Introduction to the Magnetic Motor Starter

This article explores the defining features of the motor starter. This includes aspects of construction, the family of accessories, and the challenges of purchasing. It also touches on the complexity of the wire diagram and hints at integrating the device into a larger control panel. If you are new to motor starter, you may need to read this post a few times as the motor starter is very important and a surprisingly complex device. If motor starters are old hat, you may still find a few useful gems.

Note that we will not explore the reversing motor starter as the added level of complexity demands an article by itself.

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:

What is a magnetic motor starter?

The magnetic motor starter is a compact assembly containing a contactor (high power relay) to connect a motor to the AC feeder along with an overcurrent detection circuit. A representative example is the Schneider Electric DPE09 series and accessories as shown in Figure 1 (assembled) and 2 (disassembled). The term “magnetic” refers to the electromagnet inside the primary contactor.

What is the purpose of a motor starter?

Three-phase induction motors are commonly used in industrial environments. They are popular due to their high efficiency, coupled with a long, relatively maintenance free life. While these motors provide a robust solution, they are sensitive to failures such as single-phase events. This occurs when one of the three phases is lost or present at a significantly reduced voltage (single phase brown-out). The motor starter featured in this article is an essential safety element to protect the motor from self-destructing. It provides a measure of protection against mechanical overloads and motor overheating. It will also disconnect the motor from power if the motor develops faults such as defective winding.

Figure 1: Picture of the assembled Schneider Electric DPE series motor starter. Note the wire connecting the A2 coil terminal to the normally closed contact of the overload block.

Introducing a representative motor starter

The primary Schneider Electric DPE09BL 3-phase contactor is shown in the lower left of Figure 2. This is a relatively small contactor that makes a good case study as it represents the attributes of most motor starters.

Three-phase contactor

When used as a motor starter application, the DPE09BL is capable of powering:

  • 1 hp in a single phase 240 VAC system
  • 2 hp 208 VAC 3-phase
  • 3 hp 480 VAC 3-phase
  • 7.5 hp 690 VAC 3-phase

This wide range of motor horsepower reflects the design maximum 10 A contact and 690VAC voltage rating associated with the contactor. We observe that the motor’s current is in the same ballpark for each motor / voltage combination.

Adjustable overload block

The DPER06 adjustable overload block is shown in the lower right of Figure 2. A close inspection of the image reveals a dial that may be used to set the current limit between 1 and 1.6 A. It’s important to note that many different overload blocks may be attached to the contactor. The smallest Schneider alternative is the 0.1 to 0.16 DPER01 while the largest DPER14 is designed for 7 to 10 A. In all cases, it is important to match the overload block to the motor. The overload block detection setpoint must be set high enough to prevent nuisance overloads yet low enough to provide meaningful overload protection for the motor.

Aux contact block

The DPEAN11 auxiliary contact block is placed on top of the box as shown in Figure 2. This small block attaches to the top of the contactor as shown in Figure 1. From this position, the aux block contacts are activated by the contactor’s armature.

Inductive flyback diode

The final piece shown in Figure 2 is the LAD4TBDL bidirectional diode. This part is included with the DPE09BL contactor. However, we must emphasize the importance of this diode, as we must address the inductive flyback voltage developed when the contactor opens. This is a fascinating topic that is explored in depth in this article advocating for the use of interposing relays. This consideration is especially important when the contactor is driven by a PLC with solid state outputs.

Figure 2: Picture of a disassembled Schneider Electric DPE series motor starter.

Terminology associated with a motor starter

The terms relay, contactor, and motor starter are routinely applied to the device shown in Figure 1. For clarity, we will define the terms as follows:

  • Relay: The term relay is a general-purpose term referring to a class of electromagnetic devices featuring a coil and one or more contacts. This definition may be too open for some of our readers. Many people are more comfortable describing relays on a continuum with relays on one end and contactors on the other.

  • Contactor: The contactor is a subset of the relay family. Contactors are typically found on the physically larger end of the relay spectrum. They are purpose-built devices designed to handle high-power loads. Where a relay may have normally open and normally closed contacts, the contactors primary contacts will be normally open.

  • Motor Starter: A motor starter is yet another subset in the relay family The contactor is the fundamental component of the motor starter. To this contactor we add components to detect motor faults. As a rule, the detection is designed for soft faults such as overheating on a single phase. Hard (short circuit) faults require external fuses or circuit breakers.

Purchasing a motor starter

You will find that each manufacturer has a family of components that can be assembled to produce a motor starter. Rather than producing a dedicated device, each manufacturer targets a wide range of applications thereby reducing cost. For example, the contactors are typically available as independent devices. They are suitable for general purpose loads including simple resistive heaters. By our definitions, the contactor becomes a motor starter with the addition of the overload block designed to protect a specific motor. Other accessories may be added to integrate the contactor assembly into the greater control panel. This includes additional relay based or PLC based control circuitry as well as a user interface with switches and indicator lamps.

Purchasing a motor starter from DigiKey

With regards to DigiKey, we must remember that there are many combinations and permutations associated with the motor starter. As evidence, consider the possibilities for the DPE09 family:

  • coil voltage 24, 120, 230, and 240 VAC along with 24 VDC
  • 11 unique overload blocks with current ratings from 0.1 to 10 A
  • optional aux contact block

With this simple example, we see that the DPE09 family has over 100 motor starter combinations. Consequently, we should not expect the motor starter to be sold as a unit. Instead, we must carefully select the required individual component. This is best done using the family datasheet as provided by the manufacturer.

Wire Diagram of the motor starter

The wire diagram for the motor starter plus accessories is included as Figure 3. As you review this section, please refer to Figures 1 and 2 to locate the physical connections of the various assemblies.

Primary Contactor pins

  • Power is provided to the contactor’s coil via the A1 (positive) and A2 (negative) contacts. Polarity is critical for this 24 VDC Schneider contactor. Permanent magnets interact with the electromagnet to provide a responsive relay action while saving power.

  • The primary 3-phase inputs are identified as L1, L2, and L3.

  • The primary contactor features a single normally open contact identified by terminals 13 and 14

Overload block pins

  • The overload block receives power from the contactor’s T1, T2, and T3 terminals.

  • The current for each of the three phases is sensed by overload block. These sense elements are depicted in Figure 3 as the hook like components. For a thermal overload block these elements are like low value resistors (heaters) placed in close proximity to a bimetallic strip. As the current increases, so to does the heater. At some point the heat is enough to trip the bimetallic strip thereby latching an overload and toggling the normally open and normally closed overload block contacts (97 to 98 and 95 to 96).

  • The 3-phase output is in terminals T1, T2, and T3.

  • The overload block contains a pair of “trip” contacts.

Auxiliary block pins

  • The aux block has a pair of contacts with a normally open (53 to 54) and normally closed (61 to 62).

  • These contacts are directly linked to the armature of the primary contactor.

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

Tech Tip: The wire diagram is one way to present the motor starter. Ladder logic is another. With the wire diagram, the motor starter and all its components are in one location. With ladder logic, the coil and contacts are scattered into the individual rungs according to function. Both have their place depending on the needs of the technician, assembler, and engineer. Of these three categories of people the technician should be given the highest priority to facilitate troubleshooting and repair. Always remember that equipment down time is extraordinary expensive. The cost of designing equipment and documentation for efficient repair will pay dividends over the decade’s long lifetime of the equipment.

Motor Starter circuit connections

A complete description of the motor starter is beyond the scope of this article. However, as a starting point, please consider this Introduction to the 3-Wire Start-Stop Circuit. It presents one of the most basic applications of the motor starter.

As you read that article, please note the relationship between the coil and the overload block as shown in Figure 4. The contactor’s coil is series connected to the normally closed contacts of the overload block. This is a critical connection as it ensures an overload event will open the contactor thereby protecting the motor.

Tech Tip: The overload block is not a circuit breaker; the hooked elements in Figure 3 are not contacts; they are heaters. Instead, a motor overload condition will trip (toggle) the small overload switch elements including the normally open (97 to 98) and the normally closed (95 to 96). For a typical application, the normally closed switch (95 to 96) is wired in series for the primary contactors coil (A1 to A2). Consequently, an overload will open circuit the primary contactor removing power to the motor.

Figure 4: Wire diagram for the 3-wire start stop circuit developed using KiCad.

Tech Tip: Repeated motor starts within a small timeframe can cause the overload block’s sensing elements to overheat and trip the motor starter. Recall that a motor draws significant current as it starts stressing all components including the thermal overload block.

Be patient as you attempt to reset the thermal block the block, as it takes time for the elements to cool. From one old technician to another, if you find yourself in this position, it might be a good idea to take a break and rethink your troubleshooting strategy. By the time you get back, the heater elements will be at room temperature, and you should feel the solid click of the reset button. More importantly, the motor windings will have had a chance to cool down.


The 3-phase motor starter is a common component used in many industrial environments. As demonstrated in this article, the motor starter is assembled from a family of components including a contactor, thermal overload block, and auxiliary contacts as necessary to integrate the device into a greater industrial control panel. From within this family of components, the motor starter components are selected to match a given motor to a specific voltage and current window. As with many industrial automation and control components, careful review and understanding of the family’s datasheet is necessary to select the components to match your application.

Unfortunately, we have only begun to scratch the surface of this essential component. Perhaps in future articles we can explore how to match the motor starter to a specific motor, integrate the device into a larger control panel, and then explore electrical safety aspects associated with installation.

Please leave your comments and suggestion in the space below. Ideas for future content are most welcomed.

Best Wishes,


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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