Understanding the Demand for Permanent Magnet AC (PMAC) Motors

As the world continues to ask questions such as “how are we using energy?” and “is this energy being used wisely?”, we are seeing increased governmental regulations, a focus on reducing strain on existing infrastructure, lower levels of fossil fuel emissions, and advances in materials and electronics. All these factors are causing the demand for Permanent Magnet AC (PMAC) motors to increase.

PMAC Motors vs. AC Induction motors: What’s the Difference

When comparing PMAC motors to AC induction motors, one major difference is in the rotor itself. In a squirrel cage induction motor, current is induced into the rotor from the field (stator) through the air gap and conducted through aluminum (or other material) bars, which are most often die cast in the slots of the rotor laminations. In the case of a PMAC motor, the rotor itself contains permanent magnet material, which is either surface-mounted to the rotor lamination stack or embedded within the rotor laminations. In either topology, electrical power is supplied through the stator windings.

The most obvious performance difference is that a PMAC motor rotates at the same speed as the magnetic field produced by the stator windings, meaning it is a synchronous machine. If the field is “rotating” at 1800 rpm, the rotor turns at the same speed. An induction motor, on the other hand, is considered an asynchronous machine. Its rotational speed is slightly slower than the magnetic field’s “speed”. An asynchronous motor is said to have slip. This slip is the difference between the motor’s physical speed of, say, 1750 rpm, and its stator’s magnetic speed of 1800 rpm. It cannot produce torque without this difference in speed, as the rotor is constantly trying to “catch up” with the magnetic field. The synchronization of PMAC motors results in improved efficiency, better dynamic performance, and more precise speed control.

Other performance differences include higher efficiency and power factor in a PMAC motor. Since a permanent magnet rotor lacks conductors (rotor bars), there are no I2R losses, so with everything else equal, a PMAC motor is inherently more efficient.

PMAC motors provide higher flux density than a comparable induction motor. This means more power (torque) can be produced in a given physical size, or equal torque produced in a smaller package. It is very common to see a frame size reduction with PMAC motors.

All commercially available true PMAC motors require a variable frequency drive to operate. While there is on-going research into a "line start PM" motor, all current commercially available, true PMAC motors require a variable frequency drive to operate. There are performance characteristics of this design which have to be taken into consideration.

Comparison of PMAC and Induction Motors

PMAC Motor	AC Induction Motor
Permanent Magnet rotor	Die cast rotor
VFD power only	Line or VFD power
High power density	Low power density
Wider air gap	Narrow air gap
Fixed pole count within frame size	Varying number of poles within frame size

Benefits of PMAC Motors

Because of their lower operating temperature, reduced vibration and noise, and improved reliability, PMAC motors provide benefits including energy savings, lower operating temperature, reduced vibration, reduced noise, improved reliability, and application flexibility.

1. Energy Savings - PMAC motors are inherently more efficient due to elimination of rotor conductor losses, lower resistance winding, and a flatter efficiency curve. PMAC motors are known to provide even more energy savings at reduced speed.

   Efficiency ratings are 1-3 indexes above NEMA Premium, providing electrical energy savings from day one. This translates to 10-30% fewer losses than a conventional motor and provides quick return on the investment. Since electricity is estimated to comprise approximately 95-97% of the total life cycle cost of electric motors; this energy savings significantly reduce the total investment.

2. Reliability and Longevity - Due to their lower operating temperature, PMAC motors require less frequent maintenance and offer longer motor bearing and insulation life.

3. Application Flexibility - Under most circumstances, replacing existing induction motors with PMAC motors requires no mechanical changes to the equipment. Virtually any application suitable for use of induction motors can utilize PMAC motors, with direct drop-in replacement for existing NEMA and IEC motors.

4. Performance - Due to their synchronous operation, PMAC motors offer more precise speed control. PMAC motors also provide higher power density due to the higher magnetic flux as compared with induction machines.

Which Applications Could Benefit the Most from PMAC Technology

Standard induction motors are not particularly well suited for low-speed operation as their efficiency drops with the reduction in speed. They may also be unable to deliver sufficiently smooth torque across the lower speed range. This is normally overcome by using a gearbox. The PMAC motor solution provides a high torque drive coupled directly to the load. By eliminating the gearbox, the user saves space and installation costs, as he only needs to prepare the foundations for one piece of machinery.

Since we don’t see the dropping of efficiency as we slow down as compared to induction motors, PMAC motors work well in commercial HVAC fan applications in data centers, cooling towers, and AHUs.

As more efficient motors become the industry norm and more companies look to retrofits for energy tax credits, the need for innovative and efficient motors such as PMAC will increase. Understanding your options can help ensure you achieve the lowest total cost of ownership for your electric motors.