How to Read a Motor Nameplate (and Cross Any Motor)
Every replacement motor decision starts at the nameplate. If the motor is dead, the nameplate is usually the only reliable record of what was installed - and reading it correctly is the difference between a drop-in swap and a return shipment. Here is what each field means and which ones must match.
The Fields That Must Match
| Field | What It Tells You | Match Required? |
|---|---|---|
| HP / kW | Rated output power | Yes - equal or one size up |
| RPM | Full-load speed (1725, 1765, 3450…) | Yes - same pole count |
| Frame | All mounting & shaft dimensions | Yes, for a drop-in fit |
| Voltage / Phase / Hz | Supply the windings expect (e.g. 230/460V 3φ 60Hz) | Yes |
| Enclosure | ODP, TEFC, TEXP… environment rating | Yes, or upgrade |
| Service Factor | Continuous overload margin (1.15 typical) | Equal or higher |
| FLA | Full-load amps - sizes overloads & wiring | Verify against starter |
| Design Letter | Torque curve (NEMA B is standard) | B unless the load says otherwise |
| Insulation Class / Temp Rise | Thermal capability (Class F common) | Equal or better |
| Efficiency | NEMA nominal efficiency | Higher saves energy |
Reading Speed and Poles
Nameplate RPM is full-load speed, slightly below synchronous: 3450–3550 means a 2-pole motor (3600 sync), 1725–1780 means 4-pole (1800), 1140–1180 means 6-pole (1200). Replace like with like - a fan sized for 1750 RPM moves dramatically less air at 1150. Our motor speed guide covers the trade-offs.
Dual Voltage and Connection Diagrams
A 230/460V motor runs on either supply - the connection plate shows the 9-lead wye or delta hookup for each. FLA halves at the higher voltage (e.g. 26/13A). Single-phase motors list run and start capacitor values; keep replacements matched to the correct capacitors.
Crossing to a Replacement
With HP, RPM, frame, voltage, enclosure, and SF recorded, crossing brands is mechanical: any manufacturer's motor with the same NEMA frame bolts to the same base. Start at the cross-reference hub for brand-specific notes (Baldor, Leeson, Marathon, WEG), or photograph the nameplate and call - we cross motors off nameplate photos daily.
Frequently Asked Questions
What does service factor 1.15 mean on a motor nameplate?
The motor can carry 15% over rated horsepower continuously at rated voltage and frequency without immediate damage, though at reduced insulation life. Never size a replacement so the service factor margin is consumed by normal operation - match or exceed the original SF.
What is NEMA Design B?
Design letters classify torque and current characteristics. Design B is the general-purpose standard: normal starting torque with moderate inrush, and it covers pumps, fans, conveyors, and most machinery. High-slip Design D suits punch presses and oil-well pumping units.
Can I replace a 1.0 service factor motor with a 1.15 SF motor?
Yes - a higher service factor is an upgrade, adding thermal margin. The reverse also runs, but you give up overload headroom the original installation may have counted on.
My nameplate is unreadable. How do I identify the motor?
Measure shaft height to determine the frame (3.5 inches = 56/143T family, 5.25 = 213/215T, and so on), count wires and measure voltage, and check the driven equipment's requirements. Then call (888) 203-2358 with photos and measurements - frame plus application usually narrows it to one or two candidates.
Do amps need to match exactly on a replacement motor?
No. FLA varies slightly by design and efficiency. What matters is that your overload relay and wiring are rated for the new motor's FLA - recheck the overload dial or heater table against the new nameplate at installation.
Related Resources
Send us a nameplate photo - we'll cross it.
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