What's the Difference Between AC Induction, Permanent ...
What's the Difference Between AC Induction, Permanent ...
Servocontrol, on the other hand, takes control to the next level with feedback — and is suitable for larger designs.
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Three-phase PMDC motors (brushless motors) are also commonly used for servo applications. Most brushless DC windings are interconnected in an array, and most units are fitted with a trio of Hall sensors at one stator end. These Hall sensors output low and high signals when the rotor’s south and north magnet poles pass — to allow the following of energizing sequence and rotor position.
Permanent-magnet DC motors in servo applications
- Today, many PM motors are DC and used in servo applications requiring adjustable speed. For quick stops, these can minimize mechanical brake size (or eliminate the brake) by leveraging dynamic braking (motor-generated energy fed to a resistor grid) or regenerative braking (motor-generated energy returned to the ac supply). In addition, PMDC motor speed can be controlled smoothly down to zero, followed immediately by acceleration in the opposite direction without power circuit switching. In typical three-phase brushless DC motors, energization is controlled electronically. In some designs, permanent magnets are installed on the stator. More common designs include stators with stacked steel laminations and windings through axial slots; permanent magnets are installed on the rotor. Here, the stator winding is trapezoidally wound to generate a trapezoidal back EMF waveform with six-step commutation. Brushless DC switches energize changing pairs of motor phases in a predefined commutation sequence. Most units are fitted with a trio of Hall sensors at one stator end, to allow the following of energizing sequence and rotor position. Output torque has considerable torque ripple, which occurs at each step of the trapezoidal commutation. However, due to a high torque-to-inertia ratio, brushless DC motors respond quickly to control-signal changes — making them useful in servo applications.
In their most basic form, the drive for a servomotor receives a voltage command that represents a desired motor current. The servomotor is modeled in terms of inertia (including servomotor and load inertia) damping, and a torque constant. The load is considered rigidly coupled so that the natural mechanical resonance is safely beyond the servocontroller’s bandwidth. Motor position is usually measured by an encoder or resolver coupled to the motor shaft.
A basic servocontrol generally contains both a trajectory generator and a PID controller: The former provides position setpoint commands; the latter uses position error to output a corrective torque command that is sometimes scaled to the motor’s torque generation for a specific current (torque constant.)
Servomotor capabilities for force, torque, speed, and other factors: Servocontrol exhibits less steady state error, transient responses, and sensitivity to load parameters than open-loop systems. Improving transient response increases system bandwidth, for shorter settling times and higher throughput. Minimizing steady-state errors boosts accuracy. Finally, reducing load sensitivity allows a motion system to tolerate fluctuations in voltage, torque, and load inertia.
Typically, a profile is programmed for instructions defining the operation in terms of time, position, and velocity: A digital servocontroller sends velocity command signals to an amplifier, which drives the servomotor. With the help of resolvers, encoders, or tachometers for feedback (mounted in the motor or on the load) the controller then compares actual position and speed to the target motion profile, and differences are corrected.
Servomotor limitations
Most importantly, the increased performance of servomotor designs comes at dramatically increased cost.
In addition, there are two situations in which servomotor efficiency declines — low voltage and high torque. In short, servomotors are most often employed because of their ability to produce high peak torque, thus providing rapid acceleration — but high torque often requires that servomotors run two to three times their normal torque range, which degrades efficiency.
Finally, servos are designed to operate over a wide range of voltages (as this is how their speed is varied) but efficiency drops with voltage.Comparing induction motors, AC permanent-magnet motors, and servomotors
Finally, servos are designed to operate over a wide range of voltages (as this is how their speed is varied) but efficiency drops with voltage.
Designers and motor personnel benefit from finding a supplier that’s an experienced resource of information to help in pragmatic motor selection. Involve application specialists as early as possible, as they can help develop prototypes, custom electrical and mechanical designs, mountings, and gearboxes. This also reduces costs associated with shorter lead times and rush delivery.
In the end, all industrial motor subtypes have strengths and weaknesses, plus application niches for which they’re most suitable. For example, many industrial applications are essentially constant torque, such as conveyors. Others, such as centrifugal blowers, require torque to vary as the square of the speed. In contrast, machine tools and center winders are constant horsepower, with torque decreasing as speed increases. Which motors are most suitable in these situations? As we will explore, the speed-torque relationship and efficiency requirements often determine the most appropriate motor.
Overview of the pros and cons of each motor type
Induction motor
PMAC
Servomotor
SPEED
Less speed range than PMAC motors • Speed range is a function of the drive being used — to 1,000:1 with an encoder, 120:1 under field-oriented control
VFD-driven PMAC motors can be used in nearly all induction-motor and some servo applications • Typical servomotor application speed — to 10,000 rpm — is out of PMAC motor range
Reaches 10,000 rpm • Brushless DC servomotors also operate at all speeds while maintaining rated load
EFFICIENCY
Even NEMA-premium efficiency units exhibit degraded efficiencies at low load
More efficient than induction motors, so run more coolly under the same load conditions
Designed to operate over wide range of voltages (as this is how their speed is varied) but efficiency drops with voltage
RELIABILITY
Waste heat is capable of degrading insulation essential to motor operation • Years of service common with proper operation
Lower operating temperatures reduces wear and tear, maintenance • Extends bearing and insulation life • Robust construction for years of trouble-free operation in harsh environments
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Physical motor issues minimal; demanding servo applications require careful sizing, or can threaten failure
POWER DENSITY
Induction produced by squirrel cage rotor inherently limits power density
Rare-earth permanent magnets produce more flux (and resultant torque) for their physical size than induction types
Capable of high peak torque for rapid acceleration
ACCURACY
Flux vector and field-oriented control allows for some of accuracy of servos
Without feedback, can be difficult to locate and position to the pinpoint accuracy of servomotors
Closed-loop servomotor operation utilizes feedback for speed accuracy to ±0.001% of base speed
COST
Relatively modest initial cost; higher operating costs
Exhibit higher efficiency, so their energy use is smaller and full return on their initial purchase cost is realized more quickly
Price can be tenfold that of other systems
PMAC versus servomotors
Servomotors are utilized in motion control applications where low inertia and dynamic response are important. In fact, many motors used for servo applications are similar to PMAC motors but use special controllers (amplifiers) and feedback to control position rather than just speed. However, the price for servosystems can be high — often 10 to 20 times than that of an equivalently rated induction motor. Applications requiring near-servo performance are suitable candidates for PMAC motors, benefitting from their cost-to-performance ratio. Case in point: PMACs are well suited for typical pump operations, which typically run at variable speed between 75% and 85% of maximum speed.
PMAC motors are unsuitable in typically servomotor applications approaching 10,000 rpm — out of the PMAC motor range. In addition, without feedback for the PMAC, designers can find it difficult to locate and position to the pinpoint accuracy that servomotors must often deliver.
Now compare PMAC motors to those most commonly used for servo applications — brushless dc motors. A traditional brushless-dc drive waveform is trapezoidal; here, two of the motor's three leads are used for the phases, and the third is used for hunting — so it's regularly changing fields. In contrast, the three leads of the PMAC are actively used; input waveforms are sinusoidal, to boost efficiency while minimizing noise and vibration.
As mentioned, motor stator winding patterns are typically specialized for a specific waveform shape. One cannot differentiate them by visual inspection.
A controller that produces trapezoidal waveforms is less costly than those that produce sinusoidal waveforms. However, sinusoidal controllers and motors produce more consistent shaft rotation than trapezoidal — and rotor inertia, motor rating, and specific controller characteristics magnify the difference in performance.
Basic Motor Question (PMM vs Induction) - Endless Sphere
I think it depends completely on design of the motors. In an "identical" motor (not sure what that means since they are so different) I'd expect efficiencies to be close, with the BLDC coming out on top (as long as motor is run below base speed.)dforesi said:
Click to expand...
Click to expand...True single phase induction motors are very rare, since you cannot reliably start a single phase induction motor.
You may be referring to a single phase shaded pole motor, which has a copper ring around part of the stator. That causes enough phase shift that you get a pseudo second phase, which is out of phase enough from the main field that you get a rotating field. They are horrendously inefficient since that shaded pole wastes a lot of power to create that delay; it's basically a shorted winding.
Or you may be talking about a single phase motor with a start winding. These have two phases - a "start" winding and a "run" winding. The start winding is connected via a capacitor so that you get some delay in the current; this creates a rotating field to start the motor. Sometimes a centrifugal switch or potential relay then removes the start winding once the motor is running. (Once the motor is running, the single phase creates a field in the rotor that's not aligned with the single phase, so the motor can keep running.) Those can be very efficient.
I think it depends completely on design of the motors. In an "identical" motor (not sure what that means since they are so different) I'd expect efficiencies to be close, with the BLDC coming out on top (as long as motor is run below base speed.)True single phase induction motors are very rare, since you cannot reliably start a single phase induction motor.You may be referring to a single phase shaded pole motor, which has a copper ring around part of the stator. That causes enough phase shift that you get a pseudo second phase, which is out of phase enough from the main field that you get a rotating field. They are horrendously inefficient since that shaded pole wastes a lot of power to create that delay; it's basically a shorted winding.Or you may be talking about a single phase motor with a start winding. These have two phases - a "start" winding and a "run" winding. The start winding is connected via a capacitor so that you get some delay in the current; this creates a rotating field to start the motor. Sometimes a centrifugal switch or potential relay then removes the start winding once the motor is running. (Once the motor is running, the single phase creates a field in the rotor that's not aligned with the single phase, so the motor can keep running.) Those can be very efficient.
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