In a Stepper Motor Kit, Why Is the Power Supply’s Rated Power Less Than the Motor’s Voltage × Rated Current?

When selecting a power supply for a stepper motor, you may notice a common phenomenon: the nominal power rating (in watts) of the power supply is often less than the product of the motor’s “rated voltage × rated current.” For example, using a 350 W power supply for a motor that theoretically might require 360 W or even higher calculated power is completely normal and reasonable.

Power Supply and Stepper Motor Model Included in the Kit:

• 34HS39-6004D-E1000: Nema 34 Closed Loop Stepper Motor, 6.50 Nm (920.48 oz·in), Encoder 1000 PPR (4000 CPR)
• LE-350-60: 350 W, 60 V, 5.8 A, 115/230 V Switching Power Supply

In the above kit, the power supply’s rated power is indeed lower than the product of the motor’s operating voltage and rated current. The key to understanding this lies in recognizing two different “current” states of the stepper motor and identifying which state we actually need to power. The following information is provided for your reference.

1. Logic Behind Power Supply Selection

Since the motor operates most of the time in the “working current” state, which is lower than its maximum capability (rated current), the core criterion for selecting a power supply for the entire system should be:

Meet the motor’s “average operating power consumption” and include a reasonable margin.

It is essential to understand the difference between rated current and working current.

• Rated Current – The Safe “Limit”

This is one of the most important parameters of a stepper motor. It refers to the maximum safe current that the motor’s coil windings can withstand for an extended period without overheating or damage when the motor is stalled (not rotating) or operating at very low speeds. You can think of it as the motor’s “thermal limit” or “maximum allowable operating current.” This current value directly determines the maximum torque the motor can output.

• Working Current – The Actual “Demand”

This is the current that flows through the motor during actual rotation. In most normal operating conditions, as long as the load torque does not reach the motor’s limit, the actual current in the coils will be lower than the rated current. When the motor runs at a constant speed, the current it consumes is smaller than during startup or when pushing against high loads.

2. Why Not Select Based on Maximum Power Consumption?

In normal usage, while ensuring the motor torque meets the application requirements, the system avoids prolonged stalling or continuous operation at maximum current. The power calculated as “rated voltage × rated current” corresponds to the maximum instantaneous thermal power that the motor could generate under stalled conditions—an extreme state that rarely persists.

If a power supply were selected according to this theoretical maximum, it would result in severe over-specification, increasing size, cost, and energy consumption unnecessarily.

Conclusion

Therefore, the fact that the power supply’s rated power in your stepper motor kit is “lower” than the theoretical calculated value is not a compromise in quality. Instead, it is a rational and economical design based on actual operating conditions. It accurately meets the motor’s real energy demands in the vast majority of scenarios while ensuring system reliability and cost-effectiveness. You can confidently use the kit as provided.

Updated on: 30/01/2026