Sync. motor HP vs. A?

A 230v split-phase synchronous motor is rated at 5hp and says it pulls 22A with locked rotor:
<http://www.grainger.com/Grainger/wwg/search.shtml?searchQueryta9&op=search
&Nttta9&N=0&sst=subset>
A replacement pressure switch for the compressor says it it will handle max 26A but says only 3hp motor can be used (2hp, 230v, 1ph). Other, different hp ratings at other voltages & phases are also listed.
<http://www.grainger.com/Grainger/CONDOR-USA-INC-Pressure-Switch- 3EYP6?Pid=search>
Why this discrepancy?
Thanks.
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Contacts have trouble breaking inductive loads. A resistive current rating is not analogous to a rotating inductor load.
One causes high voltage spikes when you interrupt it and one doesn't.
Your "locked rotor" current is typically never used in real life and not related to the motor hp rating.
A 230v split-phase synchronous motor is rated at 5hp and says it pulls 22A with locked rotor:
<http://www.grainger.com/Grainger/wwg/search.shtml?searchQueryta9&op=search &Nttta9&N=0&sst=subset>
A replacement pressure switch for the compressor says it it will handle max 26A but says only 3hp motor can be used (2hp, 230v, 1ph). Other, different hp ratings at other voltages & phases are also listed.
<http://www.grainger.com/Grainger/CONDOR-USA-INC-Pressure-Switch- 3EYP6?Pid=search>
Why this discrepancy?
Thanks.
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wrote:

Because you mis-read the specs. That says the 22a is "full load amps", not "locked rotor".
A 3hp switch will not last long on a 5hp motor
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Mike Cook wrote:

That sounds really low for locked rotor amps on a 5HP motor. I would expect that number to be close to 100A.
Is the motor truly 5HP? A lot of more consumer oriented air compressors and other power tools use goofy inflated horsepower ratings.
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On a compressor it would most likely be a capacitor run motor and the locked rotor current is not much higher than running current.
That sounds really low for locked rotor amps on a 5HP motor. I would expect that number to be close to 100A.
Is the motor truly 5HP? A lot of more consumer oriented air compressors and other power tools use goofy inflated horsepower ratings.
Mike Cook wrote:

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It is a cap-start & cap-run split-phase 220v replacement motor (A.O. Smith from Grainger) on a 6-year-old compressor.
As someone else pointed out, I misread the motor spec. It is "full load amps" rated at 22A, not locked-rotor.
Thanks.
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Typically "locked rotor amps" and "full load current" on a cap run syncronous motor are almost the same current, anyway.
Nice catch for the person noticing the error though.

It is a cap-start & cap-run split-phase 220v replacement motor (A.O. Smith from Grainger) on a 6-year-old compressor.
As someone else pointed out, I misread the motor spec. It is "full load amps" rated at 22A, not locked-rotor.
Thanks.
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