The Federal Trade Commission moved in U.S. District Court here yesterday to block, at least temporarily, Exxon Corp.'s $1.17 billion takeover of Reliance Electric Co., the largest cash acquisition in history.

Judge Harold Greene, after two hours of short-notice testimony last night, said he would rule today on the government's request.

In its last-minute appeal, the FTC asked the judge for a temporary restraining order to hold up Exxon's purchase until the agency could investigate it further.

Exxon is pursuing the purchase as a quick and easy way of marketing a new electronic variable-speed drive for electric motors - a device that can be used to control and adjust motor speeds.

FTC attorneys argued yesterday that the acquisition threatened to frustrate competition in the motor control industry - specifically potential competition between Exxon and Reliance, a leading maker of motor controls with a 20 percent market share.

Exxon attorney Henry Sailer told the judge that a temporary restraining order would surely kill the deal by leading to longer delays. "The transaction will be dead, dead, dead," he said at one point.

An attorney for Reliance Electric, which intervened in the case, surprised the government with a compromise offer. He suggested that Exxon and the FTC negotiate a "hold separate" agreement under which Exxon would be allowed to buy Reliance but would not be permitted to take it over fully.

FTC attorneys said they would be willing to discuss such a compromise.

In a much-publicized news conference two months ago, Exxon officials unveiled a new motor control device which, the oil giant said, would revolutionize the industry - and provide substantial energy savings.

But to produce and market the device called an "alternating current synthesizer," Exxon said it would have to acquire Cleveland-based Reliance.

In a preliminary study of the merger this week, the FTC staff reported that Exxon had in fact considered entering the electric motor business on its own and had gone so far as to locate a factory site and interview for management posts.

Exxon's attorneys strongly deni d this yesterday.

Exxon has offered to pay Reliance shareholders nearly twice the price at which their stock was selling the day the proposal was announced. Under federal law, Exxon will be free to complete the deal on Sunday, unless the judge issues a restraining order.

Exxon has said that starting from scratch would take too long. The oil company has used the energy shortage to defend its planned acquisition, stressing the potential energy savings and arguing the need for quick ation.

Others in the electric motor industry, however, have voiced doubts about Exxon's so-called breakthrough. They have speculated that the oil company may be exaggerating the innovativeness of the device as a ruse to win support for a smart diversification move.