For starting an AC electric motor of a mining vehicle, it is first accelerated to a first speed with a second AC voltage provided by an onboard battery-powered inverter of the mining vehicle. A phase of a first AC voltage taken from an external grid is compared to a phase of said second AC voltage. If the phase difference between the first and second AC voltages is larger than a predetermined limit, the speed at which said inverter rotates said AC electric motor is changed. If the difference between the phases of the first and second AC voltages is smaller than the predetermined limit, a change is made from rotating the AC electric motor with the second AC voltage to rotating the AC electric motor with the first AC voltage.