There are four major starting methods for motors. Since compressor and blower manufacturers use any number of these starting methods—some may come standard, some may be an option with a price adder—it’s important to understand the differences between them. Understanding these differences can be helpful when comparing equipment that utilizes different starting methods.
The key point to understand with these different starting methods is in-rush current. In-rush current is simply the maximum input current that is drawn by an electrical device when it is first turned on. The higher the in-rush current, the more strain it puts on the motor and also the power grid. Here is a chart showing each of the four starting methods and their in-rush currents:
Comparison of motor starting methods
Direct Online Starting (DOL)
A direct starting method utilizes a single motor contactor. When engaged, the motor sees full voltage and as much current as the system will allow. For a motor, it will pull as many amps as needed to satisfy the torque required to maintain its nominal speed. The nominal speeds range from 3600 RPM (2-pole), 1800 RPM (4-pole), and 1200 RPM (6-pole) for 60 Hz power. To satisfy this requirement, the current in-rush can be 6 – 20 times the full load amperage rating (FLA) of the motor. This in-rush current will only last a few cycles and the length of the in-rush depends on the load and the nominal speed of the motor. The higher the nominal speed, the longer the in-rush lasts. This starting method allows for the quickest starting times, but also transfers a tremendous amount of heat to the drive motor, breakers, and other electrical equipment as well as induces high levels of mechanical stress to the package. As a result, the number of starts per hour are very limited.
Soft Starting (Solid State)
If the high in-rush and system stress of Direct Online Starting are not preferred, a more controlled or current limiting starting method is available via soft starters. These devices lessen the amplitude of the in-rush discussed above by limiting the current flow to the motor, thus reducing the motor torque. Modern electronic devices can typically be programmed for different starting profiles and ramp times. While these devices do limit the in-rush current, it is not totally eliminated. Additionally, once the motor reaches full speed, it’s recommended to have a by-pass contactor to limit the number of current restrictions to the motor.
Wye-Start to Delta-Run (Y-Δ or Star-Delta)
Electric motors can be configured for a wye-start, delta-run configuration. This works by altering the motor winding configurations to change the available power output. In this case, the motor is started using the motor’s Wye configuration which limits the motors output to 1/3 of the motor’s nameplate value. In the case of a 100 Hp motor, you are starting a 30 Hp motor. At this lower power, the motor produces less torque and pulls less amps at startup. To achieve full nameplate power, the motor must transition to the Delta configuration via an open or closed transition. This transition occurs after the motor has achieved full speed and initiates a second in-rush spike to the system. This second spike is often higher than the initial in-rush, but is very, very brief.
Variable Frequency Drive (VFD)
To minimize/eliminate in-rush current at startup, a variable frequency drive can be applied. VFDs work by taking AC input voltage and building a DC bus (typically 1.35 x input). From here, rapidly acting switches (IGBTs) open and close at different rates to alter the frequency of the signal. Because the drives are able to deliver full nameplate amps at all frequencies, the in-rush current at startup can be eliminated. In reality, a small in-rush current typically remains as increasing the ramp times too far can introduce heat to the windings and rotor. This overshoot is considered negligible. While VFDs may be the best starting method in terms of in-rush current, they present other system design considerations. VFDs can offer additional energy savings benefits, but only if they are properly applied.
Each starting method has advantages. The key is to understand your system and installation needs.
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