Motor not rated for 208 volt 3 phase

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110.3B would not allow a 230 v motor on a 208 volt system. And NEMA has a voltage tolerance of +/- 10 %. 208 volt motors are labeled 200V
 
I see the disconnect
I am assuming constant speed/slip with only v decreasing

kwired is assuming constant load with v decreasing and slip increasing
since slip ~ torque and torque ~ current (power)
current would increase
 
tom baker said:
110.3B would not allow a 230 v motor on a 208 volt system. And NEMA has a voltage tolerance of +/- 10 %. 208 volt motors are labeled 200V
Click to expand...

I am generally inclined to agree with you. however, many 230V 3 phase motors are labeled for 208V. If this one is not, it may be a simple mater to just replace the motor.
 
Ingenieur said:
I see the disconnect
I am assuming constant speed/slip with only v decreasing

kwired is assuming constant load with v decreasing and slip increasing
since slip ~ torque and torque ~ current (power)
current would increase
Click to expand...
And in this case we are talking about an impeller pump so there is a lot of fudge factor allowance.
 
GoldDigger said:
:thumbsup:
Click to expand...

another take

T = P x 5252/rpm
5252 = 550 ft-lb/sec x 60 sec/min / 2Pi rad/rev
P = sqrt3 x V x I x pf x eff.
ignore pf and eff assume ~ constant
V drops 10% 230 to 208

Load increases slip and slip inversely ~ Z
as slip increases Z decreases, not linear, ref a curve I vs speed
all the way to 0 and locked rotor and for break down T etc
Therefore current increases ~ 230/208

so P is essentially constant V 208/230 x I 230/208 ~ 1
the slip may change 10% say 40 to 45 rpm or so
but rpm changes little 1760 to 1755 ratio ~ 0.99
T ( relative ) ~ V x I (1) /0.99 ~ 1

now I see said the blind man lol
 
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