BLDC Locked-Rotor Protection: Detection and Restart Policy

Engineering note. This guide is published by the Sankey engineering content team for OEM planning. It explains the design decisions and evidence a project team should review; component datasheets, the intended end product, target-market requirements, and controlled laboratory testing remain the source of truth for a released design.

A locked rotor is more than a high-current event. The controller must determine whether the motor failed to start, was blocked while running, is windmilling, has a feedback fault, or is operating through an allowed transient load condition.

What this decision changes

The protection policy affects MOSFET stress, winding heating, supply loading, audible behavior, customer recovery expectations, diagnostic information, and the number of retry attempts acceptable for the installed product.

Start with measurable inputs

Define motor starting current, normal acceleration time, load inertia, stall current, allowable winding and power-stage temperature, feedback availability, supply capability, environmental condition, required warning behavior, and safe retry interval.

Engineering review method

Use more than one indicator where practical: current, commanded versus observed speed, elapsed startup time, and thermal model or sensor information. Set thresholds from motor samples and worst operating conditions, then test controlled blockages during start and run rather than inferring performance from a current-limit setting.

Evidence to keep with the design

Store blockage test records, current and speed traces, component and motor temperature observations, retry timing, recovery outcome, fault log behavior, and the reasoning for each threshold and timeout.

Failure paths to challenge early

A simplistic current threshold can trip on an intended startup or miss a low-current mechanical blockage. Repeated automatic restarts can create heat or an undesirable user experience; no restart policy can create service issues when a temporary obstruction clears.

Before prototype release

Agree the policy with the product owner and validate it on representative motors and loads at voltage and temperature corners. Make fault reporting and reset behavior visible in the manufacturing and service documentation.

Questions an OEM team should close

How does the controller distinguish a normal high-inertia start from a true blocked rotor?
What is the allowed number and timing of restart attempts for the product?
Which test proves the thermal and electrical stress remain inside the approved limit?