How to Protect Your Motor Against Overload

May 15, 2023

If you’ve noticed a burning smell, your motor’s getting louder, or you’re replacing new motors soon after installation, then it’s time to size (or maybe resize) an overload device for your electric motor. You may also want to consider resizing the motor or reviewing recent changes to your manufacturing process to avoid future overload. The Canadian Electrical Code requires an overload device to be in place and it’s one of the best ways to get ahead of downtime. Today we’ll walk you through how to size it correctly.

In this article, we’ll cover:

How to Size an Overload Device

We do have a wide selection of overload devices at eMotors Direct. Once you know what you need, you should be able to find what you're looking for on the site or contact our team for help if needed at customerservice@emotorsdirect.ca.

What Causes Motor Overload

To recap our previous article on what causes motor overload, motor overload can happen when there’s too much load on the drive-end of the motor.

When the conveyor system the motor’s driving gets jammed, this can lead to overload. Or there could be issues with the power source, like a power surge or a short circuit. You can’t control when a power surge happens, and this is where an overload device will save your motor. The cost to replace the motor is one thing, but some custom motors can take upwards of 28 weeks to be delivered. You could incur some serious downtime costs if you don’t have an overload device in place. Downtime for some of our customers works out to hundreds of thousands of dollars per day, so losing a motor to overload can be a huge risk.

One way to spot a potential motor overload is a blown capacitor.

6 Devices That Will Protect Your Motor from Overload

An overload device will interrupt the circuit when the motor is drawing too much current.

External Overload Devices

There are a few types of external overload devices you can choose from:

Overload relays - These relays allow for the overload needed at start-up but if the overload persists, the relay trips and cuts power from the circuit. Overload relays are often part of the motor starter but can be separate as well.
Starters - Most electric motor starters have overload protection built-in in the form of an overload relay. When an overload situation occurs, the starter trips, and power is removed from the circuit. Starters are great for motors that only require start and stop control, but not speed control.
Variable frequency drives - All VFDs also have overload protection built in. And when an overload situation occurs, the VFD cuts power from the system. VFDs are ideal if your motor also requires speed control. Shop VFDs from WEG, TECO Westinghouse, KB Electronics, Leeson, and Control Techniques here: click here

Internal Overload Devices

And there are a few internal overload devices that you could request with a custom motor. These are installed during manufacturing or installed by a certified motor modification facility.

Thermistors – These sensors are built into the motor’s windings and protect against high internal and ambient temperatures. When a high temperature is detected, the thermistors will increase the resistance until the motor cools down.
Thermostats – These switches are found in the motor’s circuit box on the terminal board. They open or close based on the temperature of the circuit. Thermostats can be used to control an alarm circuit or cut power from the circuit to allow time to cool off.
Resistance temperature detectors or RTDs – These sensors are wired between the windings. As the temperature changes, the sensors will change resistance to protect the windings.

How to Select Overload Relay for Motor

How do you know which starter will properly protect your motor from overload? You must find a relay that has the correct overload rating. And your starter must be tuned to the correct overload setting. You’ll need two ratings that can be found on the nameplate of your motor:

The Service Factor or SF. Which indicates how much load a motor can withstand before it starts to cause damage. You’ll typically see ratings from 1 service factor up to 1.25. So, if your motor has a service factor of 1.15, the motor can safely be overloaded by 115% for a short period of time. I see 1.15 most often for industrial use motors. (Some motors will have two ‘SF’ ratings on the nameplate, like in the below TECO Westinghouse example. The second SF indicates the service factor when the motor is connected to a VFD. When a motor is connected to a VFD, the service factor will always be 1).
Full Load Amps or FLA. This rating indicates the motor’s current demands as amperage. Once you have those two ratings for your motor, we’ll apply them to one of these two calculations to find the maximum allowable overload setting or rating for your overload protection device. (Some motor nameplates may not indicate ‘FLA’ and you can use the ‘amps’ number in this case.)

If the motor’s service factor is 1.15 or more, you’ll multiply the full load amps by 125% or 1.25 to get the max allowable overload rating in amps. If the motor’s service factor is less than 1.15, you’ll multiply the full load amps by 115% or 1.15. If the service factor is not indicated on the nameplate, assume the motor is rated as 1 service factor and multiply the full load amps by 1.15.

If service factor is ≤ 1.15 use this formula: FLA x 1.25 = max allowable overload rating in amps
If service factor is > 1.15 use this formula: FLA x 1.15 = max allowable overload rating in amps
If service is unknown use this formula: FLA x 1.15 = max allowable overload rating in amps
If the motor is connected to a VFD, use this formula: FLA x 1.15 = max allowable overload rating in amps

Example: Calculating Max Overload Formula

Let’s go over an example by calculating the max allowable overload rating on this Nidec US Motor. In this example, the FLA is noted as a range – ie. 14.0-12.8 FLA. The 14 is in relation to the 208V and the 12.8 is in relation to the 230V. Use the voltage that matches the voltage reading at your motor while it’s operating at normal load.

Looking at the nameplate on this motor, we can see that this motor has a service factor of 1.15 and the full load amps rating is 12.8. Since the service factor is equal to 1.15, we’ll be multiplying the full load amps by 125% or 1.25.

12.8 amps multiplied by 1.25 equals 16 amps. The maximum setting or rating for the overload protection device is 16 amps. You don't want the rating to be higher than what you’ve calculated, or your motor will become overloaded and be damaged due to the heat.

Calculating Max Overload for Wye/Delta Motors

In cases where your electric motor setup has reduced voltage starting, specifically in wye/delta start motors, the max overload setting will be calculated based on the phase current and not on the line current. In wye/delta start motors, the motor starts in the wye configuration and then switches to the delta configuration once the motor gets up to speed. The current draw is different in each configuration, so the max overload will be set based on the phase current. This means that you have one additional calculation to complete before you can calculate the max overload setting. You will use the line current to calculate the phase current. Phase current is about 57.7% of the line current.

So, you’ll take the FLA and multiply it by 0.577 to get the phase current. Use the below formula.

Line current * 0.577 = X
X * 1.15 or 1.25 = max allowable overload rating in amps

With these calculations, you’ll be able to find the correct relay to keep your motor safe in those unexpected overload situations.

How to Replace an Overloaded Motor

Before replacing an overloaded motor, ensure you've identified the cause of motor failure so that your next motor does not fail shortly after start up. We cover how to replace an overloaded motor and how to test for motor overload in a previous article (how to replace an overloaded motor).