Shorted Pump

Alan (California Radiant) Forbes

I usually see fried pumps. They seize up, stop spinning and overheat the enclosure. On a job yesterday, the pump developed a short and blew a fuse on the boiler's circuit board. When I checked the fuse on the first go-around with the fuse in place, it showed continuity. It was only after I removed the fuse from the circuit board did it reveal that it was blown. 

I’ve tested fuses like this before, i.e. still connected on the circuit board and if they’re blown, they have always shown non-continuity, until now. Why?

Also, I don’t know how to test a pump for a short. Can it be ohmed out?

109A_5

Hello @Alan (California Radiant) Forbes,
How were you testing the fuse 'in circuit' ?
Voltmeter (control board powered up and the control board trying to energize the motor).
Ohmmeter (control board powered down).
Continuity test function (control board powered down).

Continuity test function is my least favorite, since there seems to be no standard between meter manufactures as to what continuity actually is, 0.5 Ohm or 90 Ohms or anywhere in-between. You have to know your meter. Since you probably don't know what else is on the circuit board that may be another path that meter can sense.

If you deal with this a lot and they are similar small single phase motors (Voltage and current draw)
An appropriately sized circuit breaker and alligator clip leads temporarily replacing the fuse may work.
An appropriately sized incandescent lamp temporarily replacing the fuse may work too. A shorted motor the Lamp will remain bright.

EBEBRATT-Ed

@Alan (California Radiant) Forbes

Ohm the two motor leads you should get some resistance if ok. (the pump mfg could tell you what it should be) If you get nothing the motor windings are open and the motor is shot. Either lead when disconnected should read nothing to ground if they do the motor is grounded

Intplm.

Sometimes when checking continuity while the fuse is installed will show a reading, due to the board that it is installed on is damaged. Giving a false reading.

Teemok

I pull fuses to test. I'm assuming 120v and a short to ground. Tests done on disconnected pump leads. Continuity to ground terminal on either L or N lead is a short to ground. The hot(L) can also be shorted to the neutral internally and an ohm test will show little resistance between them. The motor windings have an resistance / ohm spec. The resistance of a winding is related to it's gauge and length. Shorted is less ohms than there should be between L( the start of the winding) and N (the end of the winding). Test a good same model pump if you have one for a reference.

mattmia2

A glass tube fuse that is loaded just above its rating for a long period or in a holder that is making poor contact or has a bad termination to it can melt the solder in the cap of the fuse instead of the element and weird things can happen. or it could just be a manufacturing defect.

The incandescent lamp in series is a good way to test the circulator without burning something else up.

109A_5

One problem with Ohmmeter measurements on inductive loads (motors, transformers, etc.). If the defective winding has what is called a 'One Turn Short' it may leave you confused. Since the winding's measured resistance could be well within the manufactures resistance specifications and yet be defective, and not function correctly.

Teemok

I've never suspected a one turn short. Though I do understand they can happen. Maybe they've confused me and I replaced the component without fully understanding it's failure. Contrary to my nature but it happens. Intermittent, works for a time then overheats solenoid coils come to mind. I would think the amp draw would rise in addition to reading less ohms. What would symptoms be? Anyway, the existence of a blown fuse is a powerful pass fail amp draw test. I agree ohm readings are just one data point. A short to ground or an open circuit is conclusive.

109A_5

Hello @Teemok,

Teemok said:

I've never suspected a one turn short. Though I do understand they can happen. Maybe they've confused me and I replaced the component without fully understanding it's failure. Contrary to my nature but it happens. Intermittent, works for a time then overheats solenoid coils come to mind. I would think the amp draw would rise in addition to reading less ohms. What would symptoms be? Anyway, the existence of a blown fuse is a powerful pass fail amp draw test. I agree ohm readings are just one data point. A short to ground or an open circuit is conclusive.

Yes with any unintentional current path within a motor or a coil (like a relay) the input current will go up. Its like adding another winding to a transformer with the load shorted out. Or like most of continuous LRA draw for a motor.

The problem with an Ohmmeter measurement is the typical Ohmmeter does not has the resolution to see the defect.

Example;
(to keep the numbers easy)
The transformer (or motor) has 100 turns or windings, say 1 foot long each of 16 Gauge wire, so 4.016 Ohms per 1000 ft or in this example 0.4016 Ohms for 100 Feet. With a one turn short scenario can the typical Ohms function of a typical Multi-meter actually see the difference between 0.4016 Ohms and 0.3976 Ohms ? A minimum resolution of 0.004016 Ohms, most Multi-meters can't until you get into a much more expensive (Lab grade type meter).

If the windings are 14 Gauge wire or larger it is even harder to measure accurately. Also do you even have the resistance specification in the first place ? Relays maybe, motors less usual.

There are better methods for field service.

Teemok

@109A_5 Heard, ohm meters can't spot it. I wouldn't think shorts like that wouldn't last long. Wouldn't they just overheat fast and open or sort more coils till it fails. I guess if it's overload protected you'd find it still shorted. Judging by your explanation (14ga windings) you are talking industrial scale.
What would I look for as a symptom of a single coil short. Heat? Off speed out of balance motor? High amp draw?

109A_5

Hello @Teemok,
Yes, in that scenario it usually just trips an over current device. But I really depends on the quality of the internal short. It could just burn the winding so it ends up open.

Pick a wire gauge you like, it may be still out of the range of the common Multi-meter.

Teemok

@109A_5 I don't doubt your wisdom. I got your point but this statement "There are better methods for field service." runs contrary to many a request from manufactures tech support. I understand when I'm asked for ohm readings on motor windings we are not really looking for evidence of single coil shorts. Still they ask for ohm readings not continuity. An odd reading from a should be identical winding indicates a problem. I've found that valuable for service work. I've confirmed faulty coils many times with ohm readings by comparison with known goods.

109A_5

Hello @Teemok,
I'm not saying don't use Ohmmeter readings. And if Tech support wants that information, sure give it to them. Even if it is just a confidence thing on their end that a Tech actually did the test. And most Techs now have Multi-meters.

Comparing with a known good (within reason) is good too. Just when it gets below 1.0 Ohms it gets a bit useless in some cases. And there is no guarantee it is a one turn short, that is just one possibility, it could be a 50 turn short, and the device won't work correctly.

All I am saying if you are on your own, in some situations, Ohmmeter readings may not give you any true troubleshooting direction. Continuity meter function testing may be even worse. Since you may get a beep with 80 Ohms, probably because some very low amp fuses are way over 1 Ohm, so that is good continuity in that case.

With the OP "Shorted Pump" all I'm saying is there may be better ways than wasting a bunch of fuses or being confused by meaningless Ohmmeter readings.