This lesson covers understanding, measuring, and diagnosing transformers. Working on the primary side (480V) is Red risk and requires QEP status. The L2 scope here is limited to secondary-side measurement and diagnostic activities with QEP oversight.
If your diagnosis points to a primary winding fault, blown primary fuse, or any 480V-side work, stop and escalate to your QEP or foreman. Do not open the primary side as an L2 technician.
Understand transformer theory, read nameplates, measure secondary voltage, identify CPT secondary fuses, diagnose no-control-power conditions from the secondary side, and understand power supply operation. All energized work performed with appropriate PPE and QEP oversight.
Transformers are among the most common and most important devices in an industrial facility. Walk through any Motor Control Center (MCC) and you will find a Control Power Transformer (CPT) in nearly every bucket — stepping 480V down to 120VAC to power the START/STOP buttons, pilot lights, PLC inputs, and relay coils that control each motor.
Beyond the MCC, transformers appear in every DC power supply powering PLC racks and instrumentation, in variable frequency drives, in welding machines, and at the utility entrance where the service transformer steps distribution voltage down to usable levels.
Understanding how a transformer works enables you to:
Step-down transformer: 480V primary (orange, 4 turns shown) → 120V secondary (green, 2 turns shown). Magnetic flux (blue arrows) couples the windings through the iron core. Actual industrial CPTs have hundreds of turns per winding.
You should be comfortable with the following lessons before starting:
Key concepts needed: AC voltage and frequency, Ohm's Law, apparent vs. real power (VA vs. watts), and a basic understanding of magnetic fields around conductors.
A transformer operates on the principle of electromagnetic induction. When alternating current flows through the primary winding, it creates a continuously changing magnetic field in the iron core. That changing flux passes through the secondary winding and induces a voltage in it — with no direct electrical connection between the two windings.
This is called mutual inductance. The iron core provides a low-reluctance (easy) path for magnetic flux, coupling nearly all of the primary flux to the secondary.
Electromagnetic induction requires a changing magnetic field. A DC current creates a constant field — after the initial transient, flux is stable and no voltage is induced in the secondary winding. Faraday's Law: V = −N × dΦ/dt. If dΦ/dt = 0 (constant DC flux), then V = 0.
Applying DC to a transformer primary will cause it to overheat rapidly, because the only resistance limiting primary current is the low DC resistance of the winding itself — the inductive reactance (XL = 2πfL) that normally limits AC current drops to zero at 0 Hz.
The ratio of primary turns (Np) to secondary turns (Ns) determines how voltage is transformed. This is the fundamental transformer relationship:
Current behaves inversely — if voltage steps down, current steps up by the same ratio:
A transformer cannot create energy. For an ideal (100% efficient) transformer, apparent power in equals apparent power out:
This is why stepping voltage down means stepping current up by the same ratio. A 4:1 step-down transformer that receives 1A at 480V will deliver 4A at 120V — the same 480VA on both sides.
Real transformers are 95–99% efficient, losing some power as heat in the windings (copper losses: I²R) and core (iron losses: eddy currents and hysteresis). For field sizing and diagnostics, assume ideal transformer behavior — the efficiency difference is negligible at the L2 level.
A transformer has a 480V primary and a 4:1 turns ratio. What is the secondary voltage?
Vs = Vp × (Ns / Np) = 480 × (1/4) = 480 ÷ 4 = 120V
This is the most common industrial CPT — 480V primary, 120V secondary, 4:1 turns ratio.
The secondary of the transformer above draws 10A. What current flows in the primary? Verify the kVA balance.
Ip = Is × (Ns / Np) = 10 × (1/4) = 2.5A
Primary kVA = 480V × 2.5A = 1,200VA = 1.2 kVA
Secondary kVA = 120V × 10A = 1,200VA = 1.2 kVA ✓
Power is conserved — both sides show 1.2 kVA of apparent power.
A transformer must step 480V down to 24V for a solenoid valve circuit. The secondary draws 5A. Find the turns ratio, primary current, and kVA.
Turns ratio = Vp / Vs = 480 / 24 = 20:1
Primary current = Is × (Ns / Np) = 5 × (1/20) = 0.25A
kVA = 24V × 5A = 120VA = 0.12 kVA (a small transformer)
Verify: 480V × 0.25A = 120VA ✓
| Type | Description | Application |
|---|---|---|
| Step-down | Np > Ns; Vp > Vs. More primary turns than secondary. | Most industrial uses — 480→120V CPT, utility distribution, DC supply inputs |
| Step-up | Np < Ns; Vs > Vp. Fewer primary turns than secondary. | Generator output to transmission voltage; high-voltage testing equipment |
| Isolation | Np = Ns (1:1 turns ratio). No electrical connection between windings — galvanic isolation. Voltage ratio is 1:1. | Medical equipment (patient safety), sensitive instrumentation (breaks ground loops, reduces noise), GFCI-type protection |
| Autotransformer | Single winding with taps. Primary and secondary share winding conductor — there IS an electrical connection. Smaller and cheaper, but no isolation. | Reduced-voltage motor starters; buck-boost voltage adjustment (e.g., 480V→460V) |
| Control Power Transformer (CPT) | Small step-down, typically 480→120VAC. Often includes primary and secondary fuse clips built into the unit. | Every MCC bucket; panel control power for starters, PLCs, and instruments |
| Current Transformer (CT) | Ring or window-type; primary is the conductor passing through the ring (1 turn). Secondary produces a scaled-down current proportional to primary. Never open CT secondary under load. | Metering (kWh), protection relays, overload relay coils, power monitoring |
| Potential Transformer (PT) | Precision step-down for measuring high voltages safely. Scales thousands of volts to 120V or 69V for meters and relays. | Medium-voltage metering and protection (4.16kV systems and above) |
A current transformer secondary must never be open-circuited while the primary conductor carries current. With no secondary load, the primary magnetomotive force drives the core into saturation, producing extremely high voltages (potentially thousands of volts) at the secondary terminals — a lethal hazard. Always short the CT secondary before removing a burden or secondary lead.
Every transformer carries a nameplate with critical information. Being able to read and interpret it is an essential field skill.
| Field | What It Means | Why It Matters |
|---|---|---|
| kVA | Apparent power capacity — 75,000 VA maximum continuous load | Exceeding the kVA rating causes overheating and shortened life. Size the transformer to the load. |
| HV / LV Voltage | High voltage (primary) and low voltage (secondary) ratings. "120/240V" secondary means a center-tap provides two 120V circuits or one 240V circuit. | Must match facility voltage. Wrong voltage connection = wrong secondary output and potential damage. |
| Z% (Impedance) | Percentage of rated voltage needed to drive rated current through the transformer's internal impedance. Determines available fault current at the secondary. | Lower Z% = higher available fault current = larger AIC rating required on downstream overcurrent devices. Critical for system coordination studies. |
| Cooling Class | AN = Air Natural (convection cooled, no fan). ONAN = Oil Natural Air Natural (oil-immersed distribution transformers). | Never block ventilation on an AN transformer. Restricted airflow raises hot-spot temperature and shortens insulation life dramatically. |
| Temperature Rise | Maximum winding temperature rise above ambient (40°C standard). 80°C rise + 40°C ambient + 15°C hot-spot allowance = 135°C maximum hot-spot temperature. | Facilities with ambient above 40°C (e.g., un-cooled buildings in summer) require derating the transformer's kVA capacity. |
| Insulation Class | Class H = 180°C maximum winding temperature. Class F = 155°C. Class B = 130°C. | Insulation degrades exponentially above its rated class. Running hot drastically shortens life. |
| Taps | Alternate primary connections that adjust the effective turns ratio by ±2.5% or ±5%. | Used to compensate for off-nominal primary voltage. If facility runs 504V instead of 480V, use the +5% tap so the secondary sees the correct voltage. |
The impedance percentage determines how much short-circuit current the transformer can deliver to a bolted fault on the secondary:
kVA = 75 | Vs = 120V | Z% = 2.5%
Rated secondary current = 75,000 / 120 = 625A
Isc = 625A / 0.025 = 25,000A available fault current
Any overcurrent devices on this secondary must be rated for at least 25 kAIC interrupting capacity. This is why residential and light-commercial panels are commonly rated "25,000 AIC."
The CPT is the transformer an MCT encounters most often. In a standard MCC bucket, the CPT taps 480VAC from the motor power circuit (before the contactor) and produces 120VAC for the control circuit.
Simplified MCC bucket: CPT taps 480V before the contactor, steps down to 120V for the control circuit. The secondary fuse (2A FU) protects the CPT and all control wiring.
The CPT must handle all control loads simultaneously. Sum up the VA requirement of every device on the secondary:
| Control Device | Typical VA Draw |
|---|---|
| Contactor coil (NEMA 00–1, small) | 10–20VA |
| Contactor coil (NEMA 2–3, medium) | 30–60VA |
| Pilot light (LED) | 2–5VA |
| Pilot light (incandescent) | 7–15VA |
| Timer relay coil | 10–25VA |
| Control relay coil | 5–15VA |
| PLC digital input card (per card) | Per mfr. spec — typically 20–60VA |
After summing, select the next standard CPT size up from the calculated total:
50VA • 100VA • 150VA • 250VA • 500VA • 750VA • 1,000VA • 1,500VA • 2,000VA
Minimum recommendation for a typical one-motor starter with START/STOP/RUN lights: 150VA
Blown CPT secondary fuse = loss of ALL control power = motor won't start. The contactor coil won't energize. Pilot lights go dark. Nothing responds to START.
First check: Verify CPT primary has voltage, then measure secondary voltage. If you have primary but no secondary, inspect the secondary fuse. Replace with same rating. If it blows again immediately, there is a short circuit in the control wiring — chase the fault before re-fusing.
Motor starter bucket: 1× NEMA-2 contactor coil (40VA) + 3× pilot lights at 8VA each + 1× timer relay coil (20VA)
Total VA = 40 + (3 × 8) + 20 = 40 + 24 + 20 = 84VA
Next standard size up from 84VA = 100VA CPT
Secondary fuse = 100VA / 120V = 0.83A → use 1A or 2A fuse
Note: Many designers always use 150VA minimum to allow for future additions and easier troubleshooting.
PLCs, HMIs, sensors, solenoid valves, and most modern control components run on 24VDC. The device that converts AC line power to regulated 24VDC is a power supply — and understanding how it works helps you diagnose failures and specify replacements.
A linear power supply converts AC to DC through four sequential stages. It is older, heavier technology — but extremely reliable and produces very clean DC with minimal electrical noise.
| Characteristic | Value / Description |
|---|---|
| Efficiency | 40–65% (regulator wastes excess as heat) |
| Output noise | Very low — excellent for analog instrumentation |
| Weight / size | Heavy and large — big iron core and heatsinks |
| Input range | Narrow — designed for one nominal line voltage |
| Best industrial use | Sensitive analog instruments; specialty applications requiring clean DC |
A switching mode power supply (SMPS) achieves voltage conversion using high-frequency switching — operating at 20,000–500,000 Hz instead of 60 Hz. This allows magnetic components to be dramatically smaller and lighter.
| Common Rating | Output Current | Output Power | Typical Width |
|---|---|---|---|
| 24VDC / 5A | 5A | 120W | 40mm |
| 24VDC / 10A | 10A | 240W | 60mm |
| 24VDC / 20A | 20A | 480W | 80mm |
| 24VDC / 40A | 40A | 960W | 120mm |
| Feature | Linear Supply | SMPS |
|---|---|---|
| Efficiency | 40–65% | 90–95% |
| Size / Weight | Heavy, large (iron core, heatsinks) | Compact, light |
| Heat generated | High — wastes ~40% as heat | Low — loses only 5–10% |
| EMI / Noise | Very low — inherently clean | Higher — switching creates EMI (filtered internally) |
| Input voltage range | Narrow (one nominal voltage) | Wide (85–264VAC, universal) |
| Transient response | Moderate | Very fast (feedback adjusts in microseconds) |
| Reliability | Very high (few components) | High (modern units: 100,000+ hours MTBF) |
| Cost | Low for small power, high for large | Cost-effective at all power levels |
| Typical industrial use | Older analog instrumentation; specialty applications | PLC racks, HMI, field devices, all modern 24VDC control |
In modern industrial facilities, virtually all 24VDC supplies are SMPS — DIN-rail-mounted units with a green LED on the front. When one fails: check the DC OK LED, measure output under load, check input voltage. If output sags under load, the supply is undersized or failing internally. Replace with same or next size up. Common brands: Phoenix Contact, PULS, Siemens SITOP, Murr, Wago.
Enter primary voltage and either the turns ratio or secondary voltage. Provide primary current, secondary kVA, or secondary current to fully solve both sides.
Add your control loads. The tool sums the VA requirements and recommends a CPT size and secondary fuse rating.
| Failure | Symptom | Diagnostic Test | Common Cause |
|---|---|---|---|
| Open primary winding | No secondary voltage; primary fuse intact | Measure primary voltage (present). Measure secondary (0V). De-energize and measure primary winding resistance — open winding reads infinite ohms. | Sustained overload; voltage surge; age-related insulation failure |
| Open secondary winding | No control power; all control devices dead; CPT primary fuse OK | Verify primary has voltage. Measure secondary (0V). De-energize and measure secondary winding resistance — open reads infinite ohms. | Sustained overload on secondary; short followed by fuse clearing that damaged the winding |
| Shorted turns | Transformer runs abnormally hot; higher than normal primary current; primary fuse blows repeatedly; burning varnish smell | Measure secondary voltage under no load — shorted turns often pull it below nameplate rating. Use IR gun to check enclosure temperature. Measure no-load primary current vs. nameplate. Replace CPT if suspected. | Internal insulation breakdown — caused by heat cycling, moisture, over-voltage transients, rodent damage |
| CPT secondary fuse blown | No control power; motor won't start; contactor won't pull in; all pilot lights dark | 1) Verify CPT primary has correct voltage. 2) Measure secondary — reads 0V if fuse is blown. 3) Inspect or test fuse continuity. 4) Replace fuse — if it holds, test operation; if it blows again immediately, chase the short in control wiring. | Control circuit short; pinched wiring; overloaded secondary; incorrect fuse size |
| Wrong tap selected | Secondary voltage consistently high or low by a fixed percentage (e.g., 5%); equipment runs hot or malfunctions | Check tap connections on CPT primary terminal block. Compare nameplate voltage to actual facility primary voltage. If facility runs 504V and tap is at "Nominal" (480V), secondary will read 126V instead of 120V. | Tap set incorrectly; facility primary voltage changed after installation; wrong tap set during replacement |
| 24VDC SMPS output low / sagging | PLC behaves erratically; sensors unreliable; SMPS DC OK LED orange or flashing | Measure output under full load with calibrated meter. Check DC OK relay status. If output sags below 22V, supply is undersized or failing. Verify input voltage is within specified range. | Load growth exceeded supply rating; supply internal failure; input voltage low (brownout); electrolytic capacitors degraded (end of life) |
Answer all five questions, then check each one. Review incorrect answers before proceeding to Lesson 5.12.