Power and Energy

Why track power demand?

It’s not just how much energy you use, but when you use it and how you use it that effects your electric bill and your cost. Forget trying to make sense of your utility bill – here are some of the main drivers that hit your wallet:

Time of day

You pay more for electricity in the daytime then at night.

Can you schedule some of your usage for at night? Municipalities take advantage of this by pumping their water towers full at night when the power’s cheaper. Maintenace items like recirculation, filter clean (backwash), or most any non-staffed function should be able to be scheduled to take advantage of lower cost power

 

Peak demand


You are billed in incremental periods, usually 10-20 minutes by your utility company. Your demand is set by the maximum power you use in any of these periods in a billing cycle. Even if you use a large amount for only 20 minutes a month your bill will reflect an additional and costly surcharge for the entire month. This peak demand charge is in addition to the baseline energy used.

Avoid or try to minimize peak demands. If you have multiple pieces of equipment that start and stop periodically, like compressors, ovens, HVAC’s, etc and you can stagger the starting times, that is don’t allow them to start at the same time you’ll avoid exceeding the peak demand by spreading out the energy over a longer time period. Another option gaining popularity today is the use of demand shaving systems (DSS). DSS’s are basically a large energy storage device using battery or capacitor technology that senses when you’re nearing peak demand usage and supplies the additional energy form the storage bank instead of the utility. Its only purpose is the address the excess demand periods, which are typically short in duration. The bank slowly recharges in normal operation and ‘lies in wait’ for the next demand requirement.

 

Phase voltage and current unbalance

In a 3-phase power source each phase provides power to the loads connected to them. Some are three-phase loads (motors, heaters, HVAC) and some are single phase (lighting, outlets, etc). In a perfect system all the loads are equal, all are supplying the same current at the same voltage. However, real life systems are far from perfect. In reality the phase currents will vary based on loads turning on and off, and the voltage on each phase will vary at the load. It’s important to balance the system by distributing the loads on each phase.

The loss of one phase, or "leg," of a three-phase line causes serious problems for induction motors. The motor windings overheat due primarily to the flow of negative-sequence current, a condition that exists anytime there is a phase voltage imbalance. The loss of a phase also inhibits the motor's ability to operate at its rated horsepower.

Phase voltage unbalance causes three-phase motors to run at temperatures greater than their published ratings. This excessive heating is due mainly to negative-sequence currents attempting to cause the motor to turn in a direction opposite to its normal rotation. These higher temperatures soon result in degradation of the motor insulation and shortened motor life. The percent increase in temperature of the highest current winding is approximately two times the square of the voltage unbalance. For example, a 3 percent voltage unbalance will cause a temperature rise of about 18 percent.

Unbalance of a three-phase system is less extreme than a complete loss of phase, but may have similar consequences. On new installations of three-phase power systems, careful attention is given to balancing the loads on each phase. However, as single-phase loads are added to these originally balanced systems, an unbalance may occur. Thermal overloads, magnetic breakers, and other such devices will not detect this gradual unbalance and therefore will not provide adequate protection.

Power factor

In technical terms, if the voltage and current your facility demands isin- phase, that is the voltage and the current are exactly in ‘rhythm’ you have an ideal power factor of one (1). If this is you, congratulations – you are not paying one cent more for power than you really need to! The reality is, this almost never happens, for this reason – this only happens is with pure resistive loads – which are rare to non-existent.

Real world loads include the motors in your compressors, pumps, HVAC, lighting and many more. These loads as classified as inductive, which means the voltage and current you draw are no longer in phase. In lay terms, for example, if the power your equipment requires is out of phase by say 30% (which translates to your actual power factor of 0.7) you will need to draw 30% more power from the utility to compensate for this – and in turn will be charged 30% more as well. Various methods are available to help correct this including capacitors and other correction devices.

How to detect a problem?

With today’s rising utility costs, and the increasing demand put on the grid it’s imperative that you keep an eye on your utility costs, and more importantly how to maximize the money you pay into the energy you need.

By installing the Falcon™ monitor system from PhoenixSentry you’ll see within days what’s driving your utility costs. Problems such as phase unbalances, peak demand usages and power factor problems are displayed and reported in real-time via the easy to read gauges.

 

 

To download the Falcon data sheet, click here