Running amps vs fla on low RPM motor

Status
Not open for further replies.
E

ernric43

Member
Location
United States
I have a 480V 4HP motor running at 1,000rpm. The motor is running with no load, i.e. disconnected via a belt, and pulling 4.3-4.4 amps. The fla on the motor is 5.9 amps. This means that i'm running at approx 75% of fla.

All sources that I've found tell me that I shouldn't be running at more than 65% of fla. I can't find any table or source that tells me what impact the lower RPM's have on the amps. Any help?

Thanks,
Rich
 
G

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
ernric43 said:
I have a 480V 4HP motor running at 1,000rpm. The motor is running with no load, i.e. disconnected via a belt, and pulling 4.3-4.4 amps. The fla on the motor is 5.9 amps. This means that i'm running at approx 75% of fla.

All sources that I've found tell me that I shouldn't be running at more than 65% of fla. I can't find any table or source that tells me what impact the lower RPM's have on the amps. Any help?

Thanks,
Rich
Click to expand...

With essentially no slip any induction motor should run unloaded at 7200/N where N is the number of poles (always a multiple of 2).
How accurate is your 1000rpm measurement, and is the motor running off 60Hz or off a VFD?

If you have a two pole motor and it is running unloaded at 1000rpm there is something drastically wrong.

On the other hand, if it is a six pole motor the synchronous speed would be 1200rpm and slip to 1000 would be on the high side but still possible, I think.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
There are two components to the current going to an induction motor, Active Current and Reactive Current. When a motor is fluidly loaded, most of the current is doung work, what we call Active Current. When the motor is uncoupled, the Current is only there to make the magnetic fields and the power factor is very low, ie 0.4 to 0.2, so that current is a component of VARs, not Watts, so it is not being read by the kWH meter either. I’d say, similar to what Golddigger thought, that you have a 6 pole motor with a high slip, so unloaded, the power factor will be very low and the reactive current will appear high. But it’s not indicative of anything useful.

Basically, pay no attention to it, it means nothing.
 
E

ernric43

Member
Location
United States
GoldDigger said:
With essentially no slip any induction motor should run unloaded at 7200/N where N is the number of poles (always a multiple of 2).
How accurate is your 1000rpm measurement, and is the motor running off 60Hz or off a VFD?

If you have a two pole motor and it is running unloaded at 1000rpm there is something drastically wrong.

On the other hand, if it is a six pole motor the synchronous speed would be 1200rpm and slip to 1000 would be on the high side but still possible, I think.
Click to expand...




This is an EMOD 6 pole motor. The motor nameplate states 1150 rpm and it is 60Hz with no VFD.
 
I

Ingenieur

Senior Member
Location
Earth
I looked at the data sheet
it is a euro motor rated at 50 Hz 400/3 vac and 1000 rpm (slip to 960)
it is also rated to run at 60 Hz 460/3 and 1200 rpm (slip to 1150)
eff x pf is inherently low at FLA ~ 0.65
I could see NLA being that high
there is no rule of thumb for this
their data does not give NLA

if the measured 1000 rpm is accurate there is a problem
 
Sahib

Sahib

Senior Member
Location
India
Check line voltage. Low voltage beyond reasonable limit could cause abnormally low speed and higher no load/full load current.
 
Status
Not open for further replies.
Top