Air Compressor Motor Power and Current Relationships

Every electrician knows the formula. I = P / (√3 × U × cosφ × η). Power divided by voltage, square root of 3, power factor, efficiency. At 380V, power factor 0.87, efficiency 0.92, a 22 kW motor works out to about 42A. Pretty close to what's on the nameplate.

Formula is easy to memorize. Using it, a few things to watch. Power factor 0.87 is the typical full-load value. When the compressor is unloaded or a VFD machine is running at low frequency, power factor drops hard. At 30 Hz could be only 0.65 to 0.7. Efficiency too. Light load, efficiency drops. Same output power draws more current. So this formula gives you rated-condition current. The further from rated conditions, the bigger the error.

Quick Reference Table

This table is theoretical values derived from the formula. Will differ from motor nameplates. Domestic motors tend to run a bit higher. ABB, Siemens high-efficiency motors run a bit lower. No nameplate available, good enough for estimates. Have the nameplate, go by the nameplate.

Cable Sizing

First, ampacity. 35 mm² copper cable in open air can carry about 130A. 50 mm² about 160A. 70 mm² about 200A. 95 mm² about 245A. This is at 30°C ambient, single cable run.

Don't select right at the ampacity limit. Cable ampacity needs at least 25% margin. Multiply by 1.25. 55 kW motor at 103A. Times 1.25 is 129A. Table says 35 mm² is just 130A. Looks like enough.

Another easily overlooked issue is voltage drop. Cable length over 100 meters, need to calculate. Voltage drop over 5%, motor struggles to start, running efficiency drops too. Rough estimate method: 100 meters distance, per square millimeter of copper per amp, about 0.04V drop. 50 mm² cable carrying 100A over 100 meters, voltage drop roughly 8V. Out of 380V that's just over 2%. Okay. If only 25 mm², voltage drop doubles to 16V. Over 4%. Getting iffy.

Circuit Breaker Selection

Rated current at 1.2 to 1.3 times the motor's rated current. 37 kW motor at 70A. Times 1.25 is 87.5A. No breaker at that rating. Go up to 100A.

Can't go too big. Some people figure bigger margin is safer. 160A breaker on a 70A motor. Motor stalls and the breaker still hasn't tripped. Windings burn first. Breaker overload protection uses thermal trip. Higher current means faster heat buildup triggers the trip. Too big a breaker, protection is useless.

Breaker before a VFD can be C-type. The VFD's rectifier section doesn't have motor-starting type surge. To the grid it's a resistive-capacitive load.

VFD Sizing

Size VFDs by current, not power. The "suitable motor power" on the VFD nameplate is just a reference. Same labeled 55 kW, different brands' rated output current ranges from 105A to 120A.

55 kW motor at 103A. VFD output current needs to reach at least 113A. 10% margin. On the market, 55 kW VFDs output current is mostly 110 to 115A. Tight. If the factory is hot or altitude exceeds 1000 meters, VFD needs to be derated. That margin isn't enough anymore. Safe approach: pick a 75 kW frame VFD. Set the parameters for a 55 kW motor.

Check VFD overload capability. Compressor loading instant needs brief overcurrent. Specs like 150% overload for 60 seconds, 180% overload for 3 seconds matter. Cheap VFDs overstate these numbers badly. Loading triggers overcurrent faults constantly.

Output cable can't be too long. VFD output PWM waveform has high-frequency components. Long cable creates reflected waves. Voltage spikes at the motor terminals can hit over 1000V. Standard motor insulation can't handle it. Under 50 meters is generally fine. Over 50 meters, add an output reactor or use VFD-rated cable.

What Current Can Tell You

Clamp meter on three phases is the most convenient diagnostic tool.

Current running high, most common cause is discharge pressure set too high. Nameplate says 0.8 MPa but set to 1.0 MPa. Motor definitely overloads. Clogged oil separator element also causes high current. Internal pressure differential increases. Airend back pressure goes up. More work being done. Differential gauge over 0.1 MPa, time to change the element. Some people drag it to 0.2 MPa. Current is already 10% or more above rated. Mechanical faults: bearing wear, rotor clearance changes, airend binding. All increase friction. Current goes up. Usually accompanied by high vibration, abnormal noise, high discharge temperature.

Current running low means light load. Insufficient air demand and the compressor keeps unloading. That's normal. Not a fault. It's oversizing or excess capacity. Intake valve not opening also gives low current. Shows as can't build pressure or loading time gets longer. Clogged air filter increasing intake resistance also gives low current. Replace the air filter, current comes back.

Supply voltage low, current goes high. Voltage drops below 350V, current can jump 10% or more. Industrial park peak hours, voltage drops hard. That situation, take it up with the utility. Poor cooling also makes current run high. Heat exchanger blocked, fan broken, ambient too hot. Motor windings overheat. Copper resistance goes up. Efficiency drops. Same power output needs more current.

Starting Methods

Direct-on-line is simplest. Wire it up and go. Starting current 6 to 8 times rated. Downside is big impact. Same transformer, lights flicker. Sensitive equipment might have problems. Utility has limits on direct-start capacity. Generally can't exceed 30% of transformer capacity. Small machines under 15 kW, grid capacity is sufficient, can use it.

Star-delta reduces starting current to 2 to 3 times. At start, windings are connected in star, voltage becomes 57.7% of normal. After starting, switches to delta. Trade-off: starting torque is also only one-third. Loaded starting is difficult. Compressors generally start unloaded so it's okay. Note the star-delta switchover has a secondary surge. Bad timing and the current spike approaches direct-start levels. Switchover timing needs adjustment. Too short, motor hasn't accelerated enough, switching causes big surge. Too long, motor speed is already near rated, back-EMF is high when switching, also a big surge. Usually switch at 70% to 80% of rated speed. Suitable for 15 to 75 kW medium-power compressors.

Soft starter uses thyristors for voltage regulation. Starting current 2 to 4 times, adjustable. Smooth startup. Downside: thyristors have conduction losses. A 110 kW machine, losses near 1 kW. So high-power soft starters usually include a bypass contactor. After starting, thyristors disengage.

VFD start is the smoothest. Ramps up from low frequency. Current stays under 1.5 times rated throughout. Minimal grid impact. VFD compressors come standard with this starting method.

Power Supply Capacity

Transformer capacity estimate: total power divided by power factor, efficiency, and simultaneity factor. Three compressors: 55, 37, 22 kW. Totals 114 kW. Power factor 0.87. Efficiency 0.92. Simultaneity factor 0.8. Works out to 178 kVA. Pick a 200 kVA transformer.

Simultaneity factor 0.8 assumes all three might be at full load at the same time. If one is a rotating backup, simultaneity factor can drop to 0.6. Conversely, if all three often start simultaneously, take 1.0. And account for starting surge.

Low power factor wastes transformer capacity. 0.87 power factor means out of 100 kVA apparent power only 87 kW is useful work. The rest is reactive power running through the lines, taking up capacity, doing nothing. Install a capacitor compensation panel. Bring power factor above 0.95. Same transformer can carry about 15% more load. Utility penalizes low power factor. Fines.

Multiple VFDs, harmonics become an issue. Rectifier stage produces 5th, 7th, 11th harmonics. Superimposed on the grid, voltage waveform distorts. Transformer and cables heat up extra. Effective capacity drops. Several VFD compressors concentrated together, consider adding line reactors. Can suppress 30% to 50% of harmonics.

Transformer sizing, leave 20% to 30% margin. Calculated 178 kVA, selecting 200 kVA is just barely enough. 250 kVA is more reasonable. New construction, think about future expansion. Adding compressors is common. Transformer and incoming cables not sized with headroom from the start, retrofit later is a major headache.