SEED Guides Rotary Power transmission < >
3. Match the load to the supply
Figure 9 shows what would be regarded as a good match in that the torque available exceeds that demanded over the entire operating range. In order to avoid over-speed the power source curve would have to be lowered (or the load curve raised to obtain a discrete operating point). If two operating points are created close together undesirable 'hunting' will occur.
If the power source is an electric motor the power supplied may be limited by adjusting the supply voltage. Alternatively a brake may be applied to the load, though this has implications for system efficiency and energy conservation.
In the real system power is lost at all stages of the transmission. Energy loss due to friction is disspated as heat, in many cases the energy absorbed in acceleration and elastic twist is irrecoverable. It is particularly important to consider these losses when torque and speed vary with time. This occurs with all systems during starting and stopping phases and in many systems where operational torque fluctuates.
The assumption of power being conserved is often made to facilitate calculations, eg: as a first approximation in calculating the output torque available from a gearbox. However adjustment must be made later for the element inefficiency which may be significant. In the case of high rates of acceleration or high inertias the torque available for driving the final load will be reduced. Although this 'loss' can be calculated from the moment of inertia of the gearbox, such information is rarely given by manufacturers. When it is given care must be taken to ensure that it is referred to the input shaft.