Motor starting is an important issue which must be considered when applying a generator set. The high current that motors draw when starting causes voltage dips in the system. This may require oversizing the generator or applying motor starting techniques maintain this voltage dip at acceptable levels for your system and its attached components.
Motors, either loaded or unloaded, draw several times rated full load current when starting. This is termed locked rotor current or starting kVA (SkVA). SkVA is calculated from locked rotor current like this:
SkVA = V x A x 1.732 / 1000
In-rush current to the motor causes a rapid drop of generator output voltage. In most cases, a 30 percent voltage dip is acceptable, depending on the equipment you already have on line. The degree of dip must be identified by an oscilloscope, since mechanical recorders are too slow.
Motor Starting Options
If you have an application where motor starting is a concern, consider the following:
- Change the starting sequence, with largest motors first. More SkVA is available, although it does not provide better voltage recovery time.
- Use reduced voltage starters. This reduces kVA required to start a giver motor. If you’re starting under load, remember this starting method also reduces starting torque.
- Specify oversized generators.
- Use wound rotor motors, since they require lower starting current. Wound motors typical cost more, however.
- Provide clutches so motors start before loads are applied. While SkVA demand is not reduced, the time interval of high kVA demand is shortened.
- Improve the system power factor. This reduces the generator set requirement to produce reactive kVA, making more kVA available for starting.
- Use a motor generator set. A motor drives the generator which, in turn, supplies power to the motor to be started. This system runs continuously and current surge, caused by the starting of the equipment motor, is isolated from the remainder of the load.
Watch Voltage Dip
Motor starting is affected by motor and generator design, and load on the motor. Initial voltage dip depends mostly on motor and generator windings. Addition of series boost to the regulator or use of a permanent magnet exciter will not significantly decrease this dip.
The magnitude of voltage dip tolerable depends on the type of equipment on line. Motor starting contactors may open if voltage drops below 65 percent of rated.
Motors draw starting currents to eight times normal running current. Preloads on motors do not vary maximum starting currents but do determine time required from motors to achieve rated speed and current to drop back to normal running value. If motors are excessively loaded, they may not start or may run at reduced speed.
Motors connected directly to high inertia centrifugal devices or loaded reciprocating compressors will cause severe frequency excursions and lengthy motor run up. Comparing starting current between loaded and unloaded motors shows the extended time loaded motors demand high current.
When specifying motor loads, skVA and acceptable system voltage dip to a gen set specifier, it is important to ensure that you are evaluating the instantaneous voltage dip of the generator’s voltage on the system. Instantaneous voltage dip is the actual maximum voltage dip experienced when a motor load is applied.
Some gen set manufacturers state voltage dip in terms of “sustained” voltage dip. Sustained voltage dip is calculated by ignoring the actual voltage dip for the first few cycles of motor starting, where the voltage dip is highest, and then averaging the voltage dip during part of the recovery period. This significantly understands the actual voltage dip any equipment attached to the system experiences and may cause system and equipment problems due to higher instantaneous voltage dip.
If you have motor starting questions, please contact us. We can help you avoid serious equipment problems.