VFD Constant Torque vs Variable Torque Applications

When selecting a variable frequency drive (VFD), motor power is only part of the picture. A 15 kW centrifugal pump and a 15 kW conveyor may use motors with the same rated power, but their torque and overload requirements can be very different.

The key is understanding whether the machine is a constant torque or variable torque load.

This guide explains the difference, common applications, and what to check when selecting a VFD.

What Is the Difference Between Constant Torque and Variable Torque?

Constant Torque Loads

A constant torque load requires approximately the same torque across its normal speed range.

For example, slowing down a loaded conveyor does not remove the weight or friction the motor must overcome. The motor may still need substantial torque at low speed.

Typical applications include:

• Conveyors
• Extruders
• Mixers
• Screw feeders
• Positive displacement pumps

These applications often require good starting torque and higher short-term overload capability.

Variable Torque Loads

With a variable torque load, the required torque decreases as speed decreases.

The most common examples are centrifugal pumps and fans.

Typical applications include:

• Centrifugal water pumps
• HVAC fans
• Cooling tower fans
• Centrifugal blowers
• Ventilation systems

Because the power requirement can fall rapidly as speed decreases, these applications often offer good opportunities for VFD energy savings.

Quick Comparison

Feature	Constant Torque	Variable Torque
Torque at reduced speed	Remains relatively constant	Decreases
Typical applications	Conveyor, mixer, extruder	Pump, fan, blower
Starting torque	Often higher	Usually lower
Typical VFD duty	Heavy Duty	Normal Duty
Energy savings	Depends on process	Often significant at reduced speed

These are general classifications. Actual VFD sizing should always be based on the motor and load requirements.

How Torque and Power Change with Speed

The difference becomes clearer when motor speed is reduced.

Constant Torque

For a constant torque load:

Torque ≈ Constant

Power ∝ Speed

If speed is reduced to 80%, the mechanical power requirement is approximately 80%, assuming torque remains constant.

Variable Torque

Centrifugal pumps and fans generally follow the Affinity Laws:

• Flow ∝ Speed
• Pressure ∝ Speed²
• Power ∝ Speed³

For example:

Motor Speed	Approx. Theoretical Power
100%	100%
90%	72.9%
80%	51.2%
70%	34.3%
50%	12.5%

This is why reducing the speed of a centrifugal pump or fan can significantly reduce energy consumption.

However, these figures are theoretical. Actual savings depend on the pump or fan curve, system resistance, static head, motor efficiency, VFD efficiency, and operating conditions.

Common Constant and Variable Torque Applications

The following table provides a quick way to identify common industrial loads:

Application	Load Type	Main VFD Consideration
Centrifugal pump	Variable torque	Flow and pressure control
HVAC fan	Variable torque	Energy efficiency
Centrifugal blower	Variable torque	Speed and airflow control
Conveyor	Constant torque	Starting and overload torque
Extruder	Constant torque	Low-speed torque
Mixer	Often constant torque	Starting load
Screw feeder	Constant torque	Torque at low speed
Positive displacement pump	Constant torque	Pressure and load torque

One important distinction is that not every pump is a variable torque load. Centrifugal pumps are typically variable torque, while positive displacement pumps generally behave more like constant torque loads.

15 kW Pump vs 15 kW Conveyor: Does the VFD Size Change?

Consider two 15 kW motors:

Application A: 15 kW centrifugal pump

The drive mainly needs to control flow or pressure and usually operates without frequent high overloads. A suitable Normal Duty (ND) VFD may be sufficient if its continuous output current meets the motor requirements.

Application B: 15 kW loaded conveyor

The conveyor may need to start under load, provide high torque at low speed, and withstand temporary overloads. A Heavy Duty (HD) VFD may therefore be more appropriate.

So even though both motors are 15 kW, the correct VFD selection may not be identical.

The key lesson is:

Do not select a VFD by motor kW alone. Check the motor rated current and actual load requirements.

Normal Duty vs Heavy Duty VFD

Many VFD manufacturers provide both Normal Duty and Heavy Duty ratings.

Normal Duty

Normal Duty is commonly used for variable torque applications such as:

• Centrifugal pumps
• Fans
• Blowers

These applications generally require less short-term overload capability.

Heavy Duty

Heavy Duty is commonly used for constant torque applications such as:

• Conveyors
• Mixers
• Extruders
• Material handling equipment

These loads may require higher starting torque or greater overload capacity.

Some VFD families, for example, may specify overload levels around 110% for Normal Duty and 150% for Heavy Duty for a defined period.

However, these numbers are not universal. Overload current and duration vary by VFD manufacturer and model.

Always check the specific datasheet for:

• Continuous output current
• Normal/Heavy Duty rating
• Overload percentage
• Overload duration
• Applicable motor power

How to Choose the Right VFD

For most industrial applications, start with these six factors:

1. Motor Rated Current

Check the motor nameplate.

The VFD’s continuous output current should meet the motor requirements. Do not rely only on kW or HP.

2. Load Type

Determine whether the machine is:

Constant torque: conveyor, extruder, mixer, screw feeder

or

Variable torque: centrifugal pump, fan, blower

3. Starting and Overload Requirements

Ask whether the machine starts loaded and whether temporary load peaks are expected.
A loaded conveyor generally requires more attention to overload capacity than a centrifugal fan.

4. Operating Speed Range

Constant torque operation at very low speed can create a motor cooling problem because the motor’s shaft-mounted fan also slows down.

For continuous low-speed, high-torque operation, forced ventilation or an inverter-duty motor may be required.

5. Acceleration and Deceleration

High-inertia loads may need longer acceleration or deceleration times.

Applications requiring rapid stopping may also need a braking resistor, braking unit, or regenerative solution.

6. Control Requirements

Check whether the application requires:

• V/F control
• Sensorless vector control
• PID control
• 0–10 V or 4–20 mA signals
• PLC control
• RS485/Modbus communication

The appropriate control method depends on the required torque, speed accuracy, and process control.

Common VFD Selection Mistakes

Selecting Only by Motor kW

Two motors with the same kW rating can have different rated currents and load characteristics.
Use the motor nameplate current plus the load profile when selecting a VFD.

Treating Every Pump as Variable Torque

Centrifugal pumps are generally variable torque loads, but positive displacement pumps are not.

Always identify the actual pump type.

Ignoring Low-Speed Motor Cooling

Running a motor slowly does not necessarily mean it is lightly loaded.

A conveyor may operate at low speed while still requiring high torque. If this continues for long periods, additional motor cooling may be necessary.

What Information Should You Send to a VFD Supplier?

Instead of simply asking:

“Please quote a 15 kW VFD.”

provide:

• Motor power
• Motor rated voltage
• Motor rated current
• Input power supply
• Application type
• Minimum and maximum speed
• Whether the motor starts under load
• Acceleration/deceleration requirements
• Control method or communication requirements

For example:

15 kW, 380 V three-phase motor, rated current XX A, driving a loaded belt conveyor, operating from 20–50 Hz.

This gives the supplier much more useful information for selecting the correct drive.

ZHENGXI manufactures variable frequency drives for industrial motor control applications including pumps, fans, conveyors, and production machinery. For OEM or project selection, the final VFD model should be confirmed against the actual motor nameplate and operating conditions.

FAQ

Can the Same VFD Be Used for Constant and Variable Torque Loads?

Yes, provided the VFD’s current rating, overload capacity, control method, and operating conditions meet the application requirements.

A drive may have different allowable motor ratings under Normal Duty and Heavy Duty operation, so check the manufacturer’s datasheet.

Is a Conveyor Always a Constant Torque Load?

Conveyors are generally considered constant torque loads, but actual torque depends on belt design, friction, inclination, material weight, and whether the conveyor starts loaded.

For heavily loaded conveyors, starting torque should be evaluated separately.

Which Type Saves More Energy?

Variable torque applications such as centrifugal pumps and fans often offer greater energy-saving potential when operating below full speed because their power requirement decreases rapidly with speed.

Actual savings still depend on the complete system and operating profile.

Conclusion

The main difference between constant torque and variable torque loads is how the required torque changes with motor speed.

Centrifugal pumps and fans are typically variable torque loads and are commonly matched with Normal Duty VFD ratings.

Conveyors, extruders, and mixers are commonly constant torque loads and may require Heavy Duty ratings with greater overload capability.

When selecting a VFD, focus on four key factors:

motor rated current + load type + overload requirement + operating speed range.

Motor kW or HP is only the starting point.

References

• IEC 61800 Series – Adjustable Speed Electrical Power Drive Systems
• ABB – Technical Guide No. 4: Guide to Variable Speed Drives
• Schneider Electric – Variable Speed Drive Application Guides