All electrical circuits and equipment need to be protected from overloads, short-circuits and ground-faults. With a lot of circuits this can all be accomplished with a single breaker or fuse, otherwise known as an overcurrent protection device or OCPD. 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 by electricity; 50 percent of global power consumption is due to induction motors. Motors have two main parts: the stator and the rotor. The stator has copper windings passing through it, and stays stationary during the operation of the motor. Power is applied to the windings, which produce 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 two main ones, such as bearings, a terminal box, fans etc, but we won’t worry about those for the purposes of this article
So why do we need to protect the motor differently from other types of loads? Since a motor is operated with magnetic induction and operates with movement, it can have a lot of torque and 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 needs a certain amount of current to do its job. So the first reason we need to protect motors in a different way from other loads is because overheating is more of a concern with motors.
Another reason we need to separate the two fault conditions from an overload on a motor is because of what we call locked rotor — 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 causes less inductive reactance than when running at full capacity. This means that when a motor starts, it pulls more amps (current) than when the motor is running at full capacity.
What does all that mean? Well, we need to protect the motor and its windings from overheating, but we also need the motor to start. If we protected a motor the same way we protect other types of loads, one of two things would happen. Either the spike in current from the locked rotor would trip the breaker every time we tried to start it, or we would have too large of a breaker which could allow the motor to overheat during normal run time.
Here is an example of what I mean. If we were to size a breaker for a motor to protect it from all three different conditions (ground-fault, short-circuit and overload) like other types of loads, and, say, a bearing on a motor went bad, it would cause the motor to seize up. The motor would try to continue to work and would pull more amperage (current) than it 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 instead, we have the breaker or fuse protected from short-circuits and ground-fault, and a separate overload to protect the motor from heating up.
In a motor control circuit, the overload protection device is usually built into the motor itself but can be installed as a fuse instead. 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 overload protection devices we have to go to the nameplate and look for a few things. First we 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 with a temperature rise of 40 degrees celsius or less, then the overload device must be rated at no more than 125 percent of the FLA on the nameplate. If a motor doesn’t fall into any of these categories, then it would fall into the ‘all other motors’ category, and the overload device must be sized at 115 percent of the FLA. One thing that we should remember is that we can’t ‘round up’ with overload protection devices like we are allowed to with breakers or fuses. Let’s take a look at an example.
Let’s say we have a single phase, 5 hp motor that has a service factor of 1.15, and an FLA of 29 amps. We are going to use dual element fuses as our overload protection. 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 gives you a better understanding of how we protect motors.