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Calculation #2:150 hp x 8.5 kVA/hp = 1,275 kVA = 1,534 amps @ 480 volt/3 phase (amps during start

(150 hp x 0.746 kW/hp) / 0.91 (Efficiency) = 123.0 kW (running kW)

123.0 kW / 0.91 (power factor) = 135.1 kVA = 162.5 amps (represent full-load amps)

1,534 amps (amps during starting) / 162.5 amps (full-load amps) = 9.4 (times full-load current)

This illustrates that the starting of motors can dramatically affect the inrush current and associated sKVAand may exceed the maximum sKVA or the sKW of a generator set that would otherwise be large enoustate load. This could require an oversized generator set based solely on the motor starting requiremensystem.

To clarify this issue, I will use an example with the same load profile but with two different methods of msimple examples will include lighting and miscellaneous loads as well as motor starting with an across texample and a solid-state starter in another example.

Example #1: Motor with an across the line starter:

Motor Load:150-hp motor, NEMA F with a 0.28 starting power factor

Running power factor of 0.91 and an efficiency of 0.91.

NEMA Code Letter F = 5.3 kVA/hp

sKVA= 150 hp x 5.3 kVA/hp = 795 kVA

sKW= 795 kVA x 0.28 (starting power factor) = 222.6 kW

Running kVA = 123.0 kW / 0.91 (power factor) = 135.1 kVA

Running kW = (150 hp x 0.746 kW/hp) / 0.91 (Efficiency) = 123.0 kW

Lighting Load:75 kVA at 0.9 power factor

sKVA= 75 kVA

sKW= 75 kVA x 0.9 power factor = 67.5 kW

Running kVA = 75 kVA

Running kW = 75 kVA x 0.9 power factor = 67.5 kW

Miscellaneous Load:50 kVA at 0.9 power factor

sKVA= 50 kVA

sKW= 50 kVA x 0.9 power factor = 45 kW

Running kVA = 50 kVA

Running kW = 50 kVA x 0.9 power factor = 45 kW

System Totals:

sKVA =795 + 75 + 50 = 920 kVA

= =

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Running kVA = 135.1 + 75 + 50 = 260.1 kW

Running kW = 123 + 67.5 + 45 = 235.5 kW

Alternator kW = 123 + 67.5 + 45 = 235.5 kW

Using one manufacturers sizing software, the recommended generator set size is 350 kW. This is basetransient voltage dip followed by a sustained recovery of 90% of rated voltage during starting. The gene

67% of capacity (Running kW = 236, Generator Capacity = 350 kW, 236 / 350 = 67.4%).

Large motors that are started with across-the-line starters fed by generators to allow for very low transiestarting can require a greatly oversized generator set. In these cases the running capacity of the generalower than the rating of the generator set. It is critical to ensure that the running load represents at leastof the generator set or wet stacking or carboning can occur. See Part 7for definitions and a discussion

Example #2: Motor with a solid-state starter with bypass contactor:

Motor Load: 150-hp, NEMA F motor with a 0.28 starting power factor

Running power factor of 0.91 and an efficiency of 0.91.

Soft Start set at a 300% full load ampere current limit. The current limiting range is typically between 15current limit reduces the starting kVA and starting kW by almost 50%.

sKVA,= (150 hp x 0.746 kW/hp) / 0.91 (efficiency) = 123.0 kW

123.0 kW / 0.91 (power factor) = 135.1 kVA = 162 amps @ 480-volt/

3 phase.

300 % current limit = 162 amps x 3 (3 x FLA) = 487 amps = 405 kVA

sKW= 405 kVA x 0.28 (starting power factor) = 113.4 kWRunning kVA = 123.0 kW / 0.91 (power factor) = 135.1 kVA

Running kW = (150 hp x 0.746 kW/hp) / 0.91 (Efficiency) = 123.0 kW

Lighting Load:75 kVA at 0.9 power factor

Starting kVA = 75 kVA

Starting kW = 75 kVA x 0.9 power factor = 67.5 kW

Running kVA = 75 kVA

Running kW = 75 kVA x 0.9 power factor = 67.5 kW

Miscellaneous Load:50 kVA at 0.9 power factor

Starting kVA = 50 kVA

Starting kW = 50 kVA x 0.9 power factor = 45 kW

Running kVA = 50 kVA

Running kW = 50 kVA x 0.9 power factor = 45 kW

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System Totals:

sKVA =405+ 75 + 50 = 530 kVA

sKW= 113.4 + 67.5 + 45 = 225.9 kW

Running kVA = 135.1 + 75 + 50 = 260.1 kW

Running kW = 123 + 67.5 + 45 = 235.5 kW

Alternator kW = 123 + 67.5 + 45 = 235.5 kW

Using one manufacturers sizing software, the recommended generator set size for this example is 275about a 20% transient voltage dip followed by a sustained recovery of 90% of rated voltage during startrun at about 86% of capacity (Running kW = 236, Generator Capacity = 275 kW, 236 / 275 = 85.8%).

At this threshold, the engineer may want to specify the next larger generator set to allow for some futureclear from this example that reducing the sKW requirements of the motor with the use of current limitingsize of the required generator set.

The solid-state starter will cause voltage distortion across the alternator of the generator. This distortionnonlinear way the silicon-controlled rectifiers (SCRs) in the solid-state starter draw current. The generato be oversized to compensate for this voltage distortion. This issue can be avoided, as in the example bypass contactor with the sold state starter. The bypass contactor closes after startup and the SCRs arthe starting of the motor. If the solid-state starter does not have a bypass contactor, a rule of thumb is torunning kW to the running kW of the system. This calculation will estimate the total alternator kW. See c

Alternator kW with a bypass contactor: 235 kW

Alternator kW without a bypass contactor: 235 kW + 123 kW = 358 kW

If the soft starter does not have a bypass contactor, the engineer must determine if a larger alternator is

example above, the same 275-kW generator can handle either case (with and without a bypass contacalternator is required to handle the additional alternator kW if no bypass contactor is specified.

In our example above, in the across-the-line starter situation, the sKW drove the requirement for the larBelow is a breakdown of some of the critical parameters for the different starting methods as well as thesizes noted above. Three total generator set configurations have been noted below, one for 350 kW an275-kW generator set has been split into a smaller and a larger alternator. The parameters (sKVA, sKWnoted under the three generator set configurations are the maximum the generator set can provide. Theunder the form of motor starting are the requirements for the different system examples noted above wiof motor starting configuration.

Chart 1:

Small AlternatorLarger Alternator

350 kW GensetSolid-State Startwith no Bypass

Startwith Bypass

1896

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291 293 335*

300 358**

** : The solid state starter without a bypass contactor exceeded the alternator kW of thesmaller alternator. Therefore, a 275-kW generator set with a larger alternator is required forthis starting configuration.

Several factors should be evaluated prior to determining the type of starting for motors within an electricThese factors include, but are not limited to the following:

%%POINT%%Electrical system effects from not providing some form of reduced voltage starting. How current affect the components in the electrical distribution system?

%%POINT%%A cost analysis of providing alternative forms of starting should be performed. It may be provide a solid state starter, or other form of reduced voltage starter, with a smaller generator set than tline starter with a larger generator set.

All applicable utility or jurisdictional requirements have to be evaluated during the design process of the

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