In short. A starting motor draws several times its running current at low power factor. That current drops voltage across the source, transformer and cable. The study checks the dip at the motor (will it accelerate?) and at the bus (will other loads ride through?), and compares starting methods.
Physics
At standstill, an induction motor is like a transformer with a shorted secondary, drawing locked-rotor current of about five to eight times full-load current at a low power factor. As it accelerates, current falls. The voltage dip is the product of this current and the impedance between the source and the motor.
Inputs
- Motor rating, locked-rotor current and torque curve, inertia, driven load torque curve.
- Starter type and settings.
- Source data: utility fault level or generator reactances and excitation response.
- Transformer and cable impedances.
- Other loads connected and their sensitivity.
Approximate dip estimate
As a first screening, the voltage dip at the supply point is roughly the starting kVA divided by the sum of the starting kVA and the source short-circuit kVA.
Dip ≈ Sstart ÷ (Sstart + Ssc)
This ignores cable and dynamic effects and is for screening only. Full studies use dynamic simulation.
Acceptance checks
| Check | Question |
|---|---|
| Motor terminal voltage | Is torque at reduced voltage still greater than load torque all the way to speed? |
| Acceleration time | Does it remain below the motor’s safe stall time? |
| Bus voltage | Do contactors, drives, UPS inputs and PLCs ride through? |
| Source recovery | Does generator voltage and frequency recover quickly? |
| Protection | Do undervoltage and overload settings allow the start? |
Torque falls with the square of voltage, so a 20 percent dip cuts available torque to about 64 percent.
Starting methods compared
- Direct on line: highest current and torque.
- Star-delta: about a third of current and torque, with a switching transient.
- Soft starter: adjustable current and torque ramp.
- Variable frequency drive: controlled start with low inrush, adds harmonics. See harmonics.
- Autotransformer or reactor: reduced voltage tap starting.
Sequencing
Start the largest motor first when the generator is lightly loaded, and stagger others so that each transient has recovered. Consider start permissives and delays in the controller. See large-motor starting on generators.
Illustrative example
A 75 kW motor starts direct on line on a 400 kVA generator in a screening estimate. The starting kVA is about 600, and the generator’s effective short-circuit kVA is a few times its rating, so the dip is large. The study compares a soft starter, which limits starting current to about three times full-load, and shows that the dip falls to a level that other loads tolerate. See motor starting.
What happens when a motor starts
A direct-on-line induction motor draws about 6 to 8 times full-load current at low power factor, typically 0.2 to 0.3. The current flows through source and feeder impedance and pulls down the voltage at the motor and at every other load on the bus.
Voltage dip ≈ starting kVA ÷ (starting kVA + short-circuit kVA of the source)
Worked example (illustrative)
A 90 kW motor starting at 7 times draws about 7 × 160 = 1,120 kVA. On a source with 8,000 kVA short-circuit capacity: dip ≈ 1,120 ÷ (1,120 + 8,000) ≈ 12 percent. On a 500 kVA generator with 15 percent reactance (about 3,300 kVA), the dip would be about 25 percent, enough to drop contactors and stall the motor.
Limits to check
| Item | Typical limit |
|---|---|
| Bus voltage during start | Not below about 80 percent for motor acceleration; load-specific limits for others |
| Contactor drop-out | Around 70 percent in many designs |
| Lighting and electronics | Flicker and reset concerns well above 90 percent |
Mitigation options
- Star-delta, autotransformer or soft starter.
- Variable speed drive.
- Staggered starts.
- Stronger source: larger generator or lower reactance alternator.
Study outputs
Minimum voltage, acceleration time, recovery time and effect on other loads. See motor starting and large motor starting on diesel generators.
Common mistakes
- Checking steady-state kVA only.
- Ignoring load torque during acceleration.
- Assuming a stiff utility source on generator.
What to send us
Send motor datasheets, starter type and source details. Send your application details.
Questions engineers ask
What dip is acceptable?
It depends on the motor, the load and other equipment on the bus. Define limits before the study.
Why does the motor hum and not start?
Voltage may be too low for the torque required. Check supply, connections and the load.
Is a soft starter always best?
It helps many loads, but loads needing high breakaway torque may need other solutions.
Does cable length matter?
Yes. Long cables add impedance and voltage drop during starting.
Values are illustrative screening figures. Use motor data and dynamic simulation for design.