electrical motor-driven pump question

[EEatwork]

:smile:Hi there, I'm curious to know if some of you worked on electrical motor-driven pumps before? I've experienced some undercurrent trip problem; I wonder if it is always associated with the flow rate in the pump? electrically, the motor and starter are functioning well. the relay would sometimes take it out on "undercurrent trip", so we would tweek the relay setting a bit just to give a bit more tolerance. thanks all.:smile:

 

[StephenSDH]

The current of a motor will vary depending on the loading of the pump. A 1HP 480V 3-Phase motor is rated for 2.1 Full Load Amps. This is the combination of Real and Reactive(Inductive) Currents. You only pay for the Real current(sidenote). If the motor is unloaded and friction free the motor would only see the reactive current(theoretical only there are always friction losses). A .75 Power Factor motor unloaded could only have 1.4A instead of 2.1. Not sure if this helps you.

-Steve

 

[weressl]

:smile:Hi there, I'm curious to know if some of you worked on electrical motor-driven pumps before? I've experienced some undercurrent trip problem; I wonder if it is always associated with the flow rate in the pump? electrically, the motor and starter are functioning well. the relay would sometimes take it out on "undercurrent trip", so we would tweek the relay setting a bit just to give a bit more tolerance. thanks all.:smile:

Undercurrent - or more precisely underload - protection is installed in centrifugal pump circuits to protect them against dead-heading or to detect low-flow conditions.

See:
http://www.emotron.com/en/products/Shaft-power-monitors/Emotron-M20/

or
http://www.emotron.com/en/products/Shaft-power-monitors/Emotron-DCM/

 

[LarryFine]

Steve, welcome to the zoo! :smile:

Is the pump's intake under vacuum, or is it gravity-fed? Maybe there's an intake leak permitting cavitation, and causing the motor to be under-loaded.

 

[Jraef]

I agree, under current trip relays are a way of providing protection against loss of prime on pumps that are not self-priming, or dead heading (closed discharge) on those that are. But they are a poor choice for true under load, because current will also fluctuate with voltage at a given load and the current use curve is not linear, it has a steep drop-off point that can be problematic. I prefer to use Load (pf) or kW monitoring relays, because they better represent the true shaft loading on the pump, which is what you are really after. They are a bit more expensive though, so most pump OEMs tend to use the UC relays and suffer the problems you are experiencing.

 

[weressl]

I agree, under current trip relays are a way of providing protection against loss of prime on pumps that are not self-priming, or dead heading (closed discharge) on those that are. But they are a poor choice for true under load, because current will also fluctuate with voltage at a given load and the current use curve is not linear, it has a steep drop-off point that can be problematic. I prefer to use Load (pf) or kW monitoring relays, because they better represent the true shaft loading on the pump, which is what you are really after. They are a bit more expensive though, so most pump OEMs tend to use the UC relays and suffer the problems you are experiencing.

Actually the pump manufacturers we are dealing with ITT Gould, and couple of others I can't think of have the Emotron device - posted above - private labaled for them for true shaft power monitoring.

I am having second thoughts lately that current monitoring is really insufficent for simple dead-heading and dry-run protection.

 

[EEatwork]

:smile:
thanks for everyone's reply. these are some really good information.