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Devices That Control Motors

HVAC Contactors Starters Soft Starters VFDs
Summary of the article below:

Walk into almost any mechanical room, rooftop unit, pump skid, cooling tower, or air handler and you'll find one or more devices whose job is to control an electric motor. While they may look similar from the outside, contactors, motor starters, soft starters, and Variable Frequency Drives (VFDs) all serve different purposes.


For someone new to the HVAC industry, understanding the differences is important because selecting the wrong device can result in equipment damage, nuisance shutdowns, poor performance, or expensive replacement costs.

In this article we’ll explain what each device does, where it is commonly used, how it differs from the others, and what information is needed to properly identify or select a replacement.

[A Simple Analogy / Understanding the Devices That Control Motors]

HVAC Contactors, Starters, Soft Starters, and VFDs: Understanding the Devices That Control Motors 

Think of a motor like a car engine.

Devices                                                   Automotive Analogy
Contactor
Ignition switch
Motor Starter
Ignition switch plus engine protection system
Soft Starter
Automatic transmission that eases the vehicle into motion
VFD
Gas pedal and transmission combined, controlling both acceleration and speed

Each device adds another layer of functionality beyond simply turning equipment on and off.
[Contactor / Understanding the Devices That Control Motors]

What Is a Contactor?

A contactor is an electrically operated switch used to turn power on and off to a load such as a motor, compressor, electric heater, or pump.

When a control signal energizes the contactor's coil, a magnetic field pulls the contacts closed and allows power to flow to the equipment. When the control signal is removed, the contacts open and power is disconnected.

Common HVAC Applications
  • Air conditioning compressors
  • Condenser fan motors
  • Supply fans
  • Exhaust fans
  • Electric heat strips
  • Pumps
  • Refrigeration equipment
Main Components of a Contactor
  • Coil (typically 24V, 120V, 208V, 240V, or 480V)
  • Armature
  • Power contacts
  • Return springs
  • Auxiliary contacts (optional)
  • Arc suppression components
What a Contactor Does Not Do : A contactor does not protect a motor from overload conditions.
If a motor locks up or begins drawing excessive current, the contactor will continue supplying power until another protective device intervenes.

What Is a Motor Starter?

A motor starter is essentially a contactor combined with motor protection.

[The most common starter ]

ound in commercial HVAC systems is a magnetic starter, which contains:

  • A contactor
  • An overload relay
  • Auxiliary contacts
  • Optional enclosure or disconnect

[Common HVAC Applications ]

The overload relay monitors motor current and disconnects power if the motor draws excessive current for an extended period.

  • Air handling units
  • Cooling tower fans
  • Chilled water pumps
  • Condenser water pumps
  • Exhaust fans
  • Large rooftop units
Why Use a Starter Instead of a Contactor?

Imagine a fan motor whose bearings seize.

A contactor alone will continue providing power until the motor overheats or a breaker trips.

A starter's overload relay will sense the excessive current and shut the motor down before permanent damage occurs.
[Soft Starter / Understanding the Devices That Control Motors]

What Is a Soft Starter?

A soft starter is an electronic device that gradually increases voltage to a motor during startup.

Instead of instantly applying full voltage, a soft starter slowly ramps voltage up over a programmed time period.

Traditional Across-the-Line Starting with a contactor or starter:
  • Full voltage is applied instantly
  • Motor experiences high starting current
  • Equipment experiences mechanical shock
Soft Starter Starting with a soft starter:
  • Voltage increases gradually
  • Starting current is reduced
  • Mechanical stress is minimized

Benefits of Soft Starters

  • Lower inrush current
  • Reduced belt wear
  • Reduced coupling stress
  • Less bearing damage
  • Reduced water hammer in pump systems
  • Reduced voltage dips on electrical systems
Common HVAC Applications
  • Chilled water pumps
  • Condenser water pumps
  • Cooling tower fans
  • Large air handlers
  • Large compressors
  • Process equipment

How Soft Starters Work : Soft starters use electronic devices called Silicon Controlled Rectifiers (SCRs) to control how much of the AC voltage waveform reaches the motor during startup.
Once the motor reaches full speed, many soft starters bypass the SCRs and operate through an internal contactor to improve efficiency.

What Is a Variable Frequency Drive (VFD)

A Variable Frequency Drive (VFD) is an electronic motor controller that not only starts a motor smoothly but also controls its operating speed.

Unlike a contactor, starter, or soft starter, a VFD continuously adjusts motor speed by changing both voltage and frequency supplied to the motor.

This makes VFDs one of the most important energy-saving technologies used in modern HVAC systems.

Common HVAC Applications

  • Air handling unit supply fans
  • Return fans
  • Exhaust fans
  • Chilled water pumps
  • Condenser water pumps
  • Cooling tower fans
  • Variable flow hydronic systems

Why Use a VFD?

Most HVAC systems rarely need motors operating at 100% speed all the time.

For example:

  • A supply fan may only need 60% airflow on a mild day.
  • A chilled water pump may only require 70% flow during partial building occupancy.

Instead of cycling equipment on and off, a VFD allows the motor to run at the exact speed required.

Benefits of VFDs

  • Significant energy savings
  • Precise speed control
  • Reduced wear and tear
  • Soft starting capability
  • Improved comfort control
  • Lower maintenance costs

Comparison: Contactor vs Starter vs Soft Starter vs VFD

Feature                                   Contactor         Starter       Soft Starter     VFD
Turns motor on/off
YesYesYesYes               
Motor overload protection
NoYesUsuallyYes
Reduces inrush current
NoNoYesYes
Soft starting
NoNoYesYes
Variable speed operation
No                
No        No                Yes                       
Energy savings 
No
NoMinimalSignificant
Cost
Lowest
LowMediumHighest
Complexity
Low
LowMediumHigh

Where You Are Likely to Find Each Device

Equipment                                  Contactor      Starter     Soft Starter      VFD
Residential Condensing Unit
YesNoNo                    No               
Heat Pump
YesNoNoNo
Electric Heat
YesNoNoNo
Small Exhaust Fan
YesYesNoNo
Air Handler Fan                          
No           
Yes         Yes          Yes                       
Chilled Water Pump
No     
Yes    Yes         Yes
Cooling Tower Fan
No
YesYesYes
Large Compressor
NoYesYesSometimes
Process PumpNo  Yes  Yes     Yes        

Information Needed to Select a Replacement

Regardless of device type, certain information is critical*

 For Contactors

  • Coil voltage*
  • Number of poles*
  • FLA rating/LRA rating*
  • Contact configuration*
  • Voltage rating
  • Mounting dimensions

 For Motor Starters

Everything required for a contactor plus:

  • Motor full load amps (FLA)
  • Overload adjustment range
  • NEMA or IEC rating
  • Enclosure type

For Soft Starters

  • Motor horsepower
  • Motor voltage
  • Motor full load current
  • Starting method requirements
  • Number of starts per hour
  • Bypass requirements
  • Environmental conditions

For VFDs

  • Motor horsepower and/or FLA*
  • Motor voltage*
  • Single-phase or three-phase power*
  • Nema rating*
  • Bypass required
  • What type of bypass
  • Network communication requirements
  • Harmonic mitigation requirements
  • Application type (fan, pump, compressor, etc.)

Quick Takeaways - Field Identification Tips

 For Contactors look for:

  • Coil terminals
  • Power contacts
  • No overload adjustment dial

 For Motor Starters look for:

  • Contactor
  • Attached overload relay
  • Adjustable overload settings

For Soft Starters look for:

  • Large heat sinks
  • SCR modules
  • Digital settings or keypad
  • Internal bypass contactor

For VFDs look for:

  • Digital display
  • Programming keypad
  • Cooling fan
  • Input and output terminals
  • Communication ports
  • Numerous configurable parameters

Final Thoughts

Although contactors, starters, soft starters, and VFDs all control motors, they perform very different functions.

A contactor simply switches power. A starter adds motor protection. A soft starter reduces electrical and mechanical stress during startup. A VFD goes a step further by providing full speed control and energy optimization.

For someone new to HVAC, understanding these differences will make troubleshooting easier, replacement selection more accurate, and conversations with contractors, electricians, and controls technicians far more productive.

When selecting any of these devices, always start with the motor's voltage, full load current, horsepower, and application requirements. Those four pieces of information will guide you toward the correct solution and help ensure reliable system operation for years to come.

Need help with Understanding the Devices That Control Motors?

Reach out and we will be happy to help.

Email the Stromquist Team

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