Pumps and generators

Considerations for proper hardware selection, from the engine manufacturer’s perspective, to achieve a successful engine-driven generator application.

GENERATOR BASICS

• A generator may be either AC (Alternating Current) or DC (Direct Current).
• AC generators are also called alternators and are now so common that “AC generator” and “generator” have become synonymous terms.

An AC generator produces electrical current by passing a conductor through a flux field. On 15KW and smaller generator sets, permanent magnets are typically used for the flux field, while larger sets use a DC current in field windings. The AC generator’s output current is generated when the armature windings cut the flux field. Regulating the DC current in the field windings, controls voltage.

Early AC generators used a fixed field and a rotating armature. The modern AC generators use rotating field windings with a stationary armature to produce output current. The rotating field is called the rotor and the stationary armature is the stator. An exciter generates the DC current for the rotating field. The exciter can either be static or rotating.

SIZING GENERATORS

Three ways of sizing a generator set are: Peak Load, Motor Starting and Transient Response.

• When sizing for Peak Load, the installer calculates the maximum kilowatt electrical load and selects a generator set with equal or greater kilowatt capacity.
• To size an engine for Motor Starting, the installer determines the maximum motor starting current (inrush current) together with any other loads and selects a generator set with equal or greater kilowatt capacity. Since typical motor starting current is five times running current, maximum voltage dip occurs during motor starting. Smaller generator set sizes can be used when motors have sequential starting or use reduced voltage starting. Sequential starting spreads out the inrush load of several motors, while reduced voltage starting lowers individual motor starting current. Transient response sizing is based on the ability of the generator set to accept a load and recover to normal frequency and voltage within a specified time.
• Transient response is typically checked by block loading the set with a load bank.

The customer must specify the limits for maximum voltage dip, recovery time and power factor.

POWER FACTOR

When an AC circuit is composed of a pure resistive load, the current wave is in phase with the voltage wave. With a pure inductance load the wave lags the voltage wave by 90 degrees. A pure capacitance load will have a current wave that leads the voltage wave by 90 degrees. The inductance and capacitance waves do not consume engine power, but they do increase the current flow in the generator and are measured by the ammeter. For this reason inductance and capacitance are sometimes called “reactive” power or “wattles” power.

Watts or kilowatts are real power. With a resistive load, watts equal volts times amperes. If inductance or reactance is in the load, the current will be higher, so multiplying volts times amperes gives a result that is more than the real power. Therefore volts times amperes is called apparent power and is measured in kilovolt-amperes or kVA. Real power (kW) divided by apparent power (kVA) is the power factor or PF. Technically PF equals cosine q, where q is the phase angle between the voltage wave and the current wave.

• kW = Real Power= kVA x PF
• kVA = Apparent Power
• PF = kW/kVA = Real Power/Apparent Power

EXCESS CAPACITY

The typical generator set runs at less than its full capacity most of the time. As the load decreases, the operating efficiency of the engine decreases which increases cost per produced kilowatt. This is not a major concern for Standby generator sets. However, the increased cost of operating Prime and Continuous systems at decreased loads can be significant.

Part load operation also allows unburned fuel to gather in the engine exhaust and lubrication systems. This type of operation can result in unsightly leakage from the exhaust system, as well as increased maintenance costs. An oversized engine will more likely have these problems. A generator set operates best from 50% to 90% of full rated load. Long-term operation at less than 30% of full load is not recommended.

SINGLE PHASE AND THREE PHASE POWER

• Single phase (1f) AC consists of either one or two voltages in series with exactly the same phase relationship. Single phase AC supplies power over two or three lines.
• Three phase (3f) consists of three separate voltages spaced 120 electrical degrees apart, using three lines plus a neutral line. The phases are usually given letter designations (Phase A, Phase B, Phase C). Not two phases are at zero voltage at the same time. No two phases are at peak voltage at the same time.

EXERCISING

Standby generator sets are exercised on a regular basis to ensure readiness when needed. Exercising for short periods of time with no load is detrimental to the engine. The engine is best exercised when run for 30 minutes or more with 50% to 75% load. This allows the engine to run at normal operating temperatures.

BALANCED THREE PHASE LOADS

Generators should have the resistive and inductive loads balanced on each phase. A phase imbalance of more than 5% will cause unstable voltage regulation. This problem cannot be corrected with engine or generator adjustments. The distribution circuits should be rearranged until balance can be achieved.