Engine maintenance is critical to extending the life of your conveyor. In fact, the initial selection of the right engine can make a big difference in a maintenance program.
       By understanding the torque requirements of a motor and selecting the correct mechanical characteristics, one can select a motor that will last many years beyond warranty with minimal maintenance.
        The main function of an electric motor is to generate torque, which depends on power and speed. The National Electrical Manufacturers Association (NEMA) has developed design classification standards that define the various capabilities of motors. These classifications are known as NEMA design curves and are typically of four types: A, B, C, and D.
        Each curve defines the standard torque required for starting, accelerating and operating with different loads. NEMA Design B motors are considered standard motors. They are used in a variety of applications where the starting current is slightly lower, where high starting torque is not required, and where the motor does not need to support heavy loads.
       Although NEMA Design B covers approximately 70% of all motors, other torque designs are sometimes required.
        NEMA A design is similar to design B but has higher starting current and torque. Design A motors are well suited for use with Variable Frequency Drives (VFDs) due to the high starting torque that occurs when the motor is running at near full load, and the higher starting current at start does not affect performance.
        NEMA Design C and D motors are considered high starting torque motors. They are used when more torque is needed early in the process to start very heavy loads.
        The biggest difference between the NEMA C and D designs is the amount of motor end speed slip. The slip speed of the motor directly affects the speed of the motor at full load. A four-pole, no-slip motor will run at 1800 rpm. The same motor with more slip will run at 1725 rpm, while the motor with less slip will run at 1780 rpm.
       Most manufacturers offer a variety of standard motors designed for various NEMA design curves.
       The amount of torque available at different speeds during start is important due to the needs of the application.
        Conveyors are constant torque applications, which means that their required torque remains constant once started. However, conveyors require additional starting torque to ensure constant torque operation. Other devices, such as variable frequency drives and hydraulic clutches, can use breaking torque if the conveyor belt needs more torque than the engine can provide before starting.
        One of the phenomena that can negatively affect the start of the load is low voltage. If the input supply voltage drops, the generated torque drops significantly.
        When considering whether the motor torque is sufficient to start the load, the starting voltage must be considered. The relationship between voltage and torque is a quadratic function. For example, if the voltage drops to 85% during start-up, the motor will produce approximately 72% of torque at full voltage. It is important to evaluate the starting torque of the motor in relation to the load under worst-case conditions.
        Meanwhile, the operating factor is the amount of overload that the engine can withstand within the temperature range without overheating. It may seem that the higher the service rates, the better, but this is not always the case.
        Buying an oversized engine when it can’t perform at maximum power can result in a waste of money and space. Ideally, the engine should run continuously at between 80% and 85% of rated power to maximize efficiency.
        For example, motors typically achieve maximum efficiency at full load between 75% and 100%. To maximize efficiency, the application should use between 80% and 85% of the engine power listed on the nameplate.