The safest way to size a VFD is to match the supply voltage and select enough output current for the motor and its load. Motor power is useful, but it is not the only value that matters.
A correctly sized drive must start the load, run without overheating and tolerate the application’s short term overload demand.
Read the Motor Nameplate
Collect these values:
- Rated power in kW or HP
- Rated voltage
- Rated current
- Frequency
- Rated speed
- Motor connection
- Motor type
Use a clear nameplate photo when requesting help. If the label is damaged, obtain the motor data sheet or measure the required information through a qualified technician.
Match Input and Output Voltage
The VFD input must suit the site power supply. Its output voltage must suit the motor connection. A power match does not correct a voltage mismatch.
For a single phase input project, select a drive that explicitly supports single phase input or follow the manufacturer’s approved derating instructions. For a solar pump project, also check the DC input and MPPT ranges.
Select by Rated Current
Compare the motor full load current with the VFD rated output current. The VFD rating should meet the motor and application requirements under the chosen duty classification.
If the motor current exceeds the drive current, move to an appropriate larger model. Do not assume that two products marked with the same kW provide identical output current.
Identify the Load Type
Variable Torque Loads
Centrifugal fans and pumps usually need less torque as speed falls. They are often suitable for normal duty drive selection when the current and operating conditions match.
Constant Torque Loads
Conveyors, mixers and many processing machines need significant torque across their speed range. They may require heavy duty selection and closer overload review.
High Starting Torque Loads
Crushers, loaded conveyors, hoists, cranes and some positive displacement pumps can require high starting torque. The supplier should review the load before selecting a model.
Rapid Deceleration Loads
Machines with high inertia can return energy to the drive during stopping. A braking resistor, braking unit or longer deceleration time may be required.
Review the Operating Cycle
Tell the supplier how often the motor starts, stops and reverses. State how long it runs at full load and whether it operates for extended periods at low speed.
The motor’s own cooling can decrease at low speed when it uses a shaft mounted fan. A separate cooling fan or motor derating may be needed for continuous low speed torque.
Temperature Altitude and Enclosure
High ambient temperature and high altitude reduce cooling effectiveness. The VFD manufacturer may require derating or a larger model above stated environmental limits.
An enclosure also changes cooling. Confirm the cabinet size, ventilation and dust protection rather than evaluating the drive in open air only.
Multiple Motors on One VFD
One VFD can operate several motors in some engineered systems. Size the drive for the combined current and provide suitable protection for each motor. All connected motors normally follow the same output frequency, so they cannot receive independent speed commands from a single drive.
Do not switch individual motors on the VFD output while it is running unless the complete system is designed for that operation.
A Practical Selection Example
Suppose a three phase pump motor is rated 7.5kW, 380V and 15A. The site supply is 380V three phase. The first step is to find a 380V class VFD whose applicable duty rating provides at least the required output current.
Next, confirm that the load is a centrifugal pump, review the acceleration time and check the ambient temperature. If the nameplate current or operating environment exceeds the standard model limits, select the next suitable rating based on the manufacturer’s data.
This method is more reliable than ordering any product labelled 7.5kW.
Data Required for Final Selection
Send:
- Motor nameplate
- Input power supply
- Machine or pump type
- Starting load condition
- Speed range
- Acceleration and stopping needs
- Ambient temperature and altitude
- Cable length
- Quantity and destination
Frequently Asked Questions
Should the VFD be the same kW as the motor
Often it can be when voltage, current, load and duty all match. The rated current and overload requirement must still be checked.
Is it safe to oversize a VFD
A moderately larger VFD can be used in many applications when motor protection parameters are set correctly. Excessive oversizing can reduce control accuracy or increase cost, so it should have a technical reason.
What happens if the VFD is too small
The drive may trip on overcurrent or overload, overheat or fail to start the load reliably.
Do high temperatures affect VFD sizing
Yes. Derating may be required when ambient temperature exceeds the manufacturer’s rated conditions.
Request VFD Model Selection
Browse the IDEEI product range or send your motor nameplate for a model and price recommendation.
Email: info@vfdinverters.com
WhatsApp and WeChat: +86 183 5724 1165