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Understanding AC Induction, Permanent Magnet and Servo Motor Technologies: OPERATION, CAPABILITIES and CAVEATS

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Table of Contents Chapter One: AC Induction Motors ........ 2 www.cmafh.com ▪ • Operation and technology overview ...... 2 • Capabilities for force, torque, speed and 426-5480 Understanding other factors ............. 3 (800) • Caveats: Limitations, ▪ AC induction, performance challenges and potential concerns . 5 permanent magnet Chapter Two: Mechanical Permanent Magnet ▪ and servo motor AC Motors .............. 6 • Operation and technology overview ...... 6 Electrical ▪ technologies: • Capabilities for high and OPERATION, CAPABILITIES AND CAVEATS low force, torque, speed and other factors ......... 7 • Caveats: Limitations, Pneumatic ▪ performance challenges JIM MURPHY, SENIOR APPLICATION ENGINEER and potential concerns . 8 PM & Large Motors Commercial Leader LEESON Electric Corp., Grafton, Wis. Chapter Three: Hydraulic ▪ Servomotors ............ 9 Engineers today are tasked with understanding, • Operation and technology overview ...... 9 Control evaluating and applying myriad motor technologies because the majority of rotary motion is • Capabilities for force, ultimately powered by electric motors. torque, speed and other Motion factors ................. 11 ▪ In this eBook, we outline the capabilities of driven • Caveats: Limitations, performance challenges AC induction motors, permanent-magnet motors and and potential concerns . 11 servomotors — the three major technologies with partially overlapping functionalities for larger, higher-end Chapter Four: applications requiring precisely metered torque, speed or Comparisons and positioning. We then expound on some engineering caveats Conclusion ........... 11 and compare all three options for specific situations. • Overview of pros CMA/Flodyne/Hydradyne and cons of each of motor type .............. 12 • Where is each motor type suitable or Courtesy unsuitable? ............. 12 Understanding AC induction, permanent magnet and servo motor technologies: OPERATION, CAPABILITIES AND CAVEATS www.cmafh.com ▪ In this eBook, we will discuss AC ! 426-5480 induction motors, permanent magnet AC (PMAC) motors and servomotors. Then (800) ▪ we will compare each technology and explore how each motor type is most suitable for a given set of applications. Mechanical Illustrated here are magnet-induced flux and current-induced ▪ flux, upon which all electric motor operation is based. Electrical ▪ Pneumatic ▪ CHAPTER ONE: AC induced current and magnetism cause it to follow the field gen- INDUCTION MOTORS erated by the stator, rotary motion is output. Hydraulic Because an AC induction motor increases the flux enclosed ▪ In all its iterations, the induction motor induces magnetism by its stationary coils, it is a transformer with a rotating sec- that is leveraged to output rotary motion. The stationary outer ondary (rotor). The rotor current’s effect on the air gap flux stator is connected to an external electrical power source; this causes torque. Control is fed to the rotor’s poles in a rotating progression that causes AC induction motors are built by manufacturers according revolutions of the magnetic field within the motor. Conducting to established National Electrical Manufacturers Association bars in the rotor interact with the stator’s magnetic fields; cur- (NEMA) standards in myriad fractional and integral horse- Motion ▪ rent is induced in those bars, which in turn generate magnetic power ratings and associated frame sizes. These AC induction fields that are attracted to those of the stator. As the rotor’s motors are quite common — the workhorse of industry. AC induction motor technology overview An induction motor’s stator consists of a stack of thin, highly permeable steel laminations with slots; the lami- nations are either secured in a steel or cast-iron frame that provides a mechanical support. The windings that accept the external power supply are run through the slots. The AC inductor rotor assembly resembles a cage CMA/Flodyne/Hydradyne consisting of aluminum or copper conducting bars of connected by short-circuiting end rings — hence the nickname squirrel cage for induction motors. The rotor This exploded view of a LEESON Electric AC induction motor also has laminations; radial slots around the lamina- shows the stator windings, rotor (in red), support bearings, cooling tions contain the bars. As mentioned, the rotor turns Courtesy fan (in white) and other elements. 2 Understanding AC induction, permanent magnet and servo motor technologies: OPERATION, CAPABILITIES AND CAVEATS when the moving magnetic field induces current in the They’re also classified by how they are started, as these motors shorted conductors, and the rate at which it rotates is the alone develop no starting torque, but require external means www.cmafh.com ▪ motor’s synchronous speed — determined by power-supply for initial actuation. frequency and the number of stator poles. The simple split-phase (induction-start-induction-run) motor has a small-gage-start winding with fewer turns than the main winding to create more resistance and put the start 426-5480 winding’s field at a different electrical angle than that of the main — causing the motor to rotate until it reaches 75% of rated speed. Then the main winding of heavier wire keeps the (800) ▪ motor running. This inexpensive design develops high start- ing current (700% to 1,000% of rated), so prolonged starts cause overheating. Suitable applications include small grind- ers, blowers and low-starting-torque applications requiring up to 1/3 hp. Mechanical Synchronous speed is the fastest theoretical speed a motor ▪ can spin — when the rotor spins at the same speed as the motor’s internal rotating magnetic field. In practice, an AC induction motor is an asynchronous motor (in which the Electrical rotor lags field speed), so its rotor must spin more slowly than ▪ the field, orslip . This allows the induction of rotor current to flow, and production of torque to drive attached load while overcoming internal losses. Pneumatic ▪ N – Hydraulic ▪ N + Control This single-phase, capacitor-start, induction-run motor from + S LEESON Electric is an instant reversing motor designed for parking gates and door operators requiring up to 1 hp. This type of general-purpose motor has a beefed-up Motion ▪ start winding plus a capacitor for boost. Starting torque is – typically 200% to 400% of rated load; starting current to Courtesy Motion System Design 575% of rated current allows higher cycle rates and thermal S resiliency. Current is induced in the rotor’s conducting bars and Split-capacitor motors are common but slowly being associated magnetic fields interact with those of the stator. replaced with more efficient motors and variable-frequency This causes the rotor to follow the field generated by the drives (VFDs), which we’ll explore later. Split-capacitor stator, to rotate the output shaft. motors have a run-type capacitor permanently connected in series with the start winding, making the latter an auxiliary AC induction motor capabilities for winding once the motor reaches running speed. Starting CMA/Flodyne/Hydradyne force, torque, speed and other factors torques are 30% to 150% of rated load, unsuitable for hard-to- of AC induction motors are either single-phase or poly-phase. start applications. However, starting current is less than 200% Single-phase AC motors power myriad low-horsepower com- of rated load current, making them suitable for cycling or mercial and industrial applications where three-phase power frequent reversals — in fans, blowers, intermittent adjusting is impractical; they’re not efficient, but can last a lifetime. mechanisms and quick-reversing garage door openers. Courtesy 3 Understanding AC induction, permanent magnet and servo motor technologies: OPERATION, CAPABILITIES AND CAVEATS industrial applications. (For example, 3 lb-ft of torque from a four-pole motor yields 1 hp.) www.cmafh.com ▪ NEMA classifies general-purpose, three-phase motors as A, B, C or D according to their electrical design. For example, NEMA Design C motors have higher starting torque with normal start current and less than 5% slip. 426-5480 Some of these motors are connected directly to line LEESON Electric FHP Series (fractional power with a basic switch; OVERSIZING AND (800) ▪ hp) AC inverters are suitable for driving others are fitted with wye- UNDERSIZING permanent-split capacitor, shaded pole delta windings or soft starters. and AC synchronous motors. Many three-phase inverter However, roughly one-third Carefully define application manufacturers claim that they can run single-phase motors of all industrial designs are requirements before effectively by wiring only two phases; however, this method choosing a replacement may cause instability due to the lack of feedback from variable-load applications; Mechanical motor, or one for a new in these designs, motors are ▪ one of the motor connections. Furthermore, motor torque design: An undersized motor is reduced considerably because the phases are 120° increasingly being driven by exhibits electrical stresses apart. In contrast, LEESON’s FHP inverter uses a two- VFDs — which have (thanks and premature failure. phase connection but its fundamental design enables more to increasingly sophisticated, efficient operation. These solid-state inverters run in volts/ An overly powerful motor Electrical yet affordable, semiconductors (with high locked-rotor and ▪ hertz mode (which we’ll outline later in this chapter) and are and circuitry) proliferated over
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