Lenze MCH Series - Manual (Page 55)

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14 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 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
LIMIT is left at the maximum setting, which is 120% of the drive’s output current
rating. Regardless of the CURRENT LIMIT setting, the drive is capable of delivering
a maximum of 120% current for one minute before tripping into an OVERLOAD fault.
Refer to Parameter 17 – MOTOR OVRLOAD.
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 setpoint. However, if FREQUENCY
FOLDBACK cannot correct the condition and the drive remains in current limit for too
long, it will trip on an OVERLOAD fault. If the drive enters current limit while accelerating,
the time required to reach the speed setpoint will be longer than the time programmed
into ACCEL RATE (Parameter 8).
17
MOTOR OVRLOAD
The MCH 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 120% 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 120% of the MOTOR current rating for one minute. If this
parameter is set to 100%, the motor will be allowed to draw 120% 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 5.8 amps
(120% of 4.8 amps) for one minute. If the setting is left at 100%, the
motor could draw 9.1 amps (120% of 7.6 amps) for one minute before
faulting.