How to Read a Motor Nameplate and Size Your Converter

Where to Find the Nameplate

The motor nameplate is usually a metal plate stamped or printed with electrical data, attached directly to the motor frame. On larger motors it's riveted or bolted to the side of the motor. On enclosed or integrated motors (like those inside a drill press or lathe headstock), the nameplate may be on the motor housing that's partially enclosed.

If you can't find a nameplate (it may be worn off, painted over, or missing on old equipment), you can often find the motor data in the machine's manual, on a data plate inside the control panel, or by searching the machine's model number online.

Reading the Nameplate: Field by Field

HP (Horsepower)

This is the motor's continuous rated power output. This is your primary sizing input for the phase converter calculation. Use this number exactly — don't estimate or round.

On European or older equipment, you may see kW instead of HP. Conversion: HP = kW × 1.341. A 7.5 kW motor is approximately 10 HP.

VOLTS

This shows the rated voltage — what voltage the motor is designed to run at. Common values:

• 230V (or 240V) — standard US three-phase; use NL, PL, or GPX Series converter
• 460V (or 480V, 440V) — industrial US three-phase; use NLT, PLT, NLTA, PLTA, or NH Series
• 208V — some industrial three-phase systems; verify with us before ordering
• 230/460V — dual-voltage motor; can be wired for either voltage
• 400V — European standard; approximately equivalent to US 460V

PH (Phase)

PH 3 or "3Ø" means three-phase. This is what requires a phase converter if you only have single-phase service. PH 1 or "1Ø" is single-phase — you don't need a converter for single-phase motors.

FLA / AMPS (Full Load Amps)

Full Load Amps is the current the motor draws at its rated HP and voltage. This is essential for:

• Sizing the output wiring from converter to equipment
• Sizing the disconnect fuses or breaker at the machine
• Sizing the motor overload protection

For phase converter selection, you generally use HP (not FLA). But FLA is useful to estimate HP if the nameplate is unreadable. Rule of thumb for 230V three-phase: FLA / 2.5 ≈ HP.

RPM

RPM (revolutions per minute) is the rated full-load speed. Common values for 60 Hz motors: 3,450 RPM (2-pole), 1,725 RPM (4-pole), 1,150 RPM (6-pole). This doesn't directly affect phase converter sizing but tells you about the motor's design.

HZ (Frequency)

Should be 60 Hz for US equipment. European equipment may show 50 Hz — these motors run slower (20% lower speed) on US 60 Hz power but can usually be operated safely. The phase converter output is always at line frequency (60 Hz).

Service Factor (S.F. or SF)

Service factor is a multiplier that indicates how much overload the motor can sustain for short periods. An SF of 1.15 means the motor can handle 115% of its rated HP load for limited time.

Important: Size the phase converter for the nameplate HP, not the SF HP. The service factor is the motor's reserve — not something you should plan to use continuously.

DUTY (Duty Cycle)

CONT (continuous) means the motor is designed for continuous operation. INTER (intermittent) or S2, S3 etc. (IEC designations) mean the motor must cool between operating cycles. For phase converter sizing, duty cycle mainly affects how hard the converter works over time — if your motor is intermittent duty, the converter HP needed is the same, but it will run cooler since the load is cycled.

Sample Nameplate Walkthrough

Reading This Nameplate

• HP: 10 — this is a 10 HP motor
• VOLTS: 230/460 — dual voltage; wire for 230V with our converter
• AMPS: 28/14 — 28A at 230V, 14A at 460V (they're equivalent power)
• PH: 3 — confirmed three-phase motor; needs a phase converter
• HZ: 60 — US standard, compatible with phase converter output
• S.F.: 1.15 — can run at 11.5 HP for short periods, but size converter for 10 HP

Sizing the Phase Converter from This Nameplate

Say this motor powers a lathe (medium load, 1.5× multiplier):

What If You Can't Read the Nameplate?

If the nameplate is worn, painted, or missing, try these sources in order:

1. The machine's original manual or documentation
2. A data plate inside the machine's electrical control panel
3. The machine manufacturer's website (search by model number)
4. Search the motor frame number — there are online databases of motor specs by frame number
5. Visual estimate: physically large motors are higher HP. A motor the size of a basketball is typically 5–10 HP. Refrigerator-sized motors are 50+ HP.
6. Call us — (800) 417-6568. Tell us the machine make and model and we've probably sized it before.