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.