1606192215 EDT
Attached is a photo of the 3 phase induction motor curves I referenced above from Bailey nd Gault.
.
.
Note:
Full rated load is 35 HP.
Current at full rted load about 40+a small amount A.
Current at zero load is 10.5 A.
At 20 A load current, 1/2 of full rated current, output power is 15 HP.
15/35 is about 43%.
Different motor designs will have different characteristics. For any particular motor you should be able to get typical motor performance curves.
Power input to a motor is a much better way to estimate output torque and/or power.
.
.
Announcement
Collapse
No announcement yet.
Comparing Name Plate Current to a One Phase Reading Current for a 3 phase motor
Collapse
X

Originally posted by gar View Post1606191611 EDT
I took a look at one motor performance plot for a 35 HP motor. See figure 1312, p 213, of "AlternatingCurrent Machinery", Bailey and Gualt, McGrawHill, 1951.
Current vs load is not quite linear but it is somewhat. What the current curve does not do is intersect at 0,0. For this motor line current is about 25% for zero load torque. Using this curve your load was possibly about 15/35 = about 43 % of full load.
.
the pf and eff will be much lower at 40% fla vs fla
maybe 1/2
I would rate load at 1/2 x 15/35 = 2025%
motor torque is ~ linear with current
P = T w (w = 2 Pi f)
for 60 Hz in rpm
T = 5252/rpm x P
5252 = (60 sec/min x 550 ft lb/HP) / (2 Pi)
a conversion factor
typ curve 45% fla pf drops to 50% from 90
eff is similar but not as dramaticLast edited by Ingenieur; 061916, 06:35 PM.
Leave a comment:

Originally posted by LMAO View PostNo. Current you read on nameplate is the phase current measured by current clamp (ammeter) when run at full load. ...
Leave a comment:

1606191611 EDT
I took a look at one motor performance plot for a 35 HP motor. See figure 1312, p 213, of "AlternatingCurrent Machinery", Bailey and Gualt, McGrawHill, 1951.
Current vs load is not quite linear but it is somewhat. What the current curve does not do is intersect at 0,0. For this motor line current is about 25% for zero load torque. Using this curve your load was possibly about 15/35 = about 43 % of full load.
.
Leave a comment:

Originally posted by don_resqcapt19 View PostThe motor line current is based on the voltage and does not change based on the power supply to the motor being wye or delta. You directly read the current on each of the motor conductors. At full load the current reading should be very close to the nameplate rating.
Leave a comment:

Originally posted by Jason T View Postthanks for the response!
Is it correct to assume that if it was a Delta configured incoming power then we would use 1.732 to convert the phase current to line current?
Leave a comment:

Originally posted by LMAO View PostNo. Current you read on nameplate is the phase current measured by current clamp (ammeter) when run at full load. Your motor was probably not running at full load.
1.73 factor (3^0.5) applies when you are measuring the line to neutral voltage. in that case, you need to multiply it by 1.73 to get lineline voltage.
Is it correct to assume that if it was a Delta configured incoming power then we would use 1.732 to convert the phase current to line current?
Leave a comment:

Originally posted by Jason T View PostEveryone:
I seem to be very confused.
We were doing a routine maintenance check to measure the current draw on a 3 phase motor as it compares to its name plate FLC rating. The FLC is about 6A and we took an Amp probe and measured 3A on one phase. I've always thought that with a one phase amp probe reading you have to multiply the reading by 1.732 (so turns out to be about 5.196A) in order to convert it to the line current draw so we can compare it to the FLC rating on the motor name plate.
Is this a correct statement?
1.73 factor (3^0.5) applies when you are measuring the line to neutral voltage. in that case, you need to multiply it by 1.73 to get lineline voltage.
Leave a comment:

Comparing Name Plate Current to a One Phase Reading Current for a 3 phase motor
Everyone:
I seem to be very confused.
We were doing a routine maintenance check to measure the current draw on a 3 phase motor as it compares to its name plate FLC rating. The FLC is about 6A and we took an Amp probe and measured 3A on one phase. I've always thought that with a one phase amp probe reading you have to multiply the reading by 1.732 (so turns out to be about 5.196A) in order to convert it to the line current draw so we can compare it to the FLC rating on the motor name plate.
Is this a correct statement?
Leave a comment: