This is the ninth article in our series about Power Design Pro™ software and it highlights motor loads and the significant impact they potentially have on sizing the generator. Some starting loads tend to be higher and draw more power than running loads, so users have the option to adjust those settings and/or start motors in sequence to prevent specifying an oversized genset. Another benefit of Power Design Pro is that it intelligently accounts for potential issues with older motors and their starting sequences.
To enter a motor load in Power Design Pro™, you simply select add “motor” from the left column. The motor load type supports motors of various starting methods including: across the line, reduced voltage, soft starters and variable frequency drives. It also supports adjusting the motor’s mechanical load level and the loads starting torque characteristics.
Power Design Pro allows the user to customize the motor’s running kW (rkW). In real applications, motors are loaded anywhere from 75% to 115%. The adjustable “motor load level” allows for fine tuning when measured running amp data is available or when detailed combination motor/load spec sheets clearly define the running amps.
In addition to fine tuning the steady state kW requirements, Power Design Pro also allows the dynamic characteristics to be adjusted. Low starting torque loads like pumps and applications with unloading valves or disconnect clutches tend to accelerate faster and impact the engine less than high torque loads like loaded conveyer belts. Power Design Pro allows for this dynamic starting difference by reducing the starting kW (skW) for the motor which potentially reduces the peak kW requirement for the genset.
Importance of Sequencing
Like all load types within Power Design Pro, motors can be placed into starting sequences that assume all the loads start concurrently (“steps”) or in a sequence that assumes non-concurrent starting (“groups”). Given that motors tend to cycle on/off based on a controlled process (pressure, temperature, human interaction, etc.), they rarely start concurrently. Within most applications natural sequencing tends to ensure this non-concurrent starting, though simple control interlocking or timer relays can prevent concurrent starting, if necessary.
Concurrent starting of motor loads within an application can result in a significantly upsized or oversized genset. When a motor starts, it draws a large inrush of low power factor current -- lock rotor amps (LRA). Within Power Design Pro, this motor starting current is multiplied by voltage and expressed as starting kVA (skVA). When multiple motors start concurrently, each motor’s skVA become additive and the genset’s alternator must become large in order to limit the resulting voltage dip to an acceptable level. The larger the skVA for a load step, the larger the alternator and potentially the larger the genset.
Motor Starting Current
Power Design Pro uses the Association of Electrical Equipment and Medical Imaging Manufacturers (NEMA) and the International Electrotechnical Commission (IEC) data for starting codes, motor size and motor design. Power Design Pro allows the user to select a given motor’s skVA via the NEMA starting code (skVA/Hp) selection. The program defaults the motors starting code based on motor size and motor design (NEMA or IEC), but the user can always adjust this based on detailed motor data. Care should also be taken to get motor data for submersible well applications which typically have very high starting currents.
The motors starting requirements are also significantly impacted by starting method: across the line (full voltage), reduced voltage, soft starter, and variable frequency drive (VFD). Each of these starting methods has significant impacts on the motors skVA, skW, and harmonics.
Reduced Voltage Starting
If the starting method is reduced voltage, the available device types are wye-delta, various auto transformer configurations, and part winding. These settings will greatly reduce the required skVA and skW requirements.
The one issue with implementing these older methods of electrical/mechanical reduced voltage starting is the very significant possibility of the motor transitioning from reduced voltage to full voltage prior to the motor achieving full speed. An early transition to full voltage can cause near locked rotor current and may make the starting sequence seem like an across the line motor start.
The program will evaluate the available motor starting torque of the selected starting method against the loads selected mechanical characteristics. If the mechanical load characteristics (“motor load type”) are not compatible with allowing the motor to achieve full speed on reduced voltage starting, the program will assume an early transition and select appropriate skVA and skW values. This advanced feature of Power Design Pro attempts to avoid the early transition misapplication problem that is not typically accounted for by other manufacturers’ sizing programs.
Electronic Soft Starters
To overcome the limitations of electrical/mechanical reduced starting methods, the market has largely transitioned to electronic soft starters. If the motor starting method is soft starter, various current limit options are available. The current limit value is an adjustable parameter within most soft starters. The higher this setting, the higher the potential skVA load step (assuming a voltage stepped configuration) and the higher the momentary harmonic currents. Soft starter current limit settings typically vary with the load characteristics and desired motor acceleration time, but values in the 300% to 350% are common. The harmonic content will default to an appropriate value to match the current limit setting.
Two options are provided relative to how the soft starter builds output voltage: voltage stepped and voltage ramped. Most industrial soft starters support both modes. Voltage ramping gradually builds output voltage thus producing little to no skVA and skW load steps. This is the ideal configuration for a soft starter on a generator; however, this mode of operation may not be available on your particular soft starter. In the voltage stepped configuration, the soft starter steps voltage quickly up to the current limit point. This results in a skVA and skW load step in the system. This can be a significant disadvantage when coupled with a limited voltage dip tolerance. This is the case with the soft starter often utilized on hydraulic elevators.
Variable Frequency Drives
Creating the flexibility to operate a motor at various speeds, variable frequency drives (VFD) have become extremely popular in many applications. From a generator sizing standpoint, the generator powers the input rectifier stage of the VFD.
Power Design Pro has various rectifier and filtering options in support of the VFD motor option. The VFD's rectifier configuration determines the device’s harmonic current spectrum and total harmonic content. The most common type of input for three-phase drives is six-pulse, non-filtered (default value). Though this is common in general equipment drives, many engineers in municipal pumping applications will incorporate harmonic mitigation in the system design. If harmonics have been mitigated, select one of the lower (less than 15% THID) options.
One of the major operational differences between VFD’s and soft starters is the impact that harmonics have on the system. VFD’s produce harmonics all the time where soft starts only produce them while starting the motor. Assuming natural or planned motor start-up sequencing, harmonics from soft starts are typically not an application issue. Given the high harmonic current content of unfiltered drives, it is essential that the generator sizing program perform harmonic analysis. Power Design Pro’s comprehensive harmonic analysis capabilities stand alone in the competitive market.
Advantages of Power Design Pro
Power Design Pro is a highly sophisticated generator sizing program that allows users to benefit from the advanced logic involved in the default selections, while also providing the flexibility to customize user inputs to precisely match the given real world situation.
The combined aspects of ease-of-use and powerful technological design makes Power Design Pro a trusted solution among consulting engineers and system designers.
Click here for more information about Power Design Pro and its many capabilities.