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Motor Nameplate Basics: Understanding Torque, Speed, and Current

Motor Name Plate Terminology

Motor Name Plate Terminology

General Terminology

1. Service Factor

The service factor is a multiplier that indicates the amount of overload a motor can handle. For example, a motor with a 1.15 service factor can safely manage intermittent loads up to 15% beyond its nameplate horsepower.

Service Factor Amperage: This is the current the motor will draw under the service factor load condition.

2. Slip

Slip is used in two forms:

  • Slip RPM: The difference between the synchronous speed and the full-load speed.
  • Percent Slip: When the slip RPM is expressed as a percentage of the synchronous speed.

Most standard motors operate with a full-load slip of 2% to 5%.

3. Synchronous Speed

This is the speed at which the motor's magnetic field rotates. It is also approximately the motor's speed under no-load conditions.

Example: A 4-pole motor running on 60 Hz has a magnetic field speed of 1800 RPM.

  • No-load speed: ~1798–1799 RPM
  • Full-load speed: ~1745 RPM
  • The difference between synchronous speed and full-load speed is called slip RPM.

Motor Torque

1. Pull-Up Torque

This is the minimum torque developed by the motor during acceleration from zero to full-load speed (before reaching the breakdown torque point).

Some motors may not have a distinct pull-up torque value, as the lowest point may occur at the locked-rotor point. In such cases, pull-up torque equals locked-rotor torque.

For motors without a definite breakdown torque (e.g., NEMA Design D), pull-up torque is the minimum torque developed up to rated full-load speed, often expressed as a percentage of full-load torque.

2. Starting Torque (Locked Rotor Torque)

This is the torque produced when the motor is energized at full voltage with the shaft locked in place.

Importance: High starting torque is crucial for hard-to-start applications like positive displacement pumps or cranes. Lower starting torque is acceptable for applications like centrifugal fans or pumps with low start loads.

3. Full Load Torque

This is the rated continuous torque the motor can support without overheating within its time rating.

Imperial Units:

Tfull-load torque (lb ft) = (Rated horsepower of Motor × 5252) / Rated rotational speed (rpm)

Metric Units:

Tfull-load torque (Nm) = (Rated kW of Motor × 9550) / Rated rotational speed (rpm)

Full Load Torque Calculator

4. Peak Torque

This is the maximum torque requirement at any point during operation, often seen in loads with cycling torques like reciprocating compressors or punch presses.

5. Pull-Out Torque (Breakdown Torque)

This is the maximum torque the motor can develop with rated voltage and frequency without an abrupt drop in speed.

Expressed as a percentage of full-load torque. The load is increased until the maximum torque point is reached.

Motor Current

1. Full Load Amps (FLA)

This is the current the motor draws under full-load (torque) conditions, also known as nameplate amps.

2. Locked Rotor Amps (LRA)

Also called starting inrush, this is the current the motor draws under starting conditions when full voltage is applied.

Three-Phase Motor:

IL = (1000 × HP × (kVA/HP)) / (1.732 × Volt)

Single-Phase Motor:

IL = (1000 × HP × (kVA/HP)) / Volt

Locked Rotor Amps (LRA) Calculator


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