Motor overload protection (i²t) and standstill protection
What is i²t protection?
i²t protection (pronounced "I-squared-t") is a software-based method for thermal monitoring of motor windings without a physical temperature sensor. The algorithm integrates the squared motor current over time and compares the accumulated thermal energy with the rated load capacity of the motor. The principle: The power loss in a winding is proportional to the square of the current (I²·R), and the total energy is the integral over time.
Why is this important?
Without overload protection, servo drives can destroy their motors without anyone noticing:
- Winding insulation — if the winding temperature exceeds the insulation class (typically 155 °C for class F), the insulation breaks down. This results in winding short circuits and total motor failure.
- Permanent magnet demagnetization — Neodymium magnets (NdFeB) begin to irreversibly demagnetize at 150–200 °C. The motor permanently loses torque and must be replaced.
- Risk of stalling — when stationary or at very low speeds, cooling by rotor movement is not provided. The same current that is safe at normal speeds can lead to overheating when stationary.
- Fire risk — continued overheating may ignite the motor insulation or surrounding materials
i²t is superior to timer-based protection methods because it accurately reflects cyclic loads: repeated overcurrent and cooling phases are accumulated mathematically in a clean manner.
How does it work?
The algorithm:
In each current control cycle (typically every 25–40 µs), the drive calculates:
Energy accumulator += (I_current² − I_nominal²) × dt
If the current current is above the rated current, the accumulator rises—the motor heats up. If it is below, the accumulator drops—the motor cools down. At 100%, the protective reaction is triggered.
Three key parameters:
- Rated current (I_continuous) — the continuous current that the motor can withstand without thermal limitation (from the motor data sheet)
- Peak current (I_peak) — the maximum permissible current for a limited time (typically 2–5× rated current)
- Peak time (T_peak) — the maximum duration at peak current
Per-phase monitoring:
The algorithm runs independently on each motor phase (U, V, W). When stationary, the current is not distributed evenly in a sinusoidal pattern—a single phase can overheat while the total RMS current appears normal.
Stall protection:
Below an electrical frequency of 10 Hz, the algorithm switches from RMS values to instantaneous phase current values for concentrated windings — for more conservative protection in the absence of convection cooling.
How does SOMANET implement this?
SOMANET implements i²t protection and stall protection via the object dictionary 0x200A with five sub-indexes. The configuration is based on the motor data sheet:
- 0x6075 (Motor Rated Current) and 0x6073 (Motor Maximum Current) define rated and peak current
- 0x200A:2 (Peak Time) defines the peak time (3,000–3,600,000 ms)
- 0x200A:5 (Motor Winding Type) distinguishes between concentrated and distributed windings.
Four protection modes are available: Disabled (0), current limit (1) — the current is limited to 100% of the nominal value, release at 50% —, and fault trigger (2) — the drive stops and reports a fault. The current thermal status can be called up in real time via the motor heat index (0x200A:3, 0–200%).
Further links:
