Last revised: October 23, 2024
Reviewed by: Scott Orlosky, consulting engineer

AC motor starters are used to turn on and turn off electric motors and motor-controlled equipment. They use solid-state or electromechanical technologies, and are designed for single-phase or three-phase motors.                                                                               

AC motor starters adhere to ratings from the National Electrical Manufacturers Association (NEMA) and the International Electrotechnical Commission (IEC).

There are 11 NEMA size ratings: 00, 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. IEC sizes are rated at the thermal current rating: the current the contactor can withstand without overheating; or the operational current, the contactor’s ability to make or break the current.

Choices for IEC-rated products include A, B, C, D, E, F, G, H, J, K, L, M, and N. Typically, AC motor starters also carry ratings for continuous current, thermal current, motor voltage and AC horsepower.

Number of poles, control voltage, current-line acceleration, time-limit acceleration and operating temperature are also important parameters to consider when selecting AC motor starters.

Types of AC Motor Starters

Multi-speed products are designed to be operated at a constant frequency and voltage.

Reduced voltage starters (RVS) also change speeds, but use motor windings that can be reconnected to form different numbers of poles.

Reducing or reversing AC motor starters are designed for applications which must avoid overload conditions, or which need to avoid unnecessary wear and tear on equipment.

Full voltage, non-reversing AC motor starters have an overload relay and two contactors, which are either, mechanically or electrically interlocked.

Manual AC motor starters are also commonly available. They connect the incoming power directly to the motor, and are well-suited for squirrel-cage motors.

There are also many variations of reduced voltage starters. There is increased emphasis on reduced voltage starters for reduced power supplied at start-up. Reducing the power reduces potentially damaging electrical in-rush currents and mechanical shocks on the system. Here are a few strategies that make for less stressful motor starts.

• Primary resistor — Uses a resister in series with the motors power lines
• Autotransformer  — Transformer starter that can tap both primary and secondary windings
• Part winding — Only energizes a part of the winding at a time
• Wye delta  — Sometimes called delta starter; reduces voltage and current at motor start
• Solid state — A type of soft starter controlling voltage, current and torque
• Star delta — Used with three phase induction motors
• Soft Starter — Works by gradually increasing the applied voltage
• VFD — Converts line voltage to PWM (Pulse Width Modulation) control for more precise control
• Manual — Provides protection by allowing manual start control, limited to motors of less than 10 HP
• Direct on Line — Simple start/stop control used primarily with small horsepower motors.
• Stator Resistance — Feeds start current through resistors to reduce the in-rush current until motor reaches full speed

Overload Protection

Bimetallic overload relays are made of two metal strips that are joined together securely, usually being welded or riveted. Because metals expand and contract at different rates, heating a bimetallic strip causes it to bend. In turn, a spring pulls the contacts apart and breaks the circuit.

Solid-state relays for AC motor starters do not generate heat to trip a circuit. Instead, they measure current or a change in resistance.

Solid-state overload protection may also provide advanced functions such as:

• Ambient temperature sensing
• Programmable trip-time
• Current-change detection

AC motor starters with low voltage protection, phase failure (loss) trip, and phase reversal (unbalance) trip are also available.

AC Motor Starters FAQs

What are the key considerations when selecting an AC motor starter?

When selecting an AC motor starter, it is important to consider factors such as the frequency of starts and stops, the type of load, the required starting torque, and the need for overload protection. For basic motor starting with infrequent starts/stops, an across-the-line starter is a cost-effective and compact solution.

What are the benefits of using reduced voltage starters?

Reduced voltage starters are designed to minimize the inrush current and mechanical stress on the motor during startup. This can help extend the life of the motor and reduce wear and tear on connected equipment. They are particularly useful in applications where avoiding overload conditions is critical 

How do manual motor starters differ from other types?

Manual motor starters connect the incoming power directly to the motor and are typically used for squirrel-cage motors. They are simple, cost-effective, and suitable for applications where automatic control is not required.

What are some common applications of AC motor starters?

AC motor starters are used in a wide range of applications, including industrial machinery, HVAC systems, pumps, conveyors, and other equipment that requires reliable motor control and protection.

What are the advantages of using solid state starters?

Solid state starters offer several advantages over traditional motor starters. Here are some key benefits:

Reduced Mechanical Stress:

Solid state starters minimize the inrush current and mechanical stress on the motor during startup. This helps in extending the life of the motor and reducing wear and tear on connected equipment.

Smooth Starting:

They provide a gradual increase in voltage, which results in a smoother start for the motor. This is particularly beneficial in applications where avoiding overload conditions is critical.

Energy Efficiency:

By controlling the voltage and current more precisely, solid state starters can improve the overall energy efficiency of the motor system.

Compact and Reliable:

Solid state starters are typically more compact and reliable compared to traditional starters, as they have fewer moving parts and are less prone to mechanical failure.

Advanced Features:

Many solid state starters come with advanced features such as programmable settings, diagnostics, and communication capabilities, which can enhance the overall control and monitoring of the motor system.

Reduced Maintenance:

Due to the lack of mechanical components, solid state starters generally require less maintenance, leading to lower operational costs over time.

How do solid state starters compare to traditional starters in terms of overload protection?

Solid state starters and traditional starters differ significantly in terms of overload protection. Here is a comparison:

Overload Protection in Traditional Starters

Traditional starters typically use overload relays to provide overload protection. These relays measure the current flowing to the motor or detect changes in resistance .

Types of Overload Relays:

There are different types of overload relays used in traditional starters, including thermal overload relays and electronic overload relays. Thermal overload relays use a bimetallic strip that bends when heated by excessive current, while electronic overload relays use electronic sensors to detect overload conditions .

Disconnection Mechanism:

Once an overload condition is detected, the overload relay will disconnect the power supply to the motor to prevent overheating and potential damage to the motor windings and other components .

Overload Protection in Solid State Starters

Solid state starters often incorporate advanced electronic sensors and control mechanisms to provide precise overload protection. These sensors can monitor current and voltage more accurately, allowing for better protection and response to overload conditions.

Programmable Settings:

Many solid state starters come with programmable settings that allow for customized overload protection parameters. This can enhance the overall control and monitoring of the motor system .

Smooth and Gradual Voltage Increase:

Solid state starters provide a smooth and gradual increase in voltage, which helps in reducing the mechanical stress and inrush current during startup. This feature indirectly contributes to better overload protection by minimizing the chances of sudden overload conditions .

Reduced Maintenance:

Due to the lack of mechanical components, solid state starters generally require less maintenance, which can lead to more reliable and consistent overload protection over time.

AC Motor Starters Media Gallery

Related Standards

FORD EC1 — Nema ac motor starters, electro-mechanical type, requirements and tests 

References

GlobalSpec—Fundamentals of three-phase motor starters

GlobalSpec—Motor Starters and Contactors

Image Credits:

ABB Low Voltage Products & Systems | PHOENIX CONTACT USA | Control Techniques, an Emerson Industrial Automation business | Fuji Electric Corp. of America