A frequency inverter (VFD / AC drive) is a power electronics device that adjusts the speed of an asynchronous or synchronous motor by varying the supply frequency. Selection is not as simple as “if the motor is X kW, the drive should be X kW too.” The determining factor is not power but current; and current itself is shaped by the load characteristic and the duty cycle.
1. Determine the load type
The first step in selection is understanding the torque characteristic of the load.
Variable torque (VT) loads: Fans and centrifugal pumps. The required torque increases with the square of the speed, and power with the cube. Torque demand is low at low speed. Using an inverter with these loads provides a clear reduction in energy consumption.
Constant torque (CT) loads: Conveyors, extruders, mixers, positive displacement pumps, cranes. The torque demand is largely independent of speed. Full torque is required even at low speed.
Constant power loads: Winding/unwinding (coil) applications, some machine tools. As the diameter changes, speed and torque vary in opposite directions.
For the same motor power, CT loads require a drive with higher current capacity. The “heavy duty / normal duty” (HD/ND) columns in catalogs express exactly this distinction.
3. Evaluate the speed range and cooling
Standard self-cooled motors dissipate less heat at low speed. If a constant torque load is to run at low speed for extended periods, an external (forced) cooling fan or a one-size-larger motor may be needed so the motor is not thermally overloaded.
Operation above rated frequency (the field weakening region) increases speed but reduces the available torque. In this region the mechanical speed limit and bearing life of the motor must also be checked.
4. Select the control method
V/f (scalar) control: The simplest method. It is sufficient for applications such as fans and pumps that do not require precise torque control. It is also used when several motors must be driven by a single drive.
Sensorless vector control: The drive manages flux and torque separately using a motor model. It provides better torque and a faster response at low speed. It is suitable for most constant torque applications.
Vector control with encoder (closed loop): Real speed feedback is obtained. It is required for holding full torque at zero speed, precise speed control, and hoisting applications.
If vector control is to be used, a motor identification (auto-tuning) procedure must be performed during commissioning; otherwise the control will not work as expected.
5. Calculate the braking requirement
During deceleration the motor acts like a generator and feeds energy back to the DC bus. The bus voltage rises and the drive trips on overvoltage. The solution depends on the character of the cycle:
- Long deceleration time: If there is no high inertia, additional hardware may not be required.
- Braking resistor + braking unit: The returning energy is converted to heat. It is the standard solution in applications with frequent
- Regenerative unit: The energy is returned to the grid. It is considered in applications with continuous braking.
stop-start cycles and high inertia.
where continuous braking is performed.
In hoisting (crane) applications, lowering the load means continuous regenerative operation; braking sizing here is a critical safety issue.
6. Grid and EMC side
Harmonics: The inverter draws pulsed current from the grid. An AC line reactor or a DC choke reduces harmonic distortion and extends the life of the DC bus capacitors. It is recommended on weak grids and in installations with a large number of drives.
EMC/EMI filter: Limits the high-frequency noise emitted by the drive. Many drives have a built-in filter; an external filter may be required depending on the environment and cable length.
Motor cable: A shielded cable must be used, and the shield must be grounded over a large surface area at both ends. As cable length increases, capacitive leakage current and voltage reflections at the motor terminals increase; with long cables a du/dt filter or sine filter may be required on the motor side.
7. Ambient conditions
The interior of the enclosure is a thermal environment for the drive. The drive losses remain in the enclosure as heat; the enclosure internal temperature must be calculated and, if necessary, a fan or air conditioner must be fitted. At high ambient temperature and at installations well above sea level, the current capacity of the drive is reduced (derating). In dusty and humid environments, the protection class and enclosure layout must also be evaluated.
8. Communication and control interface
How the drive will communicate with the PLC is decided from the outset: analog reference and digital contacts, or a fieldbus such as Modbus RTU/TCP, Profinet, or EtherCAT? In addition to reducing wiring, a fieldbus also allows diagnostic data such as current, torque, and fault codes to be read from the drive. This data is valuable on the maintenance side because it speeds up fault resolution.
9. Safety functions
In applications requiring machine safety, the safety functions on the drive (e.g. Safe Torque Off) should be evaluated. Which function is required is derived from the risk assessment of the machine.
Checklist
- [ ] Load type (VT / CT / constant power) determined
- [ ] Motor nameplate current and voltage taken as the basis
- [ ] Overload requirement compared against the drive catalog
- [ ] Cooling requirement at low speed checked
- [ ] Control method (V/f, vector, with encoder) selected
- [ ] Braking energy and braking resistor calculated
- [ ] Harmonic and EMC measures planned
- [ ] Enclosure internal heat calculation performed
- [ ] Communication protocol decided
- [ ] Required safety functions determined
Related product families: AC Drive, VFD, Braking Resistor, EMC/EMI Filter, AC/DC Choke, Drive Accessories
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