A motor starter includes an overload relay physically connected to the three-phase contactor. Traditional overload relays depend on bimetallic strips or low-melting eutectic (solder-like) alloys that melt and release a mechanical latch. The Siemens ESP200 featured in this article replaces the electromechanical components with an electronic sensing and trip circuit. This provides motor protection capabilities beyond the traditional thermal trip.
See Engineering Takeaways
- The overload relay’s normally closed contact is wired in series with the contactor’s coil to provide a control-system-independent trip mechanism.
- The solid-state overload relay eliminates dependence on conventional bimetallic or eutectic trips.
- The solid-state technology allows sensing and trip for conditions such as loss of a phase or a ground fault that may not impact an electromechanical trips.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Circuit Protection
Difficulty: 
Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 09 Mar 2026
What components are included in a motor starter?
A motor starter consists of a contactor and an overload relay.
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The contactor is like a multipole relay with robust contacts to reliably turn the motor on and off. These contacts can withstand the motor’s startup surge current. They can also break the electrical connection in a high voltage AC environment.
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The overload relay is connected to the output (motor side) of the contactor. When properly selected and aligned, the overload relay mirrors the conditions of the motor. If the motor is overloaded, or if there is a phase imbalance, the overload relay will trip, thereby protecting the motor.
The Siemens 14EUE32AG unit shown in Figure 1 is a representative motor starter. The upper portion is the heavy-duty contactor with a solid-state overload relay below.
Figure 1: Image of a Siemens half-size motor starter featuring a solid-state overload relay.
Tech Tip: In a previous career, I was a sailor on three U.S. Coast Guard vessels including USCGC POLAR STAR, HEALY, and EAGLE. Many marine applications include an ungrounded three-phase system. The first phase-to-hull short circuit was “free”. It was the second short (phase-to-phase or phase-to-hull) that would cause the breaker to trip.
Ground fault detection and correction are important parts of power distribution integrity. This Siemens ESP200 can be programmed to detect a phase to hull fault in a floating three-phase system.
What is the relationship between the contactor and motor starter?
The overload relay will activate (trip) when abnormal motor operating conditions are detected. With regard to Figure 1, the normally closed 95 to 96 contacts will open. Observe that a red wire runs from the normally closed contact (95) to the contactor coil.
This is arguably the single most important wire associated with the motor starter.
Observe that the contactor’s coil is wired in series with the overload relay. Consequently, an overload trip will immediately disable (turn off) the motor via the contactor. This is a fail-safe connection that operates independently of any external control logic.
Tech Tip: A PLC may be configured to monitor the auxiliary contact of the primary contactor. The PLC may also monitor the overload relay’s normally open (97-98) contacts. This allows the PLC to detect abnormal operation of the contactor. It also allows the PLC to identify a motor overload condition.
For reference, there are four general failure modes for the primary contactor including failure to close, failure to open (welded contacts), momentarily open when commanded closed (glitch), or forced when not energized. Depending on the system complexity, you may want to identify and log each abnormality. Additional information is available in this short article which described a PLC technique to identify each failure mode.
What is a motor starter’s solid-state overload relay?
Now that we understand the relationship between the contactor and the overload relay, we can explore the operation of the solid-state overload relay.
Conventional Motor Overload Relay
- Conventional Overload Relay: To understand the solid-state designator, we first need to explore a conventional overload relay such as the Schneider DPER06. As described in this article, the conventional overload relay contains heaters over bimetallic strips. As the motor’s current increases, the heaters heat up and cause the bimetallic strip to bend, ultimately pressing against the overload relay’s mechanical trip mechanism.
Figure 2: Internal workings of a conventional thermal overload relay. The heaters are composed of flat wire wound over and insulated from the bimetallic strips.
Tech Tip: It takes time for a conventional overload relay to cool off. This is a good thing as it also takes time for the motor to cool off. Don’t be in a hurry as the motor’s coils could be damaged by overheating.
Repeated start and stop cycles may cause the overload relay to trip due to cumulative overheating. Again, don’t be in a rush as it takes time for the motor to cool off. Be sure to provide training to your operators to prevent nuisance tripping.
Solid-State Motor Overload Relay
The solid-state relay replaces this mechanical mechanism with electronics and adds additional (smart) protection. For example, the Siemens catalog describes the featured ESP200 overload relay as follows:
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True phase loss protection; trips within 3 seconds
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Phase imbalanced detection based on unbalanced phase currents
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Ground fault trip when selected
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Selectable trip class 5, 10, 20 or 30
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Reset trip can be selected Auto/ Manual restart
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Easy to select and use, DIP switch selectable
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Overload is self-powered, no need for external power source
How is the solid-state overload relay configured?
Like the conventional overload relay, the solid-state relay has a full-load motor current adjustment. The featured Siemens ESP200 (3UB81334HW2) as shown in Figure 3 may be adjusted from 50 to 200 A.
The featured solid-state relay has additional configuration options that are controlled via the front panel DIP switches including:
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Trip class determines the speed at which the relay trips. A trip setting of 5 acts like a fast-blow fuse, while a setting of 30 is like a slow-blow fuse. This is an important consideration for motors that start heavy loads such as conveyers or unloaded air compressors.
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Phase imbalance prevents motor overheating. Note that a small apparent phase voltage imbalance can result in a large current imbalance. This can burn a winding in a three-phase motor.
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The lost phase setting should be carefully considered. While I’m not an electrician, I’ve seen my share of motors destroyed by single phasing. You couldn’t miss them, as they were accompanied by the loud curses of an electrician.
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The featured relay has a setting for manual or automatic reset. Personally, I’m hesitant to use an undisciplined automatic reset, as certain fault conditions could cause the system to toggle on and off until indefinitely, or at least until the motor is destroyed—whichever comes first. Instead, I recommend using a PLC to monitor the system and incorporate a smart fault detecting (lockout) mechanism. For instance, three overloads in a day will cause a hard shutdown (3-strikes you’re out). Another example is two overloads in any ten-minute window.
The ground fault detection may be useful for ungrounded systems. Siemens describes this as “optimum system protection of motors against high-resistance short-circuits or ground faults due to moisture, condensation, damage of insulation or any other reason.”
Figure 3: Close-up image of the Siemens ESP200 solid-state motor overload relay.
Parting Thoughts
The motor starter is a foundational component for industrial automation. The solid-state overload relay provides next-level protection for your high value motor investment.
Please share you experience with this essential technology.
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What tips do you have for the repair technician?
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What tips do you have for the PLC programmer?
Best wishes,
APDahlen
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.