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When figuring out a power solution for an application, engineers are faced with a plethora of options, from the type of motors to how they are controlled.  This article breaks down the three main categories of options for how electric motors are controlled. As you can see below, many factors should be considered to determine the best starter including cost, functionality and requirements of the application.

  • Across-the-line Starters
    • Most Economical
    • Basic Level of Control

“Across-the-line starters” or “Direct-on-line starters” (DOL) simply connect and disconnect power to the motor. They are most often used in applications with motors of 50hp and below that are intended to run continuously. DOL starters include short circuit and overload protection and they provide ON-OFF feedback but they are not suitable for systems that must be cycled often.  However, for low horsepower, simple, low cost applications, DOL starters are an affordable option.

  • Soft Starter
    • “Middle of the Road” Cost
    • Provide ability to gradually ramp speed of motor

Soft starters can vary the speed of the motor, and are most commonly used in on/off applications which require higher frequency of cycles (turned on & off), such as compressors and conveyor belts or Hydraulic Power Units.  Soft starters usually include electronic controls which allow the motor to temporarily reduce the load and torque in the power train and electric current surge of the motor during start up.  This functionality avoids large current surges and minimizes the amount of wear on electrical contacts and motor winding in the system.  For example, in conveyor applications, a soft starter would help the conveyor to start smoothly rather than jerk to a start and place stress on drive components. In fans or other systems with belts, it’s important for drives to be started slowly to avoid belt slipping.  Soft starters are not capable of running motors at partial speeds, so if a motor speed must be adjusted at any time other than start-up the better solution to employ is a VFD (see below).

  • VFD
    • High End Cost (with many different levels of cost available)
    • Endless options for speed control and closed-loop control

Although the costliest option for motor control, VFDs enhance efficiency of the application with ability to run motors at variable speeds depending on the current demand of the application for power supply.  With 25% of the world’s power supply being used by electric motors, VFDs have become widely used to control electric motors over the past 20 years. 

VFDs also allow for applications such as dynamic braking and regenerative drives.  Dynamic braking occurs where the motor is operated faster than synchronous speed causing it to act as a generator, converting mechanical power back to electrical power or dissipated as heat.  Regenerative drives refer to the capacity of an AC drive to move faster than designated motor speed and return the electric power back to the system.  An example would be a crane, where the hoist motor stops and reverses frequently and braking is required to slow the load during lowering.  In the lowering stage, the VFD drives regenerates power to be used back in the system during the next hoist movements.

VFDs provide the most options for control, but also require the most electrical components and are generally more sensitive to electrical noise and environmental conditions making them unsuitable or less cost effective for some solutions.

Elite Controls builds and supplies with all of the above solutions – we’re happy to help you specify the best motor and control options for your next application!