If you have a four-prong 240V outlet can you switch the outlet to a three-prong 240V outlet?

The 4 prong receptacle should be more in line with the latest wiring code. It is best to change the plug or the cord on the appliance you wish to use. If you have a 4 prong outlet, the purpose is to isolate the neutral from the ground.

There are numerous examples mentioned in Wiki s but they all amount to requiring that there be no link between the neutral terminal of the device [appliance] and the frame.

It is, however, allowed [by the NEC, unless disallowed locally by the "authority having jurisdiction"] to install a link to the neutral terminal from the chassis if there is already a three prong outlet existing.

ALWAYS CHECK WITH YOUR LOCAL WIRING SAFETY OFFICE BEFORE YOU ATTEMPT TO CHANGE ANYTHING ON POWER MAINS OUTLETS

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.

If you do this work yourself, always turn off the power

at the breaker box/fuse panel BEFORE you attempt to do any work AND

always use an electrician's test meter having metal-tipped probes

(not a simple proximity voltage indicator)

to insure the circuit is, in fact, de-energized.