75HP Motor Problem

[behradk]

Hi Everyone,
I have a problem with a 75 HP motor and I hope someone could help me with that. We have installed a 75HP Gould pump in one of our car wash facilities. The model# is 66SV-6/2. Our voltage is 208V and the wiring configuration is DELTA. Our operating pressure is 275 PSI which is a normal operating pressure. The FLA for a 75 HP motor at 208V is 211A. We did measurements at various pressure and got the following results:

Line----Amperage@ No Load--------Amperage@10 psi-----------Amperage@55 psi-------------Amperage@150 psi---------------Amperage@200 psi-----------Amperage@275 psi
L1-------------40.5---------------------------99.8---------------------------127-------------------------------205--------------------------------283----------------------------314
L2--------------43----------------------------102.2---------------------------129-------------------------------205-------------------------------288----------------------------314
L3-------------41.5---------------------------99.5---------------------------127-------------------------------205--------------------------------287----------------------------314

The Line to Line voltage of the terminals @ 275 PSI is: 201V , 200V, 200V
As you see, the amperage is way too high at 275 PSI and it trips the overload every time. The manufacturer has mentioned that the Full Load Amps should be 185-Amps at 208-Volt for this Motor & at No Load, Amps should be around 50-Amps. Does anyone knows why we are getting such a high amperage? Please let me know if you need any more information and I will supply that.

Thank you all in advance.

 

[dpc]

You might consider an autotransformer to get the motor voltage up to 230 V.

 

[electricpete]

Thanks behradk and Scotty.
I'm still confused. Is pressure reported in the op measured upstream or downstream of this globe valve?

=====================================
(2B)+(2B)' ?

 

[behradk]

@electricpete: The mentioned 275 PSI is measured right after the pump. and valve is located downstream of the pump after the pressure gauge.

 

[electricpete]

Thanks. That's a logical location for the gage. And I'm still confused and probably missing something.

Paraphrased from original post:
No load 40A
10 PSI 100A
55PSI 125A
150PSI 200A
200PSI 285A
275PSI 314A

As we go down that list from top to bottom:
valve closes more resulting in discharge pressure (upstream of valve) increasing
system flow decreases
current apparently increases.

The last item (current increases as flow decreases) does not seem consistent with the curve which shows HP/stage (I assume that is BHP/stage) increasing with flow over most of the range.

I don't know why that is. Can anyone suggest a reason for this?
I also don't understand in view of this why people are suggesting to decrease flow when your data suggests that will increase current.


=====================================
(2B)+(2B)' ?

 

[electricpete]

ok, maybe we are way out to the right of the peak of the bhp curve and need to keep throttling to get it to the left.
I can buy that.

=====================================
(2B)+(2B)' ?

 

[jraef]

@dpc: The name plate just shows 230/460V. I talked to the manufacturer and they said this motor also works with 208V, but it is a very minimum voltage. When we ordered the pump, we indicated that out voltage is 208V. I don't know why they sent us a motor that they know it barely works with 208V.

Since you asked...
Because the intent of 208V systems was for smaller commercial and LIGHT industrial installations where the motor loads would be under 50HP and close (relatively) to the service entrance by virtue of it being a small facility. So motor mfrs generally don't have many off-the-shelf REAL 200V motor offerings at over 50HP, under the assumption that for the few requirements out there, the motors will likely be close to the service entrance and therefore not see much drop so the tolerance of a 230V design down to 207V will suffice, or users can special order a 200V design if not. You are in the small percentage of applications falling outside of that decision envelope on 230V off-the-shelf motors and the OEM likely did not want to wait (or pay) for a special order 75HP 200V 2 pole motor. A quick double check of Baldor's website shows that you can get 1 choice on a 200V 75HP premium efficiency motor in 4 pole, horizontal mount, but not 2 pole nor vertical, as yours is. But 230/460V vertical and 2 pole? No problem, ship it tomorrow.

An excerpt from "The Cowern Papers", an excellent treatise on motor theory from an industry expert that gives great insight into many of the design decisions that go into making motors.

There is one oddity in the mix. That is 3 phase motors for the 120/208 volt power systems. For example,
if the power system were to be 208 volts minus 5% (approximately 198 volts) and you were using a 230
volt motor, then the 230 volt motor could only go down to 207 volts (-10%) without being in trouble.
There would be a discrepancy between the 198 volt low range of the system voltage, and the 207 lowest
operating voltage of a 230 volt motor, this could spell trouble. So how can this be addressed?

There are two ways that motor manufacturers have faced up to the problem. The first is to provide
motors rated for 200 volts that can operate successfully down to 180 volts, or up to 220 volts. This is an
adequate margin to cover the normal range of voltages that could be expected on a 120/208 volt
system. But using this approach exclusively would mean that the complete inventory of motors in all
sizes, enclosures, mechanical configurations, etc. would have to be duplicated to handle the motor
requirements for the 120/208 volt power systems. This would be very expensive and cumbersome,
especially with the wide variety of small motors (under 10 HP) that exist.

So most motor manufacturers have taken a different approach to handling these smaller motors. This
approach is that by using a somewhat more conservative design on the 230 volt motors it is possible to
create a 3 phase, tri-voltage motor with voltage ratings of 208-230/460. With this approach the 230 volt
winding ( and connection diagram) is used on the 208 volt power system. When this approach is taken
the motor manufacturer is essentially saying that this motor can be successfully operated on voltages as
low as 208 minus 10% or 187 volts. This approach usually works very well since 208 volt power systems
are normally used in small buildings with relatively short distances between the incoming power service
and the utilization equipment. These short runs tend to make 208 volt power systems quite stable so that
the limit of the motor’s low voltage capability is seldom tested.

"Will work for (the memory of) salami"