How load type affects variable frequency drive selection
A variable frequency drive (VFD) provides smooth speed control for electric motors, but its performance depends directly on the load characteristics. Selecting the wrong VFD for your application leads to overloads, premature winding failure, and costly production shutdowns. In this guide, we examine three main load categories, explain how each one determines the requirements for a frequency converter, and provide real-world engineering examples.
Three load categories: constant torque, variable torque, heavy duty
Before choosing a specific VFD model, the engineer must identify the load category of the driven mechanism. This classification determines starting torque requirements, continuous load rating, permissible overload duration, and the necessary current margin.
Constant torque loads
Constant torque loads require the same torque at the motor shaft regardless of rotational speed. Power changes linearly with speed: halving the speed halves the power consumption. Typical constant torque mechanisms include:
- Belt and chain conveyors with consistent loading
- Polymer extrusion machines
- Mixers and agitators handling viscous fluids
- Hoisting equipment (elevators, overhead cranes)
- Reciprocating compressors
For these mechanisms, the VFD is selected based on the motor rated current with a 10-15% margin. The drive must deliver 150% of rated torque for 60 seconds to handle transient load peaks. If a conveyor starts under load (for example, loaded with ore), the starting torque can reach 180-200% of the rated value.
Variable torque loads
In variable torque applications, the torque at the shaft follows a square-law relationship with speed, while power follows a cube law. This means that reducing the speed by 20% cuts power consumption by 49%. This is where VFDs deliver the greatest energy savings.
- Centrifugal pumps for water supply and heating systems
- Axial and radial fans in HVAC systems
- Induced draft fans in boiler plants
- Cooling towers
For pump and fan applications, the VFD is sized to match the motor rated current with little or no margin (up to 5%), since starting torque is minimal. Many manufacturers offer dedicated pump and fan series with built-in PID controllers, dry-run protection, and automatic restart functions. For detailed VFD setup instructions for pumping stations, read our guide to configuring a VFD for a water pump.
Heavy duty loads
Some mechanisms have extremely high moments of inertia or require starting torques that significantly exceed the rated value. These include:
- Crushers and mills (flywheel effect)
- Centrifuges (extended acceleration to operating speed)
- Large ball mills
- Band saws processing heavy workpieces
For heavy duty applications, the VFD must deliver 200% of rated torque for at least 60 seconds, with some models sustaining this for up to 180 seconds. Select a drive one or two frame sizes above the motor rated power. For mechanisms that do not require continuous speed control, a soft starter provides a cost-effective alternative that delivers smooth starting without full speed regulation. We compared these two solutions in our article on top questions about VFDs and soft starters.
Load type comparison table
| Parameter | Constant torque | Variable torque | Heavy duty |
|---|---|---|---|
| Torque vs. speed relationship | M = const | M ~ n² | M at start 150-250% Mrated |
| Power vs. speed relationship | P ~ n | P ~ n³ | High inrush current |
| Starting torque | 100-150% Mrated | 30-60% Mrated | 150-250% Mrated |
| VFD power margin | 10-15% | 0-5% | 20-50% (or +1 frame size) |
| VFD overload capacity | 150% / 60 sec | 110% / 60 sec | 200% / 60-180 sec |
| Energy savings | 5-15% | 30-60% | 10-20% |
| Typical mechanisms | Conveyors, extruders, mixers | Pumps, fans, induced draft fans | Crushers, centrifuges, ball mills |
Practical VFD selection examples by load type
Example 1: Water supply pumping station
A centrifugal pump with a 7.5 kW motor, 380 V, rated current 15.6 A is installed on site. This is a variable torque load. VFD selection: the drive output current must be at least 15.6 A. No additional power margin is needed. We select a 7.5 kW model with a built-in PID controller for maintaining network pressure. Energy savings will reach 30-40% by reducing speed during low-demand periods.
Example 2: Ore processing plant conveyor
A 120 m belt conveyor driven by a 22 kW motor, 380 V, rated current 44 A. This is a constant torque load with loaded starts. VFD selection: output current at least 50 A (15% margin), overload capacity 150% for 60 seconds. We select a 30 kW model that provides the necessary margin for starting a loaded conveyor.
Example 3: Industrial workshop ventilation
A radial fan with a 5.5 kW motor requires airflow regulation based on workshop temperature. This is a variable torque load. A 5.5 kW VFD with an analog input for the temperature sensor signal is the right choice. For more details on VFD configuration for ventilation systems, see our article on frequency converters for ventilation.
Example 4: Compressor station
A screw compressor with a 15 kW motor operates in a cyclic mode: start, run at rated speed, stop. This is a constant torque load with moderate starting requirements. A 15 kW VFD with resonance frequency skip and pressure control functions is recommended. We described the step-by-step setup in our guide on VFD configuration for a compressor.
Electrical parameters to consider
Supply voltage and number of phases
Before selecting a VFD, determine the electrical supply parameters at your facility. Single-phase 220 V drives are available up to 2.2-3.7 kW. More powerful motors require a three-phase 380 V supply. For detailed wiring instructions, refer to our guide on connecting a three-phase motor to a 380 V network.
Rated and starting current
The motor rated current is the primary parameter for VFD sizing. It is listed on the motor nameplate. The VFD output current must be equal to or greater than the motor rated current. For heavy starting conditions, select a VFD output current with a factor of 1.2-1.5 times the motor rated current.
Motor cable length
When the motor cable exceeds 50 m, install an output choke or sine-wave filter at the VFD output to protect the motor winding insulation from voltage spikes caused by fast-switching IGBTs. For cable runs exceeding 100 m, also account for voltage drop and select a larger cable cross-section.
Application-specific VFD features
Modern variable frequency drives offer a wide range of built-in features that simplify integration into automation systems:
- PID controller -- automatically maintains a setpoint for pressure, temperature, or level. Essential for pump and fan applications
- Sensorless vector control -- provides accurate torque regulation at low speeds. Critical for conveyors, hoists, and extruders
- Braking module or resistor -- dissipates kinetic energy during deceleration of flywheels, centrifuges, and elevators
- Dry-run protection -- stops the pump when water is absent, preventing overheating
- Resonance frequency skip -- eliminates vibration at specific speeds, common in fans and pumps
- Communication interfaces (Modbus RTU/TCP, Profibus, EtherCAT) -- for integration with SCADA and PLC systems
Common VFD selection mistakes
Based on our engineering team experience, here are the most frequent mistakes to avoid:
- Sizing by motor power instead of current. A 7.5 kW motor from different manufacturers can have a rated current ranging from 15 to 17 A. Always reference the current on the nameplate
- Ignoring the load type. A VFD rated for pump duty (variable torque) costs less but is not suitable for a conveyor (constant torque) due to insufficient overload capacity
- Omitting input or output chokes. Without a choke, harmonic distortion can affect other equipment on the same supply and reduce motor service life
- Incorrect acceleration and deceleration time settings. Too short an acceleration time with a high-inertia load triggers the overcurrent protection
- Missing braking resistor for frequent stops. Kinetic energy from rotating masses feeds back into the VFD DC bus, raising voltage to trip level
VFD selection algorithm in 5 steps
- Identify the load type (constant torque, variable torque, heavy duty)
- Record the motor nameplate data: power, voltage, rated current, speed
- Calculate the required VFD output current with the appropriate margin for your load type
- Select a VFD model with the features you need (PID, vector control, braking module) from our frequency converter catalog
- Consult an engineer for compatibility verification and setup recommendations
