Motor Thermal Protection; Output Contactor; Brake Functions; Dynamic Braking - Danfoss VLT AQUA Drive FC 202 Design Manual

System Integration

3.4.8 Motor Thermal Protection

The adjustable frequency drive provides motor thermal
protection in several ways:
•
Torque limit protects the motor from overload
independent of the speed.
•
Minimum speed limits the minimum operating
speed range, for instance between 30 and 50/60
Hz.
•
Maximum speed limits the maximum output
speed.
•
Input is available for an external thermistor.
•
Electronic thermal relay (ETR) for asynchronous
motors simulates a bi-metal relay based on
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 Figure 3.17.
t [s]
2000
1000
600
500
400
300
200
100
60
50
40
30
20
10
1.0 1.2 1.4 1.6
Figure 3.17 Electronic Thermal Relay Characteristics
The X-axis shows the ratio between I
nominal. The Y-axis shows the time in seconds before the
ETR cut-off and trip. 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.
MG20N622
Design Guide
fOUT = 1 x f M,N
fOUT = 2 x f M,N
fOUT = 0.2 x f M,N
IM
1.8 2.0 IMN
and I
motor motor
Danfoss A/S © 09/2014 All rights reserved.

3.4.9 Output Contactor

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

3.4.10 Brake Functions

To brake the load on the motor shaft, use either a static
(mechanical) or dynamic brake.

3.4.11 Dynamic Braking

Dynamic brake is established by the following:
•
Resistor brake: A brake IGBT keeps the
overvoltage below a prescribed threshold by
directing brake energy from the motor to the
brake resistor.
•
AC brake: The brake energy is distributed in the
motor by changing the loss conditions in the
motor. The AC brake function cannot be used in
applications with high cycling frequency since
this will overheat the motor.
•
DC brake: An over-modulated DC current added
to the AC current works as an eddy-current brake.

3.4.12 Brake Resistor Calculation

A brake resistor is required to manage heat dissipation and
DC link voltage increase during electrically-generated
braking. Using a brake resistor ensures that the energy is
absorbed in the brake resistor and not in the adjustable
frequency drive. For more information, see the Brake
Resistor Design Guide.
Duty cycle calculation
When the amount of kinetic energy transferred to the
resistor in each braking period is unknown, calculate the
average power on the basis of the cycle time and braking
time (known as the intermittent duty cycle). The resistor
intermittent duty cycle is an indication of the cycle when
the resistor is active (see Figure 3.18). Motor suppliers often
use S5 when stating the permissible load, which is an
expression of intermittent duty cycle.
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