General - Motors are a common type of load used to run fans, pumps, and more. They convert electrical energy to mechanical energy, an extremely broad class of applications common in the industrial space. Motors are magnetic devices. When a voltage is applied across the terminals of a motor, current rushes in and creates a magnetic field. This magnetic field then interacts with other magnetic components of the motor to create motion. Below is an example of a typical motor circuit one-line diagram. Each component of the circuit below must be carefully chosen to ensure that a motor is adequately protected.
Typical Motor Circuit
Motor Operation - During initial energization of a motor, a large current known as inrush will flow through the system for a brief period of time. Inrush current is the maximum value to flow through the motor and can be over 20x higher than the typical operating load current. Motors are marked with an indicating letter that describes this behavior. Further details can be found in NEC Table 430.7(B) of the 2020 Code. This current will drop considerably once the motor is able to begin moving and create an opposing voltage from the alternating magnetic field inside. Motor load currents are much lower than the inrush value and are provided by manufacturers as part of the motor datasheet. The National Electrical Code has a number of tables at the end of Article 430 that provide sample full load current values.
Testing of motors is done with reference to the mechanical load supplied. The full-load current refers to the current flowing through the motor when supplying its rated mechanical torque. This current is generally the maximum operating current expected. The no-load current refers to the current that flows when the motors is allowed to rotate but has no external mechanical load to support. The no-load current is usually lower than the full-load current. The locked rotor current is the current that flows then the motor is prevented from rotating. If the motor is not allowed to rotate, no opposing voltage is created and the current that flows will be very large. Motors need to be protected from this current, but conductors don't need to be sized to carry this value continuously.
Overload Relay - Per the National Electrical Code (and good engineering design practice!) motors are required to be protected against overload. Overload protection can be provided via a standard device like a breaker or a fuse, but for typical molded case circuit breakers and fuses this usually doesn't work well. These devices are prone to tripping during startup of the motor since the inrush current is much larger than the operating current. A dedicated overload relay can be used to protect the motor instead. This overload relay isn't intended to open up the circuit during a high-current fault. The relay is only used to open up the circuit for currents slightly above standard operating values. The overload relay should be coordinated to open up based on manufacturer information. Limits to how high the overload relay can be set are provided in the NEC, but in no circumstance can it be higher than 125% of the full load current. Overload protection is only required on the branch circuits feeding motors, and not on upstream feeder circuits.
Overcurrent Protection Device - Overload relays keep a circuit and motor protected against currents that exceed the motor's recommended operating conditions. Overcurrent devices protect against fault currents. Overcurrent devices should be sized to coordinate the trip curve against the available fault current and motor inrush. For smaller installations, trial-and-error approaches may be employed, as long as the settings do not exceed the limitations set by NEC Article 430, based on the motor type. For large facilities or motors with critical applications, it is imperative that a detailed analysis of trip curves be completed. The image below is an example of how trip curves of overcurrent devices should be coordinated with relevant motor parameters.
Motor Trip Curve Overcurrent Coordination Example
Disconnecting Means - Motors need to be able to be switched in and out of service. Per the 2020 NEC, low voltage motor disconnecting means must be capable of opening up the circuit while carrying the full load current of the motor. If a circuit breaker is used as the overcurrent protection and/or overload device, this may be suitable as a means of disconnecting. Medium voltage systems are not required to be able to break a motor circuit while running at full load.
Putting It All Together - The image below is of an integrated motor starter. Devices like these are common, as they combine all of the elements listed above with more sophisticated controls and monitoring. Instead of needing separate fuses, switches, and relays to all be installed and coordinated, the designer can specify a single device.
Where several motors are required to be served at a single voltage level, it is common to use a motor control center (MCC). MCCs are enclosures designed with large numbers of motors starters to facilitate motor starting. MCCs are commonly used in industrial and generation facilities, where motors are used all over the place.
An Integrated Motor Starter
Motor Conductors - The requirements for sizing conductors to motors are straightforward. Single motor circuits (motor branch circuits) are required to be sized for 125% of the full load ampere rating of the motor. Multi-motor circuits (Feeder circuits) are required to carry 125% of the largest motor full load ampere rating and 100% of the remaining motor full load ampere ratings. The NEC is full of additional exceptions that lead to reduced ampacity requirements in special cases.
Why not 125% of all motor loads? Motors are equipped with overload protection at the branch circuit level, so conductors can be protected closer to their ampacity than with a standard thermal overcurrent element. 125% is still required for the largest motor to account for inrush currents leading to increased temperatures over steady-state operation. In other words, this ampacity requirement assumes that two motors will not be experiencing inrush conditions simultaneously.
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