Water Pump Tripping 2

[Rockwave]

We have two 40-HP water pumps that pump water from a pressurized pipe to an elevated storage tank. The pumps have been tripping arbitrarily. Each time after they tripped, the operator just reset the power and then they worked again. It happens a few times a week. We could not figure out the reason as most of the time they just work fine. Could anybody shed some light? Thanks.

 

[RRaghunath]

Rockwave,
There could be two reasons:
1. Overload relay pickup setting is below the load current
2. The motor is overloaded for some reason
3. Single phasing or unbalanced incoming power supply voltages

Knowing whether the pump centrifugal type or reciprocating type as well as the actual current drawn vs the overload setting can help.

 

[dpc]

1. What is "tripping" - the motor overload relay, motor circuit breaker, ground fault protection, etc? And how did they "reset the power"?
2. Is this motor operating from an adjustable frequency drive?
3. Did the motor seem HOT when it tripped, or did anyone bother to check?

 

[Rockwave]

raghun and dpc,

Thanks for your replies. What we found is that the pumps tripped the motor circuit breaker. When the operator reset it, they simply turned the power off and turned it back on. The pumps are closed-coupled end suction centrigugal type with constant speed drives. We did check the current and found it was within the range.

We do not know if the motor was hot or not since we have never caught it. But we do know that the motor backspins for a few seconds after the pump is turned off. We are thinking that when there is a power failure while the pump is running, the motor may backspin when the power is back on to cause the tripping. But should it trip the overload relay first? Any other reasn you could think of? Thanks.

 

[dpc]

If there is a brief power outage and the motor is spinning backwards when power is restored, that could explain the tripping of the circuit breaker. There can be high transient current when the motor is re-energized.

The overload will not respond to this because it happens too quickly.

If the circuit breaker has adjustable instantaneous (magnetic) trip elements, you may try increasing the settings, but I'm guessing they are already at maximum.

Another solution might be a time delay that keeps the pumps off for a while on a loss of power. Or Start/Stop control instead of On/Off.

 

[burnt2x]

I guess its time you check the pump's discharge check valve. Passing check valves lets some water back flow causing the pump to backpedal (reverse rotation).

 

[roydm]

I don't think it's a check valve problem, that wouldn't cause the original trip.
Do you have a recording ammeter or chart recorder that you can put on the motors to see what is causing the original trip, perhaps just plain overload.
Another possibility,
Excessive heat buildup in the starter?
Intermittent loss of suction pressure?
Good Luck
Roy

 

[jraef]

No, I think dpc is right on. PLS from me.

But we do know that the motor backspins for a few seconds after the pump is turned off.

An Overload Relay would not trip anywhere near fast enough to protect against this. Only the magnetic trips in the circuit breaker could. You need a check valve my friend. Even if the breaker occasionally trips, the torque transient that this represents is right now damaging your motor shaft and unless it is significantly over sized, it will shear off eventually.

 

[Rockwave]

Thanks for the comments.

The check valve closing speed can be adjusted. We are trying not to close it too quickly to cause water hammer. We will add a 10-second delay to the pump so that it will not restart immediately after a power loss.

We will see how it works. However, I am still not very convinced that this is the real reason for the trips as it is only related to power outage. The pumps have been working great in terms of flow and the suction pressure has been at least 20 psi over the minimum setting.

 

[davidbeach]

However, I am still not very convinced that this is the real reason for the trips as it is only related to power outage.

You don't believe power outage is the cause? You don't understand how a power outage could be the cause? Something else? I don't think I understand what you are trying to get across here.

One family of questions I see haven't be asked. Do both pumps run at the same time? If so, do they both trip at the same time? If not, can the problem happen to either pump or is it just one pump in particular? Are there power system glitches immediately preceding the trips?

If you are tripping a mag-only you have motor currents that are above locked rotor. One of the easiest ways to get there without a fault (short circuit) is to energize a reverse spinning motor. Locked rotor is associated with a slip of 1 (100%). A reverse spinning motor will have a slip greater than 1, full speed reverse is a slip of 2. When the slip gets above 1 the rotor resistance becomes negative.

 

[itsmoked]

Some real confusion going on in this thread. Let me try to reiterate.

we do know that the motor backspins for a few seconds after the pump is turned off. We are thinking that when there is a power failure while the pump is running, the motor may backspin when the power is back on to cause the tripping. But should it trip the overload relay first?

You have this answer. Unanimously we believe that the overload protection would not trip. Just about unanimously we believe the breaker would trip as this will not be looking like an overload but like a fault owing to the extreme current that would flow immediately on re-energizing a reverse spinning motor.

Now your original question does not even mention power failures but rather regular constant weekly random tripping.

The pumps have been tripping arbitrarily. Each time after they tripped, the operator just reset the power and then they worked again. It happens a few times a week.

Are these random trips in anyway associated with power failures?

Also most big motors do not instantly restart after a power failure. Someone has to do something manually most of the time since the contactors fall out. Are your systems set up to just instantly turn these motors back on? If so this is a bad set up for various reasons including the back spin problem but not limited to it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Rockwave]

I understand that power outage could be the cause. However one pump tripped again yesterday afternoon and I don't believe there was a power outage then since the weather was pretty good and there was no rain.

We only run one pump at a time and we have seen both pumps tripped. It is interesting that most of the time the pumps tripped at night.

 

[davidbeach]

Time for a recording ammeter.

 

[dpc]

So how often do these pumps start and stop normally? Are they running on automatic level control?

It's possible that the breakers are tripping intermittently on inrush current during normal starting. The inrush current is an seemingly random function that actually depends on the phase angle of the voltage when the motor is started. It's possible for the breaker to trip on 10% of the starts.

If you can provide data on the breaker size and instantaneous trip settings, that would help identify this as a possible problem.

 

[Rockwave]

These pumps alternate each time and they are running on automatic level control. I would say each pump will run once every two to three hours, 30 minutes each time.

The breaker is 100 amp with a trip setting of 600 amp.

 

[Rockwave]

The pump full load amp is 49.

 

[dpc]

You might try increasing the trip setting. The NEC allows a maximum setting of 13X FLA for standard Design B motors and up to 17X FLA for energy-efficient motors.

One of the trade-offs for high efficiency motors is increased inrush current. 13 x 49 = 637 A and 17 x 49 = 833 A.

Is this a instantaneous trip only breaker (MCP)?

If the trips are not always linked to momentary power blips, I'd say this is the most likely suspect. The breaker will trip when the motor tries to start - but not every time.

 

[burnt2x]

All,
If my memory serves me well, I remember a poster who posted a similar problem here and it went out of line from the OP to the point of posters arguing which is better: VFD control or valve control! The consensus was that the pumps were operating "deadhead" causing the ripping!
Does the handle "valvecrazy" ring any bell?
Anybody can tell me I was wrong!

 

[Rockwave]

The motor is standard design B motor. The circuit breaker is MCP. Is there a big difference if we change the setting from 600 A to 637 A?

 

[dpc]

Well, possibly. I would certainly give it a try. If the trip is happening infrequently, it could make the difference, since apparently the motor is able to start successfully the majority of the time. It would be very helpful to know if the breaker is tripping when the motor starts. If this is not the case, there is some other problem.