Induction vs. Permanent Magnet Motor Efficiency
Electric motor efficiency has a significant impact on the industrial, consumer, and automotive sectors. Increased efficiency leads to lower greenhouse gas emissions through a reduction in power consumption and increased range between charges - for everything from EV’s to your power tools. With electrification continuing to accelerate across our day-to-day living, many wonder what type of motor is best suited to meet these modern day demands.
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Historically, the induction motor was the go-to motor design because it was readily available and is a longstanding, proven technology. However, the intrinsic design of the induction motor requiring a slip between rotor and stator will always limit efficiency. With recent advances in permanent magnet materials (energy density), and manufacturing, today’s permanent magnet motor designs take performance and energy efficiency usage to new levels not possible with the induction design.
Let’s take a deeper dive into both motor designs which then supports the choice of a permanent magnet design over that of the induction motor design. Equally important, understanding how Soft Magnetic Composites (SMC) can transform not only the traditional radial flux design, but new topologies that are driving tomorrows designs and performance levels with reduced heat generation and a more efficient use of copper and magnet material.
Here are a few things to consider when exploring induction vs. permanent magnet motors:
Permanent Magnet Motor vs. Induction Motor Efficiency
The figure below shows the general layout of both the Permanent Magnet motor (on the left) and the induction motor (on the right). In the permanent magnet design, the rotor contains a series of magnets either internal or external to the OD of the rotor. The stator is wound with copper wire creating a magnetic field that interacts with the rotors permanent magnets resulting in rotation and torque. Compare this to the induction motor where rotor and stator are traditionally stamped lamination steel with the motor windings only on the stator which induces an opposing magnetic field in the rotor. This interaction results in rotational torque.
(Comparison of AC induction motor design vs. permanent magnet motor)
Modern high torque motors whether permanent magnet or induction design use three-phase applied current. The three-phase design offers inherently better efficiency and is also self-starting. If the motor is designed to operate at a fixed rotational speed, then the number of stator poles can be adjusted to give the desired speed at the typical fixed frequency of 50 or 60 Hz. For these types of applications, the laminated induction motor is probably the most frequently chosen alternative. However, what if you want to have a variable speed motor? In this configuration, you would need to incorporate a variable frequency power supply to facilitate the variable speed. Although an induction motor would work, in this design, the permanent magnet design offers enhanced performance with greater flexibility.
The fine details of electric motor design are more complex than described below, but this is a great head start for those weighing their options between an induction and permanent magnet motor design.
Permanent Magnet Motor Efficiency
The inherent efficiency of a permanent magnet motor is higher than an induction motor – eliminating the intrinsic lag of the applied and induced field. Permanent magnet motor run synchronously with the applied frequency - allowing the motor to operate at a speed set by the frequency drive. As you increase the frequency, total losses in induction motors will be far greater than in permanent magnet motors – having efficiencies up to 97.5%.
A 50 kW (about 70 HP) permanent motor typically weighs less than 30 lbs. At any given frequency, the rotational speed of the permanent magnet motor is always greater than that of its induction counterpart due to the inherent slippage necessary in the induction design. The synchronous speed can be represented by the following equation:
Ns = 120 * frequency / pole count
(Ns is synchronous speed. Pole count is the total pole count per phase, including both the north and south poles)
Today, permanent magnet motors are used in applications and platforms such as - the Ford Mustang Mach-E, BMW, Ultium Platforms, Tesla, high efficiency variable frequency HVAC motors, battery powered hand tools and drones...Did you spot the trend here – everything that is battery powered or dependent upon high efficiency, is a 3 phase permanent magnet motor.
Induction Motors:
As noted earlier, an induction motor operates by the stator winding inducing an opposing current in the rotor (thus creating a magnetic field). That opposing field results in rotor rotation. The lag between the applied stator current and resultant rotor opposing field results in slippage between the applied field and rotation. The maximum speed of an induction motor is represented by the same equation as for the permanent magnet motor. However, inherent with induction is the requirement for slippage (asynchronous operation). As shown in the figure below, when the amount of slip in an induction motor approaches zero, the torque generated also goes to zero. Thus, it is impossible to operate an induction motor synchronously. For example, a two pole AC induction motor operating at 60 Hz will have a synchronous speed of 3600 RPM but there is typically a 5% loss in speed due to the slippage; thus, the maximum motor speed will be about 3400 / 3500 RPM. This intrinsic design characteristic limits the maximum efficiency of the induction motor to about 90- 93%.
The maximum efficiency of an induction motor is 90 / 93% whereas that of a permanent magnet motor is at 97% plus. Although a 4 to 7% improvement doesn’t seem like a lot - imagine the cost of operation over a 10 year or more life span and that relatively small improvement in efficiency results in a considerable energy savings with reduced greenhouse gas emissions.
Remember when we mentioned a 50 kW (about 70 HP) permanent motor typically weighs less than 30 lbs? Well, the weight of a typical 75 horsepower induction motor can exceed 500 pounds! Think of the implications this has for an automobile – the weight reduction is significant and has a multiplying effect on the total weight of the vehicle.
Cost Vs. Performance
One major consideration in permanent magnet motors is the cost of the magnets. If you’ve used high-energy magnets (such as iron neodymium boron), you’ve felt the pain in your budget (or your boss has). The potential waste of stamping the lamination material only compounds the problem.
Opportunities for powder metallurgy are abundant in these types of motors. The rotors of a permanent magnet motor can be made via sintered powder metal, regardless of whether you’re taking the internal or external design route. The stator can also be produced via soft magnetic composites. At the high switching frequencies expected, the losses in SMCs are lower than that of laminated 3% silicon iron, further improving the efficiency of this design. Simply put, soft magnetic composites are custom-built for high frequencies.
There’s an opportunity for powdered metal to provide additional efficiency to a permanent magnet motor vs. an induction motor. The 3D shape-making capabilities of powder metallurgy allow you to form the stator to totally encase all the wire in soft magnetic composite to eliminate end turn losses.
These are some of the many advantages that powder metal -- both sintered soft magnetic materials and SMCs -- offers.
Induction Vs. Permanent Magnet Motor Efficiency: The Winner Is...
The clear winner here is the permanent magnet motor. Now, couple the permanent magnet motor, with a unique topology enabled by Soft Magnetic Composite (SMC) technology and your motor will be lighter and more efficient, with a higher torque density and lower bill of material cost – all while reducing supply chain complications and using a sustainable manufacturing process.
If you need help designing the components to fully leverage the full potential of powder metallurgy for an AC or DC magnetic applications, contact us and check out our resource hub:
(Editor's note: This article was originally published in April 2020 and was updated on November 29, 2022 and July 27, 2023)
Permanent Magnet AC Motors and Motor Ratings - Part 3 ...
Permanent Magnet AC Motors
Like an induction motor, a permanent magnet motor has a Stator configured to provide a number of poles, usually for a 3-Phase supply.
However, instead of containing Rotor bars, the Rotor has magnets mounted on, or embedded in, the laminations. These magnets, which are often made from rare earth elements (neodymium or samarium cobalt), provide a strong permanent magnetic field around the Rotor.
This field reacts with the Stator’s rotating magnets (in the same way as in an induction motor), but there is one important difference: unlike the induction motor, which relies on the rotational slip between the Rotor and Stator magnetic fields to induce a magnetic field in the Rotor, in a permanent magnet motor, the Rotor follows the Stator magnetic field directly. Furthermore, the rotating magnetic field is created by an electronic circuitry which magnetises demagnetising the appropriate motor poles. Thus by altering the timing or pattern of the process, EC motors could vary their speed infinitely and change rotational direction.
Hence, an electronic drive is required to control the operation of a permanent magnet motor. This can be separate – just like a variable speed drive used on an induction motor – or can be integrated into the motor casing, usually on the non-drive end cover.
The international description for permanent magnet motors with integral drives is a ‘Power-Drive System’ (this is the description used by Fläkt Woods); some manufacturers also use the term ‘Electronically Commutated (EC) Motor’.
Benefits of Permanent Magnet AC Motors
Because the Rotor in a PM motor doesn’t require any electrical current, it can be more efficient than an equivalent induction motor, as there will be none of the electrical losses that would be present in an induction Rotor.
This also means the heat generation is reduced, leading to improved reliability and operational life, and reduced maintenance in terms of relubrication.
Because of the permanent magnetic field, the Rotor can generate a higher torque at start-up compared with an induction Motor. The generally stronger magnetic field of the PM Rotor means more torque generated over the whole operating range, giving a higher power output for any given frame size. Thus the same-size motor can be made more powerful, or conversely the motor can be smaller and generate the same power.
This last point is important in ventilation systems, as it means reduced weight and, crucially, improved aerodynamics when the motor is mounted in the airstream.
For more information, please visit Working Principle of Pmsm Motor.
The presence of an electronic drive provides for variable speed control with no increase in wiring complexity, apart from the provision of a control signal. Some drives including the Fläkt Woods power-Drive System) can use connections from external sensors or networks for intelligent control.
Figure One is a table showing the relative advantages and disadvantages of permanent magnet motors and induction motors.
Although the initial cost of permanent magnet motors can be three or four times that of induction motors, this can be offset by the increased efficiency which delivers improved running costs.
So if speed control is required and it is intended to operate mainly at reduced speed to minimise running costs, PM motors do provide cost benefits over induction motors; however, if speed control is not required and running costs are not a primary concern, PM motors’ appeal is not so great – although their compact size and associated reduced weight may be a factor.
It is unlikely that PM motors will be suitable for us in emergency high temperature applications.
The Motor Nameplate
All general purpose motors are required to be shipped with a nameplate, which will contain specific information about the motor.
The IEC standard requires the following information to be provided:
- The manufacturer's name, model and serial number
- Rated voltage and full load amps
- Rated frequency
- Phase
- Rated full load speed
- Rated temperature rise or insulation Class
- Rated ambient temperature
- Duty rating
- Rated kW
- IE rating (efficiency) of Motor
In addition, further information may sometimes be provided on the nameplate:
- Service factor
- Enclosure type
- Frame size
- Connection diagrams
- Unique or special features
Motor Ratings
Motors are rated in terms of their mechanical output power at the shaft, in accordance with IEC 60034-1. The power is specified in terms of Watts (W), but most manufacturers express the power in kW.
The temperature rise of the Stator windings must be within the limit of temperature rise for the motor’s Thermal Class, and is assessed when the motor is operating in its intended use. IEC 60034-1 specifies the limits for operation as being under site operating conditions.
General purpose motors with integral cooling fans will be tested on a dynamometer under the standard ambient conditions (temperature between -15°C and 40°C, and altitude up to 1000 m). This motor rating is described as ‘Totally Enclosed Fan Cooled’ or TEFC.
Motors designed for use when mounted in the airstream of a Direct Drive Fan are tested when installed in the fan. Sometimes a dynamometer may be used with a supplementary fan providing cooling air at the equivalent velocity that would be experienced in the fan; these conditions are referred to as ‘Air Over the Motor’ (AOM) or airstream rated, and the motors are described as ‘Totally Enclosed Air Over’ machines (TEAO).
Generally TEFC rated motors can be used in Direct Drive Fans without adjustment to their rating, (although the direction of the motor fan compared with the main airflow should be considered).
AOM rated motors can only be used when installed in a suitable Direct Drive Fan. If the fan has a low volume flow rate (because of a low blade angle, for example), or if the motor is mounted in a drum or behind a large hub, then a de-rating factor will be required to adjust the motor’s rating accordingly.
Fans installed in Direct Drive Bifurcated Fans must have TEFC motors rated at TEFC conditions.
AOM ratings are typically 10 to 20% higher than TEFC ratings, depending on the design of the motor and the air velocity available from the fan.
Starting Current
When an induction motor is started by connecting directly to the full voltage electrical supply, the instantaneous current drawn will typically be five to seven times higher than the full load current. Known as the ‘Locked Rotor Current’, it is so high because when the Rotor is not turning, the only impedance to the electrical flow comes from the resistance of the Stator windings.
As the Rotor starts to rotate, this impedance also increases, resulting in less electrical current flowing in the Stator windings.
Alternative starting methods such as Variable Frequency Drive (VFD or Inverter), Soft Starter and Star/Delta can reduce this initial starting current.
Power Factor
Induction motors consist of resistive and inductive circuits, and this causes the current waveform to be out of phase with the voltage. The phase difference between the voltage and current waveforms is expressed as a Power Factor (PF). A PF of 1 means that the waveforms are in phase; a PF of less than 1 means that the current is leading or lagging behind the voltage.
The Power Factor can be calculated using the equation:
Power Factor = Active Power
Apparent Power
The Active power is the useful electrical power used by a motor, and is measured in Watts (W). The Apparent power consists of Active Power and Reactive Power, and is measured in volt-amps (VA). That Reactive Power is the power stored in and discharged by a motor, and is measured in volt-amps reactive (VAR).
A Power Factor of less than 1means that the circuit’s wiring has to carry more current than that which would be necessary were the power Factor equal to 1. That could mean that the supply cables and switchgear would need to be sized for a higher rating.
The Power Factor can be improved by selecting a different motor, or if this is not possible, power factor correction capacitors could be installed – although these are expensive and not always practical.
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