# Circuit to capture the motor inrush current

1. ## Circuit to capture the motor inrush current

Hi,

I am trying to capture the inrush current waveform of a pmdc motor in an oscilloscope. When the actual inrush current value is 75A, we are able to capture only 44A with set up shown below. The artificial network used is a RLC filter circuit. Please suggest any other alternative arrangement for capturing the same.

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2. ## Re: Circuit to capture the motor inrush current

Hi,

Your picture does not show the exact "capture circuit".
Thus we can't verify whether it is suitable or not. Also we don't know how you read out the peak value. A DVM or an analog voltmeter is too slow, I assume.

It shows, that is is useful for signals in the nanoseconds up to microseconds range.
But your motor current peak should be in the milliseconds range. This is off by a factor of 1000.

Klaus

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3. ## Re: Circuit to capture the motor inrush current

Inrush current can be captured with a current probe and a digital oscilloscope.

It's not clear what you exactly want to achieve and how the schematic is related to motor inrush current.
Obviously, the inrush current depends on the supply voltage and impedance. You can measure inrush current either with a specific application circuit or under certain nominal conditions, e.g. specified by the motor vendor. How do you set the conditions and why?

When the actual inrush current value is 75A, we are able to capture only 44A with set up shown below.
How do you know the actual inrush current?

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4. ## Re: Circuit to capture the motor inrush current

A schematic showing boxes identified only with numbers or letters, is not an useful schematic.
The cable distances 200+/-50, what are the units? Millimeters? Feet? Kilometers?

A wideband current probe (wideband meaning: not designed for powerline frequencies only) and any half-decent modern DSO will be more than enough.

Also as Klaus mentioned, because of the motor's inductance, the inrush current will be in the milliseconds range.

5. ## Re: Circuit to capture the motor inrush current

The same motor was tested in an industrial research association by the automotive industry. That's how i know the actual value. But these tests are very expensive. Hence , we are trying to replicate it in our lab. The circuit i posted i got it from the document International Standard ISO 7637-2. It is for measuring Electrical transient conduction along supply lines.

We are using a current probe and DSO to capture the inrush current waveform. Currently we have checked with a load of 1 Nm. What i wanted to know is whether there is any other circuit or set up like the one i posted ,which will help us to measure the inrush current accurately using a DSO and current probe.

6. ## Re: Circuit to capture the motor inrush current

Hi,

What i wanted to know is whether there is any other circuit or set up like the one i posted ,
Actually you did not show a complete circuit to measure motor current.
You showed a diagram with a couple if black boxes ... with unknown circuit inside.

Klaus

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7. ## Re: Circuit to capture the motor inrush current

ISO 7637-2 doesn't cover motor inrush currents, except for the pulse 4 motor starter scenario which is quite different from your problem.

Furthermore, the shown measurement circuit is for capturing transient overvoltage, not inrush current. The standard has no requirements or measurement circuits for inrush current.

What i wanted to know is whether there is any other circuit or set up like the one i posted ,which will help us to measure the inrush current accurately using a DSO and current probe.
Makes little sense. You don't need a specific circuit to measure inrush current. But the power supply must be able to source the current without voltage drop and the switch must have low resistance. The artificial network might reduce the inrush current, depending on the inrush current duration which wasn't yet mentioned.

It's however not clear why you are using the network for your measurement. If you want to assess transient voltage generated by motor switching, then you should use the network. If the motor characteristics are so that the network reduces inrush current then just accept the fact.

The same motor was tested in an industrial research association by the automotive industry. That's how i know the actual value.
Under which conditions these measurements have been made? Voltage source, source impedance? Are the measurements required and defined by a specific standard?

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