Motor starters are essential in industrial motor control, ensuring safe startup, operation, and protection of electric motors. Their primary role is to:

• Control motor startup and shutdown
• Protect against overcurrent and overheating
• Enable automation and remote operation

Motor starters are available in different configurations based on their application, starting method, and voltage requirements. This guide explores starter types, ratings, and selection criteria to help you choose the right motor starter.

Siemens Furnas ESP100 Starter Courtesy of SIemens

What is a Motor Starter?

A motor starter is an electrical device that manages power delivery to a motor and provides overload protection. It is composed of:

• A contactor (switch) for power control
• An overload relay for motor protection
• A disconnect mechanism (in some cases) for safety isolation

Types of Motor Starters:

Full-Voltage (Direct-On-Line) Starters

A Direct-On-Line (DOL) starter is the simplest and most commonly used motor starter. It directly applies full-line voltage to the motor, making it a straightforward and cost-effective solution for small motors (typically under 10 HP). These starters are best suited for non-sensitive loads where high starting currents are not a concern. However, DOL starters produce a significant inrush current, which can be up to 600% of the motor’s full-load current. This sudden surge can stress electrical systems and mechanical components, making them less ideal for high-inertia applications. Despite this, their simplicity and affordability make them a preferred choice for applications such as small pumps, compressors, and basic conveyor systems.

Reduced-Voltage Starters

Unlike DOL starters, reduced-voltage starters are designed to limit inrush current and voltage, preventing excessive electrical and mechanical stress on the motor during startup. These starters are commonly used for larger motors or in applications where reducing power disturbances is critical.

One of the most widely used reduced-voltage starters is the Star-Delta (Wye-Delta) Starter. This method initially starts the motor in a star (Y) configuration, which supplies a lower voltage and reduces starting torque. Once the motor reaches a sufficient speed, it transitions to a delta (Δ) configuration, providing full voltage and normal operation. This type of starter is commonly used in HVAC compressors, industrial fans, and water pumps, where a gradual increase in power helps protect the motor and electrical infrastructure.

Another type is the Autotransformer Starter, which utilizes a step-down transformer to supply reduced voltage to the motor during startup. This starter allows for different voltage tap settings—typically 50%, 65%, or 80% of full voltage—to suit various application requirements. The key advantage of autotransformer starters is that they provide higher starting torque compared to Star-Delta starters while still limiting inrush current. They are often found in high-torque applications such as blowers, large pumps, and industrial compressors.

The Part-Winding Starter is specifically designed for motors with two stator windings. Initially, only one winding is energized, allowing for a gentler startup. After a short delay, the second winding is connected, bringing the motor to full operational voltage. This method helps reduce electrical demand at startup while maintaining strong performance. Part-winding starters are commonly used in HVAC refrigeration compressors, industrial fans, and pump systems.

Another reduced-voltage method is the Primary Resistance Starter, which introduces series resistors into the motor circuit to restrict inrush current. As the motor accelerates, these resistors are gradually bypassed, allowing the motor to reach full speed. Although effective in limiting current, this method is less efficient due to energy loss in the resistors. It is primarily used in belt-driven conveyor systems, gear-driven equipment, and other applications where smooth acceleration is necessary.

Combination vs. Non-Combination Starters

Motor starters can be classified into combination and non-combination types, depending on how their protective components are integrated. A Combination Starter includes a contactor, overload relay, and disconnect switch within a single enclosure. This configuration ensures compliance with NEC safety regulations and simplifies installation by consolidating control and protection elements in one unit. Combination starters are preferred in applications requiring integrated safety measures and streamlined motor control.

In contrast, a Non-Combination Starter contains only the contactor and overload relay, requiring a separate disconnect switch to isolate the circuit. While this setup allows for greater customization, it necessitates additional wiring and panel space. Non-combination starters are often used in motor control centers (MCCs) where disconnects are installed separately for multiple motors.

Motor Starter Ratings: NEMA vs. IEC

NEMA Starter Ratings

NEMA (National Electrical Manufacturers Association) starters are rated by size and horsepower capacity. They are known for their heavy-duty construction and ability to handle harsh industrial environments.

IEC Starter Ratings

IEC (International Electrotechnical Commission) starters are compact, modular, and efficient. Instead of NEMA sizes, they use utilization categories, including:

• AC-1 – Non-inductive loads.
• AC-2 – Slip-ring motor starting.
• AC-3 – Standard motor switching.
• AC-4 – Plugging and reversing applications.

Selecting the Right Motor Starter

Conclusion

Motor starters play a critical role in industrial motor control, ensuring smooth operation, overload protection, and longevity. Whether you need a DOL starter for a small pump or a soft starter for industrial automation, understanding their types, ratings, and applications is key.

Need help selecting a motor starter? Contact Dreisilker Electric Motors for expert recommendations.