With comparatively little fanfare, engineers at the National Areonautics and Space Administration have introduced an electric motor regulator that is about as efficient as the highly publicized energy-saver trumpeted by Exxon earlier this month, with one major exception - the government is willing to give its invention away free.

The existence of the NASA device - along with a number of other motor controllers already on the market which have similar effects - has raised questions about the real worth of Exxon's achievement.

And the controversy has reinforced speculation that the oil giant's announcement may be more ploy than progress - that it is in fact a ruse by Exxon to win public approval for a major acquisition.

Exxon responds cooly to such conjecture, seemingly unperturbed by all the fuss. Company spokesmen say there is no disguised intent at the core of Exxon's interest in acquiring Reliance Electric Co. of Cleveland. The value of their discovery, Exxon engineers say, has yet to be fully appreciated. It will be in time, they assure.

Because Exxon has little experience in the electrical equipment industry, the oil company maintains it needs Reliance to make and market its new device.

Reliance is one of the largest makers of electric motors in the U.S. It had sales of $966 million last year and ranked 262 on the Fortune 500 list. If Exxon pays what it offered last week for Reliance - a total of $1.165 billion, or nearly twice the price of Reliance stock May 18 when Exxon said it was interested in making the acquisition - it will be the largest cash takeover in the history of American business.

But Reliance does more today than build motors. Once heavily dependent on the sale of electric motors for much of its revenues, the company has diversified into a broad variety of electrical and electronic equipment within the last dozen years. Currently, electric motors account for only 10 or 11 percent of Reliance's revenue.

As such, Reliance appears to be an attractive diversification move for Exxon - in addition to Exxon's stated wish for making the acquisition.

In any case, the proposed purchase is a rare instance of Exxon bidding for an outside concern. The big energy company has in recent years entered new ventures mostly by providing venture capital from within the company and building up businesses more or less from scratch.

The bid also comes at an awkward time. Oil companies are being criticized by the Carter administration and others for spending money on projects that don't increase energy production. And the Senate Judiciary Committee under Edward Kennedy (D-Mass.) has prepared legislation to block takeovers by oil companies of firms valued at more than $100 million.

Exxon's Ron Ricci, project managaer for the new device, said in a phone interview that the oil company had considered several alternatives for developing its invention, including a possible joint venture with another firm. But Exxon chose to go the acquisition route because it was "the only practical alternative," Ricci said.

Quick development of the device is essential, Ricci added, and he repeated corporate claims that the device could result in substantial reductions in U.S. oil consumption - the equivalent of 1 million barrels a day by 1990, or nearly equal to the daily oil flow from Alaska.

Ricci further noted that the planned acquisition isn't a large one for a company of Exxon's enormous size - it would add less than 2 percent to the oil gian't revenue.

All the same, the Federal Trade Commission is expected to review the deal under federal laws that prohibit anti-competitive actions. It no doubt will study carefully Exxon's own logic for the move. Among the questions likely to be considered: Just how original is Exxon's invention, and how feasible will it be?

Part of the problem in sorting out the conflicting claims at this point is the technicality of the subject. While electric motors are used by many people every day, their specific operation is not commonly understood nor easily explained.

What both the Exxon and the NASA devices do is allow electric motors to run on as little as three-quarters the energy they now consume. This is done by throttling down and controlling the amount of electric power that goes into a motor - instead of controlling the power that comes out, the method commonly used today.

The two devices take different approaches. NASA's device, called a "power factor controller," reduces the voltage applied to a motor when the motor is not being used fully. It functions like a dimmer switch for lights.

Exxon's device, called an "alternative current synthesizer," regulates not only the voltage but also the speed at which a motor runs.

NASA's device is being applied mainly to common household motors and small motors, while Exxon expects to use its device on larger motors, though engineers on both sides say their devices can be used on motors large and small, from less than 1 horsepower to 200 horsepower.

"It's not new," Frank Nola said of Exxon's touted invention. "I'm kind of surprised they're making such a big deal about the whole thing."

Nola is the NASA engineer who invented the government's device several years ago in the process of designing solar heating and cooling systems at the Marshall Space Flight Center in Huntsville, Ala.

NASA began quietly last winter to grant licenses to private businesses to manufacture and sell Nola's invention. The licenses are free under the 1958 space act which encourages dissemination of government-held patents for commercial use. So far, NASA reports, 22 firms have been licensed to make the motor controllers.

In addition, a number of companies already sell alternating-current motor controls which, like Exxon's device, regulate both voltage and speed. But these devices are also more expensive than the simpler NASA model.

"Right now, there are a number of devices around capable of performing the same function as the Exxon device, but their costs are too high for more general application in industry," said William Healey, president of Parametrics division of Zero-Max Industries of Minneapolis, a producer of motor speed controls. A one-horse-power control device Healey makes sells for $540.

Healey said that from what he has seen of Exxon's patents on its new device, "it's not obvious Exxon has a significant breakthrough in price."

Exxon believes differently. Company researchers concede the technique used in their new device is not a scientific breakthrough, but they insist it is a clever adaptation of known principles. The oil company has spent $10 million since 1974 developing the device, according to Ricci. Four patents have been allowed on it so far, another 16 have been applied for.

Exxon's chief innovation, said Ricci, is the use of solid-state technology in place of old-fashioned electromagnetic components such as transformers and chokes.The result is an iron-free device that is both efficient and cheaper - and will be cheaper still as the price of semiconductor elements continues to drop.

"We have learned how to use computer components in a way that's never been used before," Ricci said.

However, Ricci said he could not estimate the selling price for Exxon's new device.