Sync. motor HP vs. A?

[Mike Cook]

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?searchQuery=1ata9&op=search
&Ntt=1ata9&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.

 

[Josepi]

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?searchQuery=1ata9&op=search
&Ntt=1ata9&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.

 

[James Sweet]

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?searchQuery=1ata9&op=search
&Ntt=1ata9&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.

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.

 

[Josepi]

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]

On a compressor it would most likely be a capacitor run motor and the

locked rotor current is not much higher than running current.

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.

 

[Josepi]

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.

On a compressor it would most likely be a capacitor run motor and the
locked rotor current is not much higher than running current.

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.