Frequency Drive Impeller Pump Question

[CaptnYellowShirt]

So I have a Frequency Drive Impeller Pump that's on the fritz now.

480V 3phase. Toshiba inverter.

When I power it up and raise the frequency the ramp up stops at around 7Hz and stays there. 20 sec later or so the inverter faults with an over load code.

If you command a frequency less than 7hz it will stay there and just keep powering the motor.

However, the motor isnt turning one bit.

My first thought was the impeller was stuck. But now I am wondering if this condition could be caused by an electrical problem.

Any ideas?

[IanB]

You can find out if the impeller is stuck by seeing if the motor spins freely when you turn it by hand. If it does turn freely, the problem is more likely electrical than mechanical.

[strantor]

Check for an open phase

[CaptnYellowShirt]

Check for an open phase

Reading about this now. Seems to match with my observations on the problem.

[SeanB]

Check motor turns, check windings have continuity. If so then try with another motor, same voltage but not necessarily same power, so you can use a smaller motor no problem. If it turns inverter is fine. If not then inverter has blown an output stage, and will need the driver fixed, typically they are a single module on the heatsink.

[CaptnYellowShirt]

There are two pumps and two inverters and they both developed the same problem at the same time. That's another clue to this problem.

To be honest, the 480v circuits worry me a little. I'm a little afraid to go poking around in something I don't fully understand. 

[peter.mitchell]

both pumps at the same time? you sound like you've lost a phase somewhere.

[CaptnYellowShirt]

Yeah, basically the same motors, pumps, and inverters. Basically the same symptoms. I'll start snooping around with my Fluke.

[CaptnYellowShirt]

No joy on the phase theory. All the fuses check out and the terminals at motor are all showing good voltages...

Looks like I'm going to have to pull the impeller housing.

[strantor]

No joy on the phase theory. All the fuses check out and the terminals at motor are all showing good voltages...

Looks like I'm going to have to pull the impeller housing.

Just because you have voltage present at all 3 motor phase terminals does not mean you have current flowing through all 3 phases. You need to put a clamp meter on the T-leads and verify all 3 phases are working.

[CaptnYellowShirt]

I'll be sure to do that as soon as I can afford one.

[CaptnYellowShirt]

I was able to get in contact with the company that installed the system. Their tech was (obviously) much more knowledgeable than I am on this stuff.


Here's what we decided:

1. Disconnect the motor from the inverter and attempt to spin it up again. If it trips, its probably something in the inverter, if not, its probably something with the motor side.

2. Pull the impeller. (booo) Because it could be...

(a) crap that's settled from disuse in the bottom of the impeller housing  or...
(b) the seal between the motor and the impeller.

[NiHaoMike]

Check the current on each outgoing phase to make sure they're all connected properly.

[CaptnYellowShirt]

So it looks like something's jammed or rusted up inside the impeller. The company has suggested I attempt an across the line start of the motor.

Which I'm a little worried about doing. Switching 480v like that seems like it might cause problems?

[SeanB]

Not really, you will want to have a inline 3 phase soft starter if you have a motor that is high power, as most switchgear will trip with the start current draw. Simplest is to get a soft starter and place it in line, and set the ramp up and speed to the longest time and least boost it can do, and have a contactor and protection relay rated for the motor run current. That way you will start it up with a minimum current, and if it has debris or a seized seal it will trip out on the motor overload and not hopefully tear itself apart. If you have access to the motor windings see if it is wired correctly, and that the wiring is delta connected for 480V. If so temporarily connect it in star for testing, as this lowers the phase current. You could then if it runs use a star delta contactor to switch it over if you want to remove the inverters completely, though your wiring and line breakers might not be correctly sized for the star delta rating, as inverters allow you to use a lower current cable and breaker as the switch on surges are a lot lower.

If it is a simple motor under about 2kVA you can do Direct On Line control, as the motor impedance will limit the switch on current to a safer level. Bigger would need either star delta, soft starter or will be arranged for other starting methods.

Biggest motor I have in regular use is 7.5kVA, and it was originally miswired, started in delta and switched to star. ;( Wondered why it kept pulling the floor bolts out, and investigated and basically rewired the panel correctly. It later did burn out a winding from the initial abuse. Bit hard to undo a 130kg motor and lower it down off the machine, I had to mount an anchor in the roof to attach a chain block to hold it. New one was only 70kg, almost hand holdable. Chain block though was handy to take the gearbox off when it needed seals done though.

[retrolefty]

Are both motor/pumps in the same process line? If so plugging/blockage is a likely root cause.

[CaptnYellowShirt]

Wondered why it kept pulling the floor bolts out

Heh...


Here's a picture of the culprit: http://drive.google.com/file/d/0B_c9c-jzSK16T2RTR25JUzFGMDA/edit?usp=sharing

The motor is rated at 41 Amps - 15 HP. I couldn't get a great look at the wiring, but I wouldn't say its any more than #6 feeding in from the load center.

Here's a picture of the impeller with the inlet flange off: http://drive.google.com/file/d/0B_c9c-jzSK16Tm9BNzdqMXN4SUE/edit?usp=sharing

On recommendation from the factory rep, I've already tried applying torque to the 3/4" retaining bolt. 80 ft-lb (best guess by 'feel') wasn't enough to even budge the impeller. I was worried about applying any more to the bolt.

I think I'm going to try to fashion a 3-pin spanner socket to fit into those holes. The dia there is around 3" so I should be able to apply a good amount of torque -- at least something between 80 ft-lb and .... the shaft breaking torque that 41 amps is going to apply.

[CaptnYellowShirt]

The load center for this thing looks a little like...



I keep thinking I'm going to end up like poor Timmy in that movie...

[SeanB]

You are going to have to undo that volute and clean the impeller, looks like it has a giant ball of rust sitting in the bottom of the volute. Seal it back up with non setting gasket maker and it will go again.

Hope you have some help, that thing is pretty heavy. At 15HP I would not recommend trying a DOL or even a star delta start on it, it is likely to be pretty impressive when it tears itself to bits.

I had a 22kW motor that munched the output shaft of the worm extruder it drove, it pretty much turned the bearings ( 4 inches in diameter) into metal paste, and proceeded to turn the worm into a solid welded mass with the outer worm housing. It then finally tripped on overcurrent. New bearings and shaft and a fill with plain molyslip oil additive in the gear oil and the bearings only turned to powder slowly, so I put in a magnetic plug that picked up only about a gram of powdered metal a month. that one needed a 1 ton chain block and a steel rail to separate the parts when it went in for the bearings.

[retrolefty]

Cool pictures. Before retirement I use to work with a VSD/Motor that powered a 5MW motor that drove a large process compressor at a oil refinery. Had 14.2KV 3 phase feed and output 7.5KV 3 phase variable frequency output to the motor. One could stand right next to the motor when it started up and not even hear the motor slowly ramping up to speed, talk about soft start! The Drive control/power took up a whole steel building the size of a sea land trailer. Amazing tech, with lots of automatic controller switch over to a redundant VFD controller if one failed. All driven from a simple 4-20ma PID control loop output from the process control system for the plant.

 The high power drive electronics used de-ionized water cooling and care and maintenance of the self-contained water cooling system required more care and maintenance then any other part of the whole system. Water cooling and 14.2Kv is a big challenge. This had to be a 24/7/365 system with only scheduled shutdowns every 4 years or so. Unscheduled shutdowns could cost hundred of thousands of dollars a day in lost profits. I don't miss the stress and attention during such unscheduled shutdowns. 

[IanB]

Cool pictures. Before retirement I use to work with a VSD/Motor that powered a 5MW motor that drove a large process compressor at a oil refinery.

Fun stuff. As a chemical engineer, that's the kind of equipment that falls in my field. Unfortunately my job is office based, so I don't get to see the hardware up close very often.

If the compressor was the centrifugal kind I imagine you would have a chance to hear the motor. If it was the reciprocating kind you would be wearing ear defenders and would have trouble hearing yourself think, especially if it was inside a building 

[SeanB]

That big probably a scroll or an other type of variable displacement compressor.

[IanB]

That big probably a scroll or an other type of variable displacement compressor.

In chemical industry terms a 5 MW machine is not exceptionally large. It would be somewhat mainstream in terms of the size of machinery encountered.

You will find centrifugal compressors in a whole range of sizes from small to very large. The upper limit on the use of such compressors tends to be the maximum discharge pressure required rather than the amount of gas to be compressed. Such compressors are very good at compressing large volumes of gas to moderate pressures. Since the power requirement is equal to the compression work per kilogram times the number of kilograms moved per second, you can get a massive power demand from a big gas flow and lowish pressure ratio.

Instead of having huge electric motors, though, you will often find such machines powered by steam turbines.

[retrolefty]

Cool pictures. Before retirement I use to work with a VSD/Motor that powered a 5MW motor that drove a large process compressor at a oil refinery.

Fun stuff. As a chemical engineer, that's the kind of equipment that falls in my field. Unfortunately my job is office based, so I don't get to see the hardware up close very often.

If the compressor was the centrifugal kind I imagine you would have a chance to hear the motor. If it was the reciprocating kind you would be wearing ear defenders and would have trouble hearing yourself think, especially if it was inside a building 

 Yea, some kind of centrifugal compressor, you could hear it once it got up to speed but nothing like the older reciprocating compressors which we also had in some other plants. It was named the 'Alky Plant' using sulfuric acid in a cracking process, output product was isobutane. This compressor was part of a refigeration cycle for the plant. Lots of expensive acid resistance piping and components in this plant.

 And we had tons of C.E.s working at the refinery and almost 99% the management positions were filled with C.E. types, all the way up to the CEO of the whole company, Chevron. Very few MBA types would be hired or promoted too far up the chain of command. Oil companies are all about tech and making long term (40+ year payback) decisions. Personnel costs are small potatoes for a large integrated oil company.

[IanB]

It was named the 'Alky Plant' using sulfuric acid in a cracking process, output product was isobutane.

Right, that would be a sulfuric acid alkylation plant. It actually takes isobutane as one of the feeds and converts it into high value products for use in gasoline.

Some of these alkylation plants use hydrofluoric acid (HF) instead of sulfuric acid. You have to really hope such a plant does not develop any leaks when you are near it as HF is one of the most dangerous substances around...