Motor FLA Calculator
Electric motors are essential devices in many industries today. They convert electrical energy into mechanical energy (in the form of rotational torque on a shaft) based on the principles of electromagnetism. The electrical energy could be grid-supplied single-phase or poly-phase alternating current or direct current from an alternative source.
Although several factors affect the performance and longevity of electric motors, full load amperage (FLA) remains one of the most crucial parameters that engineers must consider when specifying electric motors for a particular application.
This article will get back to the basics of electric motors. It will cover full load amperage of single- and three-phase motors. It will also present helpful information about how motor FLA forms the basis for overload sizing.
Figure 1. The performance of an electric motor is affected by full load amperage
© [kosanperm] / Adobe Stock
What is full load amperage?
Full load amperage (also called full load amps or full load current) is the largest current that a motor is designed to carry under particular conditions. It is simply the motor’s rated current at rated load and voltage conditions.
Subjecting the motor to an input current greater than the full load current causes the motor to produce additional heat, which might end up damaging the electrical motor's windings and insulation.
Full load amperage calculation forms the basis of selecting fuses, cables, circuit breakers, overload relay cables, and other switchgear necessary to protect the motor from damages.
Motor full load amps calculator
The following sections present formulas for estimating motor full load current in single-phase and three-phase circuits.
Single-phase full load amps calculator
For single-phase AC motors, when the motor power rating is known in kilowatts (kW) , FLA can be calculated as follows:
For single-phase AC motors, when the output power is known in horsepower (hp), FLA can be calculated as follows:
Where:
Pout=mechanical power output
PF=Power factor
V=Line voltage
E=The efficiency of the motor
Three phase full load amps calculator
For three-phase AC motors, when the motor power rating is known in kilowatts (kW) , FLA can be calculated as follows:
For three-phase AC motors, when the output power is known in horsepower (hp), FLA can be calculated as follows:
Where:
P=mechanical power output
PF=Power factor
V=Line Voltage
E=The efficiency of the motor
[Learn more about AC motors with Engineering360]
Approximate full load amperage of single-phase motors
Table 1 presents the approximate full load current of single-phase motors with approximately 1450 rpm of average efficiency and power factor.
Table 1. Approximate full load amperage of single-phase motors
Reference: https://www.rm-electrical.com/technical-resource/motor-current-charts/
Approximate full load amperage of three-phase motors
Table 2 presents the approximate full load current of three-phase motors with approximately 1450 rpm and average efficiency and power factor.
Table 2. Approximate full load amperage of three-phase motors
Reference: https://www.rm-electrical.com/technical-resource/motor-current-charts/
How to read motor full load amps and other parameters from the motor nameplate
Figure 2 shows a typical nameplate of an induction motor. It contains information about the motor horsepower, kW, frequency, and full load amperage.
Figure 2. Nameplate of a typical three-phase induction motor
Source: [Olgailga2 /Public domain]
For example, from the motor nameplate in Figure 2, the full load amperage of this motor is 0.97A at 400V. The “/” indicates that the motor can operate at more than one line voltage value. It means engineers can wire this motor to operate using low or high voltage (230V or 400V in this case). The current will be 1.67A at 230V line voltage.
Other parameters that can be obtained from this nameplate include:
- The mechanical output power of 0.6 hp or 0.36 kW
- the motor’s rotational speed of 2730 rpm
- frequency of 50 Hz
Overload protection and motor amperage calculation
The condition “overload” is when the operation of a motor (in excess of its normal, full-load rating) persists for a long enough time that it may cause overheating and damage to the motor components. As the name suggests, overload protection helps to protect the motor from damages and fire hazards and ensures the motor performs as expected. Overload protection devices include fuses and circuit breakers.
Overload protection devices can be sized using the full load amps rating found on the motor nameplate. As a rule, NEC 430.32 standards states that for continuous duty motors marked with a service factor of 1.15 or greater and a temperature rise of 40°C, engineers should size the overload device at no more than 125% of the motor full load amperage rating. All other motors should be sized at 115% of the motor FLA rating.
So consider the example of a motor that has a service factor of 1.15 and a full load amperage rating of 24.5A . The overload protection device should be sized as follows:
Since the overload protection device rating must not exceed 125% of the FLA, engineers can use an overload protection device with a rating of 30 amps.
Motor amperage calculation: Engineers should reach out to motor manufacturers
While this article presents helpful information about motor full-load current, there are still many factors that engineers must consider when specifying electric motors for a particular application. As a result, engineers are advised to reach out to motor manufacturers to discuss their application requirements.
