AC current measurement and motor overcurrent protection

Thread Starter

8947135

Joined Sep 24, 2020
9
Greetings to anyone willing to help,

I am an intern and trying ,In the schematic bellow, to find a logic and build a small system that will take place of the fuse highlighted.

*230v VAC PS;
* the current need to be limited into these 2 values below, if in circumstances, the current exceeds 1.7 amps the motor will be shut off and after short time it will be turned on again with the right current.
*1.3 -> 1.7 amps for the motor. (1.5 ideal constant value)

Because of customer reasons I can not use contactors such as ASL16 or any viable component that can merge together.
I researched into: measuring the current by using a current transformer and amplify/sustain the desired voltage and current. I have found some other solutions that were going around using transistors. Now I am confused of how to approach this and therefore posting this thread.
All my gratitude for any help related to this overcurrent protection issue.


IMG_20200924_095806.jpg
 

Thread Starter

8947135

Joined Sep 24, 2020
9
I.m reffering to the fact that the customer allready made his prototype and he wants (because of costs related and his desired functionality) to be transplanted into a PCB board. Now the fuse that worked as an overcurrent protection for the motor, I need to replace it with another type of system(a system that measures the current and supply the motor) . I hope I have explained what you where asking.
 

Ian0

Joined Aug 7, 2020
503
You mean something possibly like a triac that is controlled by a 555, and the 555 is triggered by an over-current event.
Is that an instantaneous value of current, or rms value? If it's rms then how long is it averaged over?
You'll still need a backup fuse in case the triac fails.
 

panic mode

Joined Oct 10, 2011
1,856
in which country is this supposed to be installed and used? are you familiar with local Electrical Code? as long as you meet the code in terms of protection, you can add own circuit to do some fancy control or electronic resettable fuse etc. what you build may be perfectly function, but unless it is lab-tested and certified to meet current standards, you will have to use primary protection that meets the standards.
 

MaxHeadRoom

Joined Jul 18, 2013
21,441
I am an intern and trying ,In the schematic bellow, to find a logic and build a small system that will take place of the fuse highlighted.

*230v VAC PS;
* the current need to be limited into these 2 values below, if in circumstances, the current exceeds 1.7 amps the motor will be shut off and after short time it will be turned on again with the right current.
*1.3 -> 1.7 amps for the motor. (1.5 ideal constant value)

Because of customer reasons I can not use contactors such as ASL16 or any viable component that can merge together.
If you are asking for a replacement for the fusing F1, then it has to be a recognized means of disconnect and cannot be automatic in resetting.
Also if the neutral shown is a grounded neutral, it should not be fused, just the (L) live side.
F2 is a current O/L detection and cannot be automatically be reset either.
Max.
 

panic mode

Joined Oct 10, 2011
1,856
yes... but to be sure correct standards are applied, still need to know where is this to be deployed. 230V 1P is used pretty much all around the world. maybe the local standard is ... no standard... and everything goes... maybe there is a place where burning down building or killing someone is not a big deal...
 

LowQCab

Joined Nov 6, 2012
56
There is no Fuse in the schematic that you provided, only a Thermal "Circuit-Breaker", and a
Motor Protection Relay, (or Thermally Protected Motor-Starter).
What problem are you trying to solve ?
What is the purpose of the Motor ?
What is the Motor Starting Current, and "Ramp-Up" to full speed Time period ?
What conditions could cause it to draw too much current ?
Does any other part of the machine rely upon the Motor for safe operation ? (cooling fan maybe ? )
Could unexpected, automatic, starting of the machine cause a dangerous situation ?
If Motor overload is a common problem in this machine, maybe a larger Motor is needed.
Why is it acceptable for the Motor to Stop, and then Start-Up again automatically ?
Does the Motor need a "Cool-Down" period before re-starting after an overload event ?
Why is a common Motor Protection Relay, (called a "Motor Starter" in the States), not acceptable ?
Why is the existing Motor Protection Relay not providing adequate overload protection ?
Are the Motor Protection Relay "Heaters" sized correctly for the Motor being used ?
Why do the controls need to be placed on a Printed Circuit Board ?
Why not simply mount a thermostatic switch inside the motor, and put it in the Relay Control Loop ?
 

Thread Starter

8947135

Joined Sep 24, 2020
9
Hello again!

I have came to the conclusion that the best way to implement this current protection is to use a current transformer and a comparator that compares the current from the secondary winding and if it exceeds the 1.7 amps, after( 30 mils) it will break the circuit.
I have though about using and hall effect based linear current sensor(specially with the 2.1kVrms isolation) but I think I am complicating myself by using this chip.
I am just trying to replace the Thermal "Circuit-Breaker" (sorry for the fuse confusion), that had the role of limiting the current at (1.3 - 1.7) amps and if it exceeded it will have been breaking the circuit, protecting the motor. IMG_20200926_191701.jpg
 

MaxHeadRoom

Joined Jul 18, 2013
21,441
What about taking care of and ignoring the motor inrush current which will be much greater than 1.7a until motor comes up to operating RPM.
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
21,441
the motor is operating in the 1.3 -1.7 amps margin. What do you want to say?
Is this allowing for the up-to-speed operating current?
The inrush is way higher for a AC induction motor, followed by the zero to operating rpm current, which can last for several cycles.
Max.
 

Thread Starter

8947135

Joined Sep 24, 2020
9
Is this allowing for the up-to-speed operating current?
The inrush is way higher for a AC induction motor, followed by the zero to operating rpm current, which can last for several cycles.
Max.
the motor is fully operating in this amps margin. I just need to make sure that if the amps increase then there will be a break to protect the motor.
 

MaxHeadRoom

Joined Jul 18, 2013
21,441
the motor is fully operating in this amps margin. I just need to make sure that if the amps increase then there will be a break to protect the motor.
What is the motor plate rating of the motor? and just because the motor may be operating at this level, there exists a very high initial current due to the reasons already stated, I am just trying to confirm these have been allowed for.
O/L's and fuses allow for this time period of high current.
Max.
 

Thread Starter

8947135

Joined Sep 24, 2020
9
What is the motor plate rating of the motor? and just because the motor may be operating at this level, there exists a very high initial current due to the reasons already stated, I am just trying to confirm these have been allowed for.
O/L's and fuses allow for this time period of high current.
Max.
Unfortunate I don't have any access to the motor information. Been told that it operates in these margins and that is all. I am aware about the initial current that is present. This is why the limit of the current is processed before the relay and before the motor.
 
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