Ten Key Facts You Need to Know About Rotary Screw Air Compressor Motors
Kaishan USA | April 16, 2025| Uncategorized
Our independent distributor, Iowa Pump and Compressor, recently installed this KRSP2 compressor, which is driven by a large 350-HP motor.
A motor is the driving force behind a rotary screw air compressor. So much so that the motor’s capacity is used to describe the compressor’s. After all, we call them 25-HP and 50-HP compressors.
The motor's horsepower rating tells us how much work the compressor can perform—the higher the motor's horsepower, the greater the compressor’s capacity.
You need that power to generate all the energy needed to deliver compressed air at the required flow and pressure.
The nameplate on your compressor motor gives you all the information you need to operate the motor within established parameters. One key piece of information is the voltages the motor can be wired to, especially if you have a main motor offering several voltage options. (Courtesy, Plant Engineering.)
It’s All in a Nameplate
According to Plant Engineering magazine, all the critical information is on your air compressor motor nameplate, including the following data:
- Frame size
- Power (in HP or kW)
- Voltages, including both a delta (low-voltage) and a star (high-voltage) connection
- Motor speed (RPM)
- Efficiency standard, usually IEC or NEMA
- Ingress protection (IP) rating, protection against foreign material entering the motor
- Service factor, the motor’s power rating
As a result, it helps to know a lot about air compressor motors when acquiring, operating and maintaining an air compressor. Following are ten critical pieces of information that will help. We’ll start with the revelation that the main motor is not the only motor in your compressor.
1. There’s More Than One Air Compressor Motor
Rotary screw air compressors have secondary motors driving key components like cooling the fans used in air-cooled units. Water-cooled machines also have very small motors, keeping the hot air circulating out of the package. We’ll discuss the types of cooling approaches below.
The different uses of these motors are spelled out in the compressor’s CAGI sheets. For more on reading and interpreting the information in these important documents, read our blog post, “A Beginner’s Guide to Reading Rotary Screw Compressor Performance Curves.”
So what kinds of motors are used in rotary screw air compressors?
2. Different Types of Air Compressor Motors Are Used
While most rotary screw air compressors currently use traditional induction motors, permanent magnet motors are considered the wave of the future. Here’s a brief description of each.
Induction Air Compressor Motors
Induction motors use alternating current to convert electrical energy into mechanical energy through electromagnetic induction. Since Nikola Tesla invented them more than a century ago, they have been applied in a wide range of applications because they are efficient, reliable and flexible.
They are asynchronous—the rotor spins more slowly than the magnetic field created by the stationary stator’s wire coils, causing a lag or slip.
Single- and three-phase induction motors are used most often in industry today, especially in air compressors. However, many manufacturers are considering permanent magnet motors because of their efficiency, small size and greater reliability.
Permanent Magnet Air Compressor Motors
Unlike induction or asynchronous motors, permanent magnet motors rotate at the same speed as the stator’s magnetic field, so there is no slip.
The PM motor’s magnets are made of rare earth elements and are at least twice as powerful as those used in induction motors, allowing them to exceed the power density efficiency of the more traditional induction units. They are more expensive as a result.
You can spin the motor at higher and lower speeds than an induction motor, improving the turndown capability. They are designed to work with variable-speed drives.
And while there have been some notable failures, we believe PM motors are the wave of the future. The electric vehicle industry is all in on PMs, most likely leading to a higher adoption rate as the obvious advantages overcome the initial skepticism stemming from the early failures.
PM motors are the only way the industry will be able to comply with the evolving global efficiency standards.
3. You Can Measure Air Compressor Motor Efficiency with Different Standards
Several different groups or agencies have developed efficiency standards for motors:
- The National Electrical Manufacturers Association
- The International Electrotechnical Commission
- Guobiao (GB), China’s national efficiency standard.
Not surprisingly, NEMA ratings are commonly used in the Americas, while IEC standards are followed in Europe and GB is more commonly enforced in Asia and many other international locations.
4. Air Compressor Motors Are Rated for Protection/Insulation Class
Insulation class establishes the temperature that your motor can endure. Class F is the most common rating used in motors above 5 HP, while Class H is less common, used for heavy-duty, hot or high-altitude conditions.
The key thing to remember is that if you go too far beyond the rating, you’ll risk burning out the motor. The rule of thumb is that every 10°C you operate a motor above the insulation rate cuts the insulation’s service life in half, according to Machine Design magazine.
5. Air Compressor Motor Enclosures Protect Your Equipment
All our compressors are equipped with totally enclosed, fan-cooled motors, preventing debris and contaminants from entering the unit. In addition to cooling the unit, the fan boosts the efficiency and equipment life of TEFC motors compared with open drip-proof motors.
6. The Service Factor Measures the Resilience of Your Air Compressor Motors
The service factor is the power rating a motor can handle for short periods. It helps you know how far beyond the rated capacity you can expect to run the motor. A typical reading of 1.15, for instance, indicates you can operate the motor at 115% of its capacity for short periods.
7. Your Air Compressor Motors Needs To Be Grounded
For most variable-speed drive applications, the motors must be grounded, usually with a grounding brush, a consumable item that often needs replacement. You’ll need to work with your distributor to replace it properly.
8. Supplemental Air Compressor Motors Play a Crucial Role in Cooling
Cooling is critical in a rotary screw air compressor. Heat is the sworn enemy of an air compressor. It will shorten the life of the oil and the compressor itself. It will increase all your costs, including electricity bills and maintenance expenses. It can even result in poor air quality, including pass-through of oil and water.
There are two types of auxiliary motors in your compressor that assist in cooling:
- Air cooling. Fans push air across a fin-and-tube air compressor to remove heat and moisture. The compressor is usually designed assuming the doors are shut, so it’s essential not to remove the doors or enclosures. The motor for the oil cooler is usually the largest of these auxiliary motors.
- Liquid cooling. While a pump does most of the work in liquid cooling, a small fractional-horsepower motor drives a fan that pulls air out of the package to ensure heat buildup does not damage the main motor and the electronics. Our new portable unit has liquid cooling on its variable-speed drive and electric motor. The objective is to provide extra cooling to increase efficiency and prolong service life.
Visit our blog post to read about “All the Ways Heat Gets into Your Compressed Air System (And How To Get Rid of It)."
9. Bearings Ensure the Performance and Longevity of Air Compressor Motors
We use premium quality SKF bearings machined to tight tolerances in all our compressors. They absorb the radial and axial loads from the rotors, ensuring the performance and the life of the airend.
10. Air Compressor Motors Connect to the Airend in Several Different Ways
Rotary screw air compressors have a main motor driving the rotors. Most have electric motors:
- Direct-drive motors are directly coupled to the airend. A coupling usually connects the main motor to the airend (although some companies use a gearbox or other device between the motor and the airend to eliminate the need for couplings).
- Belt-drive units, such as our KRSB belt-drive rotary screw air compressors, are driven by a belt-and-pulley system.
Our KRSB belt-drive rotary screw air compressors are driven by a belt-and-pulley system.
A related issue is the drive function fulfilled by variable-speed drives. Many companies have saved significant amounts of energy by using rotary screw air compressors equipped with VSD technology. And energy savings are not the only benefit. VSD compressors can earn rebates, allow soft starts and offer better control of your compressed air system. They are also an energy-efficient way to address rapid cycling.
Companies have saved significant amounts of energy using rotary screw air compressors equipped with VSD technology.
But they are not a good choice at the extremes—we encourage customers not to use a VSD compressor when the machine runs at 20% of its total capacity most of the time or more than 80%. At the higher ranges, they are not cost-effective because there are energy losses associated with the drive itself, and the savings will be minimal. At usage rates below 20%, the compressor will not get warm enough to evaporate moisture in the air, creating the potential for rust and corrosion.
For more on the pros and cons of VSDs, see our blog post, “How Variable-Speed Drive Rotary Screw Air Compressors Save You Money.”
As with most motor-related issues, it’s wise to seek help from a knowledgeable compressed air consultant when determining whether a VSD is right for your operation.
Get Expert Help from a Local Professional
Your local compressed air consultant is an essential resource who can address any concerns about air compressor motors. Our nationwide network of independent distributors, can provide on-site help and consultation on almost any issue. These factory-trained air compression experts have deep expertise in addressing air compressor motors or broader compressed air system concerns.
We choose to partner with these independent, local businesses because there’s no better way to ensure you have expert guidance, fast response times and personalized support tailored to your needs. They don't just sell compressors—they build relationships that ensure you get the right system, reliable service and quick access to parts when you need them most.
So, when you buy through Kaishan, you're getting more than a product—you're getting a local partner, a compressed air expert who cares about your business and seeing it succeed.
Key Takeaways
Ten Key Facts You Need to Know About Rotary Screw Air Compressor Motors
1. There’s More Than One Air Compressor Motor
2. Different Types of Air Compressor Motors Are Used
3. You Can Measure Air Compressor Motor Efficiency with Different Standards
4. Air Compressor Motors Are Rated for Protection/Insulation Class
5. Air Compressor Motor Enclosures Protect Your Equipment
6. The Service Factor Measures the Resilience of Your Air Compressor Motors
7. Your Air Compressor Motors Needs To Be Grounded
8. Supplemental Air Compressor Motors Play a Crucial Role in Cooling
9. Bearings Ensure the Performance and Longevity of Air Compressor Motors
10. Air Compressor Motors Connect to the Airend in Several Different Ways
Let Us Help
Understanding how your air compressor motor works is extremely helpful in selecting, operating and maintaining a rotary screw air compressor. If you need help with motor-related issues or any other compressed air concerns, get in touch with the experts at Kaishan. Contact us today.
