Vibrational energy harvesters offer an alternative to batteries for the autonomous powering of low power electronic devices, such as wireless sensors. Velocity amplified electromagnetic generators (VAEGs) utilise the velocity amplification effect in order to increase the output power and operational bandwidth of an electromagnetic generator, compared to linear resonators. This effect is achieved through sequential collisions in a system of free-moving masses. The velocity amplification achieved is controlled by the number of masses or degrees-of-freedom (dofs) and the mass ratio between them. An experimental investigation into the influence of mass ratio and number of dofs on the operational bandwidth of a VAEG at low frequencies (＜ 30 Hz) is presented herein. VAEG systems with 2-, 3- and 4-dofs are analysed for mass ratios in the range of 3:1 to 20:1 under sinusoidal forced excitation. It is shown that 3- and 4-dof configurations offer broader bandwidths than 2-dof configurations, particularly at higher mass ratios. At lower mass ratios, the bandwidths of the 3- and 4-dof configurations decrease, while the bandwidth of the 2-dof configuration increases. At a mass ratio of 3:1, the bandwidth of the 2-dof configuration approaches that of the 3-and 4-dof configurations. The highest half-power bandwidth of 9.3 Hz was achieved by the 3-dof ass ratio 20:1 system, at an acceleration of 1 g. The total voltage harvested over the frequency range 7-30 Hz is at a maximum at a mass ratio of 5:1 for the 3- and 4-dof configurations, and at 3:1 for the 2-dof configuration. The findings of this investigation will be significant in the development of future reduced scale VAEGs.
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