Motor Starting Analysis
A motor starting study is performed to determine the voltages, currents, and starting times involved when starting large motors. Such a study is critical before installing a large motor to make certain that your system can start the motor successfully. It may also be performed anytime a change in the power supply is implemented.
When directly connected induction motors are started the current required to overcome the break-away torque and sync slip can be as high as five times the full load current. This surge in current drops the voltage on the nearby buses and in the case of large motors can affect the entire electrical system. The most serious effect of a large voltage drop or brown out is an increase in current of all devices servicing a constant load. Static power supplies such as computers and ballasts for lighting must increase the current to deliver the same amount of power resulting in overheating and eventually burn-out. Other induction motors operating at sync speed will increase their current draw until it reaches maximum current and may slip out of sync further increasing the current demand.
Motor starting studies vary from instantaneous startup of one or more of the largest motors to detailed simulation of the current, voltage and speed with respect to time. This allows engineers to test different slow start mechanisms ranging from auto-transformers to variable frequency drives and determine the optimum settings for start-up time and affect on the system. If a starting device is needed, its characteristics and ratings can be easily determined. Arc Flash Advisors uses the latest software, – SKM Power Tools – to calculate speed, slip, electrical output torque, load current, and terminal voltage data at discrete time intervals from locked rotor to full load speed. Also, voltage at important locations throughout the system during start-up can be monitored. The study can help select the best method of starting, the right motor design, or the required system design for minimizing the impact of motor starting on the entire system.