Air Compressor Motor Types – Air Compressor Manufacturers

Sales guys pushing permanent magnet machines like to say 30%, 40% energy savings. Permanent magnet VFD compared to asynchronous VFD, both variable frequency speed control, the savings percentage isn't that dramatic. Depends on load conditions, maybe 5% to 15%.

5–15%

30%

The energy savings are real. The 30% is fake. That 30% number comes from comparing permanent magnet VFD to fixed-speed machines.

Fixed-speed machines use load/unload to regulate air output. Need air, load up. Don't need air, unload. In unload state the motor spins idle, all the electricity turns into waste heat. If a compressor spends 40% of its time unloading and idling, that 40% of runtime is basically burning money. VFD machines adjust motor speed to match air demand. Don't need that much air, spin slower. Supply what's needed, no wasteful idle running.

Going from fixed-speed to VFD, saving 20% to 35% is normal. These savings are from VFD technology, not from permanent magnet motors. Asynchronous VFD machines can do the same thing.

Put permanent magnet VFD and asynchronous VFD side by side, both variable frequency, the savings percentage doesn't look so good anymore. Full load operation, permanent magnet is 2 to 3 percentage points more efficient than asynchronous. Load drops to 50%, gap widens to 5 to 8 percentage points. Lower the load, bigger the gap. That's the characteristic of permanent magnet motors.

Why is this?

Asynchronous motors rely on stator windings energized to produce a rotating magnetic field. The rotor gets induced current from this field, induced current produces another magnetic field, two fields interact and the rotor spins. Key point is the stator must stay energized to maintain that rotating magnetic field, regardless of load. 100% load, stator needs current to generate the field. 30% load, stator still needs current to generate same strength field. This excitation current produces losses in winding resistance. It's a fixed expense.

At full load, excitation losses as percentage of total power isn't high, like 5%, 6%. Load drops to 30%, output power is smaller, excitation losses stay the same, percentage becomes 15%, 18%. Efficiency naturally drops.

Permanent magnet motor rotors have neodymium magnets mounted on them, come with their own magnetic field, don't need stator current to induce it. Stator current only produces torque. High load uses more power, low load uses less. Without that fixed excitation loss floor, efficiency curve is flatter, doesn't drop noticeably at low loads like asynchronous motors.

This is the fundamental reason permanent magnet motors are more efficient, also why their advantage is obvious under partial load conditions.

Sounds like permanent magnet motors are wonderful.

Now the not-so-wonderful parts.

Industrial motor — Permanent magnet motor technology

Neodymium magnets fear heat. Very temperature sensitive. Temperature rises, magnetism weakens. If it's just brief overheating then cools down, magnetism can recover. If sustained high temperature exceeds a certain critical point, magnets undergo irreversible demagnetization. Even after cooling, magnetism won't return to original levels. Part of the rotor is permanently ruined, motor output drops, can't fix it, can only replace the rotor.

Compressor manufacturers obviously know this problem, so permanent magnet compressor control systems all have motor temperature monitoring and protection. Too high temperature triggers alarm. Keeps rising, forced shutdown. This is the protection mechanism to prevent magnet damage.

Protection mechanism exists doesn't mean problem doesn't exist.

Compressors themselves are heat-generating things. Compressing air produces massive amounts of heat. Discharge temperature 190-200°F is normal. This heat needs the cooling system to carry it away. Cooler gets dirty, heat dissipation efficiency drops. Cooling fan breaks, not enough airflow. Machine room ventilation is bad, heat can't get out. Summer ambient temperature hits 100°F, the whole system's temperature margin gets eaten up.

These situations are too common in industrial settings. How long since the cooler was cleaned? Six months? A year? Many factories' compressor coolers are caked with thick layers of oily sludge. Are ventilation openings blocked by debris? Did anyone think about ventilation capacity when designing the machine room? Any other heat-generating equipment next to the compressor?

One factor alone might still hold up. Several factors stacked together, motor temperature might approach the red line. Permanent magnet compressor temperature protection triggers, machine reduces load or shuts down, production line air supply cut off. Peak summer when orders are highest, compressor shuts down from overheating protection. How do you calculate that loss?

Asynchronous motors don't have demagnetization problems. High temperature makes insulation age faster, 20-year design life might become 15 years, but won't suddenly lose magnetism causing output to cliff-dive. For sites with bad environmental conditions, foundries, forging shops, cement plants, steel mill blast furnace areas, mines, asynchronous motors are much more reassuring than permanent magnet motors.

• • •

Now repairs.

Asynchronous motors are the most common industrial motors. Motor repair shops everywhere can fix them. Replace bearings, rewind coils, change fan blades, routine stuff. Small-town motor shops can handle it. Parts are universal, prices transparent. A 100 hp asynchronous motor, local repair maybe $300-400, fixed and installed, keep running.

Permanent magnet motors are different.

Those neodymium magnets on the rotor have very strong magnetic force. Bare-hand disassembly has safety risks. Pulling the rotor out, magnets pull it back toward the stator. One careless moment and fingers get caught. Reinstalling needs precise magnet positioning. Didn't mark things properly during disassembly, reassembly might be wrong. These operations need specialized fixtures and trained technicians.

Regular motor repair shops get a permanent magnet motor, most won't dare touch it. Afraid of being held responsible if they mess it up, afraid of safety incidents during the work. Standard practice is contact manufacturer or authorized service center, ship the motor there, fix it, ship it back.

How long does this process take? Remove from site, contact manufacturer, confirm repair plan, ship it there, wait in queue, repair, test, ship back, install, commission. Smooth process, two weeks. Not smooth, maybe a month.

A month without a compressor, what does that mean?

Motor repair — Motor maintenance considerations

Some users got smart. Buying permanent magnet compressor, also buy a spare motor. Breaks, swap it in directly, send the old one out for slow repair. That's the right approach, but that spare motor costs $3,000-5,000 or more, often not in the original purchase budget. Sales won't include this when calculating ROI for you, but you'll have to spend it.

Another situation. Compressor past warranty, motor breaks, contact OEM, they say that model is discontinued, parts need to be ordered, lead time two months. Two months. Wait or not?

This problem doesn't exist with asynchronous motors. Countless asynchronous motor models on the market. Same power, same speed, same mounting dimensions, grab a handful. OEM doesn't have stock, get another brand, available same day.

Done with the troublesome stuff, let's talk about how to choose.

Sales pushing permanent magnet machines will show you an ROI calculation sheet. Equipment price difference, annual electricity savings, years to payback. This sheet isn't fake, but those numbers are calculated the most optimistic way. Operating hours pushed high, load rate pushed low, electricity price pushed high.

You need to calculate with your own numbers.

How many hours does your compressor actually run per year? Some factories say 6,000 hours, actual might only be 4,000. What's your compressor's actual load rate? How much fluctuation? Lots of time at low load? What's your industrial electricity rate per kWh? How are peak and off-peak rates distributed?

Figure out these numbers, then you can calculate how much permanent magnet machines actually save.

Here's a rough decision method.

Long annual runtime, high load fluctuation, permanent magnet VFD advantage is obvious. What counts as high load fluctuation? Day shift full production, night shift half. Or weekends only running part of production line. Or distinct slow and peak seasons. All count. Compressor spending lots of time below 60% load, permanent magnet efficiency advantage translates into real electricity savings. Recovering equipment price difference in two to three years is possible.

Short annual runtime, stable load, permanent magnet advantage is limited. Only running 3,000 hours per year, or running 6,000 hours but load consistently above 85% without much fluctuation. In these cases efficiency gap between the two motors is only two to three percentage points. Equipment price difference might take five years or more to recover. Factor in cost of capital and equipment depreciation, math doesn't work out.

Poor environmental conditions, prioritize asynchronous. High temperature, dust, bad ventilation, inconvenient repairs. Add these together, asynchronous reliability advantage is worth more than those few percentage points of efficiency. Stable equipment operation is more important than saving some electricity.

Remote areas, be careful with permanent magnet. Many industrial projects in the northwest and southwest are far from big cities. Equipment breaks, hard to find repair help. Asynchronous motors, local people can fix. Permanent magnet motors might need shipping to the provincial capital. Add in downtime waiting for repairs, electricity savings don't cover it.

Tight budget projects, asynchronous has better value. Equipment purchase money has to be paid today. Electricity savings happen slowly over the next few years. Some companies just can't come up with that much money today, or don't want to tie up capital in equipment. Then buy asynchronous, save the difference for other things.

Compressor selection — Motor type selection factors

Don't get led around by marketing talk.

Permanent magnet VFD is good technology. In the right applications, really can save substantial electricity costs. But it's not a cure-all, not suitable for all conditions, not guaranteed to pay off. Those claims of 30% savings, two-year payback, need to be understood with a discount.

Atlas Copco and Ingersoll Rand product lines have both permanent magnet VFD and asynchronous VFD. If permanent magnet machines truly dominated asynchronous across the board, these two companies couldn't possibly still keep asynchronous product lines.