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Measuring stall current

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How To measure the stall current of an AC Motor 3 phase ? the problem is i cant lock the shaft , its so hard for us to do that , can i use the method for measuring the stall current for a DC motor like measuring the resistance and then divide voltage by it ?
 
How To measure the stall current of an AC Motor 3 phase ? the problem is i cant lock the shaft , its so hard for us to do that , can i use the method for measuring the stall current for a DC motor like measuring the resistance and then divide voltage by it ?
Inject low voltage AC from a bell xformer and measure V & I for the motor.
Or you could use a scope to capture the initial current spike while powered by rated voltage.
 
The only thing available to me now is to capture the intial current using a scope but how to capture it ?? i think scopes only works with voltage signals ? and what do you mean by a rated voltage ? the motor's min voltage is AC 180V ..
 
Apply an external torque load to the motor .... using a lever arm and a friction belt of some sort.

For a given external torque that is applied, measure the motor current, noting the respective motor rpm.

After two, three or more test points you should be able to get a line or curve on a graph .... The stall current will be a limit or asymptote on the graph .... probably associated with a specific rpm.
 
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The only thing available to me now is to capture the intial current using a scope but how to capture it ?? i think scopes only works with voltage signals ? and what do you mean by a rated voltage ? the motor's min voltage is AC 180V ..
You need a shunt, and you monitor voltage across it and convert to amps. A 0.1 or .01 ohm resistor would work. You switch on the 180v and look for peak value on the decaying waveform.
It's easier just to put some small AC voltage into one phase and measure impedance.
 
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Rather than using a shunt, which requires connection of the scope to the mains voltage, I would recommend using a current transformer for safe measurement of the current. They can be purchased for various current ranges.
 
Apply an external torque load to the motor .... using a lever arm and a friction belt of some sort.

For a given external torque that is applied, measure the motor current, noting the respective motor rpm.

After two, three or more test points you should be able to get a line or curve on a graph .... The stall current will be a limit or asymptote on the graph .... probably associated with a specific rpm.

It is not so easy for AC (induction) motors. The current-RPM is not a straight line.

You will need a low voltage three phase voltage source (transformer, may be a 3 phase Variac - about 10 % to 20 % of the rated voltage), a means to lock the shaft (it will output only a little torque), voltmeter and ammeter.

Make shure that your current is less that the motor's nominal (rated) current.

The full voltage stall current may be found as:

full voltage stall current = low voltage stall current x full voltage / low voltage
 
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The rpm of a three phase motor normally depends on the number of poles and mains frequency.

The motor current is steadily measured in industrial applications. (Using current sensors similar to a clamping ammeter for each phase)

Connecting a load too heavy for the motor power rating it will stop rotating, hum very loudely and finally go up in flames - if not fused properly.

Fuses are thermal sensors distributed in the packet windings and connected to the thermal fuse and an overcurrent fuse.

So stalling a three phase motor is like playing Russian Roulette. :D

Boncuk

P.S. I'v seen a burnt out 135KW (183hp) motor. Doesn't look too good anymore. :)
 
As ecerfoglio and Boncuk have suggested, it may not be a good idea to completely stall a 3 phase AC motor, at full rated current and voltage.

My thought was that a plot of several data points, for different values of external load torque, could provide some useful information regarding the objective. .... Employing graphical extrapolation, rather than a physical destructive test. ... This is only speculation, and may not be practical.

The method suggested by ecerfoglio seems to be the most promising ... and is unlikely to damage the motor.

You may want to verify that the three phase currents are all the same, or approximately so, when you take measurements. .... This is not always the case.
 
Measure the DC resistance and work out the current using Ohm's law.

The motor will be nearly a purely resistive load when stalled.
 
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