How to Select a VFD for Constant Torque vs Variable Torque Loads (With Examples)
Selecting the correct Variable Frequency Drive (VFD) is not just about matching motor power. One of the most critical — and most commonly misunderstood — factors is whether your application is a constant torque or variable torque load.
Choosing the wrong VFD can lead to overheating, nuisance trips, poor performance, and shortened equipment life. In this guide, we explain the difference between constant torque and variable torque loads, how they affect VFD selection, and how to size a VFD correctly using real-world examples.
What Is a Constant Torque Load?
A constant torque load requires the same amount of torque at all speeds. As the motor speed changes, the torque demand remains essentially constant.
Typical Constant Torque Applications
- Conveyors
- Mixers and agitators
- Extruders
- Crushers and shredders
- Screw feeders
- Positive displacement pumps
These applications usually require high starting torque and strong overload capability from the VFD. In most cases, a heavy-duty inverter drive paired with a correctly sized motor is required.
What Is a Variable Torque Load?
A variable torque load requires torque that changes with speed. As speed decreases, torque demand drops significantly.
Typical Variable Torque Applications
- Centrifugal pumps
- Fans and blowers
- HVAC systems
- Cooling towers
Variable torque loads are ideal candidates for energy-saving VFDs, as power consumption reduces rapidly at lower speeds.
Why Load Type Matters When Selecting a VFD
VFDs are designed with different current limits, thermal capacity, and overload ratings depending on the intended load type.
- Constant torque loads require high current at low speed
- Variable torque loads draw much less current as speed reduces
- Incorrect VFD selection often causes overcurrent or thermal trips
This is why most manufacturers provide separate ratings for heavy-duty (constant torque) and normal-duty (variable torque) operation.
VFD Overload Ratings Explained
Overload capability is one of the biggest differences between constant torque and variable torque VFDs.
- Constant torque (heavy duty): typically 150% current for 60 seconds
- Variable torque (normal duty): typically 110–120% current for 60 seconds
If your application requires frequent starts or high inertia loads, selecting a higher-rated inverter drive is essential.
Why VFD Power Ratings Change by Load Type
Many VFDs have two different power ratings depending on the application.
- 11 kW VFD rated for variable torque
- The same drive rated only 7.5 kW for constant torque
Always check the manufacturer’s heavy-duty and normal-duty rating tables before selection.
Constant Torque VFD Selection Example
Application: Industrial conveyor
Motor: 7.5 kW, 4-pole, 400V
Load: Fully loaded at start-up
Key considerations:
- High starting torque required
- Frequent starts and stops
- Possible shock loading
Correct selection: A 7.5 kW constant torque inverter drive. For severe duty cycles, upsizing to an 11 kW drive may be required.
Variable Torque VFD Selection Example
Application: Centrifugal fan
Motor: 11 kW, 400V
Load: Light load at low speed
Key considerations:
- Torque demand reduces rapidly with speed
- Low starting torque
- Energy efficiency is critical
Correct selection: An 11 kW variable torque VFD rated for normal duty operation.
Low-Speed Operation and Motor Cooling
At low speeds, motor cooling becomes less effective because the shaft-mounted fan slows down.
- Constant torque at low speed increases motor heating
- External cooling fans may be required
- Inverter current limits become critical
In demanding applications, pairing the drive with an inverter-rated electric motor improves reliability.
Using a Gearbox to Reduce VFD Stress
Adding a gearbox allows the motor to operate closer to base speed, reducing torque demand on both the motor and the VFD.
- Improves efficiency
- Reduces motor heating
- Extends drive lifespan
For many applications, a factory-matched geared motor offers a more reliable long-term solution than running at very low speed on a VFD.
Common VFD Selection Mistakes
- Using variable torque drives on conveyors or mixers
- Ignoring overload ratings
- Not accounting for low-speed cooling
- Selecting based on kW alone
Frequently Asked Questions
Can I use a variable torque VFD on a constant torque load?
No. This often results in overcurrent trips and overheating.
Is it safe to oversize a VFD?
Yes — moderate oversizing is common for heavy-duty applications.
Do pumps always use variable torque VFDs?
Centrifugal pumps do, but positive displacement pumps usually require constant torque drives.
Does a gearbox allow a smaller VFD?
In many cases, yes — by reducing torque demand at low speed.
Need Help Choosing the Right VFD?
At Drive Outlet Megastore, we stock a wide range of inverter drives, electric motors, gearboxes, and geared motors for industrial applications.
If you’re unsure which VFD is right for your application, contact our team and we’ll help you select the correct solution first time.
