A generator can handle a motor’s normal running load and still stumble when the motor starts. Startup briefly demands more from the system: the alternator must supply starting kVA without an excessive voltage dip, while the engine must deliver starting kW without an excessive frequency dip. To assess the problem, use the motor’s nameplate data, account for the loads already running, and model the actual start sequence—not just the sum of running watts.
Why does my generator stall when the AC starts?
Starting a motor is a transient event, not simply a larger version of its steady running load. A compressor can draw a high current as it begins turning. That creates an apparent-power demand on the alternator and a real-power demand on the engine. The resulting voltage and frequency dips may disrupt the motor or other connected equipment. Rehlko’s generator-sizing guidance emphasizes predicting voltage dip and its effect on connected loads; EGSA training distinguishes alternator starting kVA and voltage dip from engine starting kW and frequency dip. Rehlko EGSA
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That is why a generator’s continuous kW rating alone does not establish whether it can start a particular air conditioner or pump. The relevant question is whether it can handle the largest credible starting event, with the rest of the expected loads considered, while keeping voltage and frequency within acceptable limits. The answer depends on the specific motor, generator transient performance, site conditions, and start sequence.
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LRA means locked-rotor amps: a motor-current figure associated with the motor being energized while its rotor is held still. For compressors, Copeland’s Starting Methods technical information defines the measurement as current after four seconds with the motor fixed and energized at the upper nominal voltage limit. It says LRA may be treated as the compressor’s maximum starting current, while also noting that transient phenomena are not considered. This is a compressor-specific technical definition, not proof that every motor’s real-world inrush waveform stays at its LRA continuously. Copeland
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Where to find the useful figures
Start with the equipment data plate and manufacturer documentation. Record the motor or compressor’s LRA or starting code where given, along with running current. Record the generator’s continuous rating and its motor-starting or transient data. Siemens advises checking actual running amps on the equipment data plate; Rehlko describes using motor nameplate information and NEMA code letters to determine starting kVA. Siemens Rehlko
Do not use LRA alone as a generator-selection formula. It does not tell you how the generator will respond, what loads will already be operating, whether starts overlap, how quickly the motor accelerates, or every transient feature of the event. Copeland’s qualification about transients and Rehlko’s focus on voltage dip both matter when interpreting the number.
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Why voltage dip and frequency dip are different
Starting kVA is important to the alternator’s voltage response; starting kW is important to the engine’s response. EGSA’s training material treats these as separate effects. A motor-start assessment should therefore consider both, rather than treating a single running-watt total as a complete measure of capability. EGSA
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The amount of dip that is acceptable depends on the generator application and the equipment connected to it. Rehlko’s guidance frames motor-start sizing around predicting voltage dips and their effect on other loads. Use the generator manufacturer’s application data and guidance for the actual installation instead of assuming that any particular dip is harmless.
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Do I add the starting amps for every motor?
Not automatically. If motors truly start at different times, model each credible start step with the loads that remain on at that moment. The largest event may be one motor starting while other motors and non-motor loads are already running. But do not call starts “non-coincident” merely because they usually happen separately: the controls or operating practice must make that sequence credible.
Siemens’s multiple-motor guide configurations assume sequence starting. EGSA’s examples show that generator capacity needs can change depending on whether the largest motor starts first, last, or alongside other motors. Those examples illustrate why sequence matters; they are not a substitute for the equipment data and generator application information for a specific installation. Siemens EGSA
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Build the sequence from actual operating conditions
- List the loads expected to be on. Include the non-motor loads and motors that may already be running when the target motor starts.
- Write down every credible start order. Include simultaneous or overlapping starts if the equipment controls and operating conditions could allow them.
- Evaluate each start step. For each motor, consider its starting data together with the loads already running. Obtain generator-manufacturer guidance on the resulting transient demand and system response.
- Focus on the most demanding credible step. Do not add every motor’s peak as though all starts are simultaneous if controls genuinely separate them; do not assume separation if it is not assured.
What can help when a generator fails at startup?
Separate motor starts where practical
Load controls or operating procedures may reduce the chance that multiple large motors start together. The useful change is the one that reliably changes the actual sequence and leaves the generator able to handle each resulting step. Siemens’s sequential-start assumptions and EGSA’s alternate-sequence examples show why a real sequence—not a label applied to the load list—belongs in the assessment.
Check transient capability, not just continuous capacity
Ask the generator manufacturer or a qualified installer to evaluate the motor-start event using the equipment nameplates, the existing load, and the expected sequence. The comparison should address alternator voltage response and engine frequency response alongside continuous capacity. Rehlko’s motor-start guidance is specifically concerned with starting capability and voltage dip, rather than relying on a single kW rating.
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Consider a compatible compressor soft starter
A compressor soft starter can control power delivered to compressor windings during startup. Micro-Air describes this approach for its EasyStart product and reports a 50–70% peak-current reduction, as well as an example of an RV rooftop 16 KBTU, 120 V compressor starting below 20 A. These are Micro-Air’s product claims; the cited support article does not establish that the same result applies to every compressor or electrical system. Check compatibility and follow the compressor and soft-starter manufacturers’ instructions. Micro-Air
What data should I take to an installer?
- Motor and compressor nameplate information, including LRA or starting code when present, plus actual running current.
- The generator’s continuous rating and manufacturer data on motor-starting or transient performance.
- A list of other loads expected to operate during each motor start.
- The normal and worst credible motor-start sequence, including any controls that prevent overlap.
- Relevant site and installation conditions, so the equipment manufacturer or qualified installer can assess the actual application.
Use actual nameplates and generator application guidance ahead of generic appliance tables or unqualified rules such as multiplying running amps by a fixed factor. Published examples can orient a discussion, but they do not determine a suitable generator size for an unspecified installation.
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