New "motor" starting problem after Y-Delta conversion

[jraef]

For those of you who revel in odd problems, here is a new one, for me anyway.

Customer called me for consult on problems starting a 20HP 460V motor on a dust collection blower (squirrel cage). They had been using a Y-Delta starter, but transition spikes damaged the motor so they converted to X-Line (DOL). Motor was now drawing LRA and tripping the OL relay before the fan got up to full speed. It had started fine with the Y-D starter, so I knew the motor must have enough torque to get it started, but I was puzzled as to why it no longer worked just from conversion to X-Line. Thoroughly checked motor and connections (thinking they had configured the windings wrong), no errors. Motor started and ran fine uncoupled, pulled 7.0A at full speed which seemed relatively normal for an unloaded 3550RPM motor. Recoupled and tripped again on start attempt. Choked off the outlet thinking that X-Line coupled the fan load quicker than Y-D had, so the load was possibly coming on too soon. Wrong again. Tried increasing OL to Class 20. No help. Finally bypassed OL all together and got it to start, but noticed that it was sucking in the inlet ducts!



When they had removed the motor for rework, the mechanics put the belt drive sheaves in reverse position, so instead of a 2:1 ratio from fan to motor so as to make the fan run at 1775 RPM, it was attempting to drive the fan at 7100RPM! The poor little motor just didn't have the kahounas to do it...

Occam's razor strikes again.

 

[edison123]

Why did they had to remove the motor for Y-D to delta ? They could have just linked the 6 terminals for delta.

Both sheaves having same shaft dia is news to me. I had thought for the same HP, the shaft dia would increase with decreased speed.

 

[rbulsara]

Did you replace or re-adjust the O/L relay to atleast 1.25 itmes FLA? In Y-delta starter the O/L relay is set for 0.58xFLA only.

 

[jraef]

edison123,
They rewound the motor (not replaced) due to damage from Y-D transition spikes. The decision to go X-Line was done in spite of my attemp to convince them to use a soft starter, but oh well. I have no idea why the mechanics had removed the sheave from the fan when supposedly all they did was remove the motor for rewinding! Maybe there was other damage that we were unaware of. And yes, strange that both shafts were exactly the same size, but the blower OEM had ordered a custom diameter shaft on that motor for some unknown reason, leaving them with the ability to screw it up, and Murphy's Law dictates that if it is possible to screw it up, someone will.

rbulsara,
Yes, it now has the correct OLR, in fact they had to replace the entire starter because there was damage to the original Y-D starter as well.


"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[edison123]

gotcha. I missed the part about the 1st winding failure.

Yes, Murphy's law always works.

 

[aolalde]

Incredible!
That load increased the inertia reflected in the motor shaft for 16 times and at 7100 rpm the HP could increase 64 times!

The motor was not burnt by a miracle.

It is sad, but we can not take for granted that people is making always their part properly.

 

[rbulsara]

OK...so now I gather the problem is fixed by correcting the sheave issue?

I should have stopped to think when i saw the bulb sign !! also should have noticed the poster!

 

[jraef]

Actually, I didn't let the motor get to full speed when the OL was bypassed, I shut it down manually when I saw the inlet ducting suck in. It was dramatic in that it was twisting from the forces on it and popped a couple of seams. The motor was drawing locked rotor for about 25 seconds or so, only once.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[mc5w]

jraef,

You should not need to change out the motor overload relays - best practice is to run 6 wires plus ground from the controller to the motor so that the overload relays can be in series with the windings. This way, the overload relays can sense any circulating current zero sequence current that might result from a shorted turn or voltage imbalance. You have to remember that zero sequence current in a motor or generator creates almost zero magnetization of the iron resulting in almost zero sequence impedance.

If you have the motor overload relays in series with the line currents then you can have circulating current due to voltage imbalance that the relays will not sense.

Kind of give me a kick that someone got the sheaves swapped but that happens. I one time encountered a brand new machine ( hooked up for only a week ) that kept tripping the overload for one of the pumps. This machine had 2 small pumps in parallel and somehow one was running forward and the other running backward! The one running backward was overloading the motor even though the pumps only raised water about 2 feet. Turned out that doing a lot of other test showed that 1 out of 4 ultrasonic transducers was flooded with water. This machine was a ultrasonic parts washer and you have 2 water tanks. The upper tank is the actual ultrasonic cleaning area and the lower tank is for separating oil and then taking out the oil with a skimming disk and squegee. The leak in the ultrasonic transducer could have been caused by shipping vibration.

It is bad enough that with new machines shipping vibration will loosen wiring connections. My experience is that machine builders do not do thorough quality control and that I have to do it for them.

 

[jraef]

Thanks Mike, but the decision to change out the starter was based upon the fact that the end user had no electrician who even understood what a Y-Delta motor was, let alone why there would be 6 leads going to the motor!

Funny story on the pumps. I also had one where they complained that a hydraulic pump was not starting well with a soft starter, and if they increased the settings enough to get it to start, there was no oil pressure out of it anyway. When I arrived and witnessed it, I asked them "Are you sure you put oil in thereservoir?" They all looked at one another and shrugged their shoulders, apparently assuming that someone had done it before the machine shipped form the factory. I looked at the fill cap and it still had the bright red warning tag saying that it ships without oil due to transporation regulations!

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[mc5w]

YES!

 

[mc5w]

jraef,

This customer that had all of these problems with the dust collector must be one of those cheapskates who will not hire smart people because smart is "too expensive". I get this on a lot of job interviews - they think that they have to pay extra for my knowhow.

How much money I get is determined by the market among other things. My adjusted gross income for last year was less than $4,000 dollars because the work was not there. The electrical business in the US is in bad shape right now.

I get the "too expensive" excuse a lot just because somebody last year paid me X dollars and hour for 6 days of work. I found a way to calculate my average wage rate for last year to be $1.50 an hour. Nothing creative just a matter of getting retail extablishments to believe that I really do want to get out of the electrical business for a few years.

 

[itsmoked]

mc5w; I must confess I and your fellow mates here have conspired to have you black listed so that you would have enough spare time to weigh in here, often...

:]

 

[Skogsgurra]

Hi all,

Interesting thread - in many ways. One of the Gurus (yes, he is really one of them) once said: "Don't be too smart - it will scare your customers" He also said: "It is not about what you know and can do - it is about what impression you give"

I do not know if I agree with him. Or if he was serious about what he said. But it is an interesting way of looking at things.


About pulleys and motors not doing their job: I was asked to visit a customer that had problems with a DC motor for an extruder (plastic bags). It did not get up to speed as it used to do. I found no problems at all. Armature current and voltage, field current and voltage, all windings correctly connected, instrumentation in working order, extruder and tool temperature OK. Everything was just fine, but the extruder did not get up in speed.

We were discussing this when the warehouse guy arrived and asked where he could find the pulley. He was supposed to take it back and he couldn't find it.

"What pulleys?" I asked. And got the solution served on a silver platter: They were installing a new conveyor system in the factory. And the cooling air duct for the motor got in the way. So they moved the duct away about one foot - and then they moved the motor to fit the new duct position. And then they had to fit a new pulley - because the ropes were too short for the new position. Changing the ropes had never been thought of! The new pulley with the smaller diameter was installed on the extruder. The diameter reduction was about 50 percent - with obvious implications. I told the warehouse guy to find suitable ropes instead. And leave the pulley.

Yes, I know, it sounds unbelievable. But this was not the first time I ran into a problem that was not entirely, but almost, unlike an electric problem. And probably not the last time either.


Gunnar Englund

http://www.gke.org

 

[reandy]

The 2 shaft sizes do not have to be identical for the the 'pulleys to be reversed'. A lot of pulleys are split-hub type. The hubs would only fit where they were, on the motor or on the blower. But, during instilation, the pulleys could have been switched, because either one would have fit on the 'standard hub'.

We keep these 'standard hubs' in different shaft borings in stock. Then we have a supply of different sized pulleys that fit to these hubs. This way, we can keep minimal stock, and 'make up' a pulley with the correct shaft size.

Another advantage is in removing the pulley from the shaft. You remove the bolts that hold pulley to the hub, and there are additional 'pushing' holes in the pulley, you put the bolts into the pushing holes, and it pushes the pulley off the tapered hub. Once the pulley is off of the hub, the hub is split (to allow for compression from the tapered hub/pulley, pulling together, to get a tight bond to the shaft) that the hub can be 'opened' up a little bit with a screw driver or similar tool, and easily removed from the shaft. Don't open it too much, or you can split the hub, destroying it. I know!

 

[electricpete]

Yup, pretty increible and interesting story.

Why did they even have the fan sheave removed? Separate maintenance going on with the fan?

Also at our plant we have pulley belt guards shaped around the belts and you wouldn't be able to get the fan guard on with sheaves swapped (attachment points would be different if you tried to rotate the fan guard).

Hard to believe that kind of stuff can happen until it does.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[mc5w]

One reason why I do not get involved with rental housing is that about 50% of real estate snots are moronic enough to put up drywall before calling and electrician or plumber. Very hard to explain that the wires have to go in first and they have to wait for the inspector to come over.

Also, 99% of real estate snots want their electrical work done for FREE! What they get for zero money is zero volts and zero amps.

 

[jraef]

Why did they even have the fan sheave removed? Separate maintenance going on with the fan?

You know something E-Pete, that is a VERY good question. As I think of it, there was no scheduled maintenance going on for the fan itself, only the motor. Now I will have to investigate why the mechanics removed the sheave from the fan end.

FYI, this was on an enclosed unit so the "belt guard" was essentially the maintenance access hatch which was locked so that only the mechanics had access.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla