overspeeds (for example, due to excessive windmilling effects), equip the drive with a brake resistor.
The overvoltage can be handled either via using a brake function (
parameter 2-10 Brake Function
) and/or using overvoltage control
(
parameter 2-17 Over-voltage Control
).
Brake functions
Connect a brake resistor to dissipate surplus brake energy. Connecting a brake resistor allows a higher DC-link voltage during brak-
ing. AC brake is an alternative to improve breaking without using a brake resistor. This function controls an overmagnetization of
the motor when running regenerative, which can improve the OVC. Increasing the electrical losses in the motor allows the OVC
function to increase the breaking torque without exceeding the overvoltage limit.
N O T I C E
An AC brake is not as efficient as dynamic braking with a resistor and should not be used on frequently repeated braking applica-
tions as it may overheat the motor.
Overvoltage control (OVC)
OVC reduces the risk of the drive tripping due to an overvoltage on the DC link. This is managed by automatically extending the
ramp-down time.
N O T I C E
OVC can be activated for PM motors with control core, PM VVC
+
, Flux open-loop control, and Flux closed-loop control.
N O T I C E
LOSS OF HOIST CONTROL
Do not enable OVC in hoisting applications. If OVC is used with a hoist, the OVC will try to regulate the DC bus by spinning the
motor faster, resulting in loss of hoisting control and/or damage to the hoist.
6.1.4 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 a faster detection.
6.1.5 Mains Phase Imbalance Detection
Operation under severe mains imbalance conditions reduces the lifetime of the drive. Conditions are considered severe if the motor
is operated continuously near nominal load. The default setting trips the drive if mains imbalance occurs (
parameter 14-12 Function
at Mains Imbalance
).
6.1.6 Service Switch on the Output
Adding a service switch to the output between the motor and the drive is allowed. When
parameter 4-58 Missing Motor Phase Func-
tion
is set to
[5] Motor Check
, the drive automatically detects when the motor is disconnected. The drive then issues
Warning 3, No
Motor
and resumes operation when the motor is connected again. Danfoss recommends not to use this feature for 525–690 V drives
connected to an IT mains network.
AJ300847815559en-000101 / 130R0337
38 | Danfoss A/S © 2020.09
Product Features
VLT® AQUA Drive FC 202
Design Guide