DC offset with motor inrush current

Status
Not open for further replies.

philly

Senior Member
Can someone explain to me why there is a DC, and DC offset component associated with motor inrush current? I believe it may possibly have something to do with the motors L/R ratio? Is this correct, if so what does this mean?

Does the residual magnetisim in the motor have something to do with with this DC offset?

Is this DC component seen by breakers and other OCPD's?

Can this DC component be measured or seen with a clamp on meter or other current measuring device?
 
Inrush current

Inrush current

When any system starts or stops, transients occur. If you hook up an oscilloscope across the input leads of a motor (or any other machine or circuit), you will see that when you turn the switch on, the wave is not a nice sinusoidal wave immediately but it "jumps" first and then it settles down. The initial jump that you see lasts only a couple of milliseconds and it is due to capacitance, inductance and resistance that exist between all the components that make up the circuit. For example, between any two of the wires that make up the windings of the motor, there exists capacitance. When you start the motor, this capacitance must charge, like a battery until it reaches the maximum charge that it can hold. The charging current is DC and it flows only while the capacitance is charging. Once all the "parasitic" components charge or discharge, as needed, the wave settles to the value that it will have during normal operation.

There is an infinite number of setups that you can make to measure the DC component but they are not simple because there is also an infinite number of DC components that you can measure. Some will be due to capacitance between the coils, some to capacitance between the coils and the metal parts of the motor, some due to inductance, and so forth. Every part and every environmental factor such as temperature, humitity, etc. affect the transient behavior.

Engineers can design circuits and devices that can isolate and measure any of the above components as needed.

For a layman, the best way to see a transient is to look at the waveform generated by a circuit simulator. Then the waveform can be seen at any instant of time.
 
Thanks for the reference.

The article says the motor like all other inductive loads has an initial transient response that drawa excess current.

Can you refresh my memory why inductive loads exhibit this transient response?

An inductor is an energy storage device. When you connect it to a power source, it builds a magnetic field. Building the magnetic field requires energy and that energy is stored in the magnetic field. When you remove the power from the circuit, all the energy that was used to build a magnetic field makes the inductor look exactly like a charged, regular DC AA or AAA battery.
That battery is charged only while power is holding its charge. Once the power is removed, the battery discharges as the magnetic field collapses and sends electrical current back in a directio opposite to that of the charging current.

Other things happen but this is the basic operation of the inductor.
 

philly

Senior Member
Great information guys thanks!

A couple of follow up questions.

Can this DC offset value be seen by a typical clamp on ammeter or do you need a special meter that can meaure DC?

Does the instantaneous breaker see this DC offset value? I assume that it can and that the breaker is looking at a peak value. Since the DC offset shifts the waveform upwards, then is shifts the magnitude upwards as well?

Can harmonics that are a part of inrush (usually on transformers) increase the offset or magnitude of the inrush?

I also saw that phase angle on voltage waveform when closing effects inrush? Is inrush worst when each phase is furthest from zero?

Why is the transient response a DC response? I understand that there is capacitance and inductance to consider for transient response, buy why is this a DC response and not an AC response?
 

philly

Senior Member
One thing that has me thinking is why a breaker can see this aysmetrical current but it does not show up on a meter or measuring device.

Is this because most meters measure and display the RMS current? So when we look at current on a meter are clamp on, are we seeing the RMS current and not necessarily the peak?

What do breakers look at, do they look at the peak current or the RMS current?

If they look at the peak current then I assume that they are able to see the aysmetrical DC offset and therfore trip while the current measuring device may not since it converts its ouput to an RMS value?
 
Status
Not open for further replies.
Top