BEHLMAN BL10000 Series, User'S Manual And Technical Reference Manual

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16
T10 &T10D UNITS ( continued )
For the reasons stated above, the output voltmeter accuracy and voltage regulation of these units will be
degraded by this option. All specifications apply only when measured at the output terminals of the control
chassis. If higher accuracy is required external metering should be considered.
It is recommended that the output be set to zero before switching ranges on T10D units. This will prevent
nuisance tripping of protective circuits in the power chassis.
UNITS WITH MT ( MOTOR TESTER ) OPTION
All BL series units provided with the MT option , require special operating considerations. These units are
equipped with “ oversized” output devices to allow higher than rated output current for short periods of time. This
is done to allow the starting of high in rush loads such as motors , contactors, and switching power supplies.
Current limiting in the BL is provided by two methods. Protection from a shorted output is provided by fast
current limiting . This involves sensing of the output switching transistor’s saturation voltage and shutting down
the drive circuit should this voltage exceed a safe value ( as under a shorted output ). This circuit responds
within tens of microseconds. The current at which this occurs depends on many internal and external factors.
Some of these are:
1. Resistance of short circuit
2. Inductance of output wiring
3. Frequency of output current
4. Size of output transistor ( smaller devices will trigger at lower currents )
5. Point in the AC waveform where the short occurs ( highest current at max peak)
A second current limit circuit responds to the long term ( a few hundred milliseconds ) current. When the preset
current limit trip point is reached, the output voltage will decrease to maintain a constant current and sinusoidal
output. This condition will exist until the overload is removed. This is a very useful feature for testing “Locked
Rotor” motor current as well as other special loads.
Units with the MT option extend the amount of “fast” current limit by increasing the size of the output transistors
utilized. This fact is transparent to the user and requires no special attention. The “slow” current limit circuit is
modified to provide a higher trip value than standard units. This is done to keep the supply from jumping into
the “constant current” mode during inrush testing.
IMPORTANT !! It is possible to overheat the output
devices in an MT option unit if it is allowed to remain in current limit ( over 110 % rated ) for extended
periods
. This unit has no means for limiting the amount of time it will operate in the constant current mode. If
the equipment is to be left unattended, a method for limiting constant current operation should be provided.
Typically the unit may be run in the constant current mode for several minutes with no ill effects. This time varies
with the ambient temperature and duty cycle. Questions regarding MT units and safe operation in the constant
current mode should be referred to the factory. All other BL series units may be run in the constant current
mode indefinitely.