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The Question & Answer (Q&A) Knowledge Managenet

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Table of Contents

- What will create a locked rotor condition in a motor?
- What is a locked rotor motor?
- How do you find the locked rotor current of a motor?
- What does locked rotor amperage mean?
- What is locked rotor time?
- Is Locked Rotor Amps the same as full load amps?
- What does Fla mean on a motor?
- What is full load current of motor?
- What is FLA and LRA of a motor?
- What’s the difference between LRA and FLA?
- Is RLA and FLA the same?
- How is motor Fla calculated?
- How do you size a motor?
- How many amps is a 5hp motor?
- How many amps is 3 horsepower?
- How many amps is 60 horsepower?
- How many amps is a 1hp motor?
- What size breaker do I need for a 3 HP motor?
- Can I use 12 gauge wire on a 30 amp breaker?
- What size breaker do I need for a 100 hp motor?

Something as simple as a broken wire or weak capacitor can generate a lock rotor condition. Blocked rotor test. A blocked rotor test is conducted on an induction motor. It is also known as short circuit test, locked rotor test or stalled torque test.

Locked rotor current is basically the current drawn by the motor at its rated voltage when its rotor is kept stationary or in other words rotor is not spinning or rotating. So when we start a motor, its rotor is already at rest. This means, starting current and locked rotor current should be same.

To find LRA and starting kVA enter nominal operation voltage (such as 120 or 240), HP value, code letter from the nameplate label, and hit “Calculate” button. The general equation for single-phase devices is the following: LRA=1000*(kVA/HP)/Voltage. Note that this defines starting current at full nominal voltage.

locked-rotor amperage (LRA) The very high current or amperage a motor draws when power is first applied, due to low counter emf. It is usually five to six times the running load amperage.

The locked rotor time of the motor refers to the time it will take for the motor’s electrical components to reach a critical temperature when the motor is supplied with voltage to its stator (normally assumed to be full voltage) and with the rotor completely locked, i.e. unable to turn.

RE: Relationship: Locked rotor amp and full load amp The locked rotor current is the measured current with the rotor locked and with rated voltage and frequency applied to the motor. The amount of current a motor can be expected to draw under full load (torque) conditions is called Full Load Amps.

Full Load Amps

Definition of ‘full load current’ A full load current is the largest current that a motor or other device is designed to carry under particular conditions. A full load current is the largest current that a motor or other device is designed to carry under particular conditions.

FLA= Full load amps. LRA= Locked rotor amps.

LRA – Locked Rotor Amps: The current you can expect under starting conditions when you apply full voltage. It occurs instantly during start up. RLA – Rated Load Amps: The maximum current a compressor should draw under any operating conditions. FLA – Full Load Amps: Changed in 1976 to “RLA – Rated Load Amps”.

Full Load Amperage (FLA). As it relates to HVACR, Rated Load Amperage (RLA) is a mathematical calculation used to get Underwriters Laboratories (UL) approval for a certain compressor motor. It should not be confused with Full Load Amps (FLA).

The motor FLA calculator uses the following formulas:

- Single Phase AC Motor FLA (Amperes) = (P [kW] × 1000) / (V × cos ϕ)
- Single Phase AC Motor FLA (Amperes) = (P [HP] × 746) / (V × cos ϕ × η)
- Three Phase AC Motor FLA (Amperes) = (P [kW] × 1000) / (V × 1.732 × cos ϕ)

The quickest method to closely estimate motor horsepower is to use a digital clamp meter to measure current and voltage to the motor, and then perform a simple calculation. Use this formula to estimate motor horsepower. Horsepower(hp)= Voltage x Amperage x % EFF x power factor x 1.73/746.

A 5 HP motor at 230V, 3PH is12. Using the formula above, multiply 12.4 amps by 10% for a result of 1.24. Add this total to 12.4 for a total of 13.64 amps.

Common Horsepower to Amps Conversions

Horsepower | Amps | Voltage |
---|---|---|

3 HP | 20.7 A | 120 V |

3.5 HP | 24.2 A | 120 V |

4 HP | 27.6 A | 120 V |

4.5 HP | 31.1 A | 120 V |

Here are the Full Load Amp (FLA) values for typical 230VAC and 480VAC 3 phase motors.

Motor horsepower | Full load amps 230VAC | Full load amps 460VAC |
---|---|---|

60 | 149 | 75 |

75 | 180 | 90 |

100 | 245 | 123 |

125 | 310 | 155 |

AMP DRAW BASED ON HORSEPOWER

HP | 1 Phase Amps | 3 Phase Amps |
---|---|---|

115 volt | 208 volt | |

1 | 16 | 4.6 |

1-1/2 | 20 | 6.6 |

2 | 24 | 7.5 |

Full Load Current: Three Phase AC Motors

Motor Horsepower | Size Breaker | |
---|---|---|

1-1/2 | 230V 460V | 15 15 |

2 | 230V 460V | 15 15 |

3 | 230V 460V | 15 15 |

5 | 230V 460V | 15 15 |

A 30-amp breaker does not operate safely with a 12-gauge wire. The minimum wire size that is allowable for use with a 30-amp breaker is 10 gauge. A smaller gauge of wire indicates a larger diameter and the ability to carry more current safely.

Electrical motor wiring data – NEMA amps, starter size, HMCP size for motors ranging 1/2 to 500 hp

Motor HP | NEMA amps | Motor Circuit Protector type HMCP (ampere) |
---|---|---|

60 | 77 | 150 |

75 | 96 | 150 |

100 | 125 | 200 |

125 | 156 | 250 |