How to calculate current draw when fan motor fails.

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Besoeker said:
Let me try this again in a little more detail for your benefit.
All of the even the most basic motor overload units I've used in the past 40 some years have had current unbalance protection that would offer single phase protection.

Have you used any that don't?
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Being that your experience is outside of the US, you may have not come across this. Older style NEMA bi-metal or eutectic melting alloy overload relays that have the replaceable heater elements do not have differential tripping, the feature in IEC OL relays that will bias the trip point in order to help prevent motor damage from single phasing. But over here, standard off-the-shelf NEMA designed motors in sizes where people tend to get cheap about things like OL relays are designed with a 1.15 Service Factor, which makes them more tolerant to the added heating effects of negative sequence currents that take place in a 3 phase motor that loses a phase while running, a concept that doesn't exist with IEC motors. So basically, our basic OL relays don't have it so our motor designs are made to be more tolerant, or vice versa. IEC motors have no Service Factor built-in so even basic OL relays have to cover that potential problem.

This also raises a semantics issue for all: standard IEC bi-metal OL relays that do provide this differential protection technically do NOT protect against the loss of a phase, they only protect against that additional heating in a 3 phase motor that results from losing one phase while running, and only IF the motor is fully loaded. What the differential bar does is move the trip curve pickup point lower than normal so that the load current only needs to be somewhere around 75-80% of the setting instead of 100% before it begins it's trip process. But if your motor is only loaded to 50% when the phase is lost, it may run forever and not cause that OL relay to trip. So it is not truly protecting against a loss of a phase, it is protecting against the DAMAGE that may happen on a fully loaded motor if a phase is lost while running. Another important issue is that it will not PREVENT a motor starter from turning on in a lost phase situation, which can damage a motor and cause other operational issues as well because the starter will say the motor is running but it will not spin. So if there are other more important issues to guard against resulting from a phase loss, one should NOT rely upon the often misquoted "single phasing protection" of a standard IEC bi-metal OL relay, use a solid state OL relay that has single phase protection (most do) or use a CURRENT based phase loss relay (voltage based phase loss relays can be fooled too easily).
 
Jraef said:
Being that your experience is outside of the US, you may have not come across this. Older style NEMA bi-metal or eutectic melting alloy overload relays that have the replaceable heater elements do not have differential tripping, the feature in IEC OL relays that will bias the trip point in order to help prevent motor damage from single phasing. But over here, standard off-the-shelf NEMA designed motors in sizes where people tend to get cheap about things like OL relays are designed with a 1.15 Service Factor, which makes them more tolerant to the added heating effects of negative sequence currents that take place in a 3 phase motor that loses a phase while running, a concept that doesn't exist with IEC motors. So basically, our basic OL relays don't have it so our motor designs are made to be more tolerant, or vice versa. IEC motors have no Service Factor built-in so even basic OL relays have to cover that potential problem.

This also raises a semantics issue for all: standard IEC bi-metal OL relays that do provide this differential protection technically do NOT protect against the loss of a phase, they only protect against that additional heating in a 3 phase motor that results from losing one phase while running, and only IF the motor is fully loaded. What the differential bar does is move the trip curve pickup point lower than normal so that the load current only needs to be somewhere around 75-80% of the setting instead of 100% before it begins it's trip process. But if your motor is only loaded to 50% when the phase is lost, it may run forever and not cause that OL relay to trip. So it is not truly protecting against a loss of a phase, it is protecting against the DAMAGE that may happen on a fully loaded motor if a phase is lost while running. Another important issue is that it will not PREVENT a motor starter from turning on in a lost phase situation, which can damage a motor and cause other operational issues as well because the starter will say the motor is running but it will not spin. So if there are other more important issues to guard against resulting from a phase loss, one should NOT rely upon the often misquoted "single phasing protection" of a standard IEC bi-metal OL relay, use a solid state OL relay that has single phase protection (most do) or use a CURRENT based phase loss relay (voltage based phase loss relays can be fooled too easily).
Click to expand...
As it happens, my experience predates the general adoption of IEC standards - yes I am that old.......:)
In the early days, we had dashpot relays. Obviously sustained overloads would trip them. Differential loading caused a connecting bar to tilt and that also the trip.

From memory, the manufacturer was George Ellison.
 
Besoeker said:
As it happens, my experience predates the general adoption of IEC standards - yes I am that old.......:)
In the early days, we had dashpot relays. Obviously sustained overloads would trip them. Differential loading caused a connecting bar to tilt and that also the trip.

From memory, the manufacturer was George Ellison.
Click to expand...
I'm not so young that I don't remember dash pot relays. In fact I remember that toward the end of their reign, FedEx came into being and started offering "Next Day Air" package services, which because nobody stocked dash pot oil anywhere, seemed like a great solution to its shelf life problem. But alas, FedEx considered it a hazardous material and would not accept it, something I didn't discover until the wolves were howling at the gate wanting to know why the machine was not back in service when I had promised.

5 or 6 years ago I replaced some old "one-armed bandit" manual compensator starters on large pumps with soft starters. The old starters had dash pots on them, nobody on the job had any idea what they were until I took over, in fact the previous engineer on the project was stumped by them and held up the project trying to figure out how to replace them, not knowing what they were or why they were there. These old starters also had ammeters on them with 4:1 CTs on 400A circuits, so they were running 100A through them. I had never seen anything like that before. I have pics of them somewhere, I'll have to post them someday.
 
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