By Neal Pillsbury 1998 U270 36′
It’s a little more complicated than just P=IE(PF). However, especially with motors, overvoltage conditions should be regarded with even more respect than under-voltage conditions.
In increasing orders of complexity, the following are some things that we would do well to remember:
1. Don’t place stress on electric motors and other electrical equipment as a result of staying connected to any power system at or near the ends of voltage limits. The best life and most efficient operation occur when you operate motors at voltages very close to the nameplate ratings. When supplying voltage to motors, stay as close as possible to 120 VAC ± 5% (114 to 126 VAC) and as far away from the “outer limits” of 120 ± roughly 10% VAC (104 to 132 VAC) however and whenever possible.
2. Rules of Thumb for High and Low Voltage
- The amount of work (Power) required is dictated by the load. Thus, for a given cooling demand, a motor on a Rooftop AC has a FIXED Power demand at any instant. Then, if the voltage goes down, the current must go up (Power = Current x Voltage x (PF)). The Power Factor will only help a small bit in an under-voltage condition. In an overvoltage condition, however, PF will increase exponentially once a motor core becomes saturated by the stronger flux lines.
- Small motors, like ours in our FT motorcoaches, tend to be more sensitive to over-voltage and saturation than under-voltage conditions.
- Single-phase motors like ours tend to be more sensitive to over-voltage than do large industrial motors.
- Overvoltage can drive up amperage and temperature, even on lightly loaded motors. Thus, high voltage can drastically shorten motor life, even on lightly loaded motors.
- Motor efficiency drops with either high or low voltage, and motors overheat more rapidly in the reduced efficiency condition.
- Motor power factor improves (helps) with lower voltage, but drops sharply with higher voltage due to motor core saturation induced by the stronger magnetic flux.
- Motor inrush current goes up rapidly with higher voltages.
3. Most of the time, unpredicted and/or unknown electrical power source faults do damage before we ever realize something is wrong. Therefore, because we frequently have the opportunity to experience damage, a high quality, automatic, the electrical monitoring system is very cost-effective.
I installed a 50 amp, permanently wired EMS system to protect against both shore and Power Tech Gen faults (the ’98 U270 had no EMS) and it has saved my bacon numerous times while coaches all around me, including FT’s, had varying amounts of damage. IMHO it has been well worth the investment and only one or two times out of a dozen, did I know that something had gone wrong before the EMS told me.