The currents supplied to the motors in any given installation will produce losses in the distribution cables and transformers of the consumer. Correct sizing of the cables will not only allow a cost-effective minimization of those losses but also helps to decrease the voltage drop between the transformer and the motor. The use of the standard national codes for sizing conductors leads to cable sizes that prevent overheating and allow adequate starting current to the motors, but can be far from being an energy-efficient design. Ideally the cables should be sized not only taking into consideration the national codes but also considering the life-cycle cost.
The machinery manufacturers, who are responsible for choosing the motor in the first place, as well as the users who should influence the buying phase or the replacement of broken down motors should consider that the design criteria leading to oversizing may have strong consequences on the energy bill. Since motors are designed to withstand short periods of overload, there is no reason to oversize a motor just because the maximum required power is over the rated power of the motor, during some of the operation time.
The transmission system transfers mechanical power from the motor to the final end-use. The choice of transmission is dependent upon many factors, namely: the desired speed ratio, motor powers, layout of the shafts, type of mechanical load, etc. The most important kind of transmission types available include direct shaft couplings, gearboxes, chains, and belts.
Most motors are connected to their loads through a transmission system, very frequently
through a belt. About one-third of the motor transmissions in the industry use belts. Belts allow flexibility in the positioning of the motors in relation to the load. Additionally, belts can also increase or decrease the speeds using pulleys of suitable diameters.
because it has low flexing losses and no slippage. Synchronous belts have no slippage because they have meshing teeth on the belt and pulleys. Unlike standard V-belts that rely on friction between the belt and the pulley grooves to transmit the torque, synchronous belts are designed for minimum friction between the belt and the pulley. Due to their positive drive, these belts can be used in applications requiring accurate speed control. Synchronous belts stretch very little because of their construction, do not require periodic retensionings and they typically last 4 times longer than standard V-belts. Retrofitting synchronous belts requires installing sprocket pulleys that cost several times the price of the belt. In cases where pulley replacement is not practical or cost-effective,
The motor will deliver even better value. A variable speed drive motor can be started softly with a low starting current, and the speed can be controlled and adjusted to suit the application demand without steps over a wide range. The use of a frequency converter together with a squirrel cage motor usually leads to significant energy and environmental savings.
At low-speed operation, the cooling capacity of the fan decreases, which may cause highers temperature rises in the motor. A separate constant speed fan can be used and increase cooling capacity and loadability at low speed. It is also important to check the performance of the lubrication at low speeds.
The voltage (or current) fed by the frequency converter is not purely sinusoidal. This may increase the losses, vibration, or noise of the motor. Furthermore, a change in the distribution of the losses may affect the temperature rise of the motor. In each case, the motor must be correctly sized according due to the instructions supplied with the selected frequency converter.