Industrial motor and VFD planning
A practical planning guide: define the job, work through the energy and compatibility constraints, then prepare a supplier brief.
Sources and methodology
Source-based desk research and explicit calculations; no hands-on testing claimed. Missing values remain unknown. Installation compatibility needs a separate assessment.
Define the requirement
Record the motor nameplate, supply phases, rated current, speed, duty and driven load before considering a replacement or VFD. Motor output kW is mechanical output; electrical input also depends on efficiency and power factor. Starting a loaded compressor is a different duty from accelerating a fan. The process requirement determines the necessary torque and control behaviour.
Work through the system
For three-phase planning, current is approximately input watts divided by square root of three, line voltage and power factor. If starting from mechanical output, divide by motor efficiency as well. Use the nameplate and drive duty rating for selection, not the estimate alone. VFD input phases, overload capability, braking and low-speed motor cooling can constrain an otherwise plausible kW match.
Before selecting equipment
Document EMC measures, cable routing, shielding, earthing, enclosure conditions and protection coordination. A drive's stop command is not automatically a safety function. Emergency stop, safe torque off and restart behaviour require a machine-level safety assessment by competent professionals. For backup power, decide whether the goal is continued production or an orderly controlled stop; the two lead to different equipment requirements.
Worked case and decision
For an illustrative 7.5 kW mechanical-output motor on a 400 V three-phase supply, assuming 90% efficiency and 0.80 power factor, running current is approximately 15 A using P / (√3 × V × efficiency × power factor). Replace those assumptions with the nameplate and manufacturer data; the result does not establish starting current or cable protection. Direct-on-line, soft-start and variable-frequency operation create different supply and control requirements. A drive also has input-current, harmonic, cooling and motor-compatibility constraints. For backup planning, decide whether maintaining the control system is sufficient while the large motor remains stopped. That decision can reduce the continuity load substantially, but process and machine-safety requirements govern it. Document the safe state on power loss, permitted restart sequence and who authorises restart. An online current estimate cannot validate protective coordination or the safety of a production machine.
Turn the explanation into a procurement brief
Record included and excluded loads, system voltage, simultaneous power, starting demand and required duration. Keep efficiency and seasonal assumptions beside the result. Compare only documented values for the exact model; a missing catalog field means it needs verification, not that a feature exists. Ask the supplier for the manual, Romania delivery terms and warranty procedure before ordering.
After selecting components, a qualified installer should verify cables, protection, source isolation, ventilation and site conditions. Do not use a worked energy example as wiring instructions. When changing a load or battery, revisit both the energy budget and power check. A supervised operating test should confirm assumptions without exceeding manufacturer limits.
Sources and technical reading
Independent editorial explanation. Numerical examples are illustrative calculations, not specifications attributed to a product.