Brake Resistor Cabling; Brake Resistor And Brake Igbt; Energy Efficiency - Danfoss VLT AQUA Drive FC 202 Design Manual

System Integration

3.4.13 Brake Resistor Cabling

EMC (twisted cables/shielding)
To meet the specified EMC performance of the adjustable
frequency drive, use shielded cables/wires. If non-shielded
wires are used, it is recommended to twist the wires to
reduce the electrical noise from the wires between the
brake resistor and the adjustable frequency drive.
For enhanced EMC performance, use a metal shield.

3.4.14 Brake Resistor and Brake IGBT

Brake resistor power monitor
In addition, the braking energy monitor function makes it
possible to read out the momentary power and the mean
power for a selected time period. The brake can also
monitor the power energizing and make sure it does not
exceed a limit selected in 2-12 Brake Power Limit (kW). In
2-13 Brake Power Monitoring, select the function to carry
out when the power transmitted to the brake resistor
exceeds the limit set in 2-12 Brake Power Limit (kW).
NOTICE!
Monitoring the braking energy does not fulfill a safety
function. The brake resistor circuit is not ground leakage
protected.
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. Use a
relay or digital output to protect the brake resistor against
overloading in the event of a fault in the adjustable
frequency drive.
Overvoltage control (OVC) can be selected as an alternative
brake function in 2-17 Over-voltage Control. If the DC link
voltage increases, this function is active for all units. The
function ensures that a trip can be avoided. This is done
by increasing the output frequency to limit the voltage
from the DC link. It is a useful function, e.g., if the ramp-
down time is too short since tripping of the adjustable
frequency drive is avoided. In this situation, the ramp-
down time is extended.

3.4.15 Energy Efficiency

Efficiency of the adjustable frequency drive
The load on the adjustable frequency drive has little effect
on its efficiency.
This also means that the adjustable frequency drive
efficiency does not change when other U/f characteristics
are selected. However, the U/f characteristics do influence
the efficiency of the motor.
MG20N622
Design Guide
The efficiency declines a little when the switching
frequency is set to a value above 5 kHz. The efficiency is
also slightly reduced when the motor cable is longer than
30 m (100 ft).
Efficiency calculation
Calculate the efficiency of the adjustable frequency drive at
different loads based on Figure 3.19. Multiply the factor in
this graph with the specific efficiency factor listed in
chapter 7.1 Electrical Data.
1.01
1.0
0.99
0.98
0.97
0.96
0.95
0.94
0.93
0.92
0%
Figure 3.19 Typical Efficiency Curves
Example: Assume a 55 kW (75 hp), 380–480 V AC
adjustable frequency drive with 25% load at 50% speed.
The graph is showing 0.97 rated efficiency for a 55 kW (75
hp) adjustable frequency drive is 0.98. The actual efficiency
is then: 0.97 x 0.98=0.95.
Motor efficiency
The efficiency of a motor connected to the adjustable
frequency drive depends on magnetizing level. The
efficiency of the motor depends on the type of motor.
•
•
•
System efficiency
To calculate the system efficiency, multiply the efficiency of
the adjustable frequency drive by the efficiency of the
motor.
Danfoss A/S © 09/2014 All rights reserved.
50% 100%
% Speed
100% load 75% load 50% load
In the range of 75–100% of the rated torque, the
efficiency of the motor is practically constant,
both when it is controlled by the adjustable
frequency drive, and when it runs directly on line
power.
The influence from the U/f characteristic on small
motors is marginal. However, in motors from 11
kW (15 hp) and up, the efficiency advantage is
significant.
The switching frequency does not affect the
efficiency of small motors. Motors from 11 kW (15
hp) and up have their efficiency improved 1–2%.
This is because the sine-shape of the motor
current is almost perfect at high switching
frequency.
3
150% 200%
25% load
63
3