Hi!
I need some simple schematic(s) for the design of an ammeter that can measure the current in one phase of a three phase power supply for an induction motor. My motivation is to have an arrangement that will turn a buzzer on when the motor is about to stall (pull-out torque point). At that point the current drawn is maximum. Motor is a LabVolt three phase squirrel cage induction motor module. Its a semester project. Please tell me if my approach is correct and if so then I need help with the ammeter design. Thanks in advance.

 

Ever hear of a clamp meter? It measures current by inductive coupling. A very safe way to go.

 

Thank you. I have heard of it.But I need a voltage output proportional to the current as an input to a comparator which will compare the input against a set voltage level (corresponding to a current value) for the buzzer to turn on. Can you please help me on this? As far as I know the clamp meter does not have an output port. Just an LCD screen.

 

Hello,

You can also use a hall effect sensor for current sensing.
There are sensors where the output voltage is dependent on the current of the lead it is mounted close to.
Here is a page with links on halls sensors:
http://www.educypedia.be/electronics/sensorshall.htm

Greetings,
Bertus

 

Thank you Bertus!
Am I wrong in assuming that a network of low impedance, connected in series with one of the phase lines, sensing the current drawn and producing an output voltage proportional to the current will do the job? I am thinking of making the circuit for that network on a PCB. Things like Hall-effect sensors and Clamp meters will be difficult for me to make won't they? The maximum current drawn won't be more than 0.8 Amps (Actually the motor stalls at 0.55 Amps). I know this because of the AC ammeter readings from the LabVolt trainer's in-built meters.

 

Hello,

There are hall sensors that are like a bridge.
This bridge can be connected to an opamp.
This way you can measure the current isolated from the circuit you want to monitor.

Just take a look at the attached PDF of the KMZ10

Greetings,
Bertus