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APEX615n Installation Guide
Current Foldback (I
2
T Limit)
The purpose of the current foldback circuit is to protect the motor from overheating due to
prolonged high currents. The eight switches of DIP Switch#2 are used to set the parameters for
the current foldback circuit. These parameters are:
• PEAK CURRENT—the highest current that the APEX615n will produce. Peak
current can be set between 6.5A and 16.0 A for the APEX6151, between 9.0A and
24.0A for the APEX6152, and between 15A and 40A for the APEX6154..
• CONTINUOUS CURRENT—the APEX615n reduces its current to this level when
it goes into current foldback. Continuous current can be set between 1.8A and 8.0A,
between 3.0A and 12.0A for the APEX6152, and between 5A and 20A for the
APEX6154..
• TIME CONSTANT—the motor’s thermal time constant, which is a physical
parameter usually specified by the motor’s manufacturer. The time constant can be set
between 10 minutes and 40 minutes on the APEX615n.
The APEX615n uses an internal circuit to model the motor’s thermal behavior, and predict
motor temperature. Heat dissipated in the motor’s windings is directly proportional to I
2
, the
square of the motor current, and the length of time the current flows.
The APEX615n monitors motor current, and uses its internal microprocessor to simulate a
capacitor being charged by the motor current. The result is a number, similar to voltage on a
capacitor, that represents an average, over time, of the motor’s temperature.
The following equation gives an approximate time before foldback occurs, for a motor that
operates from a cold start, when I
actual
> I
continuous
.
time
(minutes)
= Time Constant − ln 1−
I
continuous
I
actual
2
Three variables affect this equation:
• I
continuous
is the continuous current (set by DIP switches)
• Time Constant is the motor’s time constant (set by DIP switches)
• I
actual
is the current that actually flows in the motor. It can be as low as Ø amps, or
as high as the peak current (which was set by DIP switches).
The shortest time until foldback occurs will be when I
actual
= I
peak
. Notice that this can be
much shorter than the time constant in the equation above.
When current foldback occurs, the APEX615n clamps its output current at the I
continuous
level,
and illuminates the LED labeled I
2
T Limit, located on the drive’s front panel. The drive does
not put out a fault signal on its fault output. However, because torque will be reduced as a
result of the lower motor current, excess position or following error may result.
To recover from current foldback, there are three options:
• WAIT—allow a period of time to pass for the motor to cool. Usually, several minutes
will be required.
• REDUCE COMMAND INPUT—lower the commanded current to a level below
continuous current. This will bleed off the voltage on the simulated capacitor, and clear
the foldback condition.
• RESET—Pull the RESET input low (the RESET input is located on the APEX615n’s
DRIVE AUXILIARY connector). An alternative is to issue the DRESET command or cycle
power on Control L1/L2. This will reset the drive, and clear the foldback condition.
However, this method is not recommended if the motor is actually hot, because the
motor temperature information in the controller will be lost. The motor should be
allowed to cool before the APEX615n is reset, and operations continue. (RESET input
and DRESET command resets drive functions; RESET command resets controller.
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