electric motor rated 575V running @ 630V supply

[lombardy1]

I have been requested to install a multiple tap autotransformer in the 3 phase line which is feeding a 51KW, 575V, 72.8 FLA water resevoir pump to reduce the 630V supply line to 600V or less. This 630V is believed to be what has been causing this pump motor to burn out on at least 3 occasions over the last few years. Could this high supply voltage be causing this motor to fail?

 

[jraef]

It depends. Although a 575V motor can tolerate a 10% overvoltage (638V) with reasonable success, it puts additional stress on the insulation because the motor will run hotter than normal. If the load is already close to full load capacity of that motor, and/or the ambient temperature is high, and/or you have a high duty cycle etc. etc. etc., you may have a combination of problems. In other words, running it at a high voltage takes away one of the built-in "safety factors" in motor design.

"Venditori de oleum-vipera non vigere excordis populi"

 

[lombardy1]

Thank-you jraef. Now to really show my knowledge. With the utility supply Xformers approx. 100'away from the MCC, the motor within 30' of the MCC, and very little other loads on the service: I would set the motor supply voltage @ 575V????????

 

[mpparent]

I'd tap it to 600V (or as close as possible).

Mike

 

[jraef]

I agree, 600V

"Venditori de oleum-vipera non vigere excordis populi"

 

[busbar]

 
That is an interesting question.

What is the voltage at the point of delivery by the utility to the electricity user? 630 volts is a ‘legally’ established upper limit for 60-Hertz electric power on the North American continent. Are you sure it’s not 629.9 volts? What about 630.1 volts?

There is sort of a “peace treaty” negotiated between electric utilities and appliance manufacturers that is usually referenced—ANSI Standard C84.1-1995 (or similar document)—that dictates voltage limits.

It is not practical to have exactly 575 volts constantly at the motor terminals. The way the standard is written, 630 volts at the point of delivery is probably legally acceptable. It may be administered differently by utilities, but that is why contracts and policies exist. A regulatory body may have to decide.

I hope the work is profitable for your organization.

 

[lombardy1]

Thanks busbar but it would probaly take 2 years for the regulatory body to decide if they had juristiction, then.........???? The Utility has + or - 10% they claim. I have not read their obligations to have the monopoly they do, but they set the rules. That the Utility has 3P, 630V at this location is a real good thing. We just have to make it work with 630V + or - and be happy it is not 660V or 546V. heh?????????

 

[Walden]

Using an autotransformer introduces another danger. If the transformer winding becomes open circuit, the full supply voltage can appear on the motor terminals. A double wound transformer would eliminate this problem.

 

[aolalde]

Before attempting any modifications, monitor the voltage and currents at least for 24 Hrs at the motor conduit box terminals.
How close is the load to the motor nameplate?
Are the currents balanced?
Did you record nameplate operating temperatures?
To burn your motor you need a voltage well in excess of 632.5 volts. If possible read the no load current and compare it to the full load nameplate current, this will give an idea of the magnetic circuit saturation.
In general, motors burn out more frequently due to low voltage, unbalanced voltage, transient surges, cooling obstructions and overload, rather than 10% over voltage.

 

[buzzp]

The high voltage would not burn your motor out IF the overload protection was set properly. If this is the case, I doubt that this high voltage caused the motors to burn out.
Do you have underload protection as well (dry run protection?).
The only result of increased voltage (assuming all phases equal) is an increased current (above ~5% of voltage nameplate) so the overload should have protected your motor. NEMA says your motor can run up to 10% over nameplate voltage. If the motor is already working near capacity, then this increased voltage could result in issues with the motor constantly tripping on overload. If you have no overload protection, get some. I would also recommend current unbalance protection (negative sequence currents included).

 

[busbar]

Some utilities have the C84.1 reference {or equivalent} buried in tarriffs or conditions at their websites. Is the pump station above or below the 49th parallel?

Are there taps on the 600V-secondary transformer? If utility owned, are the taps there but the utility won’t change them as a policy issue?

A couple of other possible things to review with multiple motor failures is the type of system grounding…solid (wye or delta), resistance or ungrounded. Are there any local capacitors?

It’s understandable that an outboard autotransformer may be the cheapest/fastest fix, but if you’re still loosing motors after the addition, that can be a little embarrassing.

 

[jraef]

Busbar brings up some good points, especially about the transformer taps. That may be the easiest place to affect a change.

This brings up another issue as well, far fetched as it is. Over the past few years I have seen a number of discussions in internet groups (some bordering on conspiracy theories) accusing the utilities of lowering voltages in order to extract more money from users by increasing current (or the paralell theory that one can raise voltage to save money). This of course is pure bull excrement, put it has received enough attention that I have seen 2 end users change taps on their transformers thinking it had merit. Maybe someone did that on your system and your utility is unaware of it!

Just FYI, here is a good piece on the effects of power quality on motors, including overvoltage. It is a sales piece for a testing system, but is a good compendium of information I have seen in other sources, some of which is not otherwise readilly available on-line. Figure 3 on page 3 in particular is a great old chart that was put out by EASA (Electrical Apparatus Service Assoc.) years ago, and this is the only on-line source for it that I have found.

http://www.pdma.com/PowerQualityFaultZone.pdf

"Venditori de oleum-vipera non vigere excordis populi"

 

[lombardy1]

There are no taps on the BC lower mainland utility Xformers. The muncipality's electrical crew have tried many tests prior to bringing us in to quote on the auto Xformer solution. It costs them large to bring in the equipment to re and re this pump motor. The utility will not change the 75 KVA Xformers on their pole. The municipality is also pricing replacing at their cost the pole mounted Xformers to ones with taps. I am very appreciative of everyones input and am exploring all these scenarios.

 

[mc5w]

A chronically excessive voltage will increase the hysteresis losses and eddy current losses in the iron in a motor. This can cause the insulation between the laminations to cook enough that the motor has even more excessive eddy current losses read the motor is ruined (FUBAR).

You may also have a marginally overloaded motor on top of your voltage problem. I had an instance of an alledgedly 3 horsepower pump that needed a 5 horsepower motor to run it. Turns out that under certaina flow and pressure conditions that pump was drawing 3.5 horsepower from the motor. This was partly a matter that somebody thought that they could drop a decimal point in order to save money. An example of how if you design for typical conditions your machine will typically work.

If you do upgrade the motor you likely need a heftier shaft coupling or even remounting of the pump to accomodate the larger motor frame. A soft start of variable frequency drive could reduce the need for a shaft coupling upgrade.

If you need to upgrade the motor then consider using a drive isolation transformer to step down to 480 volts. You could then using a variable frequency drive or soft start if doing so will help. A soft start or VFD would also reduce stresses on the shaft coupling when starting much like how large air compressors use a wye-delta starter because that is cheaper than heftier gears.

The municipal utility's voltage regulators may be out of whack or they may have a voltage support capacitor that is not being turned off during light load or is otherwise misapplied. A school in East Palestine, Ohio had this problem for a year until Ohio Edison fixed their voltage regulators back at the substation.

If an adjacent user on another transformer is having the same problem then the primary voltage level is at fault. If you are close to the voltage regulators the line drop compenstion for the voltage regulator could be set too high. This is a semibogus device that tries to produce say 102% voltage at full load so that customers at the far end of the line will have more voltage during peak periods without having to stick another voltage regulator halfway down the line.

You did not mention in which country this is located or the system frequency.

Mike Cole