Measuring Running Current

The use of a lab-quality digital multimeter to measure the DC running current will present a small error due to the AC ripple component of the DC motor. While the measured DC current value is an acceptable indicator of running measurement would be to measure the current's true root mean square (TRMS), ie: to measure both the AC and DC current components, for example:

TRMS = (DC2 + AC2 )1/2

Peak Starting Current

The peak in rush/peak starting current of a brushless DC fan typically will be a function of circuit resistance and power resource. However, many brushless DC fans incorporate additional filter capacitance for electromagnetic interference (EMI) suppression. Depending on the circuit location, the capacitance may represent a very high instantaneous in-rush current spike. Measuring the peak starting current of a brushless DC fan requires that the motor stator be at ambient room temperature prior to the application of power, the rated operative voltage selected and the storage oscilloscope set to trigger on the leading edge of the current wave form using the test circuit.

Current Limiting

The power supply limitations must be considered when DC fans are used. many power supplies incorporate current limiting current fold-back, or current shutdown protection circuits. Knowing the peak starting currents and maximum ripple currents during motor operation is essential in determining the power supply reserves necessary to maintain other peripherals which may be sharing the same power bus, and to avoid nuisance problems associated with the power supply protection circuitry. Depending on the size and design of brushless DC fan motors, the ratio of peak starting current to running current can be quite large, eg. 4:1 or 5:1. To overcome the stress that would be placed on the power supply, many brushless DC motors incorporate a form of current limit, usually linear of pulse width modulated (PWM). With current limiting, peak current draw typically will be limited to values of 2.5 to 1 or less. The limiting of current to the brushless DC fan motor will also limit the starting torque of the motor, which will extend the time needed for the fan to reach full speed.

Since DC fans have much higher starting torque that their AC counterparts, the time to reach full speed with the use of current limit will be less than equivalent DC models.

When operating more than one fan on the same power bus, the imposed ripple current can become significantly more complex as the currents of each fan add and subtract with each other. Under certain operating conditions, this complex ripple current could pose a potential for interference within the system. Bus isolation filtering may be required to assure adequate buffering.

In brushless DC fans, ripple current is a function of the motor design, electronic switching circuitry, operating voltage and current. The motor does not represent a constant load and many vary considerably for IAVG.

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