Question about a starting current and running current

[alphacat]

When its said on a device that it has a starting current of 22Amps (rms) and a running current of 16Amps (rms), what exactly does it mean?

How long does the starting current (22Amps) last?
How does it decrease down to 16Amps?

Could you give an example for such a device please?

Thanks in advance.

 

[ke5frf]

An AC motor is a good example if I understand your question correctly.

When the motor is at rest and a sinusoidal waveform energizes it, the magnetic field has not yet been established and the windings appear as a short to the power source. At this stage there is, as of yet, no power transfer taking place between the power source and the load in the form of mechanical movement. Once the magnetic field establishes itself and the rotor begins to spin, power conversion takes place in inverse proportion. Current and torque goes down as motor speed goes up. The inertia of the rotor contributes to the reduction of necessary current to drive it.

Another way to look at this is to "measure" or take note of the force required to move a physical object that resists your efforts, then once the object's opposition is overcome the reduction of that force. For instance, imagine yourself breaking loose a very stiff bolt with a wrench. You exert a lot of force upon the bolt to get it to break free. Once it is broken free, it suddenly turns with ease and the force required to act upon it is reduced. Or consider another example. Think of the stationary automobile. Think about the torque that is required, and the demand put upon the motor, to get it in motion, but once in motion the inertia of the motion relieves the power required to keep it moving and thus a higher gear ratio with less torque can be used to keep it moving.

 

[mbarazeen]

since you mean device it may be some electronic stuffs too, for example when you switch on a device thats having a large capacitor in it, then it instantly draws a large current and then come to steady state. the time varies depends on the size of the cap and the load connected but its just few milli seconds.

in motors normally starting current goes upto 8-10 times of rated value to accellerate the rotor inertia with the load inertia.

since you say 22A & 16A it may be of an electronic device i hope.

 

[alphacat]

Thank you very much.

The motor example is what i was looking for.

I got a PCB which conducts current from the mains to the appliance through its traces and a relay.
The traces and the relay are designed to handle up to 16Arms.

I was wondering, could the PCB (including the relay it owns) handle an appliance (an AC motor for example) which its starting current is 22Amps? (but its running current is 16Amps).

How long does it take to the starting current of an AC motor to drop to its running current?

Thank you again for the help.

 

[Sceadwian]

Depends on the application, the motor has to get up to it's running speed driving whatever mass it's attached too so you'd have to use a tach on the motor and time out long it takes after power up till it's at it's running RPM. If you don't have access to the motor then you need to measure the current and time it yourself. Most motors get up to running speed within a few seconds.

 

[alphacat]

I see, thank you.

If its a matter of seconds, could the PCB and relay handle a few seconds of over current (22Amps) until it goes down to the level they are designed to, 16Amps?

 

[mbarazeen]

yes for few seconds you can load them, but make sure its only few seconds for your motor to pick up and come to nominal speed.

 

[Sceadwian]

If it's an inductive load you need to make sure the relay is rated for switching inductive loads at the full load current (starting current) or the contacts may weld during contact bounce.

 

[alphacat]

Thank you for you help guys.