View Full Version : locked rotor
02-24-2007, 07:55 AM
I am an apprentice for residential hvac and was using a cd troubleshooting
simulation on my computer.
The symptom was: no cool, fan runs continuously, compressor not running
you then take your simulated meter and measure outputs etc at various connection points on the schematic for various components
i could not find the problem, and asked the program to give me the answer, which was locked rotor.
Can someone explain what this is and how it would create this condition, as well as how to check for this condition?
02-24-2007, 09:00 AM
What is the one thing, electrically speaking, that will skyrocket when a motor has a locked rotor?
Were you merely checking voltage across each component?
Since the fans were running but the compressor was not, what component is the most likely one to show a locked rotor condition?
If you can answer these questions correctly, you will solve the scenario yourself and the explanation will be obvious.
02-24-2007, 09:31 AM
LRA, locked rotor amp. is the rated amp draw of a motor on start ups and lasts for 2 to3 seconds and is checked by a amp meter. the LRA rating can be found on the data plate of the motor.
FLA full load amp. or RLA running load amp. is the rated amp draw during normal operation.
a high LRA or FLA will open a overload that is wired in series with the common. it maybe an intenal (that cannot be replaced) or an external overload.
some common causes of a high LRA is, compressor is stuck, low power under a load due to loose wiring, voltage drop across the contactor, a bad starting relay, and capcitors.
02-24-2007, 09:34 AM
typically during a seized rotor, the thermal limit inside the compressor would open to keep the system from completely nuking itself. you should've been able to check the pins with your meter set to continuity at the comp to see if you got an open pair ( S and C I think ). you would see the same thing happen when, on an older condensing unit, the top of the compressor is scorching hot due to a very low refrigerant charge and no LPS in the system. the thermal will open due to the windings getting hot. thermal will eventually close and the system will start after it cools completely. you can speed the cooldown up with a garden hose but its not instant. I hope this helps.
02-24-2007, 11:08 AM
To understand locked rotor, you must understand the difference between:
1) DC resistance & 2) AC Impedance
At start-up, the votage applied to the motor only 'sees' the DC resistance of its windings. Using Ohms Law, the current will be very high.
The AC impedance part happens only when the rotor begins to turn.
If the rotor doesn't turn ( maybe stuck ), then the only thing that limits the flow of current is the DC resistance, which, as stated above, will allow a very high current.
In a normally operating motor, it is the AC Impedance that brings the high start-up current down to its FLA rating.
You cannot 'see' this with a voltmeter, only with an ammeter.
Hope this helps,
02-25-2007, 08:59 PM
Actually RLA is rated load amps. This basically means that is the largest amount of amps the component, ie. compressor, can pull continuous duty without tripping the overload protector. Any more it will trip
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