Motor Control Coils

Motor controllers — like contactors and starters — drive and regulate the electrical load for equipment like fans, pumps, and large motors.

These control coils are a small but essential component for any motor or piece of electrical equipment. Despite being used interchangeably, starters and contactors are different in a few key ways. Understanding these differences is essential if you need to know how to identify a particular controller in an existing system or find the right controller for a new one.

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What Is a Magnetic Starter and a Contactor Coil?

A motor controller is a device that activates, deactivates or reverses the direction of an electrical motor. In the same way that a motor control relay operates, the motor control switches an engine on or off. Unlike a relay, the magnetic starter and contactor coils may also protect against power supply interruptions, low voltages and engine overloads or overcurrents.

Unlike a relay or switch, contactors will immediately release and open the closed contacts in case of a power supply interruption. As a result, they safeguard the motor against extreme electrical loads or unusual behavior from the power supply.

There are two main types of motor controllers — magnetic starters and magnet contactors. The two are similar in function and construction, but starters add some additional protective elements.

How Motor Control Coils Work

Contactors have three essential components — an electromagnet or “coil”, an enclosure, and contacts. 

The electromagnet drives power to the contacts, forcing them to close and create a circuit when activated. 

The contacts carry a current to the motor. Some contractors have spring-loaded contacts, which ensure the circuit is broken when the coil is de-energized, plus auxiliary contacts attached to the main contacts. These auxiliary contacts can be normally closed (NC) or normally open (NO) and typically change states with the main contacts. Auxiliary contacts can provide visual information to a user — for example, by driving power to a pilot light that signals the motor is on.

The enclosure, built from insulating materials, protects the contacts and coil — plus anyone working with the contactor or starter. Some open-air contactors may also have enclosures to reduce potential environmental damage or contamination. They can prevent dust, oil and other common industrial contaminants from interfering with the coil or contacts.

Contactors and starters can both include additional safety features, like magnetic arc suppression, mirror contacts and encapsulated coils. Coil encapsulation encases the electromagnetic coil in materials like resin and thermoplastic polymers to offer superior protection from oil, vibration and the elements. 

The Difference Between a Starter and Contactor

Motor starters are similar to contactors in construction, containing all the same essential components. However, a starter will also generally include additional safety features in the form of thermal overload relays. A starter is also more likely to feature support for three-wire control systems, which contactors typically aren’t built for.

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Despite the similarity of the two components, they’re typically classed differently. Contactors are grouped by voltage capacity and motor starters are categorized by current capacity and the horsepower of motors with which the starter will be compatible.

The Types of Magnetic Starters

There are two main types of motor starters — manual and magnetic starters. Magnetic models are operated using an electromagnet. Within that category, there are several common types of magnetic starters, classified based on how they send power to the motor.

Across-The-Line or Direct-On-Line Starters

The direct-on-line (DOL) starter is the least complex form of electromagnetic starters — only a manual model is simpler. A DOL starter is typically controlled by a push-button system or another basic switch. When the “on” or start button is pressed, the coil energizes, closing the contacts and creating a circuit. This action drives power to the motor. Pushing the off button or turning the switch to the off position will de-energize the coil, opening the contacts and breaking the circuit.

Because these starters are so simple, they have the advantage of being both cost- and space-efficient. They are often the most effective choice when your company needs a no-frills magnetic starter in situations where speed control and output torque aren’t particularly important.

Soft Starters or Reduced Voltage Starters

If speed and torque are important, however — as in an elevator or lift, where you generally don’t want sudden jolts when the starter switches on — a DOL starter can be a poor choice. 

In a case like this, you’ll want a soft starter, which provides a gentler startup for your company’s motor. These are similar in construction to DOL starters. However, they incorporate additional technology or circuit adjustments that enable the starter to limit inrush current and gradually start or stop a motor.

There are several commonly used categories of soft starters. Each uses a slightly different combination of technology and circuit design to bring the motor to full power in a controlled fashion:

  1. Primary resistor starters: These are some of the simplest soft starters. This model reduces the voltage delivered to the motor by installing series resistors in lines from the starter to the motor. As the motor reaches full speed, the current through the resistors decreases, gradually increasing the voltage.
  2. Autotransformer: These are a type of reduced voltage starter that uses autotransformers to gradually increase the voltage during motor starting. Once the motor is at full speed, the starter will deliver full voltage.
  3. Part winding: With this type, the starter’s winding is divided into two or three parts. As the motor speed increases, the starter activates additional part windings to step the engine up to full power.
  4. Digital or solid state starter: This is one of the most common — and advanced — soft starters. Digital starters use semiconductors — like diodes, thyristors and silicon-controlled rectifiers (SCRs) — to adjust motor voltage, current and torque with a high level of control. These starters are best in situations where motor acceleration needs to be carefully managed.

Star-Delta or Wye-Delta Starters

These starters have three windings that they physically reconfigure on a set timer to deliver two different states — one high voltage and one low voltage. This gradually ramps up the voltage sent to the motor terminals. Some star-delta starters will completely open the circuit, disconnecting the starter from the motor temporarily when changing states. Others will maintain the connection, providing just a decrease in voltage instead.

While similar in function to soft starters, these are generally in their own category due to the fixed number of states the star-delta controller switches between.

While soft starters and star-delta starters vary in complexity, they are all slightly more intricate in design than DOL controllers. They may cost more as a result. However, the gradual start they provide makes them a good fit for certain industrial applications where controlling motor speed and torque is essential.

Contact Global Electronic Services for Your Motor Control Coil Needs

Choosing the right magnetic motor starter is essential — especially when you need fine control over how much voltage you deliver to your engine on startup. 

Global Electronic Services is a leader in industrial electronics. Our technicians have the expertise to help you select the best motor control coil for the job, repair a starter or replace the coil in an existing system. 

Interested in working with us? You can reach us 24 hours a day, seven days a week by calling 877-249-1701 or requesting a quote here.

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