LEESON Micro Series Compact Inverters - Manual (Page 58)

14 DYN BRAK (DYNAMIC BRAKE)
This parameter enables the dynamic braking circuit. Set this parameter to ON only if the optional
dynamic braking circuit board and resistors are installed.
Dynamic braking is used in applications where high-inertia loads need to be decelerated quickly. When
this is attempted, the motor regenerates voltage back into the drive, causing the DC bus voltage to rise,
eventually resulting in a HI VOLTS fault. With the dynamic braking option, the DC bus voltage is
monitored, and when it reaches a certain level, a transistor is switched on that connects an external resistor
bank across the DC bus. This allows the regenerated energy from the motor to be dissipated through the
resistors as heat, which keeps the DC bus voltage below the trip level.
16 CURRENT (CURRENT LIMIT)
This parameter sets the maximum allowable output current of the drive, which also determines the torque
capability of the motor. For most applications, CURRENT is left at the maximum setting, which is 150%
or 180% (of the drive’s output current rating), depending on whether the input voltage to the drive is low
or high (see Parameter 0 - LINE VOLTS). Regardless of the CURRENT setting, the drive is capable of
delivering a maximum of 150% current for one minute, and 180% current for approximately 30 seconds,
before tripping into an OVERLOAD fault. See Parameter 17 – MOTOR OL.
The drive will enter current limit when the load demands more current than the drive can deliver, which
results in a loss of synchronization between the drive and the motor. To correct this condition, the drive
will enter FREQUENCY FOLDBACK, which commands the drive to decelerate in order to reduce the
output current and regain synchronization with the motor. When the overcurrent condition passes, the
drive will return to normal operation and accelerate back to the speed set point. However, if FREQUENCY
FOLDBACK cannot correct the condition and the drive remains in current limit for too long, it will trip
into an OVERLOAD fault. If the drive enters current limit while accelerating, the time required to reach
the speed set point will be longer than the time programmed into ACCEL (Parameter 8).
17 MOTOR OL (MOTOR OVERLOAD)
The MICRO Series is UL approved for solid state motor overload protection . Therefore, a separate thermal
overload relay is not required for single motor applications. The MOTOR OVERLOAD circuit is used
to protect the motor from overheating due to excessive current draw. The trip time for the MOTOR
OVERLOAD setting is based on what is known as an “inverse I2t” function. This function allows the
drive to deliver 150% of the rated output current for one minute, and even higher current levels for shorter
periods of time. Once the overload circuit “times out”, the drive will trip into an OVERLOAD fault.
The MOTOR OVERLOAD should be set to a value which is equal to the ratio (in percentage) of the motor
full load current rating to the drive output current rating . This will result in an overload capacity of 150%
of the MOTOR current rating for one minute . If this parameter is set to 100%, the motor will be allowed
to draw 150% of the DRIVE output current rating for one minute . This distinction is important in cases
where the motor full load current rating is significantly less than the drive output current rating, such as
applications where the drive is oversized to meet torque requirements.
Example 1: A 5 Hp, 480 Vac drive is operating a 3 HP motor with a full load current rating of 4 .8 amps .
Divide the motor current rating by the drive output current rating: 4 .8 / 7 .6 = 63% . Entering this value
will allow continuous operation at 4 .8 amps, and will also allow the motor to draw 7 .2 amps (150% of 4 .8
amps) for one minute. If the setting is left at 100%, the motor could draw 11.4 amps (150% of 7.6 amps)
for one minute before tripping the drive .
The MC Series drive has two options for thermal overload protection . One depends on the speed of the
drive, and the other does not. The diagram below illustrates the difference between “speed compensated”
and “non-compensated” thermal overload protection.
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