Danfoss VLT HVAC Drive FC 100 Series Design Manual page 44

2. Introduction to VLT HVAC Drive
NB!
Check that the brake resistor can cope with a voltage of 410 V, 820 V or 975 V - unless Danfoss brake resistors are used.
2
Danfoss recommends the brake resistance R
) of 110%. The formula can be written as:
(%)
η is typically at 0.90
motor
For 200 V, 480 V and 600 V frequency converters, R
107780
200 V : R rec =
P motor
375300
V R rec
480 : =
P motor
630137
600 V : R rec =
P motor
1) For frequency converters ≤ 7.5 kW shaft output
2) For frequency converters > 7.5 kW shaft output
NB!
The resistor brake circuit resistance selected should not be higher than that recommended by Danfoss. If a brake resistor with a higher
ohmic value is selected, the braking torque may not be achieved because there is a risk that the frequency converter cuts out for safety
reasons.
NB!
If a short circuit in the brake transistor occurs, power dissipation in the brake resistor is only prevented by using a mains switch or
contactor to disconnect the mains for the frequency converter. (The contactor can be controlled by the frequency converter).
NB!
Do not touch the brake resistor as it can get very hot while/after braking.
2.12.3. Control with Brake Function
The brake is to limit the voltage in the intermediate circuit when the motor acts as a generator. This occurs, for example, when the load drives the motor
and the power accumulates on the DC link. The brake is built up as a chopper circuit with the connection of an external brake resistor.
Placing the brake resistor externally offers the following advantages:
- The brake resistor can be selected on the basis of the application in question.
- The brake energy can be dissipated outside the control panel, i.e. where the energy can be utilized.
- The electronics of the frequency converter will not be overheated if the brake resistor is overloaded.
The brake is protected against short-circuiting of the brake resistor, and the brake transistor is monitored to ensure that short-circuiting of the transistor
is detected. A relay/digital output can be used for protecting the brake resistor against overloading in connection with a fault in the frequency converter.
In addition, the brake makes it possible to read out the momentary power and the mean power for the latest 120 seconds. The brake can also monitor
the power energizing and make sure it does not exceed a limit selected in par. 2-12. In par. 2-13, select the function to carry out when the power
transmitted to the brake resistor exceeds the limit set in par. 2-12.
44
, i.e. one that guarantees that the frequency converter is able to brake at the highest braking torque (M
rec
at 160% braking torque is written as:
rec
Ω
Ω 1 )
Ω
MG.11.B7.02 - VLT
2
U dc
R rec
Ω =
P motor x M br
η is typically at 0.98
428914
Ω 2 )
V R rec
480 : =
P motor
® is a registered Danfoss trademark
®
VLT HVAC Drive Design Guide
br
x
100
x η x η
( % ) motor