In terms of construction, how do Permanent Magnet AC (PMAC) motors differ from AC Induction motors?
In the broadest sense, the 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.
What are the primary benefits of PMAC motor versus AC Induction?
Permanent Magnet AC motors are inherently more efficient due to elimination of rotor conductor losses, lower resistance winding and “flatter” efficiency curve. Due to their synchronous operation, PMAC motors offer more precise speed control. PMAC motors provide higher power density due to the higher magnetic flux as compared with induction machines. Finally, Permanent Magnet motors generally operate cooler, resulting in longer bearing and insulation life.
What is the difference between “axial” and “radial” flux motors?
In an axial flux motor, the magnetic force (through the air gap) is along the same plane as the motor shaft, i.e. along the length of the motor. A radial flux motor is the more traditional design, in which the magnetic force is 90° (perpendicular) to the length of the motor/shaft. Think about axial flux like the disc brakes in your vehicle, where the disc rotates like the rotor in an axial flux design. Axial flux is not peculiar to permanent magnet motors.
What applications are more suitable for axial- versus radial-flux designs?
The answer depends upon which engineer you speak with, as some are "axial fans" and others favor radial. Seriously, it most often comes down to "form factor": does the customer require a longer, skinnier (radial) motor or is a "pancake" (axial) design more appropriate for the application? The "tie breaker" may be in the form of cost, as the axial design, once tooled for production, provides equivalent torque but uses less active material…i.e. it's more "power dense".
What are some of the major differences in performance between AC Induction and PMAC?
The most obvious performance difference is that a PMAC motor rotates at the same speed as the magnetic field produced by the stator windings; i.e. 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, as its rotational speed is slightly slower than the magnetic field's “speed”. An asynchronous motor is said to have “slip” (the difference between the motor's physical speed of, say, 1750 rpm, and its stator's magnetic speed of 1800 rpm) and 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 results in improved efficiency, better dynamic performance and more precise speed control…a major benefit in positioning applications.
Is the winding different in a SyMAX™ motor?
SyMAX™ IHP motors utilize a “concentrated winding” …essentially a bobbin winding. Several notable benefits are derived from this:
What’s the difference between PMDC, PMAC, Brushless AC, PMSM (Permanent Magnet Synchronous Motor) and BLDC (Brushless DC)?
PMDC motors typically employ permanent magnets affixed to the inside of the motor frame, rotating wound armature and commutator (brushes). PMAC, PMSM and Brushless AC are synonymous terms. They are PM machines that operate on a PWM AC drive or control similar to an induction motor but with software to control a PM machine. BLDC motors are very similar to PMAC machines in terms of construction, but utilize DC (trapezoidal) drives rather than AC (“sine”) drives, which are used to control PMAC motors.
Are SyMAX™ motors suitable for Variable or Constant Torque applications?
Yes. The same motor can be used in either mode. The VFD and application parameters will dictate to the motor how much torque to produce at any given speed. The flexible design makes SyMAX™ the logical choice when variable speed operation and ultra-high motor efficiency are key customer needs.
What is the useful speed range for PMAC motors, as compared to AC Induction?
PMAC machines typically have a wider speed range than AC Induction machines. However the number of poles may be different for the motors being compared and speed range is also a function of the drive being used so it is best to check with the manufacturer about your specific speed range. In general, SyMAX™ motors are rated for variable- or constant-torque to 20:1 without feedback (open loop) or 2000:1 closed loop (with encoder).
Explain “Back EMF”.
Back EMF is the voltage generated by a rotating permanent magnet machine. As the rotor spins…either with or without power applied to the stator windings…the mechanical rotation generates a voltage, i.e. becomes a generator. The resultant voltage waveform is either shaped like a sine wave (AC) or a trapezoid (DC), depending upon the power supply from the drive. This characteristic is the major difference between “Permanent Magnet AC” (a.k.a. “Brushless AC”) and “BLDC” (Brushless DC). The faster the rotor spins (again, with or without power), the higher (BEMF) voltage is generated.
Why is the SyMAX™ motor's maximum speed limited by Back EMF?
(Re-read the previous question/answer first). Motor BEMF increases directly with motor speed. The motor is connected to the electronic drive. The electronic components in the drive are designed for a maximum voltage above the rated voltage of the drive. Normally the motor and control are designed to operate well below the maximum voltage of the components. However if the motor speed exceeds the design speed range (either being powered from the control or being driven by the load) it is possible to exceed the maximum voltage the drive components and failures would result. Note that the drive is capable of controlling or “limiting” motor BEMF when it is operating properly. However if the drive faults and losses control during this over-speed condition it cannot protect itself.
Should the user exercise extra caution when the shaft is rotated, even with power off?
Absolutely! Touching the terminals while the shaft is rotating (wind-milling) will produce a shock hazard. If the shaft is turning fast enough…and generating higher voltage to the terminals…touching the connections may result in serious injury or death.
What other safety considerations are peculiar to PMAC motors?
Rare earth permanent magnets, such as Neodymium or Samarium Cobalt, have very strong magnetic properties (the reason for selecting these materials for our motor, as they produce high flux levels in the motor), and must be handled very carefully. Beyond the potential “pinching hazard”, those with pacemakers or other medically-implanted devices (including hearing aids) should exercise extra caution when working around these strong magnetic fields. Cell phones and credit cards may also be at risk. The good news is that, when the rotor is secured within the enclosure, radiated magnetic energy is no higher than that of an induction motor. A new Installation, Operation and Maintenance manual, Form 5968M, has been developed for 180-280 (IEC 112-180) frame SyMAX™ motors, and is shipped with every new motor.
What is “cogging torque”, and is this an important consideration in selecting a PMAC motor?
The most basic source of cogging torque is the interaction or attraction of the permanent magnets and the steel structure of the stator as the motor rotates. These attractions and overcoming them prevent the rotor from turning smoothly. Another source is the interaction of the rotor magnets and the stator winding when it is energized, due to harmonics. Cogging is often an undesirable feature, causing noise, vibration and non-uniform rotation, so during product development, minimizing this effect was a design “CTQ” (Critical To Quality). As a result, SyMAX™ motors have extremely low cogging torque, resulting in smoother operation at all speeds, virtually eliminating torque and speed “ripple”.
Are permanent magnets subject to “demagnetization”?
High current or high operating temperatures can cause magnets to lose their magnetic properties. The drive reduces the risk of high current “demag”, as these devices are equipped with over-current protection. The motor design minimizes the possibility of excessive temperatures causing magnet failure, due to the selection of high temperature magnets, incorporation of thermostats and low operating temperature of the motor. Permanent magnets, once demagnetized, cannot recover, even if the current and/or temperatures return to normal levels.
Can PMAC motors be operated without a drive?
No. All commercially available true permanent magnet motors require a variable frequency drive to operate. There is on-going research into a “line start PM” motor. There are performance characteristics of this design that have to be taken into consideration to determine if this type of motor is suited for your particular application.
What is a “switched-reluctance” (S/R) motor, and how is it different from PMAC?
Switched Reluctance motors…which also require a drive…do not utilize permanent magnets in the rotor; rather the topology embodies a rotor core with protruding “poles”…the protrusions are strongly magnetically permeable, while areas surrounding these protrusions are weakly magnetically permeable due to the slots cut into them. Coils in the stator act as an electromagnet that attracts the nearest rotor pole. Stator coils are synchronously energized with rotor rotation, with overlapping phases. S/R motors are known for producing high efficiency and good motion-control, producing 100% torque at “stall” indefinitely. Due to their winding design, and resultant decrease in end turn height, S/R motors can also be used in applications requiring a smaller “form factor” (size). Finally, S/R motors can generally be operated at higher speeds, as they lack the “BEMF” constraint of their PMAC brethren.
How many magnetic “poles” does the SyMAX™ motor have?
SyMAX™ FHP (fractional horsepower, meaning 48-56/140 frame) motors are a 6-Pole design.
Why does a PMAC motor have more “poles” than an equivalent AC Induction machine?
It doesn't have to, but we chose this topology to reduce cogging torque.
Since the SyMAX™ product line has a fixed number of poles, how do you get various base speeds?
Speed is a function of frequency…the same as it is with induction motors. The higher the input frequency from the drive, the faster the motor rotates. As is the case with most synchronous motor manufacturers, the pole count is most often held constant while the input frequency dictates the motor's speed. In the case of the 48 frame motor, which has 6 poles, the motor’s input frequency from the drive must be 90 Hz in order to obtain 1800 RPM. In order to obtain the same speed (1800) in the 10-Pole 180 frame motor, input frequency must be 150 Hz. Here is a simple formula to calculate required input frequency (Hz) when # poles and speed is known:
How do the performance curves compare between PMAC and IM (induction motor)?
There is a link on www.symax.com that will bring you to a series of performance curves, comparing SyMAX™ PMAC with an equivalent induction motor. These include Constant Speed Load Curves, Variable Speed Variable Torque, and Variable Speed Constant Torque.
What impact does speed (input frequency) have on the efficiencies of induction and PMAC?
Generally speaking, a PMAC motor has a “flatter” efficiency curve than its IM counterpart, providing even more energy savings at reduced speed.
How much additional efficiency should a user expect to obtain from a PMAC motor?
In general, PMAC motor losses (inverse of efficiency) are 15-20% lower than NEMA Premium induction motors. Since each efficiency index represents 10% fewer (or greater) losses than its neighbor, efficiency ratings will be 1-3 indices higher. Depending upon motor size, electric utility rate and duty cycle, customers could see as little as a one-year payback by using SyMAX™ PMAC motors.
What are servo and stepper motors, and how are they different from PMAC?
Servo and stepper motors are utilized in many motion control applications, where low inertia, fast response and high dynamic performance are important. Servo motors are very similar to PMAC motors but use special controllers (called “amplifiers”) and special feedback to control position rather than just speed. Step motors are somewhat similar to Switched Reluctance, and “step” to each defined rotor position, resulting in high repeatability and accuracy. The price for servo systems is quite high…often 10-20 times that of an equivalent rated induction motor. Applications requiring “near servo” performance may be excellent candidates for PMAC motors, as the cost/performance ratio may be much more beneficial to the user.
What is “power density”, and how does it relate to PMAC motors?
Power density is simply the ratio of output power or horsepower to physical size or volume of the motor. There are many factors such as material characteristics and temperature constraints that limit how much power a machine can deliver of a certain size. Different topologies and machine configurations address these limitations in various ways. For example, rare earth permanent magnets produce more flux for their physical size than the magnetic energy (and resultant torque) produced by an induction motor’s “squirrel cage rotor”. As such, a PMAC motor can have higher power density than an equivalent rated IM.
What is “form factor”, and how does it relate to PMAC motors?
Form factor is a different way of looking at power density. In this context, form factor refers to the physical (primarily dimensional) properties of the motor, which may be defined as the active materials or the overall envelope size. The simplest view of this is the frame size reduction potential of PMAC versus IM…assuming the same power rating. Another way of looking at this is the ability to “down-frame ” the same power rating into a physically smaller package (frame size).
What is the motor’s Service Factor, and how does this differ from “Reserve Torque Capability”?
SyMAX’s Service Factor (SF) is rated 1.0 on VFD power, which is in line with all other inverter-duty induction motors. SF is an oft-times misunderstood concept, and its use is limited to NEMA motors (the IEC doesn’t recognize SF). Operating any motor beyond its rated power will result in additional heating. Heat is the enemy. Intermittent operation above rated power is most normally acceptable, so long as its components can withstand the additional thermal stress. “Reserve Torque Capability” is an expression of the motor’s ability to safely deliver increased torque, due to higher peak torque capability, and is subject to the drive’s ability to deliver increased current. The SyMAX™ motor has a Reserve Torque Capability of 150% for 60 seconds.
What is “saliency”, and why is it important?
Saliency typically refers to the difference in motor inductance at the motor terminals as the motor rotor is rotated. This difference corresponds to an “alignment” and “misalignment” of the rotor in the stator. This difference is one way motor drives use to determine the rotor position during operation and is used to control the motor.
Are all AC drives suitable for operation of PMAC motors?
The drive used should be designed for use with permanent magnet machines. This is included in the specification for the drive and there is often a parameter to set to tell the drive that the motor attached is a PM motor. Some drives, not specifically designed for PM machines, will run and control a PM motor. Performance would typically not be as good in this case and this should be approved by the drive manufacturer before attempting. It is possible to damage the motor or drive if the drive is not set up properly or are mismatched.
What is the operating temperature of SyMAX™ PMAC motors, how does this compare with induction, and what is the benefit?
SyMAX™ motors, because they are more efficient than induction motors and run cooler under the same load condition. This results in longer insulation and bearing life and reduces the amount of heat that goes into the operating environment. A general rule-of-thumb is that for every 10°C increase in operating temperature, insulation life is reduced by half; conversely every 10°C reduction in temperature doubles the insulation life. SyMAX™ IHP (integral HP) motors are equipped with a full Class H insulation system but the design limits operating temperature to no more than Class B rise, providing very generous “thermal headroom” …and much longer insulation life.
Can SyMAX™ motors accept the same modifications and accessories as their induction motor counterparts?
How do sound and vibration levels compare, between PMAC and Induction?
The sound and vibration of PM motors varies widely from manufacturer to manufacturer. This tends to depend on the type of application on which the motor is intended to be used. SyMAX™ motors are designed for most general purpose applications with similar sound and vibration as an induction motor. In some cases the SyMAX™ motor runs quieter and smoother than an induction machine. A sound and vibration comparison of a typical induction motor versus SyMAX™ is available upon request.
How should I sell SyMAX™ motors?
This innovative new product should be presented to the customer as a means to reduce costs in the following critical areas of operation:
In short, this is a highly featured, Ultra Efficient™ innovation in motor technology, and should be sold based on the value it brings to the customer… not as a commodity/price sale. Target customers understand total life cycle cost and appreciate the product's many benefits.
What other supporting documentation is available to sell this new product?
A variety of sales support materials are available on line or by request, which include product brochures, technology comparison with induction, performance comparison curves, payback calculator (soon to be on line), pre-written customer specification and PowerPoint presentation. Of course, we offer the ability to produce prototypes to provide added assurance that the product will perform as stated. Beyond that, a “Birth Report” is supplied with every production motor, to provide baseline performance.
What are some typical applications for SyMAX™ that I should be on the lookout for? Any I should stay away from?
Virtually any application suitable for use of induction motors can utilize SyMAX™ motors. If the customer is primarily interested in increased motor efficiency, look for centrifugally-loaded variable speed applications like fans and pumps. On the other hand, if they're using belts, chains or gearboxes, they will be more intrigued by this motor's higher power density, as it's in those applications where the benefits of reducing or eliminating power transmission devices becomes apparent, resulting in increased system efficiency and reduced maintenance cost. Sale of this type of motor into a “fixed speed” application will require a major effort, since the motor requires a VFD in order to operate.
Is a PMAC motor immune from voltage spikes from the drive, causing insulation breakdown and shaft voltages?
SyMAX™ motors are designed, in much the same way as inverter duty induction motors, to withstand voltage spikes from drives and wiring conditions. A major component is the patented Max Guard® insulation system. The lower operating temperature also contributes toward longer insulation life on a drive.
What is commutation and how does the motor accomplish it?
The term commutation comes from the world of DC motors. It refers to how current is routed to the right coils, in the DC rotor, at the right time to generate the torque needed by using brushes and a commutator. A commutator is round, mounted on the rotor shaft and has conductive pads on the O.D. on which the brushes rest. Current is conducted from the brushes to the commutator and then to the connected coils as the rotor spins. This term has been carried over to brushless DC motors where the brushes and commutator have been replaced by electronics and a sensor on the rotor shaft. In this case current is still “switched” but by electronics. The term loses meaning in the world of AC motors but still sometimes refers incorrectly to how the AC voltage is generated in the drive.
What agency approvals does the SyMAX™ motor carry?
UL approval has been obtained. CSA, cUL and CE approvals are pending at this time.
Where are SyMAX™ motors manufactured?
“FHP” (48 frame) is produced in West Plains, MO. “IHP” (180-larger) is produced in Wausau, WI.
What is the warranty term for SyMAX™ motors?
36 months from date of first use, or 42 months from date of manufacture, whichever occurs first.
Are spare parts available?
Yes. However, certain components of the rotor…particularly the magnets…are not available separately.
How are we handling repairs?
All motors are to be returned to the plant of manufacture (West Plains for 48 frame, Wausau for 180 frame and larger) until instructed otherwise.