In the context of my job, I sometimes review sizing files to determine whether the recommended gen-set can be replaced by a smaller gen-set. Opportunities for downsizing the specification are possible at times. For example, I had a case in which paralleled 2500kW gen-sets have been replaced by a single 3000kW gen-set. Most recently, a review of a customer sizing file and its specifications, and utilization of some of the competitive sizing options in our gen-set sizing tool, resulted in reducing eight paralleled 3000kW gen-sets to six 3000kW gen-sets.
One of the competitive sizing options that I have used is reduced voltage starters for motors. The following examples provide information on reduced voltage starters and how they reduce the kW/kVA required for a gen-set’s loads. I am interested in applications in which you’ve been successful using reduced voltage starters versus across-the-line starting, and the costs of the reduced voltage starters you specified or installed with your gen-set. As you can see, reducing a customer application by one gen-set can provide a significant financial benefit.
Full voltage motor starting (across-the-line or direct on line) is the most severe starting method, but may be required by specification. A typical motor will draw six times its full-load current on an across the line start. When a motor is energized by a full voltage starter, mechanical and electrical problems can occur. Potential mechanical problems include broken belts, gears or couplings. Potential electrical problems include deterioration of motor insulation or brown-outs causing nuisance problems with other electrical equipment.
A reduced voltage starter helps to lessen these problems by reducing inrush line current and/or starting torque of a motor applied to the drive load. This is done by either reducing voltage applied to the motor during starting or by using only part of the motor windings during starting. Voltage reduction starters will reduce a generator sets starting requirements (SkW/engine and SkVA/alternator). However, there is a cost-benefit trade-off between the cost of the voltage reduction starter and the sizing of the generator set. Various types of voltage reduction starters and their starting torque % of full voltage starting torque are illustrated below:
Reduced Voltage Starters
* Variable Frequency Drive (60 Hz)
** Variable Speed Drive (50 Hz)
The Current Limit (C.L.) % is adjustable for both soft starter and VFD/VSD. Increasing C.L.% directionally increases the size of the generator set.
Here is an example sizing of a one-step load scenario with two 420 HP motors and one 150kW resistive load that indicated the following kW/kVA reduction in the gen-set nameplate rating:
Reduced Voltage Starters
1-Step Load Scenario: (1) 100 HP Motor, (1) 100 kW Resistive load
- What is your field application experience with voltage reduction starters?
- What are the applications you commonly use for each type of voltage reduction starter?
- As a consulting/specifying engineer, what type of voltage reduction starter would you recommend to your client to reduce the nameplate rating for the generator set you will specify for their site application?
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