Motor starting current

[m4tty]

Hi

When a motor starts up and it draws maybe x 5 normal running current why don't the cables have to be rated for the initial startup. As it's pulling more than the cable can handle?

Sad I know but I clamped my dads garage when his compressor started up to see what it pulled and was turning things on and off just to try to understand abit more.

Cheers

 

[Andy™]

the rating of a cable is continous. for the short time to get motor to speed, it will be over its rating, but not long enough to let it get too hot

 

[a1]

As Andy Says. ^^^

 

[Sidewinder]

In an effort to help explain:

This is known as the locked rotor current LRC and can be up to 20x FLC depending on the design of the motor and the external load.

To trap this and understand it in a current/time situation you will need a very fast analysing power quality analyser capable of measuring current, and voltage against time for the load.

This can then be graphed and you can see the response.

It is almost impossible to understand with a normal clamp meter.

An induction motor has very low torque at low rpm, thus external loads have a significant affect on the current draw.

They attempt to make the motor stall.

Typically this current is only seen for around 100ms (very ish). I have a Fluke 337 (I think can't remember now) which has a 100ms inrush check. This will measure the peak current for the first 100ms it starts to flow.

Not perfect but does give an indication.

After this it decays almost exponentially to a more normal level.

For "normal" use motors this can be ignored, HOWEVER, for motors with frequent stop/start cycles under high external mechanical load it can be a factor that must be considered.

These applications can be things such as hydraulic motors, lifts, air and other gaseous compressors, machinery with large rotating inertia coupled to the motor with frequent start/stop cycles, especially if there is dynamic braking fitted, e.g. plugging or d.c. injection.

However as Admin & Andy

 

[Evans Electric]

As Andy says , high current for a few seconds ,hence there is often a Type C MCB also, as Type B will trip .

Another example to illustrate it. I found this a bit strange myself when I came across it, on overhead cranes on tracks in factories , the travel motors are quite small, the traverse motors quite small but the hoist is a very big slipring motor, but the cables feeding it are approx a third of the size you would expect.

So where us guys would be looking at using ,say,10mm2 cable its actually working with 4mm2 , reason is that it lifts a load ,takes about 15 seconds and stops , cables never have time to get warm.