Mitsubishi Electric FR-A700 Series, Technical Manual

Page: 455 / 550

489
SELECTION
S
E
LECTION
3
3.1.6 Deceleration characteristic of the inverter
(common)
To shorten the acceleration time, the torque boosts is
raised or the inverter capacity or motor capacity is
increased. To shorten the deceleration time, add a brake
unit or increase the capacity of the built-in brake (increase
the inverter capacity).
When the motor is decelerated by the inverter, the inverter
gradually lowers the output frequency at a slope of the set
deceleration time.
If you try to decelerate the motor in a shorter time than
when it is coasted to a stop, the motor runs faster than the
synchronous speed of the given frequency. Hence, the
motor acts as an induction generator and its rotating
energy is partly consumed in the motor winding and partly
accumulated in the capacity inside the inverter.
This energy is consumed by the discharge resistor. As a
result, it provides braking force for the motor.
[Built-in brake resistor protection]
Any inverter of 7.5K or less is provided on its rear surface
with a discharge resistor for regenerative braking. When
the brake resistor is used up to its rating, the temperature
rises considerably. The inverter has a protective function to
protect this resistor against use beyond its rating.
When this protective function is activated, the built-in brake
circuit shut-off to disable the braking action. Therefore, the
terminal voltage of the capacity rises to activate the
regenerative overvoltage shut-off (OV1 to OV3). Since this
protective function returns to the initial state when the
inverter is reset, the brake resistor is regarded as cold at
the time of restart. Be cautious that operation performed by
repeating reset will cause the brake resistor to overheat.
• Technical note No.30 : Capacity selection II (data part)
• Technical note No.31: Capacity selection II (selection
part)
(1) Built-in brake resistor
The built-in brake can provide 100 to 150% of the
rated motor torque but must be used within 5s (duty:
less than 2 to 3%ED). Select the inverter which
satisfies the conditions after finding the permissible
duty time, as in the
Technical note No.30 (data part).
When a higher brake duty is required, select the
external brake resistor or brake unit in accordance
with paragraphs 2) or 4). In this case, disconnect the
jumper from the built-in brake resistor.
1) The maximum braking torque (% relative to the
motor output used with the inverter) value of the
built-in brake resistor is shown on page 2. When
the braking torque is required is above this value,
the inverter capacity must be increased. The
addition of the external brake resistor does not
increase the braking torque.
2) The regenerative braking duty ([ Pr. 70 ]) value of
the built-in brake resistor is indicated on page
331. If the duty required is above this value, use
the external brake resistor (FR-ABR type) or
brake unit (BU type, FR-BU type).
(2) External brake resistor
When the inverter is used in excess of the maximum
duty of the built-in brake resistor, the external brake
resistor may be installed for 0.4K to 22K. In this case
for
, the jumper (across terminal PR and PX)
on the built-in brake resistor must be disconnected.
(0.4 to 7.5kW)
(3) DC injection brake
The DC injection brake is applied during
deceleration. Since the braking torque generated by
the DC injection brake depends also on the winding
resistance of the motor, it cannot be determined
uniformly, but an average of more than 50% braking
torque is provided in the factory-set state.
In addition, the setting of the DC injection brake time
([
Pr. 11 ]), DC injection brake voltage ([ Pr. 12 ]), and DC
injection brake frequency ([ Pr. 10 ]) allows the
positioning accuracy to be adjusted according to the
load. (Refer to page 328 for details)
(4) Selection of brake unit
The brake unit is used when the brake capability
required is greater than that of the built-in brake
resistor and external brake resistor. In this case, the
capability of the built-in brake resistor is not added to
that of the brake unit.
To select the brake unit, determine the required
deceleration pattern as shown below.
Deceleration pattern
common
* The brake discharge resistor on the rear surface of the
inverter unit will rise to a high temperature. Carefully
select the installation place of the inverter.
For full information on how to select the brake resistor,
brake unit and other braking units, refer to any of the
following technical notes according to the operation
pattern:
A700
A700
* When further larger braking capability is required, fit
the power supply regeneration common converter
(FR-CV)
* The 30K or more and 0.2K
or less can not use an external brake resistor.
A700 E700 D700
A700
A700 E700 D700
common
common
TB: Brake torque (N m)
TL: Load torque (N m)
TM: Rated motor torque (N m)
JT: All inertia
Speed
(r/min)
N
1
N
2
t
Time (s)