Unidrive M / HS Frame 7 to 10 Power Installation Guide 73
Safety information Product information Mechanical installation
Electrical installation
Technical data UL listing information
resistance. Larger resistance values may give a cost saving, as well as a safety benefit in the event
of a fault in the braking system. Braking capability will then be reduced, which could cause the drive
to trip during braking if the value chosen is too large.
Thermal protection circuit for the braking resistor
The thermal protection circuit must disconnect the AC supply from the drive if the resistor becomes
overloaded due to a fault. Figure 4-11 shows a typical circuit arrangement.
Figure 4-11 Typical protection circuit for a braking resistor
See Figure 4-1 on page 55, Figure 4-2 on page 56 and Figure 4-3 on page 57 for the location of the
+DC and braking resistor connections.
4.9.2 Braking resistor software overload protection
The drive software contains an overload protection function for a braking resistor. In order to enable
and set-up this function, it is necessary to enter three values into the drive:
• Braking Resistor Rated Power (10.030)
• Braking Resistor Thermal Time Constant (10.031)
• Braking Resistor Resistance (10.061)
This data should be obtained from the manufacturer of the braking resistors. The braking resistor
thermal time constant can be calculated from resistor data sheet values using the following equation:
Pr 10.039 gives an indication of braking resistor temperature based on a simple thermal model. Zero
indicates the resistor is close to ambient and 100 % is the maximum temperature the resistor can
withstand. A ‘Brake Resistor’ alarm is given if this parameter is above 75 % and the braking IGBT is
active. A Brake R Too Hot trip will occur if Pr 10.039 reaches 100 %, when Pr 10.037 is set to 0
(default value) or 1.
If Pr 10.037 is equal to 2 or 3, a Brake R Too Hot trip will not occur when Pr 10.039 reaches 100 %,
but instead the braking IGBT will be disabled until Pr 10.039 falls below 95 %. This option is intended
for applications with parallel connected DC buses where there are several braking resistors, each of
which cannot withstand full DC bus voltage continuously. With this type of application it is unlikely the
braking energy will be shared equally between the resistors because of voltage measurement
tolerances within the individual drives. Therefore with Pr 10.037 set to 2 or 3, then as soon as a
resistor has reached its maximum temperature the drive will disable the braking IGBT, and another
resistor on another drive will take up the braking energy. Once Pr 10.039 has fallen below 95 % the
drive will allow the braking IGBT to operate again.
See the Parameter Reference Guide for more information on Pr 10.030, Pr 10.031, Pr 10.037 and
Pr 10.039.
Optional
RFI filter
Stop
Start /
Reset
Thermal
protection
device
Braking resistor
Drive
Main contactor
power supply
+DC
BR
Pr 10.031
Resistor pulse power rating x Braking time
Resistor continuous power rating
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