Sizing a shunt ammeter

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Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
I have a customer that I am trying to help. I have several pumps that are identical. Each is rated to draw 9.6 amps and they have a very high failure rate. This is expected and accepted for the application. I want to install an ammeter to monitor the current at a glance. I am thinking that a shunt meter similar to this one is economical and functional. 15A shunt meter.jpg So my questions and concerns are:
  1. Is there any advantages that would justify the largely increased cost of proposing CT'd metering instead. physical installation is not an issue.
  2. I was thinking to install 0-15A range. Given that the circuit is likely to overload often is the meter more prone to damage than say a 0-30A meter? I prefer the 15A because the sensitivity is greater?
Thanks in advance for the help.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
I have a customer that I am trying to help. I have several pumps that are identical. Each is rated to draw 9.6 amps and they have a very high failure rate. This is expected and accepted for the application. I want to install an ammeter to monitor the current at a glance. I am thinking that a shunt meter similar to this one is economical and functional. View attachment 8003 So my questions and concerns are:
  1. Is there any advantages that would justify the largely increased cost of proposing CT'd metering instead. physical installation is not an issue.
  2. I was thinking to install 0-15A range. Given that the circuit is likely to overload often is the meter more prone to damage than say a 0-30A meter? I prefer the 15A because the sensitivity is greater?
Thanks in advance for the help.

Perhaps a better way for me to ask the second question is, does spiking the meter with greater than 15 amps affect the meter or is it OK?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Perhaps a better way for me to ask the second question is, does spiking the meter with greater than 15 amps affect the meter or is it OK?
It varies with the meter.
You are not going to burn out the meter coil or anything like that. But there is some risk of mechanical damage when the meter needle hits the stop too hard. I have seen meters of that general type which have a 15 degree bend in the needle or have had their shaft bent from a gross overload.
I don't think that a 2x or even 4x overload would be a problem even for the cheapest meter you could get.
 

SG-1

Senior Member
Can you place a contact in parallel with it during start up ?

Is this a true RMS meter ? No, so it may miss some of the current that is causing heat in the motor. I have seen thermal analog ammeters used in the past. The indicator pushed a pointer that recorded the highest reading. As I recall they were used for recording demand.

Going digital could get you true RMS & other features, like alarm contacts, high/low, but the displays tend to be jumpy.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
It varies with the meter.
You are not going to burn out the meter coil or anything like that. But there is some risk of mechanical damage when the meter needle hits the stop too hard. I have seen meters of that general type which have a 15 degree bend in the needle or have had their shaft bent from a gross overload.
I don't think that a 2x or even 4x overload would be a problem even for the cheapest meter you could get.

Thank you and SG-1. I was actually an Instrument Systems Specialist in the Air Force many moons ago, so I have some ancestral memory of instrumentation. Your answer is close to what I was thinking. I didn't expect burnout, but a bent needle is exactly what I was wondering. These motors have a 9.6 amp draw, but they fail about once every 3 months via the pump freezing. The ammeter will give them real time monitoring of problems, including indication that the rebuilt one just installed is not adequate, the pump is overworking, etc. But I am worried that they may exceed the 15 amps. I don't even know if they will. I guess I can go with the 15 and if there is a problem, to change them to 30A is not a bank breaking proposition. Thanks again.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
Can you place a contact in parallel with it during start up ?

Is this a true RMS meter ? No, so it may miss some of the current that is causing heat in the motor. I have seen thermal analog ammeters used in the past. The indicator pushed a pointer that recorded the highest reading. As I recall they were used for recording demand.

Going digital could get you true RMS & other features, like alarm contacts, high/low, but the displays tend to be jumpy.

Not really the information I am looking to give them. Analog is a better display for the purposes I need.
 
The ampmeter which you want to be shunted generally are suited for DC useage.

Now the other part is the starting surge or overload., That part it will depending on the meter manufacter to see how much leeway it can be used I know some case like starting up the motour load useally are not too bad but the key question is repeating overload can ruin it.

This part I am not too sure with related to the NEC part but I know some case it can have direct load connected to the ampmeter only if they are listed for that purpose.

Is those pump as you mention is single phase or triphase AC motour or DC motour if so some of the ampmeter may have addtional protection if go over the specfic level it will make the contractor or other type of OCPD to drop out.

So there are few differnt way you can do this.

Merci,
Marc
 

SG-1

Senior Member
Have you considered an ammeter switch ?
The switch can be placed in the off position during start up, then the operator changes it to view each phase of current.

As I recall the greatest acccuracy lies in the 60-90% range of the analog meter.

The CT option is still going to slam the needle against the stop, unless the CT saturates the force will be about the same. If you can find a suitable CT that will saturate at 15 amps or so...

An MOV ( metal oxide varister ) across the meter might help to save the meter from overcurrent, if they or simular devices come with that low of a breakdown voltage.

Have you checked with your ammeter vendor ?
 

kwired

Electron manager
Location
NE Nebraska
My thoughts are if you have so many pump motor failures, maybe the wrong pump or motor has been selected or there is too frequent starting or something like that, which would still likely require different selection.

If motors are indeed overloaded there should be complaints of overloads tripping if proper overload protection has been provided.

I can almost promise you if this meter is going to be used to set a valve or something like that to limit the load on the pump, that as soon as the user (or their supervisor) sees what kind of flow they have at full load rating they are going to say something like "we need to run faster than that". Seen that one many times and only possible response is something like "you either need a larger motor or plan to replace the existing one more often"

Does this pump always pump the same media, or does it pump different level of solids or different viscosity media at different times?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
130127-1940 EST

Strathead:

Are the motors single or three phase? Probably does not matter because one leg of a three phase is probably adequate.

How many motors? How often does the current need monitoring? What are your cost constraints? What is the voltage level relative to earth?

My suggestion is going to be based on low cost clamp-on current transformers.

.
 

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
My thoughts are if you have so many pump motor failures, maybe the wrong pump or motor has been selected or there is too frequent starting or something like that, which would still likely require different selection.

If motors are indeed overloaded there should be complaints of overloads tripping if proper overload protection has been provided.

I can almost promise you if this meter is going to be used to set a valve or something like that to limit the load on the pump, that as soon as the user (or their supervisor) sees what kind of flow they have at full load rating they are going to say something like "we need to run faster than that". Seen that one many times and only possible response is something like "you either need a larger motor or plan to replace the existing one more often"

Does this pump always pump the same media, or does it pump different level of solids or different viscosity media at different times?

The design of the pumps being used is by others, and has been an established method for years. The "failure rate" is built in to the operation, so I assume the multitude of chemical engineers the company has on staff has decided over time that this is the best option. I am just trying to give them information that will help identify badly rebuilt pumps, improper wiring when replacing etc. They like the idea, and that is why I am asking about damage to the meter. From the answers I am getting, I am thinking I will be best to go with the 30A model.
 
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