Vanguard Instruments LTCA-10, User Manual

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LTCA-10/LTCA-40 USER’S MANUAL
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1.0 INTRODUCTION
1.1 General Description
The Vanguard LTCA-10 and LTCA-40 are microprocessor-controlled winding resistance meters.
These devices are designed to accurately measure large transformer winding resistance, motor
winding resistance, or the resistance of large inductive devices. This manual addresses the
LTCA-10 and LTCA-40 as one device. Any differences between the LTCA-10 and LTCA-40 are
addressed separately.
The LTCA-10 uses a 60Vdc/10 Amperes Direct Current (DC) power supply while the LTCA-40
uses a 60VDC/40 Amperes DC power supply. Both devices are capable of reading winding
resistances ranging from 1 micro-ohm to 500 ohms. Three voltage sensing channels allow the
LTCA-10/40 to read three resistance values in the same test. To ensure operator safety, the
LTCA-10/40 automatically discharges the stored energy in the transformer at the end of each
test.
The LTCA-10/40 has one resistance reading channel (V1) dedicated for dynamic resistance
testing. The dynamic resistance test feature can be used to monitor the transformer LTC or
Voltage Regulator contact resistance while changing tap positions. A resistance plot shows the
transformer LTC contact and winding resistance and can be used to detect LTC/Voltage
Regulator contact problems.
The transformer LTC or Voltage Regulator AC motor current can also be monitored by the
LTCA-10/40 using an AC clamp on the current sensor. This feature can be used to monitor the
LTC motor “On-Time” at each tap change operation.
The LTCA-10/40 is rugged, portable, and easy to use, requiring little training for first-time users.
A 16-key membrane keypad is used to control the unit. The LTCA-10/40 features a back-lit LCD
screen (64 x 128 dot graphic) that is viewable in bright daylight as well as low-light conditions.
A built-in thermal printer can print results on 4.5-inch wide thermal paper.
The LTCA-10/40 can store 128 static test records (48 tests per record) and 11 dynamic resistance
test records in Flash EEPROM. Test records can be recalled locally or transferred to a PC via the
available interfaces (RS-232C port, USB port, USB Flash drive port). A built-in USB Flash drive
interface provides a convenient method for transferring test records to or from a USB Flash
drive. Test records can also be transferred directly to a PC via the RS-232C or USB interface
ports. If using a USB Flash drive, test records stored in the LTCA-10/40’s internal memory can
be transferred to the drive, and then the supplied PC software can be used to view the test records
stored on the drive. Up to 999 test records can be stored on a USB Flash drive.
1.2 Functional Description
The LTCA-10/40’s operation is based on the electrical relationship described by Ohm’s law:
R=V/I, where I is a known current and V is the DC voltage measured across the unknown
resistance. The value of the unknown resistance is calculated by dividing the measured voltage
by the current which is calculated by the microprocessor. Calculated resistance readings are then
displayed on the unit’s back-lit LCD screen.
A special current source allows the LTCA-10 to output 1, 5 and 10 Amperes of test current. The
LTCA-40 can output 1, 5, 10 or 40 Amperes of test current. For added safety, the current source