AC stall?

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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
generally it involves the motor being unable to deliver enough torque to be able to turn.

there can be various reasons why that might happen.

might be a defective motor, low voltage, too much torque being required by the load, all kinds of things.

is this happening on startup?
 

drbond24

Senior Member
The motor is on a pump. The pump is supposed to provide 1700 psi. When the pressure reaches ~1000 psi, the motor/pump stop turning completely and the motor starts pulling over 3 times normal current.

This happens on startup only if the system pressure is already above ~1000 psi. If there is no pressure, the motor starts fine. There isn't a problem until there is pressure.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
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retired electrician
I assume this is a positive displacement pump. Are there any valves that would block the flow out of the pump?
 

drbond24

Senior Member
Only one outlet discharge valve, but it is open. Or, at least, the handle indicates it is open. I could disassemble the valve and make sure the handle is correct. I've seen them messed up before.
 

drbond24

Senior Member
Have a bit of time now.

In discussing this problem internally, the mechanical people say there is no reason the pump shouldn't turn so it must be an electrical problem. The electrical people say there is no reason the motor should turn, so it must be a pump problem. :)

We have this problem on more than one motor/pump, so if the problem is electrical it is a buss/system problem. I was wondering if the voltage being too high or too low on the system would cause a problem like this with the motor, or what other potential issues there are that I haven't thought of. The motor is rated 575 VAC. I haven't checked the buss voltage, but with all of the equipment we have out of service right now the voltage should be 600 VAC or even a bit more than that. I don't see where that is a problem, but I want to make sure.

All I know for sure is that something is wrong. I don't care which discipline it is, I just need the pumps to run because their hydraulic pressure operates other machinery that I need to test. I can't do my tests until these pumps are straightened out, so I'm attempting to help with that in order to further my own agenda. :D
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The motor is on a pump. The pump is supposed to provide 1700 psi. When the pressure reaches ~1000 psi, the motor/pump stop turning completely and the motor starts pulling over 3 times normal current.

This happens on startup only if the system pressure is already above ~1000 psi. If there is no pressure, the motor starts fine. There isn't a problem until there is pressure.

I would have to guess that the motor can't produce enough torque at the slower speeds it sees during startup.

Have you considered some kind of vent to let the pressure off first? Or a recycle valve to let the pump motor get up to speed?
 

zog

Senior Member
Location
Charlotte, NC
Have a bit of time now.

In discussing this problem internally, the mechanical people say there is no reason the pump shouldn't turn so it must be an electrical problem. The electrical people say there is no reason the motor should turn, so it must be a pump problem. :)

Gee, have never heard that before :)
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
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Electrical Engineer
If you lose voltage, the starting torque that the motor can develop drops at the square of the voltage drop. So if this is happening before the motor finishes accelerating, that will be the case and you will eventually stall. If truly stopped, the current would go to 600%, not 300%, so that indicates it is still turning, just not getting to speed.

One electrical condition that could be happening is if the supply to this motor is seriously deficient, the LOAD that increases with speed as it tries to overcome the existing 1000PSI system pressure is actually causing a severe voltage drop on the circuit feeding the motor. This could be caused by a number of things, none of which we can see from here; conductors are too small, transformer feeding this motor is too small, bad connections that heat up, the list can go on and on. If it were a failing motor, such as a broken rotor bar in the motor or failing motor insulation, it would not likely make any difference that the pressure was already over 1000 PSI as you indicated, so I would rule that out.

Have you taken any voltage measurements while starting?
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
... the motor/pump stop turning completely and the motor starts pulling over 3 times normal current.

.... There isn't a problem until there is pressure.
I going to guess single phase, cap start, cap run. Good so far?

If so, I have a couple of suggestions.

ice
 

kwired

Electron manager
Location
NE Nebraska
Check voltage and current when it is heavily loaded and when it stalls.

Is this new or existing? Potential for mechanical problems increases if existing. Could even be motor bearings.

If existing, did this just start to happen or has it always been this way?

Has anything been replaced? Is it the correct motor to drive the pump?
 

drbond24

Senior Member
Have you considered some kind of vent to let the pressure off first? Or a recycle valve to let the pump motor get up to speed?

The pump is supposed to have these features. I will see if the mechanical people can verify their proper operation.

Jraef said:
If you lose voltage, the starting torque that the motor can develop drops at the square of the voltage drop. So if this is happening before the motor finishes accelerating, that will be the case and you will eventually stall. If truly stopped, the current would go to 600%, not 300%, so that indicates it is still turning, just not getting to speed.

One electrical condition that could be happening is if the supply to this motor is seriously deficient, the LOAD that increases with speed as it tries to overcome the existing 1000PSI system pressure is actually causing a severe voltage drop on the circuit feeding the motor. This could be caused by a number of things, none of which we can see from here; conductors are too small, transformer feeding this motor is too small, bad connections that heat up, the list can go on and on. If it were a failing motor, such as a broken rotor bar in the motor or failing motor insulation, it would not likely make any difference that the pressure was already over 1000 PSI as you indicated, so I would rule that out.

Have you taken any voltage measurements while starting? .

The motor is definitely stopped. I can see the pump shaft and when this happens there is no movement. We will be doing some electrical investigation today hopefully to check voltage and current during startup. Our safety rules do not make doing this easy, but we can get it done with enough planning.

iceworm said:
I going to guess single phase, cap start, cap run. Good so far?

If so, I have a couple of suggestions.

No, sorry. Wrong on all counts. Three phase induction motor.

ptonsparky said:
New system? Motors undersized?

It is a new system. The motors are supposed to be oversized, but I do not know who did the size calculations. I could verify them. I will when I get a chance.

kwired said:
Check voltage and current when it is heavily loaded and when it stalls.

Is this new or existing? Potential for mechanical problems increases if existing. Could even be motor bearings.

If existing, did this just start to happen or has it always been this way?

Has anything been replaced? Is it the correct motor to drive the pump?

Will try to check voltage and current today. It is a new installation. Well, it is a new motor and pump installation on an existing hydraulic system. The motor and pump were a package deal from the vendor. They are supposed to be made for each other is all I can say.

Thank you for the suggestions. I will hopefully be able to gather more data today.
 
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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
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Electrical Engineer
If the motor is STOPPED yet the current is only 300%, not 600%, then I have a real good idea what the problem is.

This is a dual voltage motor, i.e. 230/460V, maybe slightly over sized to allow it to run on 208V. But someone (not saying who...:angel:) connected it for 460V and you are feeding it 208V. The loss of torque is why it is stalling, the mis-connection is why it is drawing only 1/2 the Locked Rotor Current.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
It is a 575 VAC motor. It does not have dual voltage taps.

What difference does it make to wire the motor wye versus delta?
Sorry, got this mixed up with another post in another forum.

But it would make a similar difference if it was supposed to be connected in Wye for 575V, as is likely the case, and you connected it in Delta instead, which would be the equivalent of connecting it to 332V. Same net effect as far as the motor goes and the LRC would also be reduced.
 

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
Sorry, got this mixed up with another post in another forum.

But it would make a similar difference if it was supposed to be connected in Wye for 575V, as is likely the case, and you connected it in Delta instead, which would be the equivalent of connecting it to 332V. Same net effect as far as the motor goes and the LRC would also be reduced.
Lost me here. Looks to me like one would be connecting 332V coils across 575V Severe overvoltage. I don't see how the LRC would be reduced - should go up - saturated

ice
 

kwired

Electron manager
Location
NE Nebraska
Still sounds like there is a good chance the coils are operating at less voltage than they were designed for, whether by improper connection to the circuit or not.

I too was thinking about commenting on why only about 300% full load current is being drawn if the rotor is stalled. I would expect at least 450-500% in most cases as a minimum.
 

ronaldrc

Senior Member
Location
Tennessee
We don't know the hydraulic lines and cylinder lay out.

But before I went to deep into this I would rule out a
mechandical problem.

1000 lbs. is a lot for a hydraulic pressure for a motor to
start against.

Check for a check valve on your high pressure side,could
be leaking pressure back into your pump head when it
might not should be.

Ronald :)
 

kwired

Electron manager
Location
NE Nebraska
Going back to post 3,

The motor is on a pump. The pump is supposed to provide 1700 psi. When the pressure reaches ~1000 psi, the motor/pump stop turning completely and the motor starts pulling over 3 times normal current.


This happens on startup only if the system pressure is already above ~1000 psi. If there is no pressure, the motor starts fine. There isn't a problem until there is pressure.

is this happening anytime pressure is over 1000 or only when starting if pressure is already above 1000? If it is only when starting with high existing pressure, then there likely is either poor design or selection of motor, no unloading device, or a malfunction of an unloading device.

If it can not develop 1000 PSI when it is supposed to be able to develop 1700, there could be low voltage, improperly wired motor, or other installation issues.
 
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