Sizing a Generator for Electric Motors

On a site supplied by a generator, most problems arise not from the continuous load but from the moment motors start. A pump that draws 30 kVA when running needs several times that in its first seconds. If the generator cannot meet this sudden demand, the voltage drops, contactors drop out, the motor fails to start or other equipment on the same line stops. In this article we explain step by step how to check generator power for motor loads. For the general steps of adding up loads, see our article What size generator do I need?

What happens at start-up?

When an induction motor is at standstill, its rotor is stationary and its windings behave almost like a short circuit. It therefore draws 5–8 times its rated current at the first moment. This current has a low power factor (about 0.3–0.4), so what is demanded from the generator is largely reactive power. Reactive power is supplied by the alternator, while the engine has to respond to the sudden load increase so that speed does not fall.

As a result, a motor start has two effects:

• Voltage dip: depends on the response time of the alternator and the automatic voltage regulator (AVR).
• Frequency dip: depends on the load acceptance capability of the diesel engine and its speed governor.

From the motor nameplate to starting kVA

Three items from the motor nameplate are enough for the calculation: power (kW), efficiency and power factor. Running kVA is found approximately as follows:

Running kVA = kW ÷ (efficiency × power factor)

For example, for a 22 kW motor with an efficiency of 0.92 and a power factor of 0.85, running kVA is about 28 kVA. Starting kVA is a multiple of this value depending on the starting method:

Starting method	Starting current (relative to rated)	Approximate starting kVA for a 22 kW motor
Direct-on-line (DOL)	6–7 times	170–200 kVA
Star-delta	about 2–2.5 times	55–70 kVA
Soft starter	about 2.5–3.5 times (depending on setting)	70–100 kVA
Variable frequency drive (VFD)	about 1–1.5 times	28–42 kVA

These values are for guidance; the exact starting value is taken from the locked-rotor data given by the motor manufacturer. The striking point in the table is that, for the same motor, direct-on-line starting demands about five times more instantaneous power from the generator than a drive.

Acceptable voltage dip

When selecting a generator, the question is not “will the motor start?” but “how far will the voltage drop while it starts?” Common limits in practice are:

• General motor loads: 15–20%
• Lifts and contactor-based control circuits: around 15%
• Motors on the same busbar as sensitive electronic loads: 10% or less

If the voltage falls by more than 25–30%, contactor coils may drop out; the motor cannot start and keeps trying to cut in again. Generator data sheets give the motor starting kVA the alternator can deliver at a given voltage dip. In selection, this value is compared with the starting kVA of the largest motor.

Sequential starting: the same job with a smaller set

If there are several motors, they do not all need to start at once. With timer relays added to the control system or with the outputs of the controller, motors can be brought in 5–10 seconds apart. The rule is simple:

1. Start the largest motor first.
2. Order the other motors from largest to smallest.
3. At each step, check the sum of the load already on the generator and the starting kVA of the next motor.

Example: an irrigation plant has three pumps of 30 kW, 15 kW and 7.5 kW, all started direct-on-line. If all three pumps start together, the instantaneous demand exceeds 400 kVA. With sequential starting, the most critical step is the start of the 30 kW pump, at about 230–270 kVA. If the 30 kW pump is given a star-delta starter, the most critical step becomes the start of the 15 kW pump while the 30 kW pump is running, and demand falls to about 150–170 kVA. The set size is therefore reduced considerably. This calculation is approximate; on a real project it should be confirmed with the motor nameplates and the set data sheet.

Drive-fed loads and harmonics

Variable frequency drives largely solve the starting problem but bring a new issue: harmonic currents. The current drawn by a six-pulse drive is not sinusoidal. Because the internal impedance of a generator is higher than that of the grid, harmonics distort the voltage waveform more. Points to watch:

• If more than half of the total load is drive-fed, the alternator can be selected one frame size larger.
• A line reactor or harmonic filter can be used on the drive side.
• Alternators with a permanent magnet generator (PMG) excitation option maintain more stable regulation under distorted voltage.
• On sites dominated by UPS systems and drives, the AVR setting and the measuring accuracy of the controller should be checked during commissioning.

Checklist for motor loads

• Have the power, starting method and starting order of each motor been listed?
• Has the starting kVA of the largest motor been compared with the starting kVA on the set data sheet?
• Has the acceptable voltage dip been set according to the load type?
• Is simultaneous starting really necessary, or is sequential starting possible?
• Have the share of drive-fed loads in the total load and their harmonic effect been assessed?
• For critical motors such as fire pumps, have the requirements of the relevant standard been checked separately?

Next step

Enter your load list in the power calculator to see a first estimate straight away. For projects dominated by motor loads, send us the motor nameplates and the starting order with your quotation request, and we will select the alternator frame and the controller of the set for these loads together with you. For suitable models, see our open and canopied generator set pages.