An electric motor starting study and analysis involves assessing and understanding the motor’s behavior during its initial startup phase. Typically conducted for larger motors, this study encompasses evaluating parameters such as starting current, voltage drop, voltage dip, and the duration required for starting. Motor starting studies are particularly crucial in large commercial and industrial systems where the operation of a high-capacity motor can potentially impact the motor’s performance, the overall system, and adjacent equipment.
The size of electric motors utilized in contemporary industrial systems is steadily increasing. A motor is deemed “large” by comparing its capacity (measured in kW) to the total installed capacity of the power source within a system. Initiating the startup of a large motor can result in significant disruptions not only to the motor and locally connected loads but also to other electrical buses distant from the motor’s starting point. Employing an incorrect motor-starting method may lead to motor damage, power quality issues (such as operational breakdowns), or even a blackout. Ideally, conducting a thorough study and analysis of motor startups should precede the purchase of large motors.
There are at least five reasons why conducting motor starting analysis is crucial
Determination of Starting Method
- Motor or motor starting method selection, or both, can be aided by the study.
- Detailed studies assist in determining the size of the starting resistor in motor rotor windings.
Special Torque Requirements
- Special loads require careful and precise acceleration control without surpassing torque limits set on the equipment.
Problems Revealed
- If the motor rating exceeds 30% of the kVA rating of the transformer (in the absence of a generator).
- Starting a large motor may cause interference with locally connected motors, systems, loads, and connected buses.
- When the rating or motor capacity surpasses 10-15% of the kVA rating of the generator (in systems supplied solely by generators).
- When many motors start simultaneously.
Weak Source Generation
- Motor starting studies analyze the performance of small systems with generators.
- Limited capacity in smaller power systems often exacerbates voltage drop issues during large motor startups.
Voltage Dips
- Motor load torque is directly proportional to the square of the motor terminal voltage, meaning any voltage variations directly affect torque characteristics.
- During startup, maintaining the voltage at the motor terminals at least 80% of the rated voltage (for NEMA standard design B motors) is crucial.
- Voltage drops caused by motor starting can exceed the torque capabilities of running motors, potentially leading to significant slowdowns or crashes.
- Voltage dips also affect other loads like electronic devices, sensitive control devices, and lighting loads.
A motor-starting study aims to assess voltages, currents, and starting times associated with initiating large motors or groups of motors, either sequentially or simultaneously. These studies ensure several key outcomes
- Proper sizing/setting of motor protective devices.
- Preventing nuisance tripping of the motor during startup.
- Ensuring motor(s) start with acceptable voltage drop.
- Verifying adequate sizing of motor feeder(s).
- Accurately evaluating motor/load speed-torque characteristics and accelerating time.
- Assessing thermal damage characteristics of motors accurately.
- Preventing voltage drop during startup from disrupting other loads.
- Ensuring motor(s) accelerate within acceptable start-up times.
- Identifying the type and size of starter/drive required if direct on line (DOL) start is not feasible.
Electric motor starting studies aid in selecting the appropriate motor design, determining the optimal startup method with minimal impact on the distribution system, and mitigating voltage flicker and drop issues.
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