Starting a large electric motor is not as simple as switching on a small appliance. At the moment of startup, an induction motor can draw several times its normal operating current because the rotor is stationary and has not yet developed counter-electromotive force.

This sudden current demand can produce voltage dips, flickering lights, nuisance breaker trips, excessive heating, and stress on motors, couplings, belts, pumps, and other connected equipment. It may also create power-quality concerns or conflict with utility and facility limits on motor-starting current.

A motor starting autotransformer addresses these problems by temporarily reducing the voltage supplied to the motor. The lower starting voltage reduces the current drawn from the electrical system while still providing enough torque to accelerate the connected load. Once the motor reaches an appropriate speed, the autotransformer is bypassed and the motor continues operating at full line voltage.

This form of reduced voltage starting remains valuable in water and wastewater plants, mines, oil and gas facilities, manufacturing operations, commercial HVAC systems, and other sites that depend on large three-phase motors.

What Is a Motor Starting Autotransformer?

A motor starting autotransformer is a specialized reduced-voltage device used during the acceleration of a three-phase induction motor.

Motor Starting Autotransformer Definition

A motor starting autotransformer is a tapped three-phase autotransformer that supplies reduced voltage to a motor for a limited period during startup. It normally forms part of an autotransformer motor starter, which also includes contactors, controls, timing equipment, and motor protection.

The autotransformer is connected only during the starting sequence. After the motor accelerates sufficiently, the starter transitions the motor to full supply voltage and removes or bypasses the autotransformer.

This is an important distinction: the motor starting transformer provides the reduced voltage, while the complete starter controls the full starting and transition sequence.

Our motor starting autotransformers are available for a range of three-phase voltages, horsepower ratings, and tap configurations. The product category currently includes designs for common industrial voltages such as 208V, 240V, 380V, 400V, 416V, 480V, and 600V. 

For a broader explanation of the winding arrangement, tap points, and voltage-conversion process, review our guide on what an autotransformer is and how it works.

Primary Purpose

The main objectives of an autotransformer starter are to:

• Reduce motor-starting current
• Limit voltage drop within the facility
• Reduce stress on transformers, conductors, and switchgear
• Provide adequate starting torque for the connected load
• Reduce mechanical shock during acceleration
• Support utility and facility power-quality requirements
• Protect the motor and driven equipment from unnecessarily harsh starts

A correctly selected starter does not eliminate inrush current. Instead, it controls the magnitude of the starting demand so that the motor and electrical system can operate more reliably.

Why Large Motors Need Reduced-Voltage Starting

The larger the motor and the weaker the electrical supply, the more significant the effects of full-voltage starting can become.

Understanding Motor Inrush and Locked-Rotor Current

When an induction motor is stationary, the rotor is effectively in a locked-rotor condition. Applying full rated voltage at this point can produce a current several times greater than the motor’s full-load current.

Many induction motors draw approximately five to eight times their full-load current during direct starting. The typical locked-rotor current for common induction motors at approximately 600% to 700% of full-load current, although the actual value depends on the motor design, voltage, size, and application.

For example, consider a motor with a full-load current of 100 amperes. A full-voltage start could temporarily demand approximately 600 to 700 amperes. That surge may last only a few seconds, but it can still create serious consequences if the supply system is not designed to support it.

Our guide to what inrush current is and how to manage it provides additional background on temporary current surges and their effects on electrical equipment.

Problems Caused by Across-the-Line Starting

Across-the-line starting connects the motor directly to full supply voltage. It is simple and economical, but it is not suitable for every large-motor application.

Potential problems include:

• Voltage Sag: High starting current can lower voltage at the motor terminals and elsewhere in the facility.
• Flickering Lighting: Voltage changes may be visible in lighting systems connected to the same distribution network.
• Contactor Dropout: Other control devices may release if voltage falls below their operating threshold.
• Nuisance Breaker Operation: Protective devices may trip when starting characteristics are not properly coordinated.
• Mechanical Shock: Immediate application of full starting torque can stress couplings, shafts, belts, gearboxes, and driven equipment.
• Motor Heating: Long or repeated starts can create damaging thermal stress in the motor windings and rotor.
• Process Interruption: A failed or incomplete start can stop pumps, conveyors, compressors, and production equipment.
• Utility Concerns: Large starts may exceed permitted voltage-disturbance or motor-starting limits and require corrective measures.

Every motor start introduces both thermal and mechanical stress. The motor manufacturer’s limits for starting duration, starts per hour, and cooling time must therefore be respected.

Understanding why across-the-line starting can damage large motors helps determine when a simple full-voltage starter may expose equipment to unnecessary electrical and mechanical stress.

How Does a Motor Starting Autotransformer Work?

The starter uses transformer taps and a timed contactor sequence to reduce voltage during acceleration and then transfer the motor to full line voltage.

1. The Motor Starts at Reduced Voltage

When a start command is received, the starting contactors connect the motor to taps on the autotransformer.

Common tap settings provide approximately:

• 50% of line voltage
• 65% of line voltage
• 80% of line voltage

The selected tap determines the voltage reaching the motor during the starting period.

2. Current Draw Is Reduced

Reducing motor-terminal voltage reduces the motor’s locked-rotor current. The autotransformer action then reduces the current drawn from the supply system by an additional amount.

For an idealized starter, the approximate relationships are:

• Motor current is proportional to the selected voltage tap
• Supply line current is proportional to the square of the tap ratio
• Starting torque is also approximately proportional to the square of the tap ratio

This is why an autotransformer starter can provide favourable starting torque while substantially reducing the current seen by the upstream electrical system.

3. The Motor Accelerates

The motor begins turning and accelerates the connected equipment. As speed increases, motor current gradually falls.

The starting period must be long enough for the motor to reach a suitable transition speed but short enough to remain within the thermal limits of the motor and the starter.

Loads with high inertia or high breakaway torque may require a higher tap setting, a longer acceleration period, or a different starting method.

4. The Starter Transitions to Full Voltage

Once the timer, current relay, speed sensor, or controller determines that the motor is ready, the contactor sequence transfers it to full line voltage. The autotransformer is then bypassed, and the motor operates normally at rated voltage.

The exact sequence may use an open-transition or closed-transition design. Contactors must be correctly interlocked so that an unsafe connection or short circuit cannot occur during transfer.

Suggested Four-Step Process Diagram: Start command → Reduced-voltage tap energized → Motor accelerates with limited line current → Autotransformer bypassed and motor runs at full voltage

Choosing the right motor starter tap settings, typically 50%, 65%, or 80%, requires balancing the motor’s starting torque needs with the desired reduction in current.

Main Components of a Motor Starting Autotransformer

A complete starter depends on several coordinated electrical and control components.

Autotransformer

The three-phase tapped autotransformer provides reduced voltage during startup. It is normally short-time rated because it operates only during acceleration rather than continuously.

Main or Line Contactor

The main contactor connects the starter assembly to the incoming supply. Its current, voltage, interrupting, and duty ratings must match the application.

Start Contactor

The start contactor connects the motor to the selected transformer taps during the reduced-voltage stage.

Run or Bypass Contactor

The run contactor connects the motor directly to full line voltage once acceleration is complete. It bypasses the autotransformer during normal operation.

Timer or Transition Controller

The timer or controller determines when the transition occurs. A fixed timer may be suitable for a predictable load, while more advanced systems may use current, voltage, or speed feedback.

Transition timing is critical. Switching too early can create a second current surge because the motor has not accelerated sufficiently. Switching too late can overheat the motor or starter.

Overload Protection

Overload protection responds when motor current remains excessive for too long. It must be selected and adjusted for the motor, starter arrangement, service factor, operating conditions, and applicable electrical requirements.

Control Circuit

The control circuit manages contactor coils, interlocks, start and stop commands, timers, alarms, and protective inputs.

A properly selected control transformer may be used when the available line voltage does not match the required control voltage. Transformer Source offers control-transformer configurations for common Canadian industrial voltages and control-circuit requirements. 

Common Motor Starter Tap Settings Explained

Tap selection determines the balance between current reduction and the torque available to accelerate the load.

These values are approximate and are expressed relative to full-voltage starting values, not relative to normal full-load current or torque. Schneider Electric describes 50%, 65%, and 80% taps as producing approximately 25%, 42%, and 64% of full-voltage line current and starting torque. 

50% Tap

The 50% tap provides the greatest reduction in supply current. It may be suitable for equipment that starts with a light load, such as certain unloaded pumps or fans.

However, it provides only about one-quarter of the motor’s full-voltage starting torque. If load torque exceeds available motor torque, the motor may accelerate too slowly or fail to start.

65% Tap

The 65% tap is a common starting point for many industrial applications because it provides a practical balance between current reduction and available torque.

It supplies approximately 42% of full-voltage starting torque and draws approximately 42% of full-voltage starting current from the line.

It may suit many pumps, compressors, blowers, and general industrial loads, but it should not be selected without reviewing the motor and load curves.

80% Tap

The 80% tap provides approximately 64% of full-voltage starting torque. This higher torque may be required for high-inertia equipment, loaded conveyors, compressors, crushers, or machines with substantial breakaway requirements. The trade-off is a higher current demand on the supply system.

How to Choose the Correct Tap

Tap selection should consider:

• Motor locked-rotor current
• Motor starting-torque curve
• Load breakaway torque
• Load torque throughout acceleration
• Combined motor and load inertia
• Acceptable acceleration time
• Available system capacity
• Permitted voltage drop
• Maximum starts per hour
• Motor and starter thermal limits

The correct setting is generally the lowest tap that can accelerate the load reliably within the permitted time. Selecting a tap that is too low can be just as harmful as selecting one that is unnecessarily high.

Advantages of Motor Starting Autotransformers

Autotransformer starting provides several practical benefits in large-motor applications.

Reduced Supply Inrush Current

Because the autotransformer reduces both motor voltage and upstream line current, it can significantly limit the current imposed on transformers, feeders, switchgear, and generators.

Lower System Voltage Drop

Reducing starting current helps maintain voltage for lighting, control systems, computers, instrumentation, and other motors connected to the same electrical system.

Strong Starting Torque Relative to Line Current

Compared with some traditional reduced-voltage arrangements, an autotransformer motor starting system can deliver favourable starting torque for the amount of current drawn from the line. This makes it useful when both power-system limitations and load-torque requirements must be addressed.

Improved Power Quality

Limiting current demand can reduce the depth of voltage sag and the risk of contactor dropout, control-system resets, or disruption to neighbouring loads.

Reduced Mechanical Stress

Lower starting torque can create smoother acceleration and reduce shock on:

• Shafts
• Couplings
• Belts
• Gearboxes
• Valves
• Piping systems
• Conveyed materials

The starter still applies fixed steps of voltage rather than a continuously controlled ramp, so it is not as smooth as a correctly configured soft starter or variable-frequency drive.

Longer Equipment Service Life

Reducing severe starts may help limit cumulative stress on the motor and driven equipment. Actual service life still depends on loading, operating temperature, maintenance, alignment, ventilation, and the number of starts.

Support for Utility and Facility Requirements

An industrial motor starter may be required where a direct start would create an unacceptable voltage disturbance or exceed the available capacity of the electrical system.

Common Motor Starting Autotransformer Applications

These starters are commonly considered for large three-phase motors that require dependable torque without imposing full locked-rotor current on the supply.

Water and Wastewater Facilities

Typical equipment includes:

• Raw-water pumps
• Distribution pumps
• Lift-station pumps
• Aeration blowers
• Sludge-handling equipment

Mining and Aggregate Operations

A three-phase motor starter may support:

• Crushers
• Conveyors
• Screens
• Pumps
• Ventilation fans
• Material-handling equipment

Oil and Gas Facilities

Applications may include compressors, transfer pumps, process fans, and other fixed-speed equipment operating in demanding environments. Equipment selection must account for hazardous-location requirements where applicable.

Manufacturing Plants

Common loads include:

• Large machine tools
• Process pumps
• Hydraulic systems
• Production conveyors
• Compressors
• Dust-collection fans

Commercial and Industrial HVAC

Large chillers, air-handling fans, cooling-tower fans, pumps, and blowers may benefit from reduced-voltage starting when full-voltage current would disturb the building’s electrical system.

High-Inertia and High-Torque Equipment

Autotransformer starting is often evaluated for:

• Pumps
• Compressors
• Crushers
• Fans and blowers
• Conveyors
• Centrifuges
• Mills
• Heavy rotating machinery

Selecting motor starters for pumps, compressors, and crushers requires careful consideration of each load’s starting torque, inertia, and acceleration demands.

Motor Starting Autotransformer vs. Other Starting Methods

The best starting method depends on current limits, torque requirements, speed-control needs, operating profile, maintenance capability, and project budget.

When an Autotransformer Starter Is the Best Choice

Consider an autotransformer starter when:

• The motor will run at a fixed speed
• Full-voltage current is unacceptable
• Relatively strong starting torque is required
• A proven electromechanical solution is preferred
• The electrical environment makes continuously operating power electronics less desirable

When a Soft Starter Is the Better Choice

A soft starter may be preferred when the objective is smooth acceleration and deceleration, adjustable current limiting, and reduced mechanical shock without continuous speed control. It is particularly useful for pumps where controlled stopping may reduce hydraulic transients.

When a VFD Is the Better Choice

A variable-frequency drive (VFD) is usually the stronger option when the process requires:

• Adjustable operating speed
• Precise torque control
• Process optimization
• Controlled acceleration and deceleration
• Potential energy savings on variable-torque pumps and fans

A VFD should not be selected solely because it produces low starting current. Harmonics, cable length, motor insulation, cooling at reduced speed, bypass requirements, and maintenance capability must also be considered.

When Across-the-Line Starting Is Appropriate

Across-the-line starting may remain the most economical option for smaller motors or facilities with a strong supply system and equipment that can tolerate full starting torque.

Comparing an autotransformer vs. soft starter vs. VFD helps determine which option best matches the motor’s starting current, torque, speed-control, and operating requirements.

How to Size a Motor Starting Autotransformer

Starter selection must be based on both the motor nameplate and the mechanical load.

1. Confirm Motor Horsepower

Record the motor’s rated horsepower or kilowatts. Do not estimate motor size from the driven equipment alone.

2. Confirm Voltage, Frequency, and Phase

Verify:

• Line voltage
• Motor voltage
• Supply frequency
• Three-phase configuration
• Available transformer taps

Transformer Source offers motor-starting models for multiple industrial voltages and horsepower ranges, but the exact starter must match the installation. 

3. Determine Full-Load and Locked-Rotor Current

Use the motor nameplate and manufacturer’s data. Full-load current alone does not reveal the complete starting requirement.

Obtain the motor’s:

• Full-load amperes
• Locked-rotor amperes or NEMA code letter
• Starting torque
• Pull-up torque
• Acceleration limitations

4. Determine Load Torque and Inertia

Identify whether the motor starts unloaded, partially loaded, or fully loaded. A centrifugal pump with a closed discharge valve may have a very different starting requirement from a loaded conveyor or reciprocating compressor.

5. Select a Preliminary Tap

Compare the motor’s reduced-voltage torque curve with the load torque curve. The selected tap must provide adequate accelerating torque throughout the entire speed range, not only at zero speed.

6. Verify Acceleration Time and Duty

Confirm:

• Expected time to reach transition speed
• Maximum allowable start duration
• Number of starts per hour
• Cooling time between starts
• Cold-start and hot-start limits
• Ambient temperature and enclosure ventilation

7. Select the Starter Rating

A motor starter transformer is not sized in the same way as a continuously loaded distribution transformer. Selection commonly considers motor horsepower, voltage, tap setting, starting duration, duty cycle, and the manufacturer’s specific design data.

Understanding how to size a motor starting autotransformer helps narrow the correct voltage, tap setting, and rating, but final selection should still be confirmed by a qualified electrical professional and the equipment manufacturer.

Common Installation Mistakes

Even a correctly sized starter can perform poorly when installation details are overlooked.

Selecting the Wrong Tap

A tap that is too low may not produce enough torque. A tap that is too high may fail to deliver the intended current reduction.

Incorrect Wiring or Contactor Sequencing

Improper power wiring, control wiring, or interlocking can create failed starts, damaging transitions, or severe electrical faults.

Undersized Conductors

Conductors must be selected for the starter arrangement, motor circuit, installation method, temperature, terminal rating, and applicable electrical code.

Poor Ventilation

The transformer and contactors produce heat during starting. Restricted airflow, contamination, or high ambient temperature can reduce allowable duty and accelerate insulation ageing.

Incorrect Transition Timing

An early transition may cause a substantial current spike. A late transition may overheat the motor or starter. Timing should be confirmed under actual load rather than assumed from an unloaded test.

Inadequate Overload and Short-Circuit Protection

Overload protection, fuses, and circuit breakers perform different functions. They must be properly coordinated with the motor, starter, conductors, and available fault current.

Ignoring Existing System Conditions

When replacing equipment, confirm:

• Motor nameplate data
• Existing tap setting
• Line voltage
• Control voltage
• Starter duty
• Start duration
• Contactor arrangement
• Enclosure dimensions
• Ventilation
• Protection settings
• Changes made to the driven load

When replacing an old motor starting autotransformer, confirm the voltage, tap settings, motor rating, duty, wiring, and enclosure requirements before selecting a direct replacement.

Maintenance Best Practices

Routine inspection can identify heat, wear, and connection problems before they cause an unexpected shutdown.

Inspect Contactors

Check contacts for pitting, erosion, overheating, welding, and uneven wear. Confirm that contactors move freely and that mechanical and electrical interlocks operate correctly.

Check Electrical Connections

Loose connections can create excessive resistance and localized heating. Inspect terminals, lugs, bus connections, and grounding points using approved de-energized maintenance procedures.

Test Insulation Condition

Insulation-resistance testing may help identify moisture, contamination, or insulation deterioration. Testing must follow manufacturer guidance so sensitive control components are not damaged.

Monitor Heat Build-Up

Look for discolouration, damaged insulation, blocked ventilation, unusual odours, or abnormal enclosure temperatures.

Infrared inspection may identify hot connections while the equipment is operating, provided it is performed under an approved electrical-safety program.

Verify Transition Timing

Observe the complete starting sequence and confirm that transition occurs consistently at the intended motor speed and current level.

A significant change in acceleration time may indicate:

• Increased mechanical loading
• Motor deterioration
• Low supply voltage
• Contactor problems
• Bearing issues
• Process changes

Inspect Control Components

Test timers, relays, interlocks, auxiliary contacts, control transformers, push buttons, sensors, and protective devices.

Maintenance intervals should reflect the environment, number of starts, motor importance, and manufacturer recommendations.

Control Starting Current Before It Disrupts Operations

A properly selected starting system protects more than the motor; it supports the reliability of the entire electrical and mechanical process. A motor starting autotransformer in Canada can help large three-phase motors start without imposing full locked-rotor current on the facility’s electrical system.

The system:

• Applies reduced voltage during startup
• Limits supply current and voltage sag
• Provides stronger torque performance than some traditional reduced-voltage methods
• Transfers the motor to full voltage after acceleration
• Supports fixed-speed pumps, compressors, conveyors, crushers, fans, and industrial machinery

Reliable performance depends on correct motor data, load analysis, tap selection, transition timing, protection, ventilation, and maintenance.

Looking for a reliable reduced-voltage motor starting solution? Transformer Source offers motor starting autotransformers designed for pumps, compressors, conveyors, HVAC systems, and heavy industrial equipment. Our team can help you choose the right voltage, tap configuration, and transformer rating for your application.

Find the Right Reduced-Voltage Motor Starter

Looking for a dependable reduced voltage motor starter for a pump, compressor, conveyor, HVAC system, crusher, or heavy industrial machine?

We provide Canadian-built motor starting autotransformers across a range of voltages, horsepower ratings, and tap configurations. You can also review our industrial transformer solutions, explore available autotransformers, or browse transformer products.

For help confirming the required voltage, motor rating, tap arrangement, and starting duty, contact a transformer specialist before finalizing the equipment selection.

Frequently Asked Questions

What Is a Motor Starting Autotransformer?

A motor starting autotransformer is a tapped three-phase transformer that temporarily reduces the voltage supplied to an induction motor during startup. After the motor accelerates, the starter transfers it to full line voltage.

Why Is Reduced-Voltage Starting Important?

A reduced voltage motor starter limits the current drawn during acceleration. This can reduce voltage sag, equipment stress, power-quality problems, and nuisance operation of protective devices.

When Should I Use an Autotransformer Starter Instead of a Soft Starter?

An autotransformer starter may be appropriate when the motor operates at fixed speed, strong starting torque is required, and a robust electromechanical system is preferred. A soft starter may be better when smooth, adjustable acceleration and deceleration are higher priorities.

What Are the Most Common Tap Settings?

The most common settings are 50%, 65%, and 80% of line voltage. A 50% tap provides the greatest line-current reduction but the lowest torque. An 80% tap provides more torque but draws more starting current.

Can Motor Starting Autotransformers Be Used with All Motors?

No. They are most commonly applied to three-phase induction motors designed for full-voltage operation after starting. Motor type, winding configuration, load torque, acceleration time, frequency, voltage, and duty must all be verified.

How Much Can an Autotransformer Reduce Starting Current?

In idealized terms, supply line current is approximately proportional to the square of the selected tap ratio. A 50% tap may reduce line current to approximately 25% of the motor’s full-voltage starting current. A 65% tap provides approximately 42%, while an 80% tap provides approximately 64%. Actual results depend on the motor, system impedance, load, and starter design.

Are Motor Starting Autotransformers Still Used Today?

Yes. They remain a practical industrial motor starter option for large fixed-speed motors where reduced line current and relatively strong starting torque are required. Soft starters and VFDs have expanded the available options, but they do not make autotransformer starting obsolete.