*
M’Ax Installation Guide
9
Issue Number: 6
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2.15
Braking resistor precautions
Electric shock risk
The voltages present on the braking resistor, its
associated components and terminals on the drive are
capable of inflicting a severe electric shock and may be
lethal.
Thermal overload protection
When an external braking resistor is used, it is essential
that a thermal overload protection device is incorporated
in the braking-resistor circuit in order to minimise the
risk of fire in the event of unexpectedly high current, or
loss of control of the braking circuit. A typical protection
circuit is shown in the following section, Thermal
protection of the braking resistor.
2.16
Thermal protection of the braking
resistor
High temperatures
Braking resistors can attain high temperatures and
should be segregated from temperature-sensitive
equipment and personnel.
When a braking resistor is to be used, ensure the following:
•
Include a lock-out circuit that will prevent the AC supply from being
re-connected to the drive until the cause of a trip has been fully
investigated.
•
An external braking resistor should be capable of tolerating thermal
shock; pulse rated resistors are recommended.
•
It is essential that the instantaneous and average power ratings of
the braking resistor are sufficient for the most extreme braking duty
that is likely to be encountered. If the internal braking resistor is
overloaded, the drive will trip (trip code: It.br).
•
When an external braking resistor is mounted inside the enclosure,
or the internal braking resistor is used, the heat dissipated by the
resistor will increase the ambient temperature inside the enclosure.
(The value of heat dissipation is used for calculating the enclosure
size or ventilation which are described later in this chapter.)
•
Always use shielded or steel wire armoured cable for connecting an
external braking resistor.
When an external braking resistor is used, a thermal-protection circuit
must be added. This must disconnect the AC supply from the drive if the
braking resistor becomes overloaded. For guidance, Figure 2-4 shows a
typical circuit arrangement (complete circuit diagrams for the power
connections appear later in this chapter).
When the internal braking resistor is used, a thermal-protection circuit is
not required since thermal-modelling in the drive causes the drive to trip
if the resistor becomes overloaded (trip code: It.br); also, the braking
resistor itself is fail-safe.
Figure 2-4
Typical protection circuit for an external braking
resistor
1. START/RESET switch (momentary)
2. STOP switch (latching)
3. Control-circuit supply
4. Contactor coil
5. Thermal overload protection relay
6. External braking resistor
7. 380 ~ 480V AC supply to the drive
8. Drive power connectors.
2.17
Braking-resistor example
calculations
Conditions
Model: M’Ax 409
Maximum peak output current (Ipk) from the drive (for 2 seconds
maximum): 19A
Full-load speed (n) of motor: 4000 RPM
Continuous stall torque (T
CS
) of motor: 12.2 Nm
Motor K
T
= 1.6 Nm/A
Motor inertia (J
M
): 3.43 x 10
-3
kg m
2
Load inertia (J
L
): 10.29 x 10
-3
kg m
2
Total inertia (J
T
= J
M
+ J
L
): 13.7 x 10
-3
kg m
2
Required deceleration time (t
d
) from full to zero speed: 0.5 second
Repeat cycle time for deceleration (t
r
): 7 seconds
Minimum permissible braking-resistor value: 40
Ω
Operating voltage (V
R
) at switch on: 780V
WARNING
WARNING
WARNING
