Compressor motor keeps blowing fuses

[Hopefuldave]

. Not the centrifugal switch. That is at the rear of the motor and makes a connection once its spun to full speed as it throws the switch outwards.

This is the area i would be checking next. Although the leccies with confirm that direction ??? They do get stuck or lazy due to dust etc

BREAKS the c9nnection for the start cap once up to speed, and they do stick, get lazy, weld the contacts closed... stuck/ welded closed the start cap usually does it's chaff-launcher bit (not rated for continuous duty), stuck open the motor doesn't see the start cap, goes slow and over-current...

Dave H. (the other one)

 

[Mannyroad]

Ok, so, from the web site re the CBB60 / CBB65 caps it appears that the difference is just that one has a plastic can (60) and one a metal can (65). So I guess either type will do if its still a 55uf, as the CBB part is identical, spec-wise. So, that would be my run cap then. Now, the big question is: Do I buy one? Reason I ask is, if the run cap is goosed, what would be the result of this on the running of the motor and would this be the cause of the blowing fuses or not? If so, perfect, but if not, why replace?

Now, ukracer, I am leaning your way re the bit about it maybe not being a faulty start cap if the motor is spinning for a good few seconds before blowing the fuse. But, if it's not a run cap problem, and assuming it might be a stuck centrifugal switch, this would present two options:
1. The switch is stuck closed, thereby not 'disengaging' the start cap, so the start cap stays 'engaged' so to speak. But, if this is the case, why would this blow the fuse all the time?
2. The switch is stuck open. In this scenario the start cap would never be 'engaged' and surely the motor simply wouldn't even try to spin up. Correct or not? And again, would this cause the fuses to keep blowing?

 

[the snooper]

. Not the centrifugal switch. That is at the rear of the motor and makes a connection once its spun to full speed as it throws the switch outwards.

This is the area i would be checking next. Although the leccies with confirm that direction ??? They do get stuck or lazy due to dust etc

It breaks the connection once upto speed

 

[eddie49]

An induction motor that requires a start winding and capacitor can instead be started by hand. With no load, spin the shaft or pulley by hand and apply mains power, or apply power and, whilst it's sitting there buzzing, carefully give it a spin. This should start it - in either direction, and may help to prove if the run circuit is OK.

 

[Mannyroad]

Ok, but as I've said, the motor does spin, but only till the fuse blows after about 10 secs. S

 

[ukracer]

Ok, but as I've said, the motor does spin, but only till the fuse blows after about 10 secs. S

Again i am not a leccy and i really dont know what a run capacitor actually does.
I might be way out here as dementia sets in but if my understanding is accurate. Volts and amps are "relational" in that if something is getting less volts than it needs to run a given load it compensates by trying to take more amps. (Within reason).
Add in the resistance (perhaps a worn switch) and you might see what i understand about the theory....lol

Of course i may well be wrong......i am not a woman.

 

[Hopefuldave]

OK... a brief intro to single-phase motors?

Induction motors work by having a rotating magnetic field (generated by the coils in the stator) which as it rotates induces a current in the rotor (induces - induction), and the induced current generates its own magnetic field which opposes that from the stator.

In a 3-phase motor three sets of coils are wired to the three incoming phases, and because the currents are delayed (120 degrees between successive pole windings) the magnetic field automatically rotates, the stator opposes it and is carried around by the rotating field - but not quite as fast as the field rotates, if it were at the same rate there would be no induced current in the stator as no conductors would be cutting the magnetic field. this is called "slip" and increases with the load, hence an unloaded motor runs at *almost* "synchronous speed, loaded it runs slower until it stalls.

What's synchronous speed? it's the rate that the magnetic field rotates at, worked out by dividing the mains frequency (50Hz or 60Hz) by the number of pole pairs per phase, so a 6-pole motor (the fastest 3-phase) would give 50/1 revs/second, 3000 RPM. A 12-pole would give 1500 by the same formula.

So.. what are these "poles"? The windings have to be arranged around the stator, and present "north" and "south" ends to the rotor, these are pairs of poles (excuse my oversimplifying a bit here, they're actually flipping between the two with the applied AC voltage). In a 6-pole,, three phase motor they'll be arranged so that the "north" pole moves from winding to winding around the stator with the "south" opposite, giving the rotating magnetic field.

In a single-phase motor, there isn't the sequence that rotates the field, so we have to create our own - so we have two sets in two windings, the "run" and "start" windings, and we need something to shift the current's phase to make the field rotate - this is where we use a run capacitor (if it has one), which delays the AC current - once the motor's spinning, it's enough to keep it rotating at nearly-synchronous speed.
A disadvantage of this is that with only two phases (the incoming mains and the shifted mains through the capacitor) the motor can run in either direction - so another capacitor's used, the start capacitor, so that the phase relationship forces the motor to run in only one direction - but it also makes the motor less efficient as it allows more through the start winding (why the start cap's usually several times the capacitance of the run cap) and draws more current - so it's only used at start-up, then taken out of circuit by either the centrifugal switch (buried in one of the motor end-bells) or a relay (buried in the connections box on the motor).

The single-phase motor will normally have two or four poles per winding (only one phase, so only one winding), like the 3-phase we do our sums with pole pairs (50/1 per second, 3000 RPM or 50/2, 1500 RPM). All RPMs are approximate, these are what we'd get with no slip and without the slip the motor doesn't work - we need the rotor to have a current induced for it to rotate.

There's also the chance that you might run into a "synchronous AC motor" - these have slip rings to apply power direct to the rotor, which means they run at mains-frequency speed because there's no need for slip to induce a current in the rotor - so they AREN'T induction motors!

If we don't get the phasing right, the motor doesn't spin, there's no current induced in the rotor to oppose the stator's magnetic field, so no opposing field which means the winding current rises and pops fuses...

Hope this helps rather than confuses!

Dave H. (the other one)

 

[ukracer]

I = 100 W/6 V = 16.67 amps

What would happen if you use a 12-volt battery and a 12-volt light bulb to get 100 watts of power?

I = 100 W/12 V = 8.33 amps

So, this latter system produces the same power, but with half the current. There is an advantage that comes from using less current to make the same amount of power. The resistance in electrical wires consumes power, and the power consumed increases as the current going through the wires increases. You can see how this happens by doing a little rearranging of the two equations. What you need is an equation for power in terms of resistance and current. Let's rearrange the first equation:

I = V/R can be restated as V = I*R

Now you can substitute the equation for V into the other equation:

P = V*I substituting for V we get P = I*R*I, or P = I2*R

What this equation tells you is that the power consumed by the wires increases if the resistance of the wires increases (for instance, if the wires get smaller or are made of a less conductive material). But it increases dramatically if the current going through the wires increases. So, using a higher voltage to reduce the current can make electrical systems more efficient. The efficiency of electric motors also improves at higher voltages.

 

[Seadog]

OK... a brief intro to single-phase motors?

Is it or isn't it?

 

[the snooper]

Is it or isn't it?

Errm it's not lol

 

[daleyd]

OK... a brief intro to single-phase motors?

Induction motors work by having a rotating magnetic field (generated by the coils in the stator) which as it rotates induces a current in the rotor (induces - induction), and the induced current generates its own magnetic field which opposes that from the stator.

In a 3-phase motor three sets of coils are wired to the three incoming phases, and because the currents are delayed (120 degrees between successive pole windings) the magnetic field automatically rotates, the stator opposes it and is carried around by the rotating field - but not quite as fast as the field rotates, if it were at the same rate there would be no induced current in the stator as no conductors would be cutting the magnetic field. this is called "slip" and increases with the load, hence an unloaded motor runs at *almost* "synchronous speed, loaded it runs slower until it stalls.

What's synchronous speed? it's the rate that the magnetic field rotates at, worked out by dividing the mains frequency (50Hz or 60Hz) by the number of pole pairs per phase, so a 6-pole motor (the fastest 3-phase) would give 50/1 revs/second, 3000 RPM. A 12-pole would give 1500 by the same formula.

So.. what are these "poles"? The windings have to be arranged around the stator, and present "north" and "south" ends to the rotor, these are pairs of poles (excuse my oversimplifying a bit here, they're actually flipping between the two with the applied AC voltage). In a 6-pole,, three phase motor they'll be arranged so that the "north" pole moves from winding to winding around the stator with the "south" opposite, giving the rotating magnetic field.

In a single-phase motor, there isn't the sequence that rotates the field, so we have to create our own - so we have two sets in two windings, the "run" and "start" windings, and we need something to shift the current's phase to make the field rotate - this is where we use a run capacitor (if it has one), which delays the AC current - once the motor's spinning, it's enough to keep it rotating at nearly-synchronous speed.
A disadvantage of this is that with only two phases (the incoming mains and the shifted mains through the capacitor) the motor can run in either direction - so another capacitor's used, the start capacitor, so that the phase relationship forces the motor to run in only one direction - but it also makes the motor less efficient as it allows more through the start winding (why the start cap's usually several times the capacitance of the run cap) and draws more current - so it's only used at start-up, then taken out of circuit by either the centrifugal switch (buried in one of the motor end-bells) or a relay (buried in the connections box on the motor).

The single-phase motor will normally have two or four poles per winding (only one phase, so only one winding), like the 3-phase we do our sums with pole pairs (50/1 per second, 3000 RPM or 50/2, 1500 RPM). All RPMs are approximate, these are what we'd get with no slip and without the slip the motor doesn't work - we need the rotor to have a current induced for it to rotate.

There's also the chance that you might run into a "synchronous AC motor" - these have slip rings to apply power direct to the rotor, which means they run at mains-frequency speed because there's no need for slip to induce a current in the rotor - so they AREN'T induction motors!

If we don't get the phasing right, the motor doesn't spin, there's no current induced in the rotor to oppose the stator's magnetic field, so no opposing field which means the winding current rises and pops fuses...

Hope this helps rather than confuses!

Dave H. (the other one)

I don’t think your quite right in the poles/speed of 3 phase motors - 2 pole motors are the fastest standard motors, increasing the number of poles reduces the speed.

 

[Turbo]

Are you sure your new pressure switch is venting when the compressor cuts out? It seems strange that the problem started when you fitted the new switch. I would suggest you try slackening off the belts & let the motor run free.

 

[roofman]

Are you sure your new pressure switch is venting when the compressor cuts out? It seems strange that the problem started when you fitted the new switch. I would suggest you try slackening off the belts & let the motor run free.

https://www.mig-welding.co.uk/forum/threads/compressor-motor-keeps-blowing-fuses.89994/#post-1315369 they have the TV too loud and cant hear

 

[Hopefuldave]

I don’t think your quite right in the poles/speed of 3 phase motors - 2 pole motors are the fastest standard motors, increasing the number of poles reduces the speed.

I think I wasn't clear, that's what I meant (too much rum...) 6 poles total = 2 poles per phase winding?

Dave H. (the other one)

 

[Hopefuldave]

Errm it's not lol

Compared to a book, it's short (I've got three or four books on motors, still trying to work through 'em all...)

Dave H. (the other one)

 

[Mannyroad]

Ok guys, firstly thanks for all your input. It's much appreciated.

Now, while I wait for my start cap to come from China (I guess I'm too tight to pay Clarke prices) I decided to reconnect the caps and re-assess what happens when I switch on. First, to eliminate potential pressure switch / unloader valve issue, I removed the check valve, so the pump discharged to atmosphere unimpeded.

So, after switching on, I got some crackling sparking at the contacts / 'terminal block' that all the wiring connects up to in the plastic box above the motor. It looked a bit like striking up a stick welder. Not quite sure if it is just a 'terminal block' or some kind of contactor, though I can't see why it would be the latter. If not a contactor I can't figure why it would spark like a direct short; all the wiring looks good with nothing feeling loose. Anyway, after the initial sparking the compressor ran properly.

I switched off at the pressure switch and fired the comp up again but this time I just got a small blue flash at the 'terminal block' in the plastic box over the motor. And the comp ran fine yet again. I switched off again, and then back on, and all I got was a loud humming, stationary motor, with no sparking or flash at the 'terminal block'. Tried again and the same, just the humming.

So, is the likely problem here my start cap? Once the motor is running it keeps running, so maybe the run cap is fine. If the start cap is duff, is that why I'm getting the sparking / blue flashes, albeit I can't figure why a duff start cap would induce sparking/flashes? To my way of thinking one only gets sparking / flashing by arcing when contacts open, but I doubt there are opening contacts going on here, though I haven't disconnected all the wiring to have a good poke around at the 'terminal block'.

I'd better mention something. When I connected up the start cap I wasn't sure which wire went to which of the two spades on the cap. I don't know if this matters. After the initial start up (with the sparking) I swapped them over. I presume they can be connected up either way around. Am I correct, as both wires are black, so no polarity identified. PS: I'm no leccy or electronics expert, as you've guessed by now.

 

[the snooper]

Sort out the arcing connection, that could well be you problem

 

[Pigeon_Droppings2]

I have the Clarke XET19/200 which I think is basically the same compressor as yours -but the industrial version.

The instructions (I'm pretty sure) said it requires a 32A supply?

 

[carbon]

Sort out the arcing connection, that could well be you problem

It could be that the incoming mains connection is secure but one of the winding connections that is below could be loose and arcing at high current load.

Worked on a machine recently that wouldn’t start even though all the connections were secure. In coming cable to the machine had blue crimp connections at the input to the contactor that weren’t securely crimped to the cables.

 

[Mannyroad]

Right, I sorted the arcing contact. It turned out that one of the lower brass nuts on the connector block was loose, even though the top nut was tight, so it was hard to spot the problem. anyway, I cleaned up the connections, retightened everything and fitted the new start cap that arrived same day. Powered it up and it ran perfectly, start after start after start. Bob on. No fuses blowing anymore. And tbh, though maybe it's just my imagination, but the comp seems to start up more positively and quicker.
Anyway, all's well, so it must be the end. Thanks for all your inputs guys, much appreciated.