Output Contactor; Energy Efficiency - Danfoss VLT Refrigeration Drive FC 103 Design Manual

System Integration
internal measurements. The ETR measures actual
current, speed, and time to calculate motor
temperature and protect the motor from being
overheated by issuing a warning or cutting power
to the motor. The characteristics of the ETR are
shown in Illustration 3.22.
3 3
t [s]
2000
1000
600
500
400
300
200
100
60
50
40
30
20
10
1.0
Illustration 3.22 Electronic Thermal Relay Characteristics
The X-axis shows the ratio between I
nominal. The Y-axis shows the time in seconds before the
ETR cuts off and trips. The curves show the characteristic
nominal speed at twice the nominal speed and at 0.2 x the
nominal speed.
At lower speed, the ETR cuts off at lower heat due to less
cooling of the motor. In that way, the motor is protected
from overheating even at low speed. The ETR feature
calculates the motor temperature based on actual current
and speed.

3.5.8 Output Contactor

Although not generally a recommended practice, operating
an output contactor between the motor and the frequency
converter does not damage the frequency converter.
Closing a previously opened output contactor may connect
a running frequency converter to a stopped motor. This
may cause the frequency converter to trip and show a
fault.

3.5.9 Energy Efficiency

Efficiency of the frequency converter
The load on the frequency converter has little effect on its
efficiency.
This also means that the frequency converter efficiency
does not change when other U/f characteristics are
selected. However, the U/f characteristics do influence the
efficiency of the motor.
The efficiency declines a little when the switching
frequency is set to a value above 5 kHz. The efficiency is
58
f OUT = 1 x f M,N (par. 1-23)
f OUT = 2 x f M,N
f OUT = 0. 2 x f M,N
I
1.2 1.4 1.6 1.8 2.0
and I
motor
Danfoss A/S © 08/2015 All rights reserved.
®
VLT Refrigeration Drive FC 103
also slightly reduced when the motor cable is longer than
30 m.
Efficiency calculation
Calculate the efficiency of the frequency converter at
different loads based on Illustration 3.23. 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%
I
Illustration 3.23 Typical Efficiency Curves
M
(par. 1-24)
MN
Example: Assume a 55 kW, 380–480 V AC frequency
converter with 25% load at 50% speed. The graph is
showing 0.97 rated efficiency for a 55 kW frequency
motor
converter 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 frequency
converter depends on magnetising level. The efficiency of
the motor depends on the type of motor.
•
•
•
System efficiency
To calculate the system efficiency, multiply the efficiency of
the frequency converter by the efficiency of the motor.
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 controlled by the frequency converter
and when running directly on mains.
The influence from the U/f characteristic on small
motors is marginal. However, in motors from 11
kW and up, the efficiency advantage is significant.
The switching frequency does not affect the
efficiency of small motors. Motors from 11 kW
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.
150% 200%
25% load
MG16G202