Why Generator Voltage Drops Under Load and How to Fix It
Generators are synonymous with ‘reliable power’…
They help our critical infrastructures, like hospitals, financial institutions, and businesses, keep functioning during a power crisis. Now, imagine the inconvenience they might cause should their power output drop. Some of the leading reasons why voltage drop occurs in generators under load have been discussed in detail here.
Common Reasons for Voltage Drop Under Load
Reason #1- Insufficient excitation (Field current)
The term ‘excitation’ simply refers to the process of supplying current to the field windings (the rotor) of the generator. Uniform magnetic field produced due to this process allows the generator to operate efficiently and engender constant voltage. In simple words... for the generator’s power output to remain stable under varying loads, balanced excitation is important.
If = Excitation current
E= Induced voltage
ᶲ = Magnetic flux
N= Number of turns in the stator windings
The generator's output voltage will fluctuate erratically if the components responsible for maintaining optimal excitation, such as the exciter or voltage regulator, fail.
How to fix insufficient excitation?
- Carefully inspect the excitation system components and rotor windings for any damage.
- Adjust excitation settings according to voltage requirement of the load.
Reason #2- Overloading the generator
Not selecting the right generator size may contribute to voltage instability among other performance issues. There is a limit to how much voltage a generator may produce. Overloading may cause the generator to work hard to meet the power demand of the connected loads… this may hinder the generator’s ability to perform smoothly and maintain consistent voltage.
So, what’s the solution?
- Select the right generator size… The following example can help you in this regard.
- Let’s imagine you have to power three loads: Device 1: 20kW, Device 2: 15kW, and Device 3: 10 kW.
Total load demand: 20 kW+ 15kW+ 10kW= 45kW
- Maintain a safety margin of 20% of the total load (20% of 45kW= 9kW) and take ‘starting surge’ into account. To calculate starting surge, multiply the motor’s rated power by 3. If the motor’s rated power is 1.5 kW, then the starting surge would be… 1.5 * 3= 4.5 kW. Add all these things up… and you have your generator size.
Generator size: 45kW + 9kW + 4.5 kW= 58.5 kW ≈ 60kW
Reason #3- Poor or loose connections
Wires and connectors linking the generator to the load may wear down or become corroded over time… which can introduce more resistance in the circuit. Added resistance will reduce the amount of current flowing through the system― impacting the generator’s output voltage. Heat buildup because of increased friction may introduce unwanted resistance to the circuit, contributing to voltage instability.
What’s the fix?
- Check wires and connections for any visible signs of corrosion, wear, or looseness.
- Properly tighten the bolts, screws, and nuts.
- Clean corroded terminals and apply a protective coating.
- Replace defective terminals and connections with high-quality substitutes.
Reason #4- Prime mover issues
How efficiently a generator will produce power depends greatly on the functionality of its prime mover… the engine or turbine. It is the central component of any generator unit, responsible for turning the generator’s rotor and inducing electric current in the stator. If it starts operating inefficiently, the generator’s voltage output will fluctuate erratically.
What’s the fix?
- Find out the culprit that has hindered the prime mover’s ability to operate smoothly and efficiently.
- Start by checking the fuel delivery, air intake, and cooling system components.
Reason #5- Load imbalance
Load imbalance is a leading reason for voltage drop in three-phase generators. Such generators work by providing three separate alternating currents in a cyclic manner… each phase offset by 120 degrees. Load imbalance resulted from uneven load distribution, improper wiring, and faulty equipment can lead to significant voltage drop, among other issues.
What’s the fix?
- There are a few things that you can do to balance the load across all three phrases.
- You can reconfigure the system wiring.
- Consider investing in load management systems or automatic phase selectors.
Wrapping Up
Some of the prominent reasons why your generator voltage might drop under load include improper excitation, overloading, loose connections, prime mover malfunction, and load imbalance (in three-phase generators). Using undersized cables, a weak voltage regulator, issues with the commutator and brushes may also result in voltage drop in generators.
