testing of overloads

[joe j]

Bear with me, I haven't seen the setup in question so I can only go on what I've been told........

We had a swimming pool pump that locked up after the bearing failed and then tripped the 63A/30mA rcd, presumably after the insulation had broken down. The pump runs through a contactor operated by a time clock, the clock had been set to permanently on for a few years so that wasn't an issue. The contactor has an overload attached which was set at 6.5 which seems right for a motor rated at 6/9A run/start up. The replacement motor has been fitted and is running fine but it is rated at 7.2/10.1 run/start up and is through the same overload set at 5.5A (to test it).

Is there a way of testing an overload or does this seem normal? How can we prevent this motor doing the same?

In case you're wondering I am a sparky and have the usual multimeter and mft.

 

[malcolmsanford]

Normally a locked motor would take the overload out on high current as you say, but we can only assume that for some reason you had an earth fault that took the RCD out first

To be honest I don't really know a way of testing an overload, unless you but high load onto it, a bit like no one on site, unless it is HV switch gear, do MCB testing.

What I sometimes did on overloads is ramp it down so that it would/should trip out at it's lowest level, and took the logical path that if it trips when set below the motors run load, chances are it was working, but realyl not a very sure way of testing.

If you have a motor rated at 7.2 and the 5.5 is happily holding in, either it's just on the edge, which it shouldn't be, it is either well out of calibration or is entirely faulty. For me depending on how critical/expensive this motor is I would look at changing the overload.

 

[Darkwood]

This is also a motor sizing thing... if your motor is bigger than needed then it wont run near FLC and overloads can be set lower to allow for this after clamping, this way any issues will be picked up and pre-empt a service. The overload may have been fine but you said the bearings failed and tripped the rcd so i suspect the overload didnt have chance to operate as the rcd saw the fault first, if the bearing collapsed the shaft could have been given enough movement to cause it to crash into its surrounds thus shorting to earth.
Talk to the motor repair engineers and ask what they found, i doubt they will say it siezed up and over-heated but in fact had winding damage caused by the shaft having a collapsed bearing.
I havent heard or had reason to believe an overload had failed in more experience although i know its plausable, but i always get a detailed report from my motor repair guys so i learn as well as been able to pass on what caused the motor failure to the customer.

You say the motor is running constanly too which is not what its original design plan was (timer inline), look into and discuss having a second motor fitted as either a bypass or a system where the motors are run alternatively a flow switch can also pick up a issue with a pump and lock it off giving other permanent run command while having an alarm operate to notify of possible fault.

 

[Guitarist]

It's one of those things we take on trust, just like MCB's. Like Malcolm said, sounds like the insulation broke down and took out the RCD rather than an O/L occurring.
Clamp the supply to the motor before assuming that it is running at 7.2A.

 

[joe j]

Thanks all.
I am concerned about the overload as the motor casing is badly damaged which would suggest that it was running for a while before it locked and burnt out and I would have thought this would have raised the loading. It ran under no load at the engineers so windings etc poss ok.
I take the points on FLC and constant running. Think I'll change the overload as a precaution and stick my clamps on it.

 

[Knobhead]

For what they cost compared to a new motor get a new contactor and O/L set up. Unless you know the inverse time / current curve of the O/L and have a reliable current injection device, testing isn’t really an option.
Like MCB’s you have to rely on the manufacturers type testing.

 

[mheyes]

Why do you not fit a thermistor relay. Most pump motors have a thermistor fitted as standard. It would detect the sudden jump in motor temperature. Just wire it in series with A1 on the contactor.

 

[Engineer54]

Swimming pool pumps are not designed to run continuously, most have a 6 or 8 hour Max continuous rating, so Put the timer back in circuit, ...end of!!... A pool pump/motor combination will cost around 250 to 350 quid for a 1.5HP unit, so as has been suggested change the contactor/overload unit. Most new pool pumps that i have seen now have klixon type motor protection, yours may, or may not.

Apart from using a standard MCCB injection test kit, or other similar current injection test kit, then your going to be stumped trying to test that O/L module!! Certainly can't be done with your multimeter and MFT!! lol!!

Whatever you do, ''Do Not'' leave this new pump motor running continuously 24 hours a day!!!

 

[joe j]

Swimming pool pumps are not designed to run continuously, most have a 6 or 8 hour Max continuous rating, so Put the timer back in circuit, ...end of!!... A pool pump/motor combination will cost around 250 to 350 quid for a 1.5HP unit, so as has been suggested change the contactor/overload unit. Most new pool pumps that i have seen now have klixon type motor protection, yours may, or may not.

Apart from using a standard MCCB injection test kit, or other similar current injection test kit, then your going to be stumped trying to test that O/L module!! Certainly can't be done with your multimeter and MFT!! lol!!

Whatever you do, ''Do Not'' leave this new pump motor running continuously 24 hours a day!!!

Have been advised by the guys on site that (they were told) motor ok for 24/7 operation. Didn't know if timer put in for energy saving or on manufacturers advice.
If you want a job doing properly ............

New contactor & ol on order. Cheaper than 650 the new motor cost.

Hopefully be swimming next week.

 

[Knobhead]

Thanks for letting us know hone:

 

[Engineer54]

At 650 quid a pop, i'd expect it to be able to run 24/7 too ...lol!!

I have installed numerous swimming pools, both indoor and outdoor, of every shape and description. But i have yet to see one where the filter pump is left to run 24/7!! Total waste of both energy and money. Every standard Pool control panel incorporates a timer of one type or another, for the control of the pump motor....

A communal outdoor heavy use pool in a hot climate during summer months, would normally be set for 8 hours continuous run time. Maybe with an additional 2 hour maintenance night period. A private pool in the same environmental conditions would normally be set-up for 2 X 3 or 4 hours run times (morning and afternoon), depending on it's usage... Winter would bring those times down dramatically to maybe 1 or 2 hours a day, ....where pool is unused!! Many variations to these times, ...again depending on, indoor or outdoor, pools being heated or not, pool being winterised or not, etc etc!!

But None, not even one have i seen run 24/7!! Total nonsense!! ...lol!!!