Motor Starting & Control Methods for Industrial Phase AC Motors

AC Motor Motor Starting & Control Methods for Industrial Phase AC Motors Industrial Knowledge Zone

Electric motors are involved in almost every manufacturing process and consume 60% to 70% of the electricity used in manufacturing. Choosing the right motor control technology can help you reduce your energy costs and operate more efficiently. The following is a discussion of the most common motor control methods:

Across-the-line Motor Starter (or Direct on Line)

Using across-the-line control is similar to turning on a light switch: when the motor starter is turned on, the motor is instantly granted full power. As a result, the motor will reach running speed as fast as the load will allow. Once it is at full speed, the motor will run at one constant pace. When turned off, the motor will coast to a stop.

Across the line starting is the least expensive starting method. However, the load may experience a mechanical shock or jolt due to the rapid acceleration to full speed from the off position. The opportunities for energy savings with this method are minimal and can mainly be achieved by turning off the motor when it is not needed. Additionally, because large motors require higher starting torques and currents, this method is not suitable for use with large HP motors as it can cause brownouts that may affect other equipment.

Star/Delta Motor Starter

Star/Delta starting utilizes a multiple motor starter/contactor to start an AC motor with Star/Delta winding connection arrangement. While the motor is initially connected in a star arrangement, it is changed to a delta connection during the starting, resulting in reduced power usage upon motor startup. This reduced energy usage means that the load is only allowed 1/3 motor torque during startup.

Star/Delta starting is a cost effective method to reduce the power, torque and current required for motor startup. This technique may be used for larger motors and reduces the potential for brownout conditions. Compared to motors that use across-the-line control, Star/Delta starting minimizes the number of shock/jolt reduction results. This method, however, does not provide running speed adjustments and only reduces energy usage when starting the motor.

Soft Start

Soft Starts utilize electronic power components and logic to control the voltage, current and torque during motor starting and stopping. Short voltage, current or torque boosts are available at initial motor startup, which allow the motor to prepare itself for higher-load applications. This method of control reduces the amount of power required to start a motor and can also better control stopping.

Soft Starting is a cost effective method to reduce motor startup power demand. Because soft starting allows operators to progressively control voltage, the frequency of shocks and jolts is reduced, exerting less stress on mechanical components. Soft starters do not provide running speed control and provide minimal motor starting energy savings.

AC Variable Speed Drive or Variable Frequency Drive (VFD)

AC Variable Speed Drives allow for better control of voltage and frequency during motor startup, usage and stopping. Of all motor control methods, AC drives provide the most flexible motor control. Because AC Drives can directly adjust motor speeds, loads can operate at optimum speeds that match the desired operation for fixed and variable load demands. This control method reduces starting current and controls motor starting and stopping time. AC drives reduce the frequency of shocks and jolts, exerting less stress on mechanical drive trains. The ability to run motors at reduced speeds results in energy savings.

Variable torque pump and fan applications provide the largest savings potential, allowing motor speed to be reduced in order to better match operational needs. Additionally, the higher initial cost of AC variable speed drives may be offset reduced motor energy consumption, lower maintenance costs and/or savings related to improved production performance.


When it comes to choosing the best method of control for your motor systems, it is imperative to account for the characteristics of your application, desired performance and energy usage/savings considerations. At Kaman Industrial Technologies, we provide solutions all starting and controlling methods that have been discussed here. Contact your Account Manager, Electrical/Automation Specialist, Customer Service Representative, or Key Supplier Partner for assistance in selecting the best method for your application.

Starting Method
Characteristic Across the Line Star Delta Soft Start AC Variable Speed Drive
Speed/Torque Control None None Start/Stop Only Full Control
Shock/Jolt Reduction None Low High High
Reduce Start Inrush Current No Yes Yes Yes
Control Motor Stop No No Yes Yes
Purchase Cost Low Low/Medium Medium High
Potential for Energy Savings Low Low Low High

To learn more, contact your local Kaman to schedule a meeting with one of our AC motor specialist.

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Tim Tiebert
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