This blog explains regenerative braking, its impact on the DC bus, and safe energy dissipation strategies in the NDrive Z1.

What is regenerative braking?

Regenerative braking occurs when the motor’s back-EMF exceeds the DC bus voltage during deceleration, direction reversal, or external forcing. In this condition, the motor behaves as a generator. The generated current flows back into the NDrive Z1 power bus, increasing the bus voltage. This regenerated energy can impose excessive voltage stress on the driver circuitry and the upstream power supply, potentially leading to component failure if the energy is not dissipated through a controlled path.

How does regenerative braking affect the NDrive Z1 and the power source?

During regeneration:

• Energy flows into the DC bus capacitors, charging them further.
• Most SMPS power supplies cannot sink current, causing the bus voltage to rise sharply.
• Excess voltage or regen current can trigger protection events such as DC_BUS_OVER_VOLTAGE or DC_BUS_OVER_REGEN_CURRENT .

When a protection event is triggered, the error LED (red LED) turns on and the NDrive Z1 transitions to the idle state to prevent further stress on the system. The user should check the corresponding error code to determine the cause.

To control regeneration safely, the NDrive Z1 allows setting a regeneration current threshold using: setRegenCurrentTripLevel() .

This defines the Maximum Regeneration Current Trigger Level. When the regen current exceeds the configured threshold, the driver activates the brake circuit and routes surplus energy into the brake resistor before a fault is reached.

What is a brake resistor?

A brake resistor is an external high-power resistor connected to the NDrive Z1’s brake output. When enabled, the driver routes surplus regenerative energy into this resistor, which converts the electrical energy into heat. Key selection parameters include resistance value, power rating, and sufficient thermal capacity for repeated braking.

How does the brake resistor help?

The brake resistor provides a controlled path to dissipate excess regenerative energy. It clamps the rising DC bus voltage by sinking regeneration current, protecting the bus capacitors and the upstream power supply from over-voltage stress. It enables higher deceleration rates and stable braking for high-inertia loads, preventing regen-related shutdowns and faults. A suitable brake resistor allows higher inertia loads and safer rapid deceleration or direction reversals.

When the regen current trigger level is exceeded, the NDrive Z1 switches the brake MOSFET to divert surplus current into the resistor until the DC bus voltage stabilizes.

Demonstration

NDrive Z1 Brake Resistor and Regenerative Braking

The demonstration illustrates how the NDrive Z1 behaves during fast deceleration and direction changes both with and without a brake resistor enabled. When the system operates without a brake resistor, the rapid increase in DC bus voltage during regeneration quickly leads to an error event. The error LED (red LED) turns on and the driver enters the idle state, showing that the bus cannot accommodate the energy returned during braking.

After connecting a brake resistor and enabling it through enableBrakeResistor(brake_resistance_value), along with configuring the regeneration threshold using setRegenCurrentTripLevel(), the same tests are performed again. With the brake path active, the excess energy is diverted into the resistor, the bus voltage remains controlled, and no error events occur. The motor decelerates smoothly, changes direction safely, and operates without triggering faults.

For additional details, consult the full documentation or reach out to the NMotion support team!