When selecting a motor transformer—typically an isolation or step-down transformer feeding an AC motor from a higher voltage shop feed—the continuous kVA rating must exceed the motor’s full-load amps (FLA), but the impedance and inrush capacity must handle the locked-rotor current (LRA) without excessive voltage drop. The baseline rule of thumb is to size the transformer kVA at 1.25 to 1.5 times the motor’s running kVA, but you must verify that voltage drop during startup stays above 85% of nominal. If the voltage sags below that threshold during the first few seconds of startup, the motor’s breakdown torque collapses, and it will stall or trip your breaker.

In this guide, we break down how different motor types impact transformer sizing, walk through a real-world inrush calculation using NEMA code letters, map out dual-voltage terminal wiring, and diagnose the physical failure signatures of an undersized or failing unit.

Motor Types, Load Profiles, and Transformer Impact

Not all motors draw current the same way. A direct-on-line (DOL) induction motor slamming a compressor online will hammer a transformer with 600% inrush, while a VFD-driven motor ramps up gently but introduces harmonic heating. Choosing the right motor type for your load profile dictates the driver it demands and the transformer capacity you must provision.

Motor Type Starting Torque Curve Control / Driver Needs Transformer Inrush Multiplier Typical Cost / kW
DOL Induction (TEFC) High starting torque (150-200% FLA torque), sharp current spike. Simple contactor/overload relay. No complex drive needed. 6x to 8x FLA (Requires massive kVA headroom or soft starter). $80 - $150
VFD-Driven Induction Constant torque or variable torque, controlled ramp-up. Variable Frequency Drive (VFD). Requires line reactors for long cable runs. 1.5x FLA (Low inrush, but requires K-factor rated transformer for harmonics). $200 - $400 (incl. VFD)
Synchronous Reluctance (SynRM) High efficiency, smooth torque, requires precise rotor angle tracking. Specialized VFD with sensorless vector control. 1.5x FLA (Low inrush, high harmonic content). $350 - $600 (incl. drive)
Universal / Brushed AC Very high starting torque, RPM drops significantly under load. Simple switch or TRIAC phase-angle dimmer for speed control. 3x to 5x FLA (Moderate inrush, highly inductive spikes at commutation). $50 - $120
Bench Tip: If you are retrofitting an old machine tool with a new VFD, you might think you can downsize the feed transformer because the inrush is eliminated. Don't do it without checking the transformer's K-rating. Standard 15kVA transformers will overheat from the 3rd and 5th harmonics generated by the VFD's 6-pulse rectifier. You need a K-4 or K-13 rated transformer for VFD loads.

Sizing the Motor Transformer: A Worked Inrush Example

Let’s size a single-phase step-down transformer for a 5 HP, 230V AC induction motor driving a reciprocating air compressor. Compressors are high-inertia loads that require high starting torque, meaning the motor will draw locked-rotor current (LRA) for several seconds before reaching operating speed.

First, we establish the running load. According to NEC Table 430.248, the FLA for a 5 HP, 230V single-phase motor is 28A.
Running kVA = (230V × 28A) / 1000 = 6.44 kVA.

If we only looked at running current, a 7.5 kVA transformer seems adequate. But transformers must survive startup. We look at the motor nameplate for the NEMA Design Code Letter. Let’s assume it’s Code G, which dictates 5.6 to 6.29 kVA per horsepower during locked-rotor conditions.

  • Inrush kVA: 5 HP × 6.0 kVA/HP (average for Code G) = 30 kVA.
  • Transformer Impedance (Z): Standard single-phase dry-type transformers in this range typically have an impedance of 4.5%.

We calculate the voltage drop during startup using the formula: Voltage Drop % = (Inrush kVA / Transformer kVA) × Z %.

Scenario A: Using a 7.5 kVA Transformer

Voltage Drop = (30 / 7.5) × 4.5% = 4 × 4.5% = 18% drop.
Secondary voltage during startup falls to 82% of nominal (approx. 188V). Because motor starting torque is proportional to the square of the voltage (V²), an 18% voltage drop results in a 33% loss of starting torque. On a loaded compressor, this motor will likely stall, overheat, and trip the thermal overload.

Scenario B: Using a 15 kVA Transformer

Voltage Drop = (30 / 15) × 4.5% = 2 × 4.5% = 9% drop.
Secondary voltage holds at 91% (approx. 209V). Torque reduction is only about 17%, which is well within the breakdown torque margin of a NEMA Design B motor. The compressor starts reliably.

For a comprehensive look at how manufacturer impedance tolerances affect these calculations, refer to the Eaton low-voltage transformer application guides, which detail how a +10% impedance tolerance on a cheap transformer could still cause a stall even at 15 kVA.

Terminal Identification and Dual-Voltage Wiring

Most single-phase control and power transformers in the 1 kVA to 15 kVA range feature dual-voltage primary and secondary windings. Miswiring these is the most common reason a new transformer blows its primary fuse on day one.

The primary (high voltage) terminals are labeled H1, H2, H3, and H4. The secondary (low voltage) terminals are labeled X1, X2, X3, and X4. The secondary also features a center tap (usually bonded to X2 and X3) to provide half-voltage for control circuits.

Configuration Primary Voltage Primary Wiring (H-Terminals) Secondary Voltage Secondary Wiring (X-Terminals)
Series / Series 480V AC H1 to Line 1, H4 to Line 2. Jumper H2 to H3. 240V AC X1 to Load 1, X4 to Load 2. Jumper X2 to X3.
Parallel / Series 240V AC H1 & H3 to Line 1, H2 & H4 to Line 2. 240V AC X1 to Load 1, X4 to Load 2. Jumper X2 to X3.
Series / Parallel 480V AC H1 to Line 1, H4 to Line 2. Jumper H2 to H3. 120V AC X1 & X3 to Load 1, X2 & X4 to Load 2 (Neutral).

Crucial Wiring Rule: When wiring primaries in parallel for 240V, you must maintain polarity. H1 and H3 must connect to the same line, and H2 and H4 to the other. If you cross them (e.g., H1 and H4 to Line 1), the magnetic fluxes in the two primary coils will oppose each other, creating a dead short that will instantly vaporize the primary fuses and potentially damage the transformer core.

Failure Signatures: Diagnosing Hum, Overheat, and Stall

Transformers don't usually fail silently. They give you physical warnings long before the insulation breaks down. If you are troubleshooting a motor circuit, listen and feel for these specific signatures.

1. Excessive Hum (Magnetostriction and Saturation)

All transformers hum due to magnetostriction—the core laminations physically expand and contract as the magnetic flux alternates at 50/60Hz. However, a loud, aggressive buzzing indicates core saturation. This happens when the primary voltage is too high (e.g., feeding a 480V primary with 520V from an unloaded solar inverter grid) or when a failing VFD rectifier is injecting a DC offset into the AC line. DC offset shifts the hysteresis loop, driving the core into saturation on every other half-cycle. Fix: Measure the primary voltage with a true-RMS meter and check for DC voltage on the AC line.

2. Overheat (Harmonic Loading and K-Factor Mismatch)

If the transformer casing is too hot to touch (exceeding 50°C rise over ambient) but the ammeter shows the motor is drawing less than FLA, you have a harmonic heating problem. Standard transformers are designed for pure 60Hz sine waves. VFDs and soft-starters chop the waveform, creating high-frequency eddy currents in the core and skin-effect heating in the windings. Fix: Derate a standard transformer by 30% when feeding non-linear loads, or replace it with a K-4 rated unit designed with thicker conductors and specialized core geometries to dissipate harmonic heat.

3. Motor Stall (Voltage Sag and Breakdown Torque)

If the motor hums, struggles to turn, and eventually trips the thermal overload without ever reaching full RPM, the transformer is likely undersized for the inrush, or the secondary wiring has excessive voltage drop. As calculated in our worked example, if the secondary voltage drops below 80% during startup, the motor cannot generate enough torque to overcome the load inertia. Fix: Measure the secondary voltage at the contactor coils during the exact moment the motor starts. If it dips below 85% of nominal, you must either upgrade the transformer kVA, install a reduced-voltage soft starter to limit inrush, or switch to a VFD to eliminate the LRA spike entirely.

For deeper diagnostics on motor starting currents and their impact on upstream distribution equipment, the EC&M guide on motor starting currents provides excellent oscilloscope captures of these exact voltage sag events.