hello guys. I just joined this site so that I could comment on this thread. I'm a retired nuclear utility engineer who for the last 15 years or so has moonlighted in my friend's electric motor repair shop and served as his field troubleshooting consultant.
I'm cringing at this thread. The central problem that no one has pointed out yet is that there is no central technical authority controlling this startup. This person should be an engineer who knows motors, pumps, utility practices and electronic controls. Trying to rely on a series of manufacturer's tech support reps' advice is a good way to burn down this installation.
- Bimr, I should point out that if this is a standard TEFC surface mounted motor driving the pump through a long shaft, the value of that motor is in the $20,000 range. If it's a submersible pump, double that amount. The starter, transformers and other apparatus are valued at least that much again. You're taking great risks with some very expensive hardware and with almost no hard data to work with.
- I don't understand the electrical architecture, first stepping the voltage down and then back up. Your system engineer's statement that a 480 volt transformer was less expensive than a 2300 volt transformer of the same kVA rating is certainly odd and contrary to my experience. Here, within a wide range, price is based on kVA regardless of the voltages involved.
- 500kVA transformers are borderline too small. If the motor is 92% efficient, that's 365kW. Figure a 0.9 pf and that works out to 405kVA. That gives you very little headroom for inrush or overload. They're probably workable but everything else is going to have to be just right. The utility transformer probably has the overload rating to handle the situation but the dry one may not unless it is also utility-service rated.
- You have essentially no test data to analyze nor to make decisions from. At the minimum, I would instrument the motor for voltage, amperage, wattage and RPM feeding into a data acquisition system. Ideally, I'd also instrument the output of the transformer feeding the motor upstream of the autotransformer, the utility transformer secondary voltage and amperage and utility primary voltage. You can probably tap the utility's revenue metering potential transformer for the primary voltage. I'd have thermal sensors on the autotransformer and the motor windings if the motor doesn't already have a sensor built in.
With as much magnetic hardware as is involved between the utility and the motor, you probably already have "reduced voltage starting" without the auto-transformer. No way to know without instrumentation. This sort of instrumentation is fairly inexpensive to rent by the month, especially when compared to damaging the motor or other apparatus.
- I do not believe the manufacturer's claim that the motor can accelerate in less than a second, even without an attached load. I've never seen an unloaded motor of that size spin up that fast in our shop.
- Has anyone done the inertial calculations on this setup yet? Especially if this is a surface mounted motor with a long shaft drive, there is a LOT of inertia involved. Depending on at what interval the shaft is supported, the start-up torque could cause it to wind up, bow and whip, adding a LOT of inertial loading. If that is happening it would be evident in the RPM and kW traces on the data acq system.
- Is this a totally new design or has one similar already been started up and operated in the area? The question I'm interested in here is whether there may be something wrong with this particular setup or whether it's an erroneous design.
- Bimr, I see that your contractor "got it running" by hitting with with a bigger power source (reminds me of a certain utility client - "hit 'er again and let's see what blows next".) That it now starts (for the time being) on utility power is easy to explain. The initial run bedded-in the bearings and seals and significantly reduced both the starting and running power required. If a 90 deg gearbox is involved then it required significant bedding-in power.
This is almost always the case for large machinery. In fact, our shop has a variety of large motors available for lease so that a larger than design motor can be used to do the initial bedding-in of the rotating machinery. Our most frequent customers are rock crusher and cement kiln operators.
- Even though the pump is running now, that does not mean that it is out of trouble. It is at its optimum point in its life. sliding surfaces like seals are bedded in, ball bearings are clean and new, the pump impeller is free of crud build-up and cavitation wear. Power demand will likely rise as normal wear and tear happens.
- Even though it's running now, I'd still want to instrument it and find out how much headroom I had to accommodate system deterioration. I predict very little. The cost of renting the instrumentation and a consultant to operate it is minor compared to what even one rewind of that motor would cost. Having presented that data to management NOW may save your job when something does happen in the future.
- I'd want to know the service factor on that dry transformer. If it is 2 or above you're probably OK as far as long-term loading goes. If it is close to 1 (1.1, 1.2, etc) then while it'll probably handle the load now, it'll run hot and will have a shortened life.
Though it's probably too late in the project to change things now, were I designing this system, I'd have around a 750 kVA 2100 volt transformer feeding the motor control panel, with a smaller 2100 to 480 or whatever feeding other smaller loads.
One last comment. As someone else mentioned, pay REAL close attention to the maximum start frequency specification of that motor. Motors that size typically are limited to 3 or 4 starts an hour.
I've seen WAY too many motors come into our shop where that specification was not respected. The process demanded more starts and stops (a clutch would have probably been appropriate) than the motor could withstand. The interior looks like it has been in a burn-out oven. Everything is burned to a char. Somethings there is molten aluminum laying around, slung from the rotor as it melted.
We generally charge half the retail price of a motor for a rewind when one is possible. Rotor melts send the remainder of the motor to the salvage yard, as do very bad in-slot arcs.
If this motor has a thermal sensor (most all motors that size do), make SURE that it is connected to the proper protective electronics. If your management doesn't want to do that then at least CYA by putting the advice in writing.
John DeArmond
