The Difference Between Inrush Current and Operating Current

Inrush spike, then operating current graphConsider inrush current and operating current — two values relating to a blower motor’s input capacity and output profile. Inrush current (sometimes called locked-rotor amps or starting inrush, depending on the context) is the current a motor draws upon starting with full application voltage.

Inrush essentially causes a freshly electrified motor (as well as electrical supplies and any connected drive components) to function as a large capacitor — one that requires charging until the circuit reaches normal operating power.

With across-the-line ac starting, the blower-motor system intakes full voltage and draws line amperage that’s often 300% to 600% more than its rated operating current — or to 800% more than operating current in the case of some high-performance setups. Motor horsepower and drive connections (if applicable) and design dictate the exact value of this inrush current.

Contrast this with operating current, which is current the blower motor draws once it’s up and running. This is current drawn once the whole system circuit is energized — with all the circuit subcomponents saturated and (in the case of brushless blower setups with drives) all capacitors charged — and the motor beginning to turn. At this point, the blower-motor system only needs steady-state current to keep the loaded motor at target rpm.

What design features accommodate these different current values to maximize blower output, efficiency, and life?

Photo of timing matters when starting on AC power graph

Let's consider the special case of brush motors for blowers. Here, back emf contributes a current-limiting function to winding resistance and inductance — but only once a motor is running. Back emf is zero upon startup because the rotor begins at zero rpm, and winding resistance is relatively low, so again — current through the windings is large upon initial power application. Such current draw can induce detrimental voltage drops in the system — even degrading the performance of other devices in the circuit and tripping overload safeties. That’s why these motors often use current limiters until rpms can sustain sufficient back emf.

In contrast, consider the special case of brushless motors for blowers — the focus of example calculations in this particular FAQ. Within this motor type, windings are spared significant exposure to inrush current. That’s because of the necessary inclusion of a power supply or control drive that also happens to bear the brunt of inrush current. The capacitive function of the power module or control drive (for converting ac to dc voltage) intervenes. Case in point: Within a control drive, the bridge rectifier and line capacitors charge first — and these subcomponents are the first in the inrush current’s line of fire.

As we explore in other AMETEK blower FAQs, power modules and control drives for brushless motors bring other the application of various drive functions can minimize or even avoid inrush issues altogether. In fact, some of these offerings are particularly useful if a system needs a blower setup to start and stop a lot. Here, some manufacturers recommend using supply or drive speed commands as a better approach to actual motor stopping and starting.

In contrast, other motor types — including the brush motors already mentioned— rely on construction features as well as power cables having conductors of sufficient AWG size to withstand these inrush currents and prevent excessive voltage drops. Many address inrush with inrush-current limiters (in the form of thermistors), transformer switching relays, or precharge circuits. Motor starters (including soft starters for ac motors) are other motion components to help address inrush current.

Keep in mind that timing matters. Location on the ac line-power sine wave at which a power switch closes affects inrush current. So if switched at the peak of the sine wave, the inrush spike is highest and of short duration.

Graph of timing matters: smaller spikes near zero crossingsConsider one example to illustrate: When switched near the peak yields, one brushless motor’s windings might see a 150-A peak but only for a duration of 3 msec. In contrast, if switched near the zero crossing, the spike is smaller yet of longer duration. For this example, switching near the zero crossing point might yield an inrush current of only 50 A … but having a duration of 7 msec and followed by a second pulse.

Note that these values are motor specific, as inrush current depends on the circuit’s amount of resistance, control-drive capacitor size, and signal-filtering settings.

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