Comparing induction motors, permanent-magnet motors, and servomotors

Designers and motor personnel advantage from discovering a supplier that's an skilled resource of info to assist in pragmatic motor choice. Involve application specialists as early as you possibly can, as they are able to assist create prototypes, custom electrical and mechanical styles, mountings, and gearboxes. This also reduces expenses related with shorter lead occasions and rush delivery.

Servo motors can offer higher performance, faster speeds, and smaller sizes. PM synchronous motors offer advantages on high-energy- consuming and high-dynamic applications, compared to induction motors. Variable frequency drives used with asynchronous motors also can be used with synchronous servo motors, producing higher efficiencies than an asynchronous motor, using perhaps 30% less energy in positioning applications.

Induction motor systems (lower cost, rugged, reliable, and well known) can offer an alternative to servo motor systems (the traditional, established solution) for certain applications. This, of course, is based on similar electronic controls being used (with the latest technology and approximately the same cost), leaving the cost of motors the differentiating issue.

Overview of the pros and cons of each motor type

Induction motor

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
EFFICIENCY - Even NEMA-premium efficiency units exhibit degraded efficiencies at low load
RELIABILITY - Waste heat is capable of degrading insulation essential to motor operation • Years of service common with proper operation
POWER DENSITY - Induction produced by squirrel cage rotor inherently limits power density
ACCURACY - Flux vector and field-oriented control allows for some of accuracy of servos
COST - Relatively modest initial cost; higher operating costs

PMAC

SPEED - 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.
EFFICIENCY - More efficient than induction motors, so run more coolly under the same load conditions
RELIABILITY - Lower operating temperatures reduces wear and tear, maintenance • Extends bearing and insulation life • Robust construction for years of trouble-free operation in harsh environments
POWER DENSITY - Rare-earth permanent magnets produce more flux (and resultant torque) for their physical size than induction types
ACCURACY - Without feedback, can be difficult to locate and position to the pinpoint accuracy of servomotors
COST - Exhibit higher efficiency, so their energy use is smaller and full return on their initial purchase cost is realized more quickly

Servomotor

SPEED - Reaches 10,000 rpm • Brushless DC servomotors also operate at all speeds while maintaining rated load
EFFICIENCY - Designed to operate over wide range of voltages (as this is how their speed is varied) but efficiency drops with voltage
RELIABILITY - Physical motor issues minimal; demanding servo applications require careful sizing, or can threaten failure
POWER DENSITY - Capable of high peak torque for rapid acceleration
ACCURACY - Closed-loop servomotor operation utilizes feedback for speed accuracy to ±0.001% of base speed
COST - Price can be tenfold that of other systems

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.

What is a Servo Control System

A servo control system is one of the most important and widely used forms of control system. Any machine or piece of equipment that has rotating parts will contain one or more servo control systems. The job of the control system may include:

* Maintaining the speed of a motor within certain limits, even when the load on the output of the motor might vary. This is called regulation.
* Varying the speed of a motor and load according to an externally set programme of values. This is called set point (or reference) tracking.



Servo motors are brushed or brushless servo motors with feedback, typically encoder or resolver. They can be rotary or linear motors. They require a complex closed loop control algorithm (such as the classic PID method). Normally the control loop has to be tuned, and servo dither can be a problem.. Due to the added control and feedback, typically servo systems are more expensive than stepper systems.

Servo motors typically have a peak torque of 3-10x the continuous torque, their torque curve is much flatter than the stepper curve, and the maximum speeds are much higher. Peak torque is a great thing; often, a system just needs extra power for a short time to accelerate, overcome friction, or such.

Our daily lives depend upon servo controllers. Anywhere that there is an electric motor there will be a servo control system to control it. Servo control is very important. The economy of the world depends upon servo control (there are other things to be sure – but stay with me on the control theme). Manufacturing industry would cease without servo systems because factory production lines could not be controlled, transportation would halt because electric traction units would fail, computers would cease because disk drives would not work properly and communications networks would fail because network servers use hard disk drives. Young people would become even more unbearable and they would complain more than they do now, because their music and games systems will not work without servo control.

Servo control systems are that important and it is vital to know about them. So pay attention and sit up straight – you are not on holiday and I am not writing this for the good of my health.

Hybrid Stepper Motor

The hybrid stepper motor uses the principles of the permanent magnet and variable reluctance stepper motors. In the hybrid motors, the rotor flux is produced by the permanet magnet and is directed by the rotor teeth to the appropriate parts of the airgap.

The main flux path is from the north pole of the magnet, into the end stack, across the airgrap through the stator pole, axially along the stator, through the stator pole, across the air gap and back to the magnet south pole via the other end stack.

There are usually 8 poles on the stator. Each pole has between 2 to 6 teeth. There is two phase winding. The coils on poles 1,3,5 and 7 are connected in series to form phase A while the coils on poles 2,4,6 and 8 are connected in series to form phase B. The windings A and B are energised alternately.

When phase A carries positive current, stator poles 1 and 5 become south and 3 and 7 become north. The rotor teeth with north and south polarity align with the teeth of stator poles 1 and 5 and 3 and 7 respectively. When phase A is de energised and phase Bis exicited, are energised alternately.

The torque in hybrid stepper motor is produced by the interaction of the rotor and the stator produced fluxes. The rotor field remains constant as it is produced by the permanent magnet. The motor torque T is proporatinal to the phase current.

Following are the main advantages of the hybrid stepper motor:
1. Very small step angles upto 1.8
2. Higher torque per unit volume which is more than in cae of variable reluctance motor
3. Due to permanet magnet, the motor has some detent torque which is absent in variable reluctance motor.

These are the various types of the stepper motors. After duscussing the various types and the operating principle, let us discuss the important parameters related to a stepper motor. The stepper motor characteristics are mainly the indication of its important parameters.

1. Holding Torque:
It is defined as the maximum static torque that can be qpplied to the shaft of an excited motor without causing a continuous rotaing.
2. Detent Torque:
It is defined as the maximum static torque that can be qpplied to the shaft of an unexcited motor without causing a continuous rotation.
Under this torque the rotor comes back to the normal rest position even if excitation ceases. Such positions of the rotor are referred as the detent positions.
3. Step Angle:
It is defined as the angular displacement of the rotor in response to each input pulse.
4. Critical Torque:
It is defined as the maximum load torque at which rotor does not move when an exciting winding is energised. This is also called pullout torque.
5. Limiting Torque:
It is defined for a given pulsing rate or stepping rate measured in pulses per second, as the maximum load torque at which motor follows the control pulses without missing any step. This is also called pull in torque.
6. Synchronous stepping rate:
It is defined as the maximum rate at which the motor can step wihout missing steps. The motor can start, stop or reverse at this rate.
7. Slewing rate:
It is deined as the maximum rate at which the motor can step unidirectionally. The slewing rate is much higher than the synchronous stepping rate. Motor will not be able to stop or reverse without missing steps at this rate.

The introduction of Planetary Gearbox

Planetary gears are very popular due to their advantages such as high power density, companctness, and multiple and large compact gear ratios and load sharing among planets. Gearing arrangement is comrised of four different elements that produce a wide range of speed ratios in compact layout. These elecments are, Sun gear, an extenally toothed ring gear co-axial with the gear train Annulus, an internally toothed ring gear coaxial with ghe gear train Planets, externally toothed gears with mesh with the sun and anulus, and Planet Carrier, a support structure for planets, co-axial with the train. Planetary gear system as shown in Figure 1 is typically used to perform speed reduction due to serveral advantages over conventional parallel shaft gear systems.

Planetary gears are also used to advantages over conventional parallel shaft gear systems. Planetary gears are also used to obtain high power density, large reduction in small volume, pure torsional reactions and multiple shafting. Another advantage of the planetary gearbox arrangement is load distribution. If the number of planets in the system are more the ability of load shearing is greater and the higher the torque density. The planetary gearbox arrangment also creats greater and the higher the torque density. The planetary gearbox arrangment also creates greater stability due to the even distribution of mass and increased rotational stiffiness.

In recent years, enhancement of interior quietness in passenger cars. Automobiles is an important factor for influencing occupant comfort. planetary gear box sets are essential components of automatic transmissions because of their compact size and wide gear ratio range. They produce high speed reductions in compact spaces, greater load sharing, higher torque to weight ratio, diminished bearing loads and reduced noise and vibration. A Despite their advantage, the noise induced by the vibration of planetary gear systems remains a key concern. Planetary gears have receive considerably less research attention than single mesh gear paris. This paper focus on the study o two PGTs with different phasing (angular positions) while keeping every individual set unchanged.

This figure shows that the basic layout planetary gear train in which there is one Sun gear. Three Planet gear and one ring gear. They can produce the high speed reduction in compact space and having greater load shearing capacity & high torque to weight ratio.

Planetary basics — ratios, helix angles, axial loads, crowning

A planetary gearhead takes a high-speed, low-torque input, say from an electric motor, then increases torque and reduces speed at the output by the gearhead ratio. This lets motors run at higher, more-efficient rpms in equipment that operates at low speeds. It also reduces inertia reflected back to the motor, increasing stability. And using a planetary gearhead often lets machine builders reduce the size and cost of motion-control hardware.

Planetary units with helical gears, rather than spur gears, have a larger contact ratio. The contact ratio is the number of teeth in mesh at any given moment. While typical spur gearing has a 1.5 contact ratio, helical gearing more than doubles it to 3.3. Benefits of higher contact ratios include:

• 30 to 50% more torque capacity than equivalent spur-type planetary gearing.
• Better load sharing, which increases life.
• Smoother and quieter operation.
• Backlash reduced by as much as 2 arc-min.

The gearhead’s helix angle also has a significant impact on performance because the greater the angle, the more teeth in the mesh at any one time. So increasing the helix angle from the typical 12° up to 15° raises torque capacity by 17 to 20%; and by as much as 40% over straight-cut spur gears. Gears with a 15° helix angle also emit less noise.

The Type of Gear Reducers

Gear drives are also known as gear reducers or gearboxes. These are rugged mechanical devices desined to transmit high power at high operating efficiencies and have a long service life. The gear reducer is an important component of the mixer drive systems, providing speed reduction and increasing allowable torque. Moreover, in some cases it provides support to the mixer shaft.

Helical gears are used in parallel shaft gear reducer motor. In helical gears, gear teeth are machined along a helical path with respect to the axis of rotation. Helical gears are commonly used with two-, three-, and in some cases even four-, five-, and six-stage speed reductions. In-line helical reducers are a variation of parallel shaft speed reducers configured such that the output and input shafts are in-line.

Spiral bevel gears are used when the input and output shafts of the gear reducers are required to be at right angles. The curve shape of the spiral bevel teeth makes gradual contacts, resulting in less noise during operation. Helical, parallel shaft, and helical bevel gear units have high operating efficiency, approximately 98% for each gear stage reduction.

Worm gears, are the most economical speed reducers, capable of providing a sizable speed reduction with a single gear set. The input and output shaft of these gears are at right angles to each other. However, becasue of the sliding contact between the worm pinion and the gear, the worm gear redcuer is less efficient. The efficiency decreases as the speed redcution ratio increases. For example, at a speed reduction ratio of 10:1, the efficiency of the worm gearbox may be approximately 90%. However, at a redution ratio of about 50:1, the efficiency of the worm reducer drops to about 70%. Gearbox manufactures offer worm gear reducer in helical bevel and helical worm design.

Helical, spiral bevel, and worm gears are external gears with the teeth on the outer periphery of the gears. In planetary gears the teeth profile is on the inside of a circular ring with meshing pinion. Planetary gears consist of an internal gear with a small pinion, known as a sun gear, surrounded by multiple planetary gears. These gears can provide high speed reduction ratios and are relatively compact in size. Gear reducer manufacturers also offer geared-motor, that consists of a factory assembled motor with the gear unit. Figure 12.34 shows a variety of gear reducer with motor configurations.

NMRV worm gear series also available as compact and flexibility. NMRV worm gear series also available as compact integral helical/worm option, has been designed with a view to modularity: low number of basic models can be applied to a wide range of power ratings guaranteeing top performance and reduction ratios from 5 to 1000.

Stepper Drive Modes

This chapter has explained how to operate steppers by energizing one or two winding pairs at a tiem, but tere are a number of different ways to drive a stepper, and this discussion touches on four of them:

* Full-step (one phase on) mode - Each control signal energizes on winding.
* Full-step (two phases on) mode - Each control signal energizes two windings.
* Half-step mode - Each control signal alternates between energizing one and two windings.
* Microstep mode - The controller delivers sinusoidal signals to the stepper's windings.

Full-Step (One Phase On) Mode

The simplest way to control a stepper is to energize one winding at a time. This is the method discussed at the start of this chapter. Figure 4.15 shows what the signaling sequence looks lide when controlling a stepper in this mode.

With each control signal, the rotor truns to align itself with the energized winding. The rotor always turns through the stepper's rated step angle. That is, if a PM motor is rated for 7.5, each control signal causes it to turn 7.5.

Full-Step (Two Phase On) Mode

In the full-step (two phase on) mode, the controller energizes two windings at once. This turns the rotor through the stepper's rated angle, and the rotor always aligns itself between two windings. Figure 4.16 illustrates one rotation of a stepper motor driven in this mode.

Figure 4.17 shows what the corresponding drive sequence looks like.

The main advantage of this mode over full-step (one phase on) is that it improves the motor's torque. Because two windings are always on, torque increases by approximately 30%-40%. The disadvantage is that the power supply has to provide twice as much current to turn the stepper.

Half-Step Mode

The half-step mode is like a combination of the two full-step modes. That is, the controller alternates between energizing one winding and two windings. Figure 4.18 depicts three rotations of a stepper in half-step mode.

Figure 4.19 illustrates a control signal for a stepper motor driven in half-step mode.

In this mode, the rotor aligns itself with windings (when one winding is energized) and between windings (when two windings are energized). This effectively reduces the motor's step angle by half. That is, if the stepper's step angle is 1.8, it will trun at 0.9 in half-step mode.

The disadvantage of this mode is that, when a single winding is energized, the rotor turns with approximately 20% less toruqe. This can be compenstated for by increasing the current.

Microstep Mode

The purpose of microstep mode is to have the stepper turn as smoothly as possible. This requires dividing the energizing pulse into potentially hundreds of control signals. Common numbers of division are 8,64,and 256. If the energizing pulse is divided into 256 signals, a 1.8 stepper will turn at 1.8/256=0.007 per control signal.

In this mode, the controller delivers current in a sinusoidal pattern. Successive windings receive a delayed version of this sinusoid. Figure 4.20 gives an idea of what this looks like.

Using this mode reduces torque by nearly 30%， but another disadvantage involes speed. As the width of a control signal decrease, the ability of the motor to respond also decrease. Therefore, if the controller delivers rapid pulses to the stepper in microstep mode, the motor may not turn in a reliable fashion.

Sychronous type AC Servo Motor

The stator consists of a cylindrical frame and a stator core. The stator core is located in the frame and an armature coil is wound around the stator core. The end of the coil is connected with a lead wire and current is provide from the lead wire. The rotor consists of a shaft and a permanent magnet and the permanent magnet is attached to the outside of the shaft. In a synchronous type AC servo motor, the magnet is attached to a rotor and an armature coil is wound around the stator unlike the DC servo motor. Therefore, the supply of current is possible from the outside without a stator and a synchronous type AC servo motor is called a "brushless servo motor" because of this structural characteristic. Because this structure makes it possible to cool down a stator core directly from the outside, it is possible to resist an increase in temeprature. Also, because a synchronous type AC servo motor does not have the limitation of maximum velocity due to recification spark, a good characteristic of torque in the high-speed range can be obtained. In additon, because this type of motor has no brush, it can be operated for a long time without maintenace.

Like a DC servo motor, this type of AC servo motor uses an optical encoder or a resolver as a detector of rotation velocity. Also, a ferrite magnet or a rate earth magnet is used for the magnet which is built into the rotor and plays the role of a field system.

In this type of AC Servo Motor, because an armature contribution is linearly proportional to torque. Stop is easy and dynamic brake wordks during emergency stop. However, because a permanent magnet is use, the structure is very complex and the detection of position of the rotor is needed. The current from the armature includes high frequency current and the high frequecy current is the source of toruqe ripple and vibration.