Korea Ever-Power · Y2 Series · Air Compressor Motor Selection Guide

Electric Motor for Air Compressor Drives:
Reciprocating, Rotary Screw and Scroll Selection Guide

Air compressors are one of the most power-intensive and continuously operated industrial drives. The motor driving a compressor must handle high starting torque on reciprocating types, sustain full rated power at S1 continuous duty for 8 to 24 hours per day, and maintain Class F insulation integrity in the elevated ambient temperature of a compressor room. The Korea Ever-Power Y2 series in 2-pole and 4-pole configurations is the correct motor specification for reciprocating piston compressors, rotary screw compressors, and scroll compressors in factory, workshop, and industrial utility applications from 1.5 to 200 kW.

S1 Continuous Duty
High Starting Torque
IE3 Energy Efficient
IP55 Compressor Room
1.5–200 kW

S1
Continuous 24hr duty
2.5× T
LRT for reciprocating
IE3
Premium efficiency
IP55
Dusty compressor room
Class F
155°C hot ambient
SF 1.15
Continuous overload margin

Three-phase motor air compressor reciprocating screw scroll Korea Ever-Power Y2 IE3 S1 continuous 2-pole 4-pole

Korea Ever-Power Y2 series IE3 three-phase motor for air compressor drives — the 2-pole Y2 at 2,900 rpm is the standard specification for rotary screw compressor direct-drive and belt-drive applications, while the 4-pole Y2 at 1,450 rpm suits reciprocating piston compressors requiring higher starting torque through a V-belt or direct crankshaft drive. Both deliver S1 continuous duty at rated load for 24-hour factory compressed air generation.

1. Compressor Types and Motor Requirements

Reciprocating Piston Compressor

Single-cylinder and multi-cylinder reciprocating compressors from 0.75 to 37 kW. The piston compressor imposes a cyclically varying torque load on the motor shaft as each cylinder goes through suction and compression strokes. The peak torque at the end of the compression stroke can be 2.0 to 3.0 times the average torque. The motor must start the compressor against residual pressure in the cylinder — requiring locked-rotor torque (LRT) of 2.0 to 2.5 times rated torque. The 4-pole Y2 at 1,450 rpm is the standard configuration, often belt-driven at 2:1 to 3:1 ratio to the compressor flywheel to provide smooth torque delivery through the flywheel inertia.

Rotary Screw Compressor

Twin-screw positive displacement compressors from 5.5 to 200 kW, producing continuous compressed air at 6 to 13 bar. The screw compressor has a smooth torque characteristic — no peak compression stroke torque — so starting torque is close to running torque. The 2-pole Y2 at 2,900 rpm is used for direct or gear-belt drive of the male rotor at 3,000 to 6,000 rpm. Rotary screw compressors with fixed speed motors use an inlet valve to load and unload the compressor under PLC control. Energy-efficient screw compressors use a YVF2 VFD for continuous pressure-regulated speed control.

Scroll Compressor

Scroll compressors from 0.75 to 7.5 kW for small compressed air systems, HVAC chillers, and medical air. Smooth torque characteristic similar to screw compressors. Direct-coupled to the scroll orbiting mechanism at 2,900 to 3,600 rpm. Starting torque requirement is low (1.0 to 1.5 times rated). 2-pole Y2 or YS small-power motor for compact scroll compressor packages. Oil-free versions require Class H insulation for the higher ambient temperatures inside the oil-free scroll housing.

Centrifugal Compressor

High-speed centrifugal (turbo) air compressors from 75 to 1,000 kW for large industrial compressed air plants. Speed 5,000 to 60,000 rpm, requiring step-up gearbox or high-frequency variable speed drive. For motor frame sizes within the Y2 range (up to 200 kW), the 2-pole Y2 with integral step-up gearbox covers the lower end of the centrifugal compressor range. Above 200 kW, consult Korea Ever-Power for dedicated large compressor motor specifications.

2. Starting Torque and Motor Pole Selection

Compressor Type Motor Poles Speed LRT Required Starting Method
Reciprocating piston (belt-drive) 4-pole 1,450 rpm 2.0–2.5× DOL or star-delta
Reciprocating piston (direct) 6-pole or 4-pole 960–1,450 rpm 2.5–3.0× DOL preferred
Rotary screw (belt or gear) 2-pole 2,900 rpm 1.2–1.5× Star-delta or VFD
Scroll (direct) 2-pole 2,900 rpm 1.0–1.5× DOL
VFD screw (variable speed) 2-pole YVF2 0–120 Hz Controlled by VFD Ramp start via VFD

For reciprocating piston compressors on star-delta starting: verify that the motor locked-rotor torque in star connection (33% of delta LRT) is still sufficient to start the compressor against residual cylinder pressure. If the star-connection LRT is insufficient, use direct-on-line (DOL) starting with a current-limiting reactor or a soft-starter that maintains full voltage torque capability during start.

3. Compressor Motor Power Calculation

Motor Power Sizing — Rotary Screw Compressor 7 bar, 1 m³/min Free Air Delivery
Theoretical power (isothermal):
P-iso = (P₁ × Q × ln(P₂/P₁)) ÷ 60
P₁ = 1 bar abs, Q = 1 m³/min, P₂ = 8 bar abs
P-iso = (100,000 × 1/60 × ln(8)) = 3,466 W = 3.47 kW

Actual screw efficiency: 65–75%
Indicated power = 3.47 ÷ 0.70 = 4.96 kW
Mechanical losses (bearings, seals): +0.5 kW
Drive efficiency (belt/gear): 0.95

Motor selection:
Shaft power required = (4.96 + 0.5) ÷ 0.95 = 5.75 kW
Service factor 1.15 for S1 continuous: 5.75 × 1.15 = 6.61 kW
Select next standard size above: Y2 7.5 kW 2-pole 2,900 rpm

Check: 7.5 kW ÷ 5.75 kW = 1.30 load factor — comfortable S1 rating
Rule of thumb: 5.5–7.5 kW motor per 1 m³/min at 7–8 bar for screw compressor

4 bar / 58 psi
Low pressure utility
3.5–5.5 kW per 1 m³/min FAD
7 bar / 100 psi
Standard factory air
5.5–7.5 kW per 1 m³/min FAD
10 bar / 145 psi
High pressure industrial
7.5–11 kW per 1 m³/min FAD
13 bar / 188 psi
Two-stage reciprocating
10–15 kW per 1 m³/min FAD

4. Elevated Ambient Temperature and Motor Thermal Rating

Compressor Room Thermal Environment

A compressor room with multiple compressors operating continuously generates significant heat from compressor jacket cooling, oil cooler rejection, and motor frame heat dissipation. Ambient temperatures of 35 to 50°C inside a compressor room are common without adequate ventilation. Standard IEC motor rating assumes 40°C maximum ambient — at 45°C the motor must be derated to approximately 92% of nameplate power, and at 50°C to approximately 83%. For compressor rooms regularly above 40°C, either derate the selected motor power or install positive mechanical ventilation of the compressor room to maintain ambient below 40°C at the motor inlet.

Class F Insulation Margin

The Korea Ever-Power Y2 motor is specified with Class F insulation (rated to 155°C winding temperature) but thermally rated to Class B temperature rise (80 K above ambient at rated power). This 25 K thermal margin between actual operation and Class F limit provides protection against short periods of ambient temperature above 40°C without accelerated insulation ageing. For compressor motors operating continuously at elevated ambient 40–50°C, the Class F insulation of the Y2 provides the necessary thermal headroom that would not be available with a Class B motor insulation system.

5. IE3 Efficiency and Annual Energy Cost Saving

IE2 vs IE3 Energy Saving — 7.5 kW Compressor Motor, 16 hr/day, 250 days/year
IE2 motor (90.4% efficiency at 7.5 kW):
Input power = 7,500 W ÷ 0.904 = 8,297 W
Annual energy = 8,297 × 16 × 250 ÷ 1,000 = 33,188 kWh
At 0.12 USD/kWh: annual cost = 3,983 USD
Motor losses = 8,297 − 7,500 = 797 W heat in compressor room
IE3 Y2 motor (91.7% efficiency at 7.5 kW):
Input power = 7,500 W ÷ 0.917 = 8,179 W
Annual energy = 8,179 × 16 × 250 ÷ 1,000 = 32,716 kWh
At 0.12 USD/kWh: annual cost = 3,926 USD
Annual saving vs IE2: 57 USD — payback on IE3 premium: typically 2–3 years
Motor losses reduced by 118 W → lower compressor room heat

For larger compressor motors (22 to 75 kW) operating 24 hours per day in continuous manufacturing plants, the annual IE3 energy saving over IE2 can reach 500 to 2,000 USD per motor, with payback on the efficiency premium within 1 year. Korea Ever-Power Y2 series motors comply with IE3 as standard — no separate efficiency class selection is required.

6. Korea Ever-Power Y2 Specifications for Compressor Drives

The Korea Ever-Power Y2 series covers the 1.5 to 200 kW range in 2-pole and 4-pole configurations for all air compressor motor applications. The 2-pole Y2 at 2,900 rpm suits rotary screw and scroll compressors requiring high shaft speed. The 4-pole Y2 at 1,450 rpm suits reciprocating piston compressors needing higher starting torque. IE3 efficiency is standard on all Y2 frames. IP55 is specified for compressor room installations with dust and oil mist. For VFD-controlled screw compressors requiring variable speed, replace the Y2 with the YVF2 IC416 from the VFD motor section. The complete Y2 range is in the three-phase motor section. Contact Korea Ever-Power for compressor motor sizing assistance above 75 kW.

Y2 — Air Compressor Motor Data
Power range 1.5–200 kW
Poles 2P (2,900) or 4P (1,450)
Efficiency IE3 standard
Insulation Class F (155°C)
IP rating IP55 for compressor room
Service factor 1.15
Duty S1 continuous
LRT 2.0–2.5× (4P) / 1.8–2.2× (2P)

7. Air Compressor Applications

Y2 three-phase motor air compressor rotary screw reciprocating Korea Ever-Power IE3 S1 continuous factory

Factory Compressed Air Plant

Rotary screw compressors 7.5 to 200 kW providing 6 to 10 bar compressed air for pneumatic tools, automation, spray painting, and process equipment in manufacturing plants. Y2 2-pole, IE3, IP55, S1 continuous. Multiple compressors on a compressed air header with lead-lag control. VFD trim compressor using YVF2 for pressure band control and energy saving at partial load.

 Y2 three-phase motor air compressor workshop reciprocating piston Korea Ever-Power 4-pole high starting torque

Workshop Reciprocating Piston Compressor

Single-cylinder and twin-cylinder reciprocating compressors 1.5 to 15 kW for workshop air tools, spray painting, tyre inflation, and service station compressed air. Y2 4-pole, IE3, IP55, S1 or S3 duty depending on compressor control mode. Belt-drive at 1.5:1 to 3:1 to compressor flywheel. DOL starting required for full LRT to start against residual cylinder pressure.
Medical and Dental Air

Oil-free scroll or piston compressors for medical grade compressed air (ISO 7396-1) supplying hospital air terminals, dental handpieces, and laboratory instruments. Y2 or YS 0.75 to 7.5 kW 2-pole. Oil-free compressors have higher ambient temperature inside the compressor housing — Class F insulation of the Y2 provides necessary thermal margin. Redundant compressors with automatic changeover.

Food Grade Air Compressor

Oil-free compressed air for food and beverage packaging, pneumatic conveying of food ingredients, and process air in contact with food products. Y2 2-pole 5.5 to 55 kW. Oil-free screw or piston compressors. IP55 for food factory washdown environment. Some installations require BXG stainless motor if motor is in the food contact zone of the production area.

Mining Air Compressor

Underground and surface mine compressed air for pneumatic drilling, rock bolting, and ventilation. Y2 4-pole or 2-pole 22 to 200 kW for large mine air compressors at 6 to 10 bar. Altitude derating for high-elevation mines. IP55 for dusty mine surface environment. Ex d YB2 required for coal mines where methane is present in the compressor room.

Nitrogen Generator Compressor

Feed air compressors for nitrogen generation by pressure swing adsorption (PSA) or membrane separation, supplying nitrogen for fire suppression, packaging, and inerting. Y2 2-pole 7.5 to 75 kW for PSA feed air compressors at 8 to 12 bar. Continuous S1 duty required as PSA systems run 24 hours per day to maintain nitrogen inventory.

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8. Frequently Asked Questions

Why does my reciprocating compressor motor trip on overload every time it starts under load?

Reciprocating compressor starting overload trips are almost always caused by one of three conditions: (1) the compressor control system is not unloading the cylinder pressure before restart — the compressor tries to start against the full discharge pressure remaining in the receiver, which requires much higher starting torque than starting from zero pressure. The correct restart sequence is: stop compressor, open unloader valve to release cylinder pressure, start motor with unloaded cylinder, then close unloader and resume loading after the motor reaches rated speed. Check whether the compressor control system has a restart delay and unload valve; if not, add an unloader. (2) The motor locked-rotor torque is insufficient for the loaded start; this is a motor sizing problem — replace with a motor with higher LRT (2.5 times rated) or use DOL starting instead of star-delta. (3) The overload relay trip current is set too low for the motor starting duration; adjust the overload relay to class 10 or class 20 trip characteristic for compressor starting, which allows the higher starting current for a longer time before tripping.

Should I use a 2-pole or 4-pole motor for a belt-drive reciprocating compressor?

For belt-drive reciprocating compressors, the 4-pole motor at 1,450 rpm is the standard specification for several reasons. First, the 4-pole motor has higher locked-rotor torque as a percentage of rated torque (typically 2.0 to 2.5 times rated vs 1.6 to 2.0 times for a 2-pole motor of the same power), which is important for starting the compressor against residual cylinder pressure. Second, the 4-pole motor at 1,450 rpm allows the compressor to run at its design speed (typically 300 to 700 rpm for a reciprocating compressor) with a manageable belt drive ratio of 2:1 to 4:1 — the compressor flywheel and crankshaft are designed for this speed range. Using a 2-pole motor at 2,900 rpm would require a higher belt ratio (4:1 to 8:1) which increases belt slip, reduces belt life, and creates higher belt tensioning loads on the motor and compressor bearings. The 4-pole motor with moderate belt ratio is the correct engineering choice for reciprocating compressor belt drives in virtually all power ranges from 1.5 to 37 kW.

Is it worth fitting a VFD to an existing fixed-speed screw compressor motor?

Yes, in most factory compressed air applications with variable demand, retrofitting a VFD to the lead compressor is one of the highest-return energy-saving investments available. A fixed-speed screw compressor controls pressure by inlet valve unloading — the motor runs at full speed but the compressor delivers no air during the unloaded period. During unloading the motor still consumes approximately 25 to 35% of its full-load power. A VFD-controlled compressor reduces motor speed to match actual air demand, consuming power proportional to actual delivery rather than running at full speed when unloaded. Savings of 20 to 35% of annual compressor energy are typical for factories with variable air demand profiles. To retrofit, replace the existing standard motor with a YVF2 IC416 motor of the same power and frame, install a compatible VFD sized for the motor rated current, and set the VFD to control compressor speed to maintain the header pressure set point. The YVF2 IC416 is essential for the retrofit — a standard motor without IC416 will overheat at the reduced speeds used during partial load operation.

 

Korea Ever-Power · Y2 Series · Air Compressor Drive Motors

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Korea Ever-Power Y2 series: IE3, IP55, Class F, SF 1.15, 2-pole and 4-pole, 1.5–200 kW. S1 continuous compressor duty. In stock for fast delivery worldwide.

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