Permanent-magnet motor rotary screw compressors

What is a permanent-magnet motor compressor?

A conventional screw compressor uses an induction motor. Its rotor carries copper or aluminium bars, and the magnetic field that turns it is induced by current from the stator. That induction process costs energy as rotor losses and heat, and the motor needs magnetising current even when it is lightly loaded.

A permanent-magnet synchronous motor (PMSM) replaces the induced field with high-strength magnets fixed in or on the rotor. The rotor turns in step with the rotating field from the stator, with no slip. Rotor losses fall sharply, and efficiency stays high even at low speed and part load. Most designs used in compressors are interior permanent-magnet motors, where the magnets sit inside the rotor laminations.

A PM motor cannot be started direct-on-line from the mains. It always needs a dedicated inverter drive that knows the rotor position and controls frequency and current. In practice, therefore, a permanent-magnet compressor is always a variable-speed compressor. Many manufacturers also mount the motor directly on the airend, sometimes sharing the oil circuit for cooling, which removes the gearbox or belt and the coupling losses that go with them.

How the motor, drive and airend work together

The compressor controller reads the discharge or header pressure and asks the drive for more or less speed. Because a screw airend delivers roughly in proportion to rotor speed, slowing the motor reduces free air delivery (FAD) to match demand, instead of loading and unloading against a fixed-speed motor.

• Drive (inverter): converts mains supply to variable frequency, sets speed and limits starting current, so there is no star-delta inrush.
• PM motor: delivers torque with low losses across the speed range. Its efficiency curve is flatter than an induction motor’s at reduced speed.
• Airend: still sets the basic efficiency of compression. A poor or worn airend is not rescued by a good motor.
• Controller: holds a narrow pressure band, often within a fraction of a bar, which lets the plant lower its set point.
• Cooling: the motor and drive both need cool air. Motors may be air-cooled, oil-cooled or liquid-cooled depending on the design.

Every screw airend has a speed range where it is most efficient. At very low speed, internal leakage back across the rotor tips becomes a larger share of the flow, and specific power rises even if the motor itself stays efficient. That is why turndown range and part-load specific power curves matter more than the motor label.

Induction fixed speed, induction VSD and PM VSD compared

The table describes general behaviour. Exact efficiencies and turndown limits depend on the manufacturer’s design and must be taken from that model’s published data.

When does a PM motor compressor pay off?

The motor saves energy in two ways: lower losses at every speed, and the ability to follow demand without unloaded running. The second effect is usually the larger one, and it comes from variable speed itself, so the question is really whether a variable-speed machine fits your demand, and then whether a PM motor improves on an induction VSD for your profile.

A PM variable-speed compressor is a strong candidate when:

• Demand swings between shifts, during batch processes or with machine start-ups.
• The existing compressor spends long hours unloaded, or cycles frequently.
• The plant runs two or three shifts, so small efficiency gains accumulate over many hours.
• Stable pressure matters to the process, for example packaging, textile machinery or pharmaceutical equipment.
• Electricity tariffs and demand charges make running cost the main lifecycle cost, which is normal for industrial compressors in India.

It is a weaker case when one compressor runs flat out all day at full load. There the gain over a good fixed-speed machine is limited to the motor and drive difference, and drive losses partly offset it. In multi-compressor plants the usual answer is fixed-speed machines on base load with one variable-speed machine as the trim compressor, sized so that its efficient speed range covers the expected swing.

How to evaluate a PM motor compressor properly

1. Measure the demand. Log flow, pressure and power for at least a representative week. Without a demand profile, any saving claim is a guess.
2. Fix the real pressure need. Identify the highest pressure any user truly needs at the point of use, then add only the real treatment and piping losses to set the compressor discharge. Each extra bar costs about 7% more power.
3. Compare specific power, not motor class. Ask for package specific power in kW per m³/min at your working pressure, at full speed and at several part-load points, measured to ISO 1217 Annex E for variable-speed machines.
4. Check the turndown range. Find the minimum and maximum FAD and confirm that your normal demand sits within the efficient part of the curve, not at the bottom edge.
5. Look at the whole package. Dryer, filters, receiver and piping losses can cancel out motor gains. A high pressure drop through clogged filters wastes as much as a poor motor.
6. Check site conditions. Ask about the rated maximum ambient temperature, drive enclosure protection and derating. Many Telangana and Andhra Pradesh compressor rooms run well above 40 °C in May.
7. Plan the service support. Confirm who can diagnose the drive, what spares are needed and how long a drive or motor replacement would take for that brand.

What we check on site

When a plant asks us whether a PM motor machine will help, or why an existing one is not delivering, we look at the following:

• Pressure band and set point, and whether the controller is holding pressure tightly or hunting.
• Speed profile over time: is the machine living at minimum speed, at maximum speed, or stopping and starting?
• Power and flow readings to calculate actual specific power.
• Compressor room ventilation and the temperature at the drive and motor cooling inlets.
• Supply voltage, voltage imbalance and harmonic issues that can trip the drive.
• Drive fault history, fan condition, dust on heat sinks and filter mats.
• Airend condition: discharge temperature (normally about 75–95 °C on an oil-injected screw), noise and bearing condition.
• Interaction with other compressors: a variable-speed machine fighting a fixed-speed one is a common cause of poor results.

Common faults and their usual causes

Most PM motor problems we see are not motor failures. They are heat, supply quality and control set-up. For detailed drive fault work, see our VFD diagnostics page in the links below.

Servicing a PM motor compressor safely

The routine service of the compressor itself is the same as any oil-injected screw: air filter, oil filter, separator element, oil change, cooler cleaning, valve checks and condensate drains. The differences sit in the motor and drive:

• A spinning PM motor generates voltage even when the drive is switched off. The airend must not be turned by hand or by air flow during electrical work, and the drive’s DC bus must be confirmed discharged.
• Removing a PM rotor needs proper tools and procedures, because the magnets pull strongly on steel parts and tools.
• Magnets can lose strength if the motor runs persistently too hot. Cooling and correct temperature protection are not optional.
• Drive firmware, parameters and motor data must match. Replacing a drive or motor from another source without the right set-up can cause poor running or faults.

Our general screw-compressor service approach is described on the rotary screw service page; this page focuses on what is different with permanent-magnet machines.

How Reddy Compressors helps

Reddy Compressors has worked with industrial compressed air since 2002 from our office at Tarnaka, Secunderabad. We are an independent multi-brand specialist, not an authorised dealer of any manufacturer. We have tie-ups with all major and reputed brands. We sell new and used compressors, including variable-speed and permanent-magnet machines from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes, and we rent, service and supply spares for them.

• Selection: we size from your measured or estimated demand profile and pressure need, and compare the published specific power of candidate machines, rather than choosing on motor type alone.
• Audits: we log flow, pressure and power to show whether a variable-speed or PM machine is likely to pay back on your site.
• Service and AMC: our team of over 20 service engineers and technicians covers all of Telangana and Andhra Pradesh, handling routine service, cooler and ventilation problems and drive fault diagnosis.
• Spares: we identify parts from the nameplate, model and serial number and confirm fit before quoting, and supply across India.
• Standby air: if a drive or motor fails and the replacement takes time, we can provide a rental compressor from our own fleet to keep production running.

What to send us

• Make, model, serial number and a nameplate photo of the existing compressor or the model you are considering.
• Working pressure at the compressor and the minimum pressure needed at the point of use.
• Typical air demand, or a description of the machines and shifts it supplies.
• Running hours, load hours and any fault codes from the controller.
• Photos of the compressor room, showing ventilation openings and ducting.
• Your electricity supply details and whether the plant runs on a DG set at times.

Frequently asked questions

Is a permanent-magnet motor compressor worth the higher price?

It is usually worth it where demand varies and the compressor runs long hours, because energy is most of a compressor’s lifetime cost. Where one machine runs at full load all day, the gain over a good fixed-speed compressor is smaller. The way to decide is to log your demand, then compare the specific power of each option at your pressure and calculate annual kWh for your real load pattern.

What is the difference between a PM motor and a VSD compressor?

A VSD compressor changes motor speed to match demand; the motor may be an induction or a permanent-magnet type. A PM motor compressor is always variable speed, because the motor needs an inverter drive to run at all. The PM motor adds lower losses, particularly at reduced speed, so it can improve part-load efficiency compared with an induction VSD of the same size.

Can a permanent-magnet motor compressor run without its drive?

No. A permanent-magnet synchronous motor cannot be started or run directly from the mains, so the inverter drive is essential. If the drive fails, the compressor stops until the drive is repaired or replaced. For production-critical air, plan a standby compressor, keep critical drive spares where practical, or arrange rental cover so a drive fault does not stop the plant.

Do PM motor compressors work in hot Indian summers?

They can, provided the compressor room is ventilated properly and the machine is rated for the ambient temperature. Drives and PM motors are sensitive to heat; running persistently hot causes drive trips and, over time, can weaken the magnets. Check the rated maximum ambient, duct hot air outside, keep cooling filters clean and avoid recirculation of exhaust air back to the inlet.

How much energy does a permanent-magnet compressor save?

There is no honest single figure, because saving depends on your demand profile, pressure, the machine being replaced and running hours. Most of the saving comes from following demand instead of running unloaded, and some from the efficient motor. Measure existing power and flow, compare against the new machine’s published part-load specific power, and calculate the difference in kWh per year.

Is servicing a PM motor compressor different from a normal screw compressor?

The compressor side is serviced the same way: filters, separator, oil, coolers and drains. The motor and drive need extra care. A turning PM motor generates voltage, the drive must be confirmed safe before electrical work, and rotor removal needs proper tools. Drive diagnosis and correct parameter set-up also need experienced engineers familiar with variable-speed machines.

Related pages

• Variable-speed drive (VSD) rotary screw compressors
• Direct-drive rotary screw compressors
• Fixed-speed rotary screw compressors
• Specific-power assessment
• Compressor VFD diagnostics and drive faults
• Rotary screw compressors hub