Motor Starters: Types, Ratings, and Applications

January 31, 2025/by Lynn Dreisilker

Motor starters play a crucial role in industrial motor control by ensuring electric motors start, operate, and shut down safely. These devices serve three key functions:

Control motor startup and shutdown to regulate power flow.
Protect against overcurrent and overheating to prevent equipment damage.
Enable automation and remote operation for improved efficiency.

Motor starters come in various configurations, each designed for specific applications, starting methods, and voltage requirements. This guide breaks down different starter types, ratings, and selection criteria to help you choose the best option for your needs.

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 offers the simplest and most widely used method for starting motors. It applies full-line voltage directly to the motor, providing a straightforward and cost-effective solution for small motors (typically under 10 HP).

DOL starters work best for non-sensitive loads that can handle high starting currents. However, they generate a significant inrush current—up to 600% of the motor’s full-load current. This surge puts stress on electrical systems and mechanical components, making DOL starters less suitable for high-inertia applications.

Despite this limitation, their simplicity and affordability make them a go-to choice for 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.

NEMA Size	Continuous Amp Rating	HP @ 230V	HP @ 460V
00	9A	1 HP	2 HP
0	18A	3 HP	5 HP
1	27A	7 HP	10 HP
2	45A	15 HP	25 HP
3	90A	30 HP	50 HP
4	135A	50 HP	100 HP
5	270A	100 HP	200 HP

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

Factor	Considerations
Motor Size (HP & Amps)	Larger motors need reduced-voltage starters.
Voltage Rating	Ensure compatibility with 230V, 460V, or 600V.
Starting Torque Needs	High-torque loads need autotransformer or part-winding starters.
NEMA vs. IEC Standards	NEMA for heavy-duty use, IEC for space-saving applications.
Duty Cycle & Frequency	High-cycling loads require AC-4 rated starters.

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.

Understanding Motor Controls: Starters, Contactors, Overloads, and Disconnects

January 31, 2025/by Lynn Dreisilker

Starters:

Motor starters control the starting and stopping of electric motors while offering built-in protection features.

Importance of Starters:

Soft Start Options: Some starters reduce inrush current, preventing electrical and mechanical damage.
Remote & Automated Control: Allows for safer and more convenient motor operation.
Integrated Protection: Most starters include overload protection to prevent overheating and excessive current draw.

Types of Starters:

Combination Starters: Include a contactor, overload relay, and disconnect switch, simplifying installation and ensuring NEC compliance.
Non-Combination Starters: Only contain a contactor and overload relay, requiring an external disconnect switch or breaker.

Example Application: Combination starters are commonly used in industrial HVAC systems, where integrated safety and compliance are required.

Siemens 14CUB32BA NEMA Size 0 NEMA 1 Enclosure Motor Starter

Contactors

Contactors are electromechanical switches that control power flow to motors, lighting, and industrial equipment.

Importance of Contactors:

Handles High Currents: Designed to switch large electrical loads efficiently.
Remote Operation: Enables control from a panel or automation system, improving safety and efficiency.
Fail-Safe Designs: Many contactors have safety features, such as arc suppression and interlocking mechanisms, to prevent unintended activation.

Types of Contactors:

3-Pole Contactors: Standard for three-phase motors, used in most industrial applications.
4-Pole Contactors: Include an extra pole to switch the neutral wire, useful in backup power and generator transfer systems.
2-Pole Contactors: Used for single-phase motors, such as in HVAC compressor control.
Reversing Contactors: Allow a motor to change direction by swapping phase connections.
Vacuum Contactors: Used in high-voltage applications, offering superior arc suppression.
DC Contactors: Designed for battery-powered equipment, such as electric vehicles and renewable energy systems.

Example: A 4-pole contactor is commonly used in standby power systems, ensuring that both phase and neutral are disconnected when switching between power sources.

Packard C440C 40 Amps 208/240 Coil Voltage 4 Pole Contactor

Overloads

Overload relays protect motors from excessive current by monitoring electrical load conditions and shutting down the circuit when necessary.

Importance of Overloads:

Motor Protection: Prevents overheating and potential motor burnout.
Adjustable Settings: Allows customization based on motor size and operating conditions.
Safety Compliance: Ensures adherence to NFPA 70 (NEC) and IEC motor protection standards.

Types of Overload Protection:

Thermal Overload Relays: Use bimetallic strips to detect excessive heat buildup.
Electronic Overload Relays: Provide digital monitoring for precise protection.
Magnetic Overload Relays: React to sudden high-current spikes, often integrated with circuit breakers.

Example: In a pump control system, an electronic overload relay prevents damage if the pump gets blocked and starts drawing excessive current.

Siemens 48ATC3S00 ESP200 Solid State Overload Relay

Disconnects

A disconnect switch provides a manual way to isolate electrical circuits for maintenance or safety compliance.

Importance of Disconnects:

Ensures Safe Maintenance: Prevents accidental re-energization of equipment.
Code Compliance: Required by OSHA and NEC regulations for industrial electrical systems.
Reliable Isolation: Helps prevent electrical hazards during repairs.

Types of Disconnect Switches:

Fused Disconnects: Provide both isolation and overcurrent protection.
Non-Fused Disconnects: Used where a separate circuit breaker or fuse provides protection.
Enclosed Disconnects: Designed for harsh environments with NEMA-rated enclosures.

Example: A disconnect switch is required in manufacturing plants to isolate conveyor motors for maintenance, preventing accidental restarts.

How These Components Work Together in a System

These four components form the foundation of an industrial motor control system. Here’s how they work together:

Starting the Motor: The operator activates the starter, energizing the contactor, which allows power to flow to the motor.
Monitoring and Protecting the Motor: The overload relay continuously monitors current. If excessive current is detected, the overload relay trips the circuit, stopping the motor before damage occurs.
Isolating Power for Maintenance: Before maintenance, the disconnect switch is used to completely cut power to the motor and control circuit. This ensures safe servicing and compliance with electrical safety regulations.

Conclusion

Understanding the role of starters, contactors, overloads, and disconnects is essential for industrial operators, electricians, and engineers. By selecting and integrating these components correctly, facilities can ensure:

Safe operation of motors and electrical systems.
Reduced downtime through proper overload protection.
Compliance with standards.

By implementing the right motor control solutions, businesses can enhance efficiency, extend equipment life, and minimize operational risks.

Electric Motor Protection: Basics of Overload Relays

March 14, 2018/by Lynn Dreisilker

Motors can be damaged by excess heat caused by current flow when there are overload conditions. Some examples include a locked shaft, too many systems on a circuit, the power supply single phasing on a three phase circuit. Installing overload relays in your applications can protect your motors.

Why are Overload Relays needed?

When a motor starts, it typically requires 6 times the full-load current rating. After the motor ramps up to operating speed, the current drops off. Motors are designed to handle this overload condition only for a short period of time. If a motor maintains this overload condition, the motor will overheat and potentially become damaged.

While fuses and circuit breakers can protect your system from short circuits, ground faults, or an overload, they are not the proper protection device for motors. As noted above, motors pull significantly more amps at startup than their full-load current rating. Any fuse used with a motor would need to be rated to handle this higher startup amp draw, therefore it would fail to protect the motor from overload conditions beyond normal startup. Overload relays are designed to allow temporary overloads for a specific period during startup. If the overload persists, the overload relay will trip and break the circuit to protect your motor. Overload relays can be easily reset after the overload is corrected.

Overload Relay Trip Classes

Overload relays have a trip class rating for different applications. The most common trip classes are Class 10, Class 20 and Class 30. The number in a trip class is simply the total number of seconds that the motor is allowed to overload before the circuit trips. For example, if you have an overload relay with a Class 10 rating, your system will allow an overload condition for 10 seconds before the overload relay trips to protect your motor.

Types of Overload Relays

A few different types of overload relays include Bimetal Overloads, Ambient-Compensated Overload Relay, and Electronic Overload Relays.

Bimetal Overloads use a bimetal strip that acts as a trip lever. When there is an overload condition, the bimetal strip becomes heated and will bend to close and trip the circuit.
Ambient Compensated Overload Relays are similar to Bimetal Overloads. The main difference is that the Ambient Compensated Relays allow for there to be an ambient temperature, such as the temperature of the surrounding environment. These relays can prevent false tripping by allowing the ambient temperature to be higher.
Electronic Overload Relays do not have heaters found in Bimetal and Ambient-Compensated Overload Relays. The Electronic Overload Relays also offer phase loss protection by detecting phase losses and disconnecting the motor from the power source. There are many types of Electronic Overload Relays to fit a lot of applications.

Installing overload relays in motor applications will prevent motors from running in overload conditions and can protect your motors from damaging heat. There are many types and settings for overload relays. If you need guidance finding the right overload relay for your application, call us today!

Motor Starters: Types, Ratings, and Applications

What is a Motor Starter?