Manufacturer derating practice5 min read

Worked Example: Sizing a VFD for a Constant-Torque Conveyor at 50 °C and 1,500 m

How duty type, ambient temperature and altitude turn a 65 A motor into a 96 A drive requirement — and why a 96 A drive is not quite enough.

Open VFD Sizing Calculator calculator

Scenario

Motor	400 V, full-load current 65 A
Load	Conveyor — constant torque (heavy duty)
Design margin	10%
Site	50 °C ambient, 1,500 m altitude
Candidate drive	400 V, 96 A rated output

Step-by-step calculation

Step 1: Apply margin and duty

A constant-torque load needs the drive's heavy-duty rating, which is roughly 85–87% of its normal-duty rating, so the requirement is raised by about 1.15 when comparing against normal-duty catalogue currents.

I required = FLA × (1 + margin) × duty factor

65 × 1.10 × 1.15

I required = 82.2 A

Step 2: Derate for temperature and altitude

Temperature factor = 1 − 0.01 × (50 − 40) = 0.90 Altitude factor = 1 − 0.01 × (1500 − 1000)/100 = 0.95

82.2 / (0.90 × 0.95)

Minimum drive rating = 96.2 A

Step 3: Check the candidate drive

Result summary

Check	Requirement	Actual	Status
Drive voltage rating	≥ 400 V	400 V	✓ PASS
Drive current rating	≥ 96.2 A	96 A	✗ FAIL

Select the next drive frame above 96 A (for example a 105–110 A unit), or check the manufacturer's own heavy-duty and derating tables, which may give the 96 A frame a little more capacity than these generic factors.

Key insight: On a hot, high site a drive can need nearly 50% more rated current than the motor's FLA. Sizing on motor kW alone would have picked a drive about a third too small.