Technical notes of interest to Marine Engineers

Sizing an Alternator

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Motor starting considerations when sizing an alternator

Electric motor starting must be reviewed when sizing a generator set. Electric motors typically draw several times full load current during startup which causes voltage dips in the system. Excessive voltage dips may cause equipment to malfunction, fall offline or operate at risk of damage. Acceptable voltage dips vary among different installations and depend on the type of load installed. A 30% voltage dip is sometimes used as a rule of thumb. Motor starting aids or oversizing of the generator may be required to achieve acceptable voltage dip levels for your system. Keep in mind that the in-rush current to the motor during startup will create a very rapid drop in voltage at the generator and recording the magnitude of this drop may require an oscilloscope.

The current consumption of electric motors during startup is termed "Locked Rotor Current" or "starting kVA (SkVA)". SkVA is calculated:

SkVA = V x A x 1.732 / 1000

Motor Starting Options

The following suggestions may help accomodate motor loads into your system:

  1. Starting large motors first takes advantage of more available SkVA from the generator.
  2. The use of reduced voltage starters reduces kVA required to start a given motor. Keep in mind, however, that torque will be reduced as well, which may be detrimental if starting the motor under load.
  3. Specify oversized generators to handle the required SkVA upon motor startup, or select generators designed for motor starting, such as the Marathon LimaMac.
  4. Wound rotor motors require lower starting current, although they typically cost more.
  5. Allow motors to start quickly and come up to speed before loads are applied by using clutches. This will reduce the length of time that the motor imposes the high current surge.

Things to Keep in Mind

  1. Voltage dip level depends on motor and generator windings. The addition of series boost to the regulator or the addition of PMG (permanent magnet generator) excitation does not contribute to better motor starting characteristics.
  2. Motor starting contactors may drop-out (open) if voltage drops below 65% of nominal level.
  3. Excessively loaded motors may not start or could run at reduced speed, resulting in overheating.
  4. Instantaneous voltage dips are different than sustained voltage dips. The latter ignores the maximum initial voltage drop that occur during the first few cycles of motor starting. These are the critical instants that should be considered when sizing generator.