Caterpillar — Transformer Polarity Verification
Testing & diagnosis · Isolation / potential transformers
From a Caterpillar service manual covering: APM 300, SR4, VR3. Check it applies to your exact model before use.
Applies to: Caterpillar SR4
Before you start
- ct = Center Tap - Do Not Use - Tape Loose End
- Transformers must be identical (same stock number) and polarized
Procedure
- Select a transformer and call it Trans #1.
- Identify the primary winding (115 V).
- Arbitrarily select one primary winding lead and mark it H1; mark the other primary lead H2.
- Arbitrarily select one secondary lead and mark it X1; mark the other X2; if a center tap exists, tape the end.
- Make the connections for the polarity test.
- Apply 115 V AC across H1 and H2 and measure this applied voltage.
- Measure voltage of Trans #1 from X1 to X2; this will be approximately 12 V AC.
- Measure voltage from H2 to X1; if equal to the sum of H1-H2 and X1-X2 voltages, polarity is correct; if the difference, reverse X1 and X2 markings.
- Select a second transformer, Trans #2, with the same stock number, and repeat Steps 2, 3 and 4.
- Connect the H1 leads together and the H2 leads together; connect the X2 lead of Trans #1 to the X1 lead of Trans #2.
- Apply 115 V AC across H1 and H2; measure voltage of Trans #2 from X1 to X2 (approximately 12 V AC).
- Measure voltage from Trans #1, X1 to Trans #2, X2; 0 V AC means incorrect polarity marking, 24 V AC means polarity is correct.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Approximately 12 V AC secondary
- 24 V AC indicates correct polarity
- 0 V AC indicates incorrect polarity marking
Source: Caterpillar service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.