How Does a Motor Starter’s Thermal Overload Block Work?

This article provides an in-depth hands-on exploration of a motor starter’s thermal overload block. It explores the intern construction (Figure 1) along with the physical connections to real world. It concludes with an experiment (Figure 2) to evaluate the overload (trip) characteristics. For this test, a constant current DC power supply is used to simulate a motor overload condition. This simple setup allows the time vs current tripping curves to be observed.

Additional Information about the motor starter

This article is part of a larger work that introduces the motor starter and explores the important and at time subtle applications. Please refer to these related posts for more information:

Figure 1: Picture showing the internal working of the thermal overload block. The heaters composed of flat wire are wound over and insulated from the bimetallic strips.

What is a motor starter’s thermal overload block?

By definition, a motor starter is composed of a contactor and a thermal overload block. The thermal overload block is used to detect “soft” motor faults. When triggered, it causes the motor starter’s primary contactor to open. Here the term soft includes conditions that cause the associated motor to overheat including mechanical overloads, brown out conditions, single-phasing, an unbalance system, and some motor winding faults. By contrast, the term “hard” would include high current faults such as a phase-to-phase or a phase-to-ground fault. Hard faults are handled by an upstream circuit breaker or fuse.

For more information about the motor starter please review this introductory article as well as this article regarding the 3-wire start stop circuit. The remainder of this note assumes you are familiar with the contents of those two articles. This is important as much of the experiment setup shown in Figure 2 is outlined in those introductory articles.

Figure 2: A constant 1.5 ADC current (left power supply) is passed through the motor starter including the contactor and overload block. The motor starter’s coil is activated by the power supply on the right.

Tech Tip: Remember that a motor starter’s thermal overload block is not a circuit breaker. Unlike a circuit breaker, the thermal overload block does not have large (arc extinguishing) contacts that open in response to an overload event. However, there are many similarities between the thermal overload block and a circuit breaker including the critical tripping curves.

What is inside a motor starter’s thermal overload block?

Many motor starters feature a thermal system to detect an overload. We can envision this system as a set of low value resistors (heaters) in close proximity to bimetallic element. As the system heats up due to soft motor fault, the bimetallic element will bend and assert pressure on a trip lever. The trip point for this mechanism is a function of both the magnitude of the current and time.

Figure 1 shows the internal working of the Schneider DPER06 (1 to 1.6 A adjustable) thermal overload block. The heaters are prominently displayed in the front of the picture. They look and act like low value wire-wound resistors composed of flat wire on mandrel. The core at the center of the “wound resistor” is a bimetallic element. The electrical insulation between the heater and bimetallic strip can also be seen in the image.

The operation of the bimetallic element is implied in Figure 1. On the top of the heaters, we can see a brown lever mechanism. As the system heats up, this lever will be pushed to the side by the action of one or more of the bimetallic elements warping in response to heaters. At some point the mechanism will mechanically trip thereby toggling the small switch contacts. These contacts are seen in Figure 3 as the normally open (97 to 98) and normally closed (95 to 96) pair.

Tech Tip: Do not disassemble components such as the thermal overload block, as it is very easy to damage or misplace the delicate internal components. It’s not worth the future risk pointed directly at your expensive motor. In the interest of education, we have done this for you and included the results as Figure 1.

What is a motor starter’s tripping curve?

The tripping curve is a graphical representation of the thermal and time relationships that cause the overload block to toggle. These curves with current on the horizontal and time on the vertical axis should look familiar as they are commonly associated with circuit breakers and fuses. However, we must reiterate that the overload block does not “open” upon fault like its cousins. Instead, the overload block signals the contactor coil to deactivate.

Figure 4 presents the tripping curves for the Schneider Electric DPE family of motor starters. The left panel includes the experimental results (circled points) that will be described later in this note. Observe that a higher current causes the overload block to tip faster than lower current. As will be shown momentarily, a 1.5 A current takes about 80 seconds to trip in a cold state 2-phase test. The same test performed at a higher current will trip the system in about 15 seconds.

Tech Tip: There is an implicit relationship between the overload block and the motor it is designed to protect. For example, is generally acceptable to momentarily overload a 3-phase induction motor. However, the motor will overheat and be damaged with a sustained overload. The time constant for a properly matched motor starter overload block will reflect the heat in the motor’s windings. If the motor is hot the thermal heaters in the motor starter will be hot. The overload block should trip well in advance of damage to the motor. This is important consideration for selecting and adjusting an overload block as a mismatch will result in nuisance overloads (set too low) or an overheated motor (set too high).

Recall that the thermal block featured in this article is adjustable. For the experiment described later in this note, the current was set to a convenient 1 A. This allows direct reading of the tripping curve’s horizontal axis. Should a different overload block be used or should the adjustment be set to anything other than 1 A, multiply the tripping curve by the setting. For example, if the block was set to 1.6 A, we would expect a 4.8 A current to trip the block in 15 seconds; calculated as 3 * 1.6 A.

Figure 3: Wire diagram of the motor starter, overload block, and auxiliary contact block.

Tech Tip: We must be patient when dealing with a thermal overload event. Recall that the bimetallic element bends in the presence of heat to cause the overload block to trip. They remain in this distorted configuration until they have cooled off. During this period, we will not be able to reset the overload block. On a related note, the corresponding motor will be hot. We should allow the system to cool. On the other hand, if the system is down, you will have pressure from folks such as the foreman to restore the equipment to full operational status. Never forget that equipment down time is very expensive.

Figure 4: The tripping curves for the Schneider Electric DPE family overload block with annotated experimental data.

Design an experiment to generate the tripping curves

Looking back to your first lessons in AC, recall that the term Root Mean Square (RMS) involved heat. The RMS measurement equate AC with the equivalent DC based on the ability to produce an equal heating (power) in a device such as a resistor. That fundamental lesson applies to the heaters (resistors) located inside the motor starter’s thermal overload block. Therefore, we can use DC to stand in for the AC current consumed by a large AC motor. With equal heating, we are free to identify the time associated with the tripping curve in a controlled and safe experiment.

The actual experiment is relatively simple. We start by wiring in a constant current power supply as shown in Figure 5. We then use another power supply to provide 24 VDC to the relay circuitry using the classic 3-wire start-stop circuit shown in Figure 6.

With regards to Figure 6, we note that the normally closed contact from the overload block (95 to 96) is in series with the contactors coil (A1 to A2). Therefore, when we push the green start button the motor starter is activated. It will remain latched until it experiences a thermal overload event at which time the normally closed (95 to 96) contact will open releasing the latch in the 3-wire circuit. With regards to Figure 5 we see that the current will pass though the L1 and L2 phase heaters while the motor contactor is latched.

Figure 5: Wire diagram for the balanced “two-phase” test with a constant current supply.

Figure 6: Wire diagram for the 3-wire start stop circuit developed using KiCad.

Experiment results support the manufactures published tripping curves

The experiment is demonstrated in the Videos 1 to 3. The constant current supply was set to 3, 2, and 1.5 A respectively. For each video, the green pushbutton was pressed at the top of the minute. The video is then sped up so that we don’t need to watch the grass grow. The video is then moved back to normal speed when the thermal trip occurs. You can hear the contactor release as a loud thumping sound. The corresponding trip time is then displayed on the face of the clock.

The results for this three-part experiment show are superimposed on the manufacture provided tripping curves. There is a very good match with 3A @ 19 seconds, 2A and 33 seconds, and 1.5 A @ 79 seconds. Note shown in another experiment conducted with a constant 1 A. As suggested by the tripping curves, the motor system did not trip. This is like a 1A fuse or circuit breaker as we expect these devices to operate continuously at their rated current.

Video 1: Experiment showing the trip time when 3 A is passing through two phases of the thermal block. Total time is 19 seconds.

Video 2: Experiment for 2A. Total time is 33 seconds. Note the system is allowed to cool between experiments.

Video 3: Experiment for 1.5 A. Total time is 79 seconds.

Next steps

The experiment describe in this note was conducted with the overload block set to a convenient 1 A. It also used a “cold state” two phase configuration with a constant current power supply. A natural follow up experiment is to verify the results using a different setting e.g., set the block to 1.5 A. It wouldn’t be too difficult to use all three heaters in a series configuration. Finally, it may be useful to learn what is meant by the “after a long period at the set state (warm state)” stipulation included in Figure 3.

If you have the time and resources, you should take the next step and operate the motor starter for its intended high-power application. You will find that this article has barely scratched the surface when it comes to this device. To learn more consider becoming part of UL’s Industrial Control Panels and Panel Shop Program.


These experiments were conducted using a relatively safe 24 VDC originating from a bench power supply. This an essential safety consideration allowing you to thoroughly explore the motor starter. Yet, I’m reminded about the old saw regarding the safety of ships in port; but that’s not what ships are meant for.

In the real world the motor starter is an industrial workhorse designed for high voltage and high-power systems. The motor starter featured in this article can operate a 7.5 hp (5.5 kW) 3-phase motor in a 690 VAC system. There is an ever-present danger is such systems in the form of electrocution and arc blast. One mistake could be your last.

Follow all local, federal, and state guidelines. Establish and maintain a Lockout/Tagout (LOTO) program. As a starting point consider this OSHA document. Use test equipment with a high CAT rating. Use appropriate PPE up to an including flash suite for high energy systems. Finally conduct your work as part of an informed team following best practices for operational risk management.

With that in mind, it’s important that we be mindful of safety and not become complacent with the 24 VDC testbed. As educators we must continue to explore safety concepts at the same time as we introduce the fundamental properties and tools of the trade.

Be safe!


About the author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this. LinkedIn | Aaron Dahlen - Application Engineer - DigiKey

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