Electric motors are the workhorses of industry. Electricity feeds these motors, allowing them in some cases to operate continuously as needed. Unfortunately the controls that are used to start, stop and monitor these motors are extremely sensitive to minor disturbances in the electric supply, such as a fleeting decrease in voltage (referred to as a "sag") or a momentary power interruption (referred to as an "outage") of as little as a few milliseconds.

Motors themselves are very robust and could be left in operation during these events, but in such situations motor controls tend to "drop" the motor off-line. This interrupts the manufacturing process (necessitating a potentially damaging restart), reduces production output, requires additional manpower and in some cases results in a very high cost in damaged product.

Techniques are available for holding conventional motor starter contacts (motor controls) "in" during momentary electrical disturbances; this is referred to as "ride-through." Special contactors also have been developed to assist in implementing this feature for momentary interruptions. This allows an induction motor to "ride-through" a momentary event, improving production efficiency and reducing manufacturing costs.

Restarting an induction motor (voltage resumption following the interruption) while it is still rotating is potentially damaging. Magnetic flux from the collapsing induction field, system capacitance and/or residual magnetism in the motor can counter the field created by the reapplied voltage. The instantaneous torque produced may exceed levels anticipated in the design of the motor and/or the driven machine.

The ride-through feature would eliminate the need for many restarts. However, some equipment manufacturers have indicated that they will not honor warranties on equipment driven by motors that employ contactor hold-in devices.

Thus, manufacturing operators are caught between accepting losses created by production disruptions, loss of warranty coverage on driven equipment and potential damage to motors and production machinery. This situation is primarily the result of the fear of possible equipment damage and the lack of definitive knowledge of motor behavior under contactor-hold-in restarts. Very little testing has been done to measure the actual peak torque or to develop simulations to predict the peak torque for common motor applications. So what is the manufacturer's best option?

Progress Energy (with project leader Scott Peele), utilizing the services of the Advanced Energy Motor Resource Center test lab, has embarked on a research project to measure peak torques on a range of motor sizes under controlled conditions representing three of the most common types of industrial motor applications; pumps, fans and air compressors. This project is a continuation of research started by NCSU graduate student John Cavaroc. So far the research has determined that there are many circumstances under-which motor ride-through can be implemented without risk of damage to the motor or driven machine.

A computer modeling program is in development based on the lab testing ; it will permit prediction of the peak torque during typical momentary electrical disturbances. Knowing the peak torque expected will give equipment and motor manufacturers as well as the industrial motor operators the assurances they need that motor ride-through devices will not damage their equipment.

The equipment developed to simulate and monitor momentary disturbances for the lab testing will be field-portable. This means the equipment can be taken to customer facilities to verify computer predicted peak torque, characterize the specific disturbance and also to verify that a ride-through enabling device performs properly during the disturbance it is designed to handle. Project completion is expected in late 2002.

E-News on Energy will publish the results of the project in a future issue.