Sizing Overload Protection for Motors

All electrical circuits and equipment need to be protected from overloads, short-circuits and ground-faults. With a lot of circuits the can be accomplished with a breaker or fuse otherwise none as an overcurrent protection device. Motors however have separate overload protection from short-circuit and ground fault protection. 

So what is overload protection? And why is it separate from the other two fault conditions for motors? Those are great questions. First let’s look at what a motor is and what makes it “special”.  

A motor is a small engine. It operates with magnetic induction produced from electricity. 50% of global power consumption is due to induction motors. Motors have two main parts. The stator and the rotor. The stator stays stationary during the operation of the motor. It has windings passing through it. The winding has power given to it. When power is applied to the winding it produces a rotating magnetic field or RMF. The rotor sits in the center of the stator and when the RMF rotates it causes the rotor to turn. A motor has other parts beside those to main parts such as bearings, terminal box, fans ect. 

So why do we need to protect the motor differently from other types of loads? Since a motor is operated with magnetic induction and a motor operates with movement and can have a lot of torque, it can create heat. Heat and electricity don’t really mix. Heat creates resistance and resistance is the opposition to current flow. Heat will also damage the winding and this could start a fire or ruin the motor. As resistance builds, current increases because the motor is trying to do its job. 

Another reason we need to separate the two fault conditions from an overload on a motor is what we call locked rotor, locked rotor is also referred to as “starting current”.  Locked rotor is when the current spikes during the start up of the motor. This happens because the motor is an inductive load. When voltage is applied to an inductive load it creates a counter voltage or a voltage that pushes back on the applied voltage. This voltage is called counter EMF. counter EMF much like resistance is an opposition to current flow, this is called Inductive reactance (XL) and is measured in ohms. When a motor starts it cuts though less magnetic lines of flux so is less inductive reactance than when running at full capacity. This means when a motor starts it pulls more amps (current) than when the motor is running at full capacity. 

What does all that mean? We need to protect the motor and its windings from overheating, but we also need the motor to start. If we protect a motor the same way we protect other types of loads, either the motor would keep tripping a breaker and not be able to start or the motor wouldn’t be able to be protected from overheating. 

Here is an example of what I’m trying to say. If motors were sized off of a breaker to protect them from all three different conditions (Ground-fault, Short-circuit & Overload) like other types of loads and a bearing on a motor went bad it would caused the motor to seize up, the motor would try to continue to work and would pull more amperage (current) than what the motor is rated for. This is what we are trying to avoid. If the breaker was sized off of the overload so that it would protect the motor itself, the locked rotor current would trip the breaker because of the starting current. So we have the breaker or fuse protected from short -circuits & ground-fault and separate overloads protect the motor from heating up.

The overloads that are generally in the motor controls, internal overloads inside of the motor or installed as a fuse. In The National Electrical code article 430 covers the requirements for motor installations. Part III of this article covers overload protection. In order to size overloads we have to go to the nameplate and look for a few things. First look for the Full load amps (FLA) on the nameplate. Next we need to take a look at the code book. Section 430.32 tells us that if the nameplate of the motor is marked with a service factor of 1.15 or greater or if the nameplate is marked with a 40 degree celsius or less than the FLA off the nameplate must be sized to 125%. If a motor doesn’t fall into any of these categories then they would fall into the “all other motors” category and the FLA must be sized at 115%. One thing that we should remember about overloads is that we can’t “round up” with overloads like we are allowed with brakes or fuses. Let’s take a look. 

Let’s say we have a 1 phase 5 hp motor that has a service factor of 1.15 and an FLA of 29 A. We are going to use Dual Element fuses as our overlords. 29 x 1.25 = 36.25. We can’t round up to a 40 amp fuse, so we would size these fuses at 35 amps.

I hope this helps you in your electrical journey and give you a better understanding of how we protect motors.