×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

75HP Motor Problem

75HP Motor Problem

75HP Motor Problem

(OP)
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.

RE: 75HP Motor Problem

You're pumping too much water?

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

RE: 75HP Motor Problem

(OP)
YEs, I'm pumping about 338 GPM.

RE: 75HP Motor Problem

Classic 208V that isn't.. I would consider putting boost transformers in to get it back up to 208 or more if that pump runs for more than a few minutes at a time.

A pump is a mass-flow device. That means the more material that runs thru it the more work it does. If you are pumping too much volume you are overloading the motor. You need to put in restrictions be they nozzles, smaller nozzles, orifice plates, or a flow limiting valve.

Keith Cress
kcress - http://www.flaminsystems.com

RE: 75HP Motor Problem

(OP)
On the discharge side of the pump I have many nozzles that they are supposed to spray on the car. The current operating point is almost in the middle of the pump and I think the motor should be able to handle this load.... If I close the downstream valve it will increase the pressure after the pump. How does this affect my amperage? Does it reduce my amperage? Besides increasing the voltage, is there any other solution?

RE: 75HP Motor Problem

Looking at the curve for your pump it appears your 338gpm and 275psi are right where they should be. This points back at the motor side. I believe you are seeing over current because the motor is undervoltaged.

Has anything changed since the system has been running? Or is this a new installation?

Is the volume you're stating actually a measured value you have?

Keith Cress
kcress - http://www.flaminsystems.com

RE: 75HP Motor Problem

(OP)
This is a new installation. The GPM is the calculated GPM. I believe the actual GPM should be almost the same.

RE: 75HP Motor Problem

Whoaaaaa wait a minute. I take it all back. Not the motor side. The values you're giving us as your operating point is fine for the '6' version. You actually have the '6/2' version which is less capable. You are using too much volume which IS requiring more hp. At 275PSI you can only have 290GPM NOT 338GPM.
So you need to reduce the flow or reduce the head at which your flow is occurring at down to 250PSI.

T'wer me I would close a valve slowly while watching the ammeter until you hit nameplate. Try it briefly and see how the current responds. Some pumps won't drop current with restricted flow.

Keith Cress
kcress - http://www.flaminsystems.com

RE: 75HP Motor Problem

We cross-posted. Calculated.. Ugh. Too bad as that doesn't help with diagnosis.

Keith Cress
kcress - http://www.flaminsystems.com

RE: 75HP Motor Problem

(OP)
You are right. Basically the design pressure is 251 PSI @ 338 GPM. When I move my nozzles (It is a huge Gantry with lots of nozzles), due to movement of they rubber hoses the pressure ramp up to 275 PSI or drops to 251 PSI. Either way, my operating point is moving on the pump curve and I'm not out of the curve. That's why I think the motor itself should be able to handle the load. I have a specific demand (Pressure and flow rate) that I have to meet at the nozzles. Do you think that if I close the discharge valve a little, it helps me to reduce the amperage ?

RE: 75HP Motor Problem

Yes! If you have the valve give a try while looking at the pressure and the amperage. That will quickly show if it's going to work. A lot of motor loads are in-constant and allow overloads that are short and sporadic however some loads like yours, air compressors, and refrigeration can be nearly continuous. In these continuous cases slight overloads can make BIG differences in operational reliability. Likewise low voltage can be very problematic in continuous situations. You may need to reduce your pump's load even down from the curve as that low voltage will put your motor at increased amp-draw continually. Again in some cases it wouldn't matter but in continuous application it can and does.

Keith Cress
kcress - http://www.flaminsystems.com

RE: 75HP Motor Problem

Looking at your pressures and currents you listed it really does look like you are FLOWING way too much. Definitely try the valve.

Keith Cress
kcress - http://www.flaminsystems.com

RE: 75HP Motor Problem

Just out of curiosity, what is the actual nameplate voltage for the motor? There are a lot of "triple rated" motors out there that are listed as 208/230/460. These motors will not perform well when operated at 208 V. It will run, but not at rated efficiency or power factor (or torque).

A motor actually intended to operate on a 208 V system will have a nameplate voltage of 200 V.

RE: 75HP Motor Problem

Try plugging off some nozzles.
Throttling a valve will drop the current but it will also drop the pressure at the nozzles.
You may get some increase in performance with a higher voltage but not enough.
By the way, do you have positive pressure on the suction side of the pump?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: 75HP Motor Problem

It seems like this is a positive displacement pump, isn't it?
Then it is expected that current goes up as pressure (I assume dp) goes up.

338 psid * 150 gpm = 22 KW fluid power
sqrt(3)*208V&205a = 74 KW electric apparent power
A surprisingly low product of pf* EffMotor* EffPump..most likely EffPump is the problem somehow.

How is the pressure adjusted?
Do you have link to this pump curve?

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

RE: 75HP Motor Problem

Quote:

Do you think that if I close the discharge valve a little, it helps me to reduce the amperage ?
Not if it is positive displacement pump. Unless I am misunderstanding something about the system..

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

RE: 75HP Motor Problem

Thanks Keith. Your link doesn’t seems to work for me.
I did find this with pump curve on 2nd page of pdf (page 41) which sure looks like centrifugal.
http://blumenauerpumps.com/commercialpressure/esv/...

So, going back to op, pressure is changing and currrent going up - what is being adjusted?

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

RE: 75HP Motor Problem

(OP)
@itsmoked: I will definitely try using a globe valve downstream of the pump and see the effect.

@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.

@waross: Yes, I have a positive pressure on the suction side. This pump is drawing the water from a 4000 GAL tank, so I have enough static head on the suction side.

@electricpete: No, this is a multi-stage centrifugal pump. Here is the link to the curve: http://blumenauerpumps.com/commercialpressure/esv/... you should use curve 6/2. I adjust the pressure by using a globe valve downstream of the pump.

RE: 75HP Motor Problem

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

RE: 75HP Motor Problem

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

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

RE: 75HP Motor Problem

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

RE: 75HP Motor Problem

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)' ?

RE: 75HP Motor Problem

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)' ?

RE: 75HP Motor Problem

Quote (behradk)

@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.

Quote (Edward Cowern, PE)


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"

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources