Coil Resistance - Pond Pump

Thread Starter

daba1955

Joined Apr 27, 2019
130
Measuring DC resistance (multimeter) of a 220-240V 50Hz 70W pond pump at about 50 ohms.

Does that sound about right ?
 

ericgibbs

Joined Jan 29, 2010
13,882
hi daba,
The DC resistance will be low, the impedance when running could be around 10 times higher.

Does the motor run OK with 230Vac.??
E
 

Thread Starter

daba1955

Joined Apr 27, 2019
130
Hi Eric, Yes it's running OK, got it on test in my bath and has been running over an hour now ....

But we think this pump may have tripped the power at it's owners bungalow, but it could have been something else, not sure.

He will have a standard ELCB, but mine is slower acting (300mS). It's a long story, but Western Power in their infinite wisdom changed it to stop MY board being tripped when the pub 200m down the road used to switch their car park and beer garden lights off !! They had just installed a new PME supply cable down the road, and for some reason the spikes preferred to use my earth rod instead of their PME. The guy who tested my installation initially said it was the lowest impedance to earth measurement he had ever recorded.
 

ericgibbs

Joined Jan 29, 2010
13,882
hi daba,
As a confidence check, you could measure the AC running current to the motor.
Off load it should around 0.3A to 0.4A.

When starting, the current will be momentarily high or when the motor is stalled due to overloaded shaft torque.

E
 

Thread Starter

daba1955

Joined Apr 27, 2019
130
Ok, I've got ....

232V 50Hz
0.48A 0.52 cos
58.98W

Looks normal to me from what you said.

It's not got much load, it's about 8 inches submerged in my bath !!
 

LesJones

Joined Jan 8, 2017
3,523
You need to use an insulation tester (Megger.) to test for earth leakage. As the pump casing is probably plastic you will need to immerse it in water. (It might be a good idea to add a small amount of salt to the water in case it is very pure and does not conduct without any added impurities.) Connect one wire from the insulation tester to a bare piece of metal immersed in the same water container that the pump is immersed in. Connect the other wire from the insulation to one or both of the pump wires. If the insulation tester has a number of voltage settings (These will probably be 250, 500 and 1000 volts.) select 500 volts. If it is a modern insulation tester press the test button note the reading. If it is an old fashioned Megger wind the handle and note the reading. If there is no problem with the pump I would expect the reading to be over 50 meg ohms. If it is less than 1 meg ohm then I would say is definitely faulty. One meg ohm should not be low enough to trip an ELCB but it is an indication that moisture has entered the pump. Values in between could mean the that the insulation could break down intermittently.
The test in your bath may not prove anything as the bath is probably plastic or enameled and probably has a plastic waste connection. For the test to be meaningfull you would need to have an earthed metal plate in the same water as the pump. Your method of testing is dangerous as the water in the bath would be at mains potential if the pump has earth leakage.

Les.
 

Thread Starter

daba1955

Joined Apr 27, 2019
130
In the absence of a Megger for now, I have measured the AC voltage from the water (only slightly salty, all I've got !), to the copper piping earth clamp.

With the pump running I get 6.0Vac.
With the pump unplugged - 4.3Vac. Don't know where that is coming from ..... Ah, wait a minute, it's a Jacuzzi bath ! Think I need to get it checked out pronto ....
 

Thread Starter

daba1955

Joined Apr 27, 2019
130
Eureka ! we have just discovered the reason for the tripping of the board .....

My mate said he ought to change the UV tube in the clarifier, so on opening it up we discover the outer casing has had water in !

And considering there's a ballast and starter in there for the tube, the evidential tell-tale signs of water ingress must have been the culprit. And, to be fair, it's the only part of the installation that has a connection to mains earth.

Anyway, said pump is re-installed, and runs quite happily without tripping his (more responsive than mine) RCD trip.

Thanks for your time guys ....

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