Convicted murderer and burglar Bernard Charles Welch Jr., who is serving a 143-year term imposed by the District of Columbia, was given another 30-year sentence in Montgomery County yesterday after pleading guilty to additional armed robbery and burglary charges.

The sentence, issued by Circuit Judge Rosalyn Bell, was the result of plea bargaining negotiations that began last December between prosecutors and Welch's court-appointed attorneys. It is to run concurrently with Welch's earlier sentence in connection with the Dec., 1980 slaying of Washington D.C. cardiologist Michael Halberstam.

The sentence was necessary, said Montgomery County State's Attorney Andrew Sonner, because it insures Welch will serve time in prison if his D.C. conviction, which is under appeal, is overturned.

Welch, the prime suspect in 21 county police cases in 1980, had been indicted by a Montgomery grand jury last May on seven charges that ranged from rape and armed robbery to burglary, said a spokesman. Under the bargain, officials dropped one burglary case, the rape charge and lesser charges on the five remaining cases and agreed Welch would receive no more than a 30-year sentence.

In return, Welch agreed to plead guilty to a January, 1980 armed robbery and four burglaries.

"I don't look at it as a victory or a defeat," said defense attorney Steven Salant after Welch had been sentenced to 10 years for armed robbery and five years on each of the burglary charges. "It's what Bernard Welch wanted."

Under Maryland sentencing guidelines, Welch could have received from 25 to 50 years for the crimes. Prosecutors said the agreement served the public because the rape case was weak and the strength of the others was uncertain.

Some of Welch's victims were dissatisfied with the sentence. Sorra-Lee Raven, former Democratic precinct chairman and the victim of a Welch burglary in November 1980, said,"I don't think five years for what he did to me was adequate." Raven says she still is attempting to recover stolen gold and silver items that police found at Welch's Great Falls home.

Welch appeared weary and exasperated. Rubbing his eyes frequently, Welch described his crimes in ragged sentences.

"I broke in through the cellar door. . . very hard to get in that window. They had taken elaborate precautions. . . went upstairs. . . There were large quantities of jewelry on a shelf," Welch said, describing the November 1980 burglary of a Rockville home.

Welch, who has been kept in solitary confinement in the Montgomery County jail since September, may face charges on a series of crimes in Fairfax County, as well as Arlington and Alexandria. But prosecutors in each of those jurisdictions have made no final decisions, and said yesterday it may be unnecessary to try Welch.

If Welch's D.C. conviction stands and "there is no likelihood of his being paroled, then I will not proceed," said chief Arlington prosecutor Henry Hudson.

If no other jurisdictions prosecute, Welch will be transferred back to a maximum security prison in Marion, Ill.