When a large motor starts, it draws much more current for a few seconds than during the rest of the shift. Generator sizing for an electric motor starts from this moment, and the required power is usually well above the sum of the nameplate powers of the motors in your plant.
This peak changes depending on how each motor starts and the order in which they come online. Read to the end to see how the type of startup affects the generator size and which data from your plant our engineering uses in this calculation.
Why motor startup defines the generator size
To start from zero, the electric motor needs to set the load in motion and also magnetize its own circuit. It is during this startup that the starting current appears. In our guide on how to choose the industrial power generator, we noted that large motors can require up to seven times the nominal current, which is the working current indicated on the nameplate.
The peak lasts only a few seconds, but the generator needs to deliver it without letting the voltage drop to the point of tripping the panel controls, inverters, or the motor itself. The nameplate power is the starting point, and the rest depends on how each motor starts and the order in which the loads come online.
kVA, kW, and power factor: what you need to have on hand before the calculation
On the motor nameplate, the power appears in kW or HP, while the generator is specified in kVA. The kVA calculation includes, in addition to the energy that turns into work on the shaft, the portion that the motor uses just to magnetize itself.
The power factor, printed on the same motor nameplate, shows how much of this total turns into work, and the lower it is, the more kVA the generator needs to deliver to move the same load. In a plant full of induction motors, kVA is what matters most in choosing the equipment.
For the calculation, gather three pieces of information from each important motor in the plant:
- Nameplate power, in kW or HP, and supply voltage.
- Nominal current and power factor, which appear on the nameplate or in the manufacturer’s catalog.
- Type of startup installed in the panel and how the motor operates throughout the shift.
Direct start, star-delta, soft-starter, and inverter: how each one changes the required power
Two plants with the same motors may need very different generators just because of the panel, as the startup method installed in it controls how much of the current peak reaches the generator.
| Type of startup | How it acts at startup | Effect on generator power |
|---|---|---|
| Direct start | Connects the motor with full network voltage | Demands the highest current peak and the largest kVA reserve |
| Star-delta | Starts with reduced voltage and switches to the final connection | Reduces the peak and eases the generator during startup |
| Soft-starter | Gradually increases voltage through electronic control | Smooths the start and allows adjustment of starting current |
| Frequency inverter | Controls frequency and voltage from zero speed | Keeps starting current close to nominal |
The soft-starter and frequency inverter significantly ease the startup but generate harmonic distortion in the installation. The generator needs to be prepared for this type of load, and our engineering takes this into account in the calculation.
In the startup sequence, turning on the larger motors one at a time makes the generator face one peak at a time, and the equipment can be smaller.
What happens when the generator is too small for the load
With a generator below the necessary capacity, the problem appears at the first heavy start. Voltage drops, frequency fluctuates, and the motor may not reach speed before protection kicks in. In an automated line, the effect goes beyond the motor. The drop resets controllers and stops production at points that take time to safely resume.
Conversely, an oversized generator operates with low load most of the time, consumes more fuel, and increases rental costs without bringing returns to the operation. Our post about generator or generator set shows the parts of the set, such as the engine, alternator, and control panel, and who is responsible for the load.
What data from your plant does our engineering team need to calculate the power
Our engineering team calculates the power at no cost to you. The more complete the survey, the more accurate the recommended equipment will be, so gather what you have from these items:
- List of motors, with power, voltage, nominal current, and power factor.
- Type of start for each motor and the planned order of activation.
- Other plant loads, such as lighting, furnaces, compressors, and IT systems.
- Period of use, operation rhythm, and whether the generator will work alone or in parallel.
- Photos of the motor plates and the distribution panel, if available.
With this material, we simulate the start of the larger motors and determine the appropriate kVA, already with a margin for your operation.
Sizing for the industry in Campinas, Sorocaba, and Piracicaba
In the factories of the Metropolitan Region of Campinas, in cities like Sumaré, Hortolândia, Indaiatuba, and Americana, the generator needs to handle heavy motor starts. Since the automated lines of the automotive and manufacturing industries feel any voltage fluctuation, the generator sizing for electric motors in these plants starts with the start of the larger motors. Meanwhile, the data center in the region requires a different calculation, as the equipment needs to run without interruption.
In Paulínia, the refinery and petrochemical plant operate continuously, with scheduled maintenance on a fixed date. If the shutdown requires cutting off the power from the grid, our generator keeps the process running.
Continuing through the interior, the manufacturing industry and sugarcane mills in Sorocaba, Piracicaba, and the Mogiana region, between 40 and 110 km from our unit, undergo the same motor start calculation. Further away, in the Ribeirão Preto and Araraquara region, between 110 and 200 km, the mill uses the generator for maintenance during the off-season. It also provides power to operations in rural areas where the grid is weak or unavailable.
Our unit is located at the Campinas Intermodal Cargo Terminal, 93 km from São Paulo, and the generator is operational within 24 hours. The on-call service is available 24 hours a day. For Marília, Ourinhos, and more distant cities, the delivery time is agreed upon in the proposal.
If you need a generator for the industry in the Campinas region, send your plant data and receive the sizing from our engineering team. To learn about other uses, also see the generator rental for industry.
Frequently Asked Questions
What is the difference between kVA and kW when choosing a generator?
kW is the part of the power that turns into work in the motor, and kVA also includes the portion it uses to magnetize itself. That’s why the generator is chosen based on kVA, considering the load’s power factor.
Can I add up the nameplate power of the motors to determine the generator size?
The sum shows how much the plant consumes with the motors already running. To determine the correct size, the calculation needs to include the starting current, the type of start, and the order in which the larger motors start.
Do soft starters or frequency inverters reduce the generator power I need?
They do reduce it because they hold the current peak when the motor starts. In return, the generator needs to support the harmonic distortion of these devices, and our engineering takes this into account in the calculation.
What information from my factory should I send to calculate the power?
Send the list of motors with power, voltage, current, and power factor, the type of start for each one, the other loads, and the expected operation pace. Photos of the nameplates also help.
