Proper Generator Sizing 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.
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).
Figure 2. Typical Code Letters for Various HP
Motors
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.”
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.
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.
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 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:
1. 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.
2.
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.
3.
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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