WO2013045978A1

WO2013045978A1 - Alternating current stepper motor / ultra low speed motor

Info

Publication number: WO2013045978A1
Application number: PCT/IB2011/054656
Authority: WO
Inventors: Pallege Mudiyanselage Nilantha Prasad Dhammika GUNARATNE
Original assignee: Gunaratne Pallege Mudiyanselage Nilantha Prasad Dh
Priority date: 2011-09-27
Filing date: 2011-10-19
Publication date: 2013-04-04

Abstract

The vibration and the noise of the stepper motor is very high compare to other motors due to discrete steps and ringing at each step. Also have the limitations of accuracy, power and torque. Ultra low speeds are very important in control systems, usually are created using gear reduction motors. Achieving high accuracy, high power with smooth operation for, stepper motors and ultra low speed motors using conventional technology is expensive and highly complicated. Uses of the alternating current motors for stepper / ultra low speed motors are not possible with available technology mainly due to two reasons <ol id="olis87"> <li id="list88"> · The speed of the AC motor directly depends on the supply frequency and low frequency motors are bulky and inefficient. </li> </ol> <ol id="olis90"> <li id="list91"> · Ac motors always provides continues rotation and no discrete steps </li> </ol> Stand still magnetic field, created out of combination of rotating magnetic fields with ability to change the stand still position, makes stepper/ultra low speed motor using alternating current possible. This also creates many advantages in design because, the speed and the angular position of the motor is completely independent form the supply frequency. Two rotating magnetic fields of opposite directions are originated by swapping two phases on one supply and leaving other supply uninterrupted. When the amplitude, frequency and the phase angle of both the supplies are identical it creates non rotating (stand still) resultant magnetic field with sinusoidal varying amplitude. The direction of the resultant field is on the axis of the pole which the identical power phase is given (the phase not swapped). When the phase difference is introduced to two supplies the the direction of the resultant magnetic field rotates by half of the phase shift angle in 2 pole machine.

Description

ALTERNATING CURRENT STEPPER MOTOR / ULTRA LOW SPEED MOTOR

Technical Field

Controlling & automation is very important areas in modern industrial and engineering processes. Electrical motors are the main tools used in the area of control & automation engineering. Almost all the movements, forces, rotations etc. are created using electrical motors. Accuracy, power, speed and the efficiency are very important factors in design of automation and control systems

Background Art

Stepper motors, servo systems with Ultra low speed motors are widely used in control mechanisms. Stepper motors are used to reach the exact position in control systems, This is fairly complicated task as an separate micro processor has to be used in generating pulse pattern and the torque can be created by stepper motor has many practical limitations. Ultra low speed motors are always made by using gear reduction mechanisms which increases the weight and cost of the motors and also reduces the efficiency.

Technical Problem

The accuracy of the angular position of the stepper motor depends on the number of poles of the motor and control electronics, increased no of poles gives higher accuracy but increases the cost and complication of the motor and control system.

The vibration and the noise of the stepper motor is also very high compare to other motors as the rotor takes separate steps from one position to another. In each discrete step the rotor overshoots and bounces back and forth before stabilize, this is called ringing. This becomes a series issue when stepping frequency get equal to the rigging frequency.

The power and the torque created by conventional stepper motor has many limitations.

The angular speed of the three phase alternating current synchronous / Induction machines depends on the supply frequency and the no of poles.

In most of the control systems the ultra low speeds are created using gear reduction motors. This makes the motors bulky and increases the loss.

Constructing ultra low speed direct drive motors using VFD is very difficult due to high flux density created in low frequencies.

Technical Solution

Combination of two parallel placed 3 phase windings with separate power supplies are used to create a resultant magnetic field which is stand still.

Two rotating magnetic fields of opposite directions are originated by swapping two phases on one supply and leaving other supply uninterrupted. When the amplitude, frequency and the phase angle of both the supplies are identical it creates non rotating (stand still) resultant magnetic field with sinusoidal varying amplitude. The direction of the resultant field is on the axis of the pole which the identical power phase is given (the phase not swapped).

Introducing a phase difference to the two supplies will make stand still axis shifted by half of the phase variation angle. This enables an ability to rotate the axis and stop at any angular position, making the motor as high accurate, smoother, low cost, high power and small stepper motor.

Continues variation in phase angle will make the magnetic field rotates and the angular velocity and the direction is purely depend on the phase difference which can be controlled very easily and smoothly.

Advantageous Effects

The angular accuracy of the motor does not depend on, no of poles, control system or supply frequency, it purely depends on the accuracy of the phase shift which is very easy to control. This gives the ability to stop the motor at any angular location.

No overshoots, bounced back or vibration in rotor due to smooth rotating magnetic field.

Higher power and torque can be generated.

The rotational speed depends only on phase angle shift and this gives the ability of controlling the speed and direction without changing the supply Frequency.

Slow change in phase angle will create ultra low angular speed, ignoring the frequency of the supply.

Ability to change the direction of rotating magnetic field very smoothly.

This allows to use higher frequency power supply and makes the motor size smaller because of low flux density in high frequency for same power output.

There is no requirement of VFD or gear reduction required for ultra low speed direct drive motors.

Description of Drawings

Fig 1 A - Stator winding for three phase machine rotating anti clock wise

Fig 1 B - Stator winding for three phase machine rotating clock wise

Fig 2 - Winding diagram for combination of clock wise and anti clock wise rotating fields which creates stand still magnetic field

Fig 3 - Variation of the resultant magnetic field with the introduction of phase shift to one supply

Fig 4 - Rotor arrangement to synchronize with changing amplitude of resultant magnetic field

Best Mode

Fig 1 shows the arrangement of three phase supply to create rotating magnetic field, Taking the axis of pole of supply 'a' as horizontal axis (x - axis),

If two clockwise & anti clockwise windings are placed parallel as shown in fig 02 ,

Resultant magnetic field is stand still on horizontal (x) axis and the amplitude is sinusoidal in the frequency of supply voltage.

The position of the magnetic field does not depend on the frequency of the supply, creating the ability use any frequency as the supply frequency. This gives enormous advantages in design especially the size of the machine. Ability to change the resultant angle according to the requirement create a stepper motor with infinite no of steps, exceptionally smooth and ability to provide more power.

Continues variation in the resultant angle will create machine where speed can be easily controlled in big range especially in ultra low speeds, including angular direction.

Stand still magnetic field is created by using two, 3 phase windings which are placed to create rotating magnetic fields in opposite directions as shown in Fig 02.

If same voltage and same phase is supplied to the windings it creates perfectly stand still sinusoidal varying magnetic field in the direction of the core which identical phases are wound.

Applying a phase change of β to one supply as in Fig 03, will make the system as below

Above results shows that introduction of a phase shift of β to one of the supply, makes the resultant magnetic field rotates by an angle of β/2 and the magnitude of resultant will get a lag of β/2

The concept used for rotor is almost same as the concept in transformer. Rotor is made out of laminated steel bar and applying the winding on it as shown in Fig 04, this is exactly as transformer secondary. This is a brushless rotor and magnetic field is created by induced current.

When rotor gets aligned with the resultant magnetic field, the field which is sinusoidal goes through the rotor (winding), and induced an EMF on the windings as transformer secondary. When windings are interconnected at the ends the current starts passing through the winding creating a magnetic field along the rotor bar. The magnetic field on the rotor gets synchronized with the resultant field of the stator, and two fields get interlocked creating an exceptionally accurate and smooth stepper motor and ultra low speed motor which can easily rotate back and forth.

The resultant magnetic field varies sinusoidal , creating a changing magnetic force .

The variation in magnetic force produce variation on the torque; that can lead to creates vibrations and instability especially in higher load situations.

Use of higher frequency power supplies or use of parallel magnetic field concept (Patent Application LK/P/1/16440) to improve the performance further.

Industrial Applicability

This application may be used in almost all the stepper motor requirements with more accuracy because the rotation steps are unlimited and provides higher power and torque.

Also this application can be used for ultra low speed motors used in satellite tracking mechanisms, focusing of cameras and telescopes etc.

Claims

Use of two AC stator windings which creates oppositely rotating magnetic fields, to create a resultant magnetic field which is stand still despite of supply frequency and number of poles and use a rotor to lock with the resultant magnetic field.
Use the phase difference of supply voltages to change the stand still position of the resultant magnetic field and create stepper motor of very high accuracy.
Continuously shifting the phase angle to create rotation in resultant magnetic field with required angular speed, to make a motor with highly flexible speed controlling especially in ultra low speeds.
Change the angular direction of the motor by changing of the phase angle of two supplies.
Use of sinusoidal varying resultant magnetic field of stator to create an EMF on rotor windings and Use of self generated sinusoidal varying magnetic field of rotor windings to synchronize the rotor with the stator magnetic field