A method of controlling a motor drive having a normal mode wherein a DC link voltage is charged using an AC (e.g. mains) power supply and an emergency mode wherein the DC link voltage is charged using a DC supply (e.g. from a battery) is described. In the normal mode of operation, if the DC link voltage falls below a threshold indicative of the AC power supply being lost, the emergency mode is entered, typically following an intervening coast mode (during which the motor is not driven). Similarly, in the emergency mode, if the DC link voltage rises above a threshold indicative of the AC power supply being restored, the normal mode is entered, typically following an intervening coast mode.