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Industrial Controls

current monitoring for 7.5kW motor

Thread Starter: ionut.b   Started: 2/24/2009 8:01 PM   Replies: 4

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  2/24/2009, 8:01 PM
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I want to protect a motor from overloading, using a current monitor 3UG4621-1AA30 and a window type transformer 4NC5113-0BC20, mounted on the first phase of the motor.

The monitor has a range of 3...500mA (24VDC/AC supply )and the transformer a ratio of 60/1 A.

If the nominal current of the motor is 13.8 A I should see a current of 230mA, half of the range of the current monitor.

According to the wiring diagram of the current monitor, A2 and M are internaly connected; this means that the 0VDC from my application will be connected to the voltage induced in the secondary circuit of the transformer.

If I choose 3UG46 21-1AW30 (power supply of 24..240VDC/AC), A2 and M are not connected internaly.

Has anyone used this configuration?
What would be the best solution?

Thank you.

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  2/26/2009, 9:21 AM
Joined 5/28/2008
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Hello,
As per manuals,

The A2 and M terminals of the 3UG4621/22-.AA30 24 V AC/DC versions are electrically connected inside the device! The load current must flow via the

M terminal.

The A2 and M terminals of the 3UG4621/22-.AW30 24 V to 240 V AC/DC versions are isolated!


First of all, I have never used this configuration as usually I use the oveload relays or motor starter to trip an AC motor at overload condition. And also I think that using motorstarter/oveloads price are lower than using that relays. I think it would be a good solution to use these relays if we are monitoring DC currents as these relays can measure both AC and DC current.

But I do not think that it would be a problem if the M and A2 is not isolated for the first relay, As its supply is 24 VDC/AC.

The second one is 230VAC Supply voltage and M & A2 are isolated.
Finally, I do not think that there is a problem.
Regards.


Ayman Elotaify
MISC Egypt
www.miscegypt.com
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  2/27/2009, 1:22 PM
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Thank you Ayman fot the time spent to read my letter.

You are right about the overload relay.
I forgot to mention that the motor has also a device like this in the main circuit.

The application is a shredder.
If there is to much material in the shredder the current will rise above a limit I chose.
If this happens I must rotate the motor in the opposite way to clear the shredder.

I'm sure now its more clear why I need the current monitor.


About the first relay with 24DC/AC supply ant A2-M internally connected:
if the transformer has a ratio of 60/1A, so the voltage should be 380/60 = 6.3 V, with just one coil for the primary (motor phase); but this is AC voltage placed together with 0 VDC from 24VDC power supply of PLC and sensors.

You think it wouldn't be a problem?

I would preffer the second version with sepparate A2-M because of the sepparation between induced AC voltage in the transformer and 24VDC power supply.


Regards.
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  2/27/2009, 1:51 PM
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Hello ionut.b
ionut.b schrieb:

I forgot to mention that the motor has also a device like this in the main circuit.

The application is a shredder.
If there is to much material in the shredder the current will rise above a limit I chose.
If this happens I must rotate the motor in the opposite way to clear the shredder.

I'm sure now its more clear why I need the current monitor.


Yes Now it's clear. And you should use current monitoring relay for your application.

ionut.b schrieb:

About the first relay with 24DC/AC supply ant A2-M internally connected:
if the transformer has a ratio of 60/1A, so the voltage should be 380/60 = 6.3 V, with just one coil for the primary (motor phase); but this is AC voltage placed together with 0 VDC from 24VDC power supply of PLC and sensors.
Regards.

Iam sorry, I do not understand well. I think we are talking here about CT "Current transformer. And its ratio is 60/1A. So, If the current in the primary is 60A, The current in the sec. is 1A. And our relay max current is 500mA but there's no problem as the primary rated current is some sort of 13 or14A.
But I am really do not understand what do you meant by 6.3V?

With best regards,

Ayman Elotaify
MISC Egypt
www.miscegypt.com
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  2/28/2009, 10:11 PM
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For a 7.5kW motor I should get around 15A nominal in the primary; this means about 266 mA, that is ok for the domain of the monitor.

I wonder what is the voltage induced in the secondary circuit of the CT ?

Has this induced voltage any negative effect on the current monitor?
Will it be damaged by this voltage ?

Is it ok when the induced voltage will be placed over M that is common to A2, and linked to 0VDC of my 24VDC power supply?

Thanks.

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