Generate Clean Jobsite Power

by Curt Bennink
Created: December 31, 2008

Not all generator sets are created equal.

To choose the right size generator you need to calculate the total wattage demand, then calculate the surge rating based on the motor loads.
To choose the right size generator you need to calculate the total wattage demand, then calculate the surge rating based on the motor loads.
Clean power with minimal voltage or frequency variation and without harmonics will extend tool life.
Clean power with minimal voltage or frequency variation and without harmonics will extend tool life.
Make sure all cables are the right gauge and length for the job, or tool life could be affected by voltage drop.
Make sure all cables are the right gauge and length for the job, or tool life could be affected by voltage drop.

?In mobile generator applications, where you have bigger units and more sophisticated control technology, we try to hold it to a half percent of voltage variation,? says Howe. ?A quarter percent frequency regulation is about the best you can do nowadays. The little portable gas- and diesel-powered machines still have mechanical governors and carburetor technology. Typically, you are looking at +3% to -5% nominal speed on those.?

Larger generators with more extensive electronic controls will provide better power quality. ?If you need precision power that closely approximates what we are used to getting from the utility grid, step up to a towable machine,? says Howe. ?The portable machines are going to have way too much voltage and frequency variation.?

Single phase vs. three phase

Both single-phase and three-phase generators are commonly found on jobsites.

?Single-phase generators contain the standard voltage that is delivered to most homes ? either 120 or 240 volt,? says Spears. ?They are adequate for running resistive loads, low power draw applications and machines with simple electric motors. Three-phase generators are more efficient than single phase because the resulting line currents are lower. Three-phase generators are used for higher power applications where losses due to inefficiencies are significant. Three-phase generators may be found running pumps, industrial motors, batch plants and other large scale electrically powered equipment.?

Three-phase power delivers greater energy density. ?A typical three-phase generator is like having three single-phase generators combined into one,? says Bernier. ?Almost three full voltage sine waves are produced in the same time period it takes for one sine wave to come from a single-phase generator. These sine waves overlap each other slightly and follow one another 120° apart. This allows three-phase motor manufacturers to install three windings in electric motors (5 hp and larger) to provide more power in a smaller package.?

The differences between single-phase and three-phase generators are not visually apparent. ?The difference basically has to do with the construction of the alternator,? says Howe. ?A single-phase alternator has one set of windings that produce one sinusoidal AC sine wave of voltage. A three-phase alternator has three independent windings that each produce an independent sine wave.?

Larger generators, such as towables, typically offer three-phase power. ?Usually, when you are getting into three-phase power, you are looking at applications that have higher amperage requirements,? says Howe. ?For example, you can get single-phase motors up to about 5 hp. Once you get 7 to 10 hp, you get single or three phase. Everything 15 hp and up is three phase.?