Points to Consider When Sizing Generator Sets for Motor Starting

How Motor Starting Affects Generator Sets?

Motor starting can have a significant impact on generator sets. When a motor starts, it draws a large amount of current from the generator set, which can cause a voltage dip or even a momentary loss of voltage. This voltage dip can lead to voltage-sensitive loads such as computers, electronic equipment, and lights flickering or shutting off.

If a motor is started using normal utility power, the high inrush current will only cause a minor voltage dip as the utility is a more resilient voltage source. However, when starting a motor using generator set power, the high inrush currents (measured in kilovolt-amperes or KVAs) can lead to a significant voltage dip that may prevent the motor from reaching its operational speed.

The inrush current during motor starting is typically six times the motor's rated full-load current, and remains high until the motor reaches about 75 percent of its rated speed.

Figure 1. Motor Lock Rotor Value | Source: NEMA

The challenge, then, is to size the genset to handle the motor-starting load, but also Manufacturers Association (NEMA) sets design standards for motors and has established a NEMA code-letter designation for classifying motors according to the ratio of locked-rotor KVAs (LRKVAs) per horsepower. These code letters range from A to V, covering motors with an LRKVA-per-horsepower ratio of 3.14 or less to a ratio of 22.4 LRKVA-per-horsepower or more (see Figure 1).

For example, a 50 hp Code F motor requires 279.5 LRKVA per horsepower upon starting (50 hp x 5.59 LRKVA per hp = 279.5 LRKVA/hp). LRKVA is also known as “starting KVA” or “SKVA.”

Figure 2. Typical Code Letters for Various HP Motors

Voltage Dip

A voltage dip, also known as a voltage sag, is a short-term reduction in the voltage level of an electrical system. It can occur when there is a sudden increase in the demand for electrical power, such as when starting an electric motor.

Figure 3. Voltage Dip

Voltage dips also reduce the torque a motor can supply to its load. A common NEMA Design B motor will develop 150 percent of rated full-load torque during starting. Torque is proportional to the KVA delivered to the motor, so a 30 percent voltage dip that reduces KVA to 49 percent also reduces torque to 49 percent of its rating. If the motor starts unloaded – as most fans, centrifugal pumps and motors used with elevators do – this torque reduction produces no problem other than a somewhat longer acceleration time. Other types of loads, such as positive displacement pumps, may require more torque than the motor can develop at reduced voltage, which prevents the motor from reaching full speed. Additional consequences could include tripping of breakers or overheating of the motor.

Figure 4. Motor Starting Current

Motor Starting and Voltage Dip

When an electric motor is started, it requires a large amount of electrical power to overcome the inertia of its rotating parts and begin moving. This sudden increase in power demand can cause a momentary voltage dip in the electrical system supplying the motor. The voltage dip can cause problems for other equipment connected to the same electrical system, as it may cause them to malfunction or shut down.

An excessive voltage dip can lead to the malfunctioning of control relays or magnetically held motor starting contactors, which can result in the motor failing to start altogether. If the relays or contactors drop out, the load is removed from the generator set, causing the voltage to increase rapidly, which can cause contactors to become damaged if allowed to continue. While most control relays and motor-starting contactors can tolerate a voltage dip of up to 35 percent, there are some exceptions. Certain relays or contactors may start to vibrate if subjected to a voltage dip as low as 20 percent. Additionally, it's important to account for other voltage-sensitive loads like UPS systems, medical equipment, and HID lighting when determining the appropriate size of the generator set. To ensure the proper operation of a standby power system, it's advisable to check the voltage/frequency limitations of control components from the manufacturers or suppliers.

Motor Starting Can Reduce Voltage Dip

When sizing generators for motor starting, applying different type motor starting method aside from full voltage starting is necessary. The downside of full voltage starting in electric motors is that it can cause a high inrush current, which can lead to a range of issues.

First, a high inrush current can result in voltage dips in the power system, which can cause other equipment to malfunction or even trip circuit breakers. If the voltage dips are severe, they can cause the motor to stall, which can damage the motor or connected equipment. Second, the high inrush current can cause excessive mechanical stress on the motor and connected equipment, which can lead to premature wear and failure. This can result in higher maintenance costs and reduced equipment lifespan. Lastly, the high inrush current can also result in high energy consumption and demand charges, as the electrical system needs to supply a large amount of power to the motor during the startup period. This can result in higher operating costs for the equipment.

To avoid these downsides, various techniques are used to limit the inrush current during motor starting, including soft starters, variable frequency drives, part winding, wye-delta, and reduced voltage starting methods. These methods can help to reduce the mechanical stress and energy consumption during motor starting, as well as prevent voltage dips in the power system.

Figure 5. Full Voltage Starter vs. Wye-Delta Starter

Figure 5 has shown the comparison between full voltage starter and wye-delta starter. Full voltage starting, as the name suggests, involves applying full voltage to the motor windings during the starting period. This results in a high inrush current, which can cause voltage dips, excessive mechanical stress, and high energy consumption. However, full voltage starting is a simple and cost-effective method that can be used for small and medium-sized motors. On the other hand, wye-delta starting is a reduced voltage starting method that involves starting the motor with reduced voltage in a wye configuration and then switching to full voltage in a delta configuration once the motor is up to speed. This reduces the inrush current and associated issues like voltage dips and high mechanical stress.

Fundamental Criteria for Motor Starting

Regardless of what sizing method is used or how manufacturers specify motor-starting performance, the following fundamental criteria for motor starting must be accomplished – and in the following sequence – to successfully start a motor:

Sufficient LRKVA at the instantaneous voltage dip for inrush current – The required LRKVA at the maximum permissible instantaneous voltage dip is considered to be the first step for motor starting by most genset and alternator manufacturers. Typical motors are designed to sustain a 30 to 35 percent instantaneous voltage dip before the motor-starting contacts drop out. Many specifying engineers prefer a maximum 20 percent instantaneous voltage dip limit to ensure the motor will start and hold in the starting contacts.
Sufficient genset torque and power – Next, the torque available from the genset must exceed the torque required by the motor load, or the motor will stall or never start.
Sufficient alternator excitation system strength – The genset must have sufficient excitation system strength and adequate response to accelerate the motor and return it to operational voltage and speed. This third and final step addresses voltage recovery

Conclusion

Proper genset sizing is critical for reliable and efficient motor starting. If the genset is too small, it won't be able to start the motor, and if it's too large, it can result in excessive costs and inefficiencies. Using proven genset-sizing software can help to ensure that the genset is appropriately sized for the specific application, taking into account all the relevant factors and dynamic conditions. By doing so, the genset can be optimized for reliable and efficient motor starting, leading to cost savings, increased equipment lifespan, and improved performance.

For this reason, when using a genset to power motor-starting loads, the interaction between the two is dynamic and complex. To ensure the most reliable and accurate results, the genset needs to be viewed as a system that includes the engine, alternator, voltage regulator, excitation system, and motor starters. It's also crucial to take into account dynamic conditions such as system inertia, motor loading, motor type, and genset preload. By analyzing the dynamic system and evaluating the functions in real-world applications, specifiers can gain a better understanding of how to predict motor-starting performance more consistently and reliably.

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Title: Sizing gensets for motor starting: A practical guide to understanding how motor-starting loads affect genset performance
Source: KOHLER Power Systems

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Points to Consider When Sizing Generator Sets for Motor Starting