circuitry unit 14 in drive control 8 limits the power supplied to the motor by reducing the current the gain circuitry can adopt a gain of 1 which does not limit motor power a gain of 0 to permit no power to be delivered to the motor and a gain between 1 and 0 to adjust the power delivered to the motor between u2018 no limit u2019 and u2018 no power u2019 accordingly the current supplied to the motor is controlled so that the motor thermal load does not exceed a predetermined operational limit i e broken line 62 is not exceeded as shown in fig3 a the gain of unit 14 is 1 when motor thermal load is below a predetermined lower limit power supplied to the motor is not limited when the gain is 1 the safety margin between broken line 62 and solid line 40 in fig4 is determined by the selected lower limit unit 14 gradually reduces the gain as the motor thermal load increases above the lower limit according to the relationship between motor thermal load and gain shown in fig3 a in the example shown the stabilisation point when i motor i rated is reached at a gain of 0 5 the motor is not allowed to operate in an overload condition when the motor thermal load is above the stabilisation point the gain is gradually reduced to zero when the motor thermal load.