A motor on a Metro subway car caught fire yesterday morning in the Metro Center station and forced a 20-to-30-minute interruption in rush-hour service on the Red Line. It was the second motor fire in as many days for Metro.

Erich Vogel, general superintendent of car equipment for Metro, said that "we're doing an investigation of both of the incidents" but that preliminarily he did not think the causes were related.

Motor problems, according to both Vogel and federal officials who did a full year of testing on the Metro subway cars, have been with Metro since it started subway operations in March 1976. Fires have been rare, however. "We've had three motor fires; it's just coincidental that two of them came in the past two days," Vogel said.

No one has been injured and the fires themselves have been more smoke than flame. Subway service was badly snarled yesterday morning, however, because the electricity for both Red Line tracks was turned off while the D.C. Fire Department extinguished the fires.

There is a 750-volt direct current 175-horsepower electric motor on each Metro subway car axle; there are four axles to a car. The motors provide both propulsion power and, when they are reversed, braking.

Complex circuitry controls and coordinates the motors. If something is wrong, the motors are supposed to cut themselves out of the system. From the early days of Metro, according to Vogel, there have been an excessive number of motor "overloads," or cutouts.

The problem is industrywide, according to Gunnar Spons, who directed a U.S. Department of Transportation testing program of Metro's cars in Pueblo, Colo. "Every vehicle we've had out here has had a bad (motor) problem," Spons said. "I think Washington went the farthest to try and resolve it" with a series of modifications.

None of Metro's 15 train fires caused by motors or other factors has penetrated the passenger compartments, although increased fire protection has been specified in a new car order.