# Which sensor to use for measuring the current of a single phase motor?

1. ## Which sensor to use for measuring the current of a single phase motor?

Hi!
I want to measure the current of a single phase motor. which sensor should i use? I read about current transformers but i cannot understand how to use them. Can you help me.
Thanx

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2. ## Re: current measurement

current transformers are similar to ordinary transformer only... the main idea is that the output voltage is directly proportional to the current we want to measure... the primary is optimised that way....

3. ## current measurement

i hope the link i'm sending might be of some help to you http://en.wikipedia.org/wiki/Current_transformer

4. ## Re: current measurement

y dont u try a adc with a voltage follower from the CT the voltage will be converted into digital signals from there i think u can measure the current

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5. ## Re: current measurement

http://www.allegromicro.com/en/Produ...rentsensor.asp

many current sensors ,also see honeywell sensors

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6. ## Re: current measurement

put a very low resistor but HW series with uour motor then measure its voltage and calculate the current. for example put a .04 ohm resistor(2 Watt) and measure its voltage.it is better to use an opamp to gain its voltage rane then measure it.

7. ## Re: current measurement

Originally Posted by hamed_sotoudi
put a very low resistor but HW series with uour motor then measure its voltage and calculate the current. for example put a .04 ohm resistor(2 Watt) and measure its voltage.it is better to use an opamp to gain its voltage rane then measure it.
I agree with you.
Another key point is that the resistor ( always we call it SHUNT) should be placed in low voltage side to avoid high common mode voltage for Op Amp.

8. ## Re: current measurement

http://www.ohmite.com/
use four lead power resistors from ohmite.

they provide 0.01 ohm 3W resistors with 4 lead for current sense applications.

The heat is very low in this type of application and insertion loss is low.

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9. ## Re: current measurement

Could you please give a schematic with the use of a CT, and a shchematic with a use of a shunt resisstor. which of 2 cases gives accuracy to the measuments? could u please give me also the schematic with the lead power resistors?

10. ## Re: current measurement

mostly CT is used in places where the voltage level is too high and direct measurement could be dangerous.... shunt resistor can be used but the output current sourcing capability and the power dissipation are to be taken into account...

both these when used need to be connected to the output and i think you can imagine this easily....

11. ## Re: current measurement

If you use resistor on low side, be carefull how you connect it to the opamp on the PCB. Best way is to put two dedicated traces, side by side from resistor pads to opamp pads and use them for current measurement only. Also put opamp as close to the resistor as possibile.

Another way to measure current is to get ICs intended for this - maxim-ic has some low side or high side current monitors that also have output for power measurement.

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