As electric carmaker Tesla prepares to release quarterly results Wednesday, it is also trying to fend off escalating worker complaints about pay and safety at its California factory, where a move to unionize is gaining steam.The union drive is a challenge for the company, which has faced production delays and is trying to speed up delivery of its mid-priced Tesla 3. An initial batch of the new models rolled off the assembly line just last week.Tesla said it expects to build 100,000 vehicles this year at the complex in Fremont, California, just south of San Francisco. In a letter to Tesla's independent board members, workers requested access to the automaker's safety plan as well as clarity on compensation and a promise of no retaliation against employees as they try to form a union. The United Auto Workers is in the process of trying to unionize the 10,000 Tesla workers at the Fremont plant, alleging the company has a poor safety record -- a charge it vehemently denies."We're tired of suffering preventable injury after preventable injury. It impacts morale, it slows down production and it's, of course, traumatizing," said Michael Catura, a Tesla production worker who signed the letter.Tesla founder and chief executive Elon Musk has used Twitter to criticize the union drive and denied the charges that working conditions at the Fremont facility are unsafe.A Tesla spokesperson pointed to a company blog from May that argued its safety record is improving. The automaker did not provide additional comment.That blog accused the UAW of conducting a misleading campaign and said despite a small number of incidents, the plant has a better safety record than the industry average. Tesla has acknowledged its employees had to work large amounts of overtime in the early days, but that also is improving and the average work week is now 42 hours."Tesla's safety record is much better than industry average, but it is not enough. Our goal is to have as close to zero injuries as humanly possible and to become the safest factory in the auto industry," the company said in the blog.But so far, Tesla has not responded to worker complaints about pay.Starting pay for production associates in the Fremont facility is $18 an hour, far below the national average for auto workers of $25.58 and even farther below the living wage in Alameda County, California, where the average wage is $28.10, according to the letter sent by workers.Renting an apartment in Fremont would require 70 percent of a worker's take home pay. In addition, the letter said the paths to promotion are not clear."Many of us have worked for years with the vague promise of a raise, with nothing to show for it," said Richard Ortiz, who works in the paint shop."We have no idea what the criteria is to move forward, and no idea of what defines success. We've raised these issues repeatedly, and have gotten no response," he added.Workers, who are currently attempting to unionize the facility, also asked the company not to retaliate against those who speak out."We should be free to speak out and to organize," the letter said.UAW President Dennis Williams has said the union is not yet prepared to call for a vote among Tesla employees. The UAW opened an office near the Fremont plant and also has two organizers working to recruit Tesla workers. Tesla is due to report earnings after the market closes on Wednesday.The company's share price slipped 0.75 per cent in early trade, but has risen 36 per cent in the last six months -- boosting its market capitalization and for a brief time making it the biggest US automaker, surpassing General Motors, even with a tiny fraction of the production.But analysts are sceptical the company can ramp up production of the Model 3 as fast as it plans, which could impact earnings.