Mechanical Engineers Guide to Motor Terminologies and Architectures
Motor Terminology
Cogging Torque
Cogging torque is a torque disturbance related to the change magnetic circuit reluctance with angle. It is caused by the non-uniform lamination tooth structure. It is customary to skew stator laminations or magnets to minimize the effects of cogging, but it is difficult to remove all the cogging torque in a motor through these methods. Motors that do not have teeth, or a magnetic change in reluctance with angle, do not have cogging torque. The cogging torque amplitude and frequency is typically related to the least common factor of the pole and slot count of the motor. The torque itself can be both opposing and helping, depending on the location of the shaft relative to stator. Cogging toque is most commonly felt when the motor shaft is turned by hand, and the shaft appears to have many locations of natural resting positions, wanting to jump to them, or resist moving away from them. Cogging torque of less than 3% of continuous torque is generally considered to be low, while cogging above 5% would be considered high. The most user-friendly motors have zero cogging or less than 1%.
Torque Ripple
While cogging torque can create torque ripple, current induced torque ripple is fundamentally different and can appear in motors that do not have cogging. Torque ripple is a torque disturbance that results from a mismatch between the controller current profile and the motor torque output with respect to rotor angle. Theory states that when the field vector and current vectors are exactly 90 degrees apart and uniform in their magnitude, uniform torque will be produced throughout the entire electrical cycle. This again assumes that the motor torque produced with angle is matched to the current versus angle curve for the appropriate phase being energized. Each motor design has a distinct relationship between pole count, slot count, skew or new skew, and saturation that result in non-linear torque production with angle (non-sinusoidal, as the drive is expecting). For some motor architectures, physical properties of the motor, such coil centroids and magnet location and strength, can be managed.
Torque ripple can also be caused by improper commutation angle settings, a configuration step that is required in all motor and drive combinations. Torque ripple values of less than 1% of continuous torque would be considered “low”, while ripple above 5% would be considered high.
Motor Stiffness
Motor stiffness is a term that is often used when discussing motors, but unfortunately is not a true motor parameter, as it is influenced by many factors that are not part of the motor itself. Motor stiffness usage is often meant to be statement about the motor’s servo bandwidth, which makes the motor behave like a spring-mass-damper system in that the shaft position has a restoring force, or an ability to resist rotation. A motor by itself, un-servoed, has no stiffness, and when servoed, has a bandwidth that could vary, depending on many factors, but primarily driven by the mass or inertia of the moving load and limitations of available drive current. Other meanings of “motor stiffness” could be regarding the stiffness or rigidity of the motor housing, the shaft rotational stiffness, the magnet track or forcer (if linear), or the stator stiffness. All of these could be the meaning but are less likely than the “stiffness” determined by the servo loop.
Hysteresis
Motor hysteresis is a term that describes loss in the laminations of the stator. As the magnetic field created by the stator switches between a north and a south pole, atoms within the lamination steel resist fully aligning to the “new” north or “new” south poles, and as such, create loss in field strength, manifesting itself in the form of heat within the laminations.
Eddy Current
Eddy currents are created when a magnet moves relative to electrically conductive material, such as copper. Within the copper itself, currents circulate in a closed loop manner, creating loss in the form of heat. These speed dependent losses can become a significant form of torque or force loss in a motor when running at high speeds.
Commutation
All permanent magnet synchronous machines require electronic commutation. Commutation is the act of changing magnetic fields in the stator with the appropriate phase and amplitude to produce torque in the correct direction. In its simplest form a brush DC motor has a commutator which is a mechanical connection between the rotor and stator that allows current to be directed to the correct winding to product positive or negative torque. Electronic commutation is achieved by using sensors in the motor (hall devices) or external sensors that can inform the driver about rotor position. With this knowledge the driver can put current into the phases in the correct amplitude and direction to create positive or negative torque. It is critical in an electronically commutated motor that the initial phase angle (relation between rotor magnets and stator phases) be set the right point. If this is not correct the torque constant of the motor will change with direction and torque ripple can be induced.
Poles
“Poles” are defined as the number of permanent magnets on the rotor of a Brushless Three Phase motor. This value is a key component to all commutation algorithms. It is customary to refer to pole-pairs which indicate the number of north + south magnets are in the motor.
Slot
The motor “slot” is defined as the space between the teeth within the stator. Historically it has been required to allow copper to be inserted into the stator to complete an electromagnetic phase, or three of them as is common in most PM synchronous motors, (AKA BLDC). The “slot-pole” count is a common parameter referenced, as it has a significant impact of cogging torque. This parameter is an indication of the number of poles
IPM versus SPM Motors
The first Brushless DC (PM Synchronous motors) were made from AC Induction motors. The AC motor rotor was removed and replaced by a rotor with permanent magnets bonded to the circumference. This became the SPM, surface permanent magnet motor. This new technology required electronic commutation because the commutator and brushes were not present. Hall devices were installed in the motor allowing DC motor drivers to switch phases at the appropriate angle to electronically commutate the motor for positive and negative torque and speed. This trapezoidal commutation put square waves of current into the synchronous motor.
Later advances in microcontrollers and DSPs enabled sinusoidal current control to allow this inherently synchronous motors to once again be run with sine waves instead of square waves (trapezoidal).
Another, and more current method of making PM Synchronous motors is using internal permanent magnets buried in the rotor core, the IPM. This method reduces the amount of magnet material required, retains the magnets internal to the rotor mechanically, and offers some improvements in motor control for a wider operating speed range.
Electrical Cycle
Electrical Cycle refers to the frequency of the applied voltage and subsequent current waveforms within the stator. The term “Electrical Cycles” is often reference to “mechanical revolution”, or “rev”. The value itself is the number of Poles divided by 2. This value is often needed in servo drives when commissioning a motor. The number of electrical cycles/revolutions is equal to the number of pole-pairs a given motor has.
Electrical Time Constant
The motor’s electrical time constant is defined as the division of phase-to-phase inductance by phase-to-phase resistance. The term is relevant in that it defines how fast current rises in the motor phase as voltage is being applied via a high-speed Pulse width modulated signal. Generically, time constants are defined as the time needed to reach 63% of the final value. For motors, the Electrical Time Constant would be the time it takes for current to reach 63% of its final value.
Mechanical Time Constant
Mechanical Time Constant is the time it takes for the motor shaft to reach 63% of its final speed value. The value is calculated as Tau = R*J/(Ke*Kt), where R is the resistance, J the inertia, Ke the back-emf and Kt, the torque constant.
Direct and Quadrature Current
These are terms used to describe the currents that exist in a brushless three phase motor when FOC algorithms are used. Direct Current is aligned to the rotor’s magnetic field whereas Quadrature Current is perpendicular to the rotor’s field. Recalling Cross Product rules, since torque is the cross product of current and magnetic field, when the angle is zero, the Sine of zero is also zero and when the angle is 90 degrees, the sine of 90 is unity. This angular difference in the vectors is why Quadrature Current is the torque producing current in motors. It is worth noting that any residual Direct Current often leads to torque ripple, which is an unwanted disturbance in a motion system. Field Orientated Control works to maximize Quadrature Current while holding Direct Current to zero.
FOC Control
Field Orientated Control (FOC) is a form of current control that implements a mathematical transformation that makes the drive more efficient. Control is done on the rotating D-Q reference frame, and when combined with the unique states of the multiple power stage FETs within the drive, the outcome is a highly efficient drive versus standard sinusoidal drive control. Traditional “Sine Drives” are susceptible to lower performance at higher speeds, as undesired Direct Torque becomes larger as the lag in the current loop grows. In an FOC drive, however, the transformations put the control on the rotated reference frame, making speed, or lag from higher speeds, a non-issue.
Field Weakening
Field weakening is an AC Induction motor control technique that changes the induced field in the rotor allowing a wider speed range to be achieved. This method is inherit in FOC control and also applied to permanent magnet synchronous machines. Injecting current into a phase at an improper torque position will reduce the effective back emf of the magnet. This achieves higher speeds than traditional no-load speed at the expense of efficiency loss (because current is going into the wrong place that wrong time). The benefit of Field Weakening is an increased available speed for the same bus voltage. The downside is a reduce level of available torque and efficiency. Field Weakening can be applied to both SPM and IPM motor architectures, but it works much better in IPM motors.
Motor Architectures
Frameless
Frameless motors are un-housed motors, meaning the stator and rotor are two unique parts that are designed to work in unison, but are not mechanically combined.
Slotted
Slotted motor architectures have the windings formed around laminations of iron, generally referred as “teeth”. The iron teeth provide a boost in the magnetic field as the filed crosses the gap between the stator and the rotor, producing a boost in the back-emf constant. The downside of these style motors is the presence of cogging torque. The iron teeth are attracted to the magnets within the rotor and create mechanical instabilities, or alternatively stated, create a desired rest position of the rotor within the stator’s field.
Slotless
Slotless architectures have no iron teeth in the stator, and therefore no cogging torque, or preferred rest position. The windings are formed, and sometimes potted, without any iron, or only iron behind the stator windings. These motors are often lighter, but generally have less motor constant given a diameter and length.
Brushed
Brushed motor bring voltage to the motor phases via mechanical connections known as “brushes”. Brushed motors do not require any form a drive commutation and are ready to function out of the box. Downsides are electrical arching and mean time before failure (MTBF), as the brushes wear over time.
Brushless
- DC
Brushless DC (BLDC) motors are not as common today, but the term lives on as the most common way people refer to all types of Brushless DC or AC motors. The term specifically describes a three-phase motor that when back-driven, produces a trapezoidal back-emf waveform, separated by 120 electrical degrees.
- AC
Brushless AC (BLAC) is the most common motor architecture, but the name is ironically, the least common. These motors are known for their sinusoidal back-emf waveforms, and when paired with Sine Drives or FOC drives, provide theoretically zero Torque Ripple.
- PMSM
Permanent Magnetic Synchronous Machines (PMSM) is the correct descriptive name for all Brushless AC motors. These motors are BLAC motors, with the more “textbook” correct descriptors in the name of “permanent magnet” and “machine”. At the textbook level, authors of electro-magnetism content regarding rotary motors, will refer to the motor as a “machine”. A little research on the origins of each will tell you a machine is a broader term describing any device that converts energy to perform a function whereas as motor is specific form of converting electrical energy into mechanical motion.
Servo Motor
A servo motor is when a PMSM is paired with a housing, an encoder, a shaft, and bearings, to provide a complete motor solution. The presence of the feedback device, meaning the encoder, is the most distinguishing aspect that defines a servo motor. There are some definitions that also include a servo drive in the housing, or attached to the housing, but that specific usage is not universally accepted.
AC Induction/Asynchronous
AC Asynchronous motors are those where the rotor also has motor phases, and the electromagnetic field created by the stator coils “induce” a field in the rotor coils, causing the rotor to turn. “Slip” is a phenomenon associated with these motors and is best described as a lg between the stator field and the rotor field. Slip makes these motors generally less efficient. AC motors generally are used for constant speed applications, but can be paired with an FOC drive, making them function as BLAC motor.
SPM
SPM stands for a surface permanent magnet rotor, meaning the magnets are bonded to the outside of the rotor. These motors differ from IPM motors in that there is no Reluctance Torque created.
IPM
IPM refers to an internal permanent magnet rotor, meaning the magnets are inside the rotor. These motors differ from SPM in that IPM motors have naturally occurring Reluctance Torque. IPM motors are often better for high-speed applications due to their ability to do field weakening and considered more durable because the magnets are internally captured.
