Induction motors are designed to work from the mains. However, a variable frequency drive produces a series of high voltage pulses that build up an approximate sinusoidal current in the motor. Most motors run from a drive without problems, and the high switching frequencies used by modern drives create a pretty good sine wave current. The voltage waveform however, can cause problems. The waveform contains many frequencies higher than the switching frequency, and these can cause interference and leakage currents if there is any stray capacitance. Cables between the motor and the variable frequency drive will always have some capacitance to ground, and screened cables are generally worse than loose or unscreened cables – which aren’t recommended as they can cause interference. The longer the cable the greater the capacitance, so drives are usually specified with a maximum cable length. If the capacitance is too great, then the drive may trip on over current or overheating as the leakage current becomes excessive.