American Superconductor Corp, which makes electrical systems for wind farms, said it will post a loss in the fourth quarter, as a slowdown in China's wind power market forced its top customer to refuse shipments.

Shares of AMSC tumbled about 50 percent in after-market trade, after China's Sinovel Wind, the world's third-largest wind turbine maker, refused to accept contracted shipments of some wind turbine components.

AMSC also said it is reviewing its books after Sinovel failed to pay for certain contracted shipments worth about $56 million, which it has already accounted for.

The company has posted a profit for eight straight quarters, and analysts were expecting it to post a 32-cent per share profit this quarter, according to Thomson Reuters I/B/E/S.

China's wind turbine sector, a stable source of growth for AMSC, experienced explosive growth in the past few years aided by hefty government subsidies. However, it is expected to be flat in 2011.

They (AMSC) are still going to have to remain very focused on China, but obviously they are going to continue to diversify to more components with this market slowing down so significantly, Gleacher & Co analyst John Hardy told Reuters.

AMSC has been trying to diversify its customer base away from Sinovel, which accounts for about 75 percent of its revenue, through new wind turbine orders and emerging power grid opportunities.

Last month, AMSC said it was buying smaller Finnish rival, The Switch Engineering Oy, for $266 million to trim its dependence on China.

AMSC said it was taking actions to cut expenses as it continues to have active discussions with Sinovel to determine when the Chinese company would accept further shipments.

The company expects full-year revenue to be less than $355 million compared with its prior estimate of $430-$440 million.

AMSC also said full-year adjusted earnings would be well below its previous forecast of $1.31-$1.35 per share.

(Reporting by Divya Sharma in Bangalore; Editing by Roshni Menon)