What is the difference between Locked Rotor Amps (LRA) and Rated Load Amps (RLA)?

Rated Load Amps (RLA) is the continuous current drawn by the compressor under steady-state operating conditions at full rated thermal load. Locked Rotor Amps (LRA) is the momentary inrush current drawn at the instant the rotor begins turning from a standstill. LRA is typically 5 to 7 times higher than RLA, producing massive voltage sag on backup power sources.

How does a microprocessor soft starter differ from a hard start kit?

A traditional hard start kit consists of a start capacitor and potential relay designed solely to increase starting torque and shorten startup duration; it actually increases peak inrush current slightly. In contrast, an electronic soft starter uses solid-state switches (TRIACs/SCRs) and software algorithms to dynamically ramp motor voltage across 100 to 250 milliseconds, reducing peak inrush current by 65% to 75%.

What size generator is needed to run a 3-ton heat pump with a soft starter?

A standard 3-ton compressor has an LRA of ~79A (requiring over 19 kW of surge capacity without soft start). With a 70% soft starter reduction, starting surge drops to ~24A (5.7 kW). A quality 7.5 kW continuous / 9.5 kW surge generator or an 8 kW solar hybrid inverter can reliably start and run the unit.

Can a soft starter damage a scroll compressor?

No. Leading compressor manufacturers (such as Copeland) approve microprocessor soft starters because reducing mechanical acceleration shock on compressor bearings, crankshafts, and motor windings reduces thermal fatigue and extends equipment lifespan.

Microprocessor Soft Start • Generator Sizing • LRA Physics

Heat Pump LRA Soft Starter & Generator Sizer

Calculate compressor Locked Rotor Amps (LRA) inrush reduction, size electronic soft starters, and determine the exact minimum backup generator and off-grid inverter surge capacity.

Compressor Inrush Physics: Single-phase residential heat pump compressors draw 5× to 7× Rated Load Amps (RLA) at startup, causing massive voltage dips on backup power systems. An advanced microprocessor soft starter (e.g. Micro-Air EasyStart, ICM, Hyper Engineering) ramps voltage across the first 100–250 ms, achieving a 65% to 75% reduction in starting surge ($I_{\text{start}} = \text{LRA} \times 0.30$). This enables a 3-ton to 4-ton heat pump to start cleanly on an 8 kW off-grid inverter or a 7.5 kW generator.

Compressor Nameplate Specifications

Standard Capacity Preset

Select typical nominal tonnage or enter custom values.

Rated Load Current (RLA in Amps)

Nameplate RLA from the outdoor condensing unit data tag.

Locked Rotor Current (LRA in Amps)

Nameplate LRA indicating across-the-line starting inrush.

Supply Line Voltage (Volts AC)

Line-to-line AC voltage at the condensing unit disconnect.

Soft Starter Inrush Reduction

Documented current reduction factor from dynamic ramp algorithm.

Surge Reduction & Power Source Sizing

Reduced Starting Surge

-- A

Down from -- A raw LRA

Peak Soft Start Surge Power

-- kW

Peak inrush watts

Min Backup Generator

-- kW

Surge / running capacity

Min Solar Inverter

-- kVA

Peak surge rating (100ms)

Click Calculate Soft Start Surge to evaluate inrush current reduction and minimum power source requirements.

Compressor Inrush & Power Source Sizing Formulas

Across-the-Line Raw Surge: P_raw = LRA × V_line

Soft Starter Reduced Current: I_soft = LRA × [ 1 - (η_reduction / 100) ]

Soft Starter Peak Surge Watts: P_surge,soft = I_soft × V_line

Minimum Generator Surge Rating: P_gen,surge = P_surge,soft × 1.25

Residential Compressor Inrush vs Soft Starter Benchmark Table

Across-the-line Locked Rotor Amps compared to 70% soft starter inrush reduction across standard tonnages.

Compressor Tonnage	Typical RLA (Amps)	Nameplate LRA	Raw Surge @ 240V	Soft Start Surge (70%)	Soft Start Power	Min Sized Generator
1.5 Ton	9.0 A	48.0 A	11.5 kW	14.4 A	3.46 kW	4.5 kW / 5.5 kW Surge
2.0 Ton	11.2 A	58.0 A	13.9 kW	17.4 A	4.18 kW	5.5 kW / 6.5 kW Surge
2.5 Ton	13.5 A	67.0 A	16.1 kW	20.1 A	4.82 kW	6.5 kW / 7.5 kW Surge
3.0 Ton	14.7 A	79.0 A	19.0 kW	23.7 A	5.69 kW	7.5 kW / 9.0 kW Surge
3.5 Ton	17.9 A	96.0 A	23.0 kW	28.8 A	6.91 kW	8.5 kW / 10.5 kW Surge
4.0 Ton	19.8 A	109.0 A	26.2 kW	32.7 A	7.85 kW	9.5 kW / 11.5 kW Surge
5.0 Ton	24.3 A	134.0 A	32.2 kW	40.2 A	9.65 kW	12.0 kW / 14.5 kW Surge

Step-by-Step Worked Engineering Example

3.0-Ton Heat Pump Compressor on Off-Grid Solar Backup

Installation Context: A homeowner with a 3.0-ton central heat pump wants to run heating and cooling during power outages from an off-grid solar battery system. The outdoor unit data tag specifies RLA = 14.7A and LRA = 79.0A at 240V AC.

Calculate Raw Across-the-Line Starting Inrush: $$P_{\text{raw}} = 79.0\text{ A} \times 240\text{ V} = 18,960\text{ Watts } (19.0\text{ kW})$$ Without soft start, this instantaneous 19 kW pulse exceeds the surge limit of standard residential 8 kW to 10 kW inverters, causing instantaneous overload shutdown.
Calculate Soft Starter Reduced Inrush Current (70% Reduction): $$I_{\text{soft}} = 79.0\text{ A} \times (1.0 - 0.70) = 79.0 \times 0.30 = 23.70\text{ Amperes}$$ The electronic soft starter limits peak current to just 23.7A during the initial 150 ms voltage ramp.
Calculate Soft Starter Surge Wattage: $$P_{\text{surge,soft}} = 23.70\text{ A} \times 240\text{ V} = 5,688\text{ Watts } (5.69\text{ kW})$$
Size Backup Generator and Solar Inverter: - **Generator:** Adding a 25% safety buffer for engine governor response gives $5.69\text{ kW} \times 1.25 = 7.11\text{ kW}$. A standard **7.5 kW running / 9.0 kW surge generator** easily handles startup without bogging down. - **Solar Inverter:** The system requires at least $5.7\text{ kVA}$ surge capacity. A single **8 kW hybrid inverter (with 12 kVA 10-second surge)** or dual stacked 5 kW inverters will easily start the compressor while powering household base loads.

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