WO2019207525A1

WO2019207525A1 - Synchronous induction motor

Info

Publication number: WO2019207525A1
Application number: PCT/IB2019/053422
Authority: WO
Inventors: Hans Alfred GRANIG; Machiel Wilhelmus ODENDAAL
Original assignee: Lorbrand (Pty) Ltd
Priority date: 2018-04-26
Filing date: 2019-04-25
Publication date: 2019-10-31

Abstract

A synchronous induction motor including a stationary three-phase AC powered stator including stator windings and a DC powered rotor located external to the stator. The rotor includes rotor windings. The rotor is arranged to rotate around the stator. A rotary transformer is used to provide DC power to the rotor.

Description

SYNCHRONOUS INDUCTION MOTOR

BACKGROUND OF THE INVENTION

This patent application relates to a synchronous induction motor.

SUMMARY OF THE INVENTION

According to one example embodiment, a synchronous induction motor including: a stationary three-phase AC powered stator; a DC powered rotor located external to the stator and arranged to rotate around the stator; a rotary transformer to provide DC power to the rotor.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic drawing of a motor in accordance with an example embodiment of the present invention;

Figure 2 shows a schematic drawing of the rotor and stator of the motor of Figure 1 ;

Figure 3 shows another schematic view of the rotor and stator of

Figure 2;

Figure 4 shows a cross section through the rotor and stator of Figure

Figure 5 shows the parts of a rotary transformer used in the motor of Figure 1 .

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Figure 1 of the accompanying drawings, a synchronous induction motor 10 is illustrated.

In this embodiment, the motor 10 is used in an application with a heavy duty idler roller 12 which has been partially cut away to show the motor 10 located inside.

As the motor illustrated has been designed to drive conveyor belts, it has been integrated into a conveyor idler roller so that it can drive conveyor belts.

It will be appreciated that the motor of the present invention has other applications in the materials handling industry.

Figure 2a shows in an exploded form for illustrative purposes that the motor includes a stationary three-phase AC powered stator 14 including stator windings.

A DC powered rotor 16 is located external to the stator 14 and is arranged to rotate around the stator 14.

It will be appreciated that this is the reverse of the standard induction motor design, where the motor stator is situated on the outside of the motor body and the rotor situated at the centre of the body.

The rotor 16 includes rotor windings.

In a prototype of the present invention, the rotor core was made from mild steel and made up of 100 x 3mm laminations and held together using 10mm bright bolts and M8 nuts. The stator core was made from non-oriented magnetic steel and made up of 600 0.5mm laminations held together using washers and circlips.

Also illustrated in Figure 2a is a motor shell 18 to accommodate accommodates therein the stator 16, rotor 14 and transformer 20.

Figure 2b shows these components assembled.

Figure 3 is a perspective view of the stator 14 and rotor 16 assembled and showing the rotor and stator windings.

Figure 4 is a cross section through Figure 3 showing the rotor windings 22 and the stator windings 24.

The motor has a fixed stator shaft 26 shown in Figure 1 and Figure 4 at the centre of the rotating motor body or shell to which the stator 14 and transformer stator 30 are connected

Referring to Figure 5, the rotary transformer 20 (Figure 1 ) is used to provide electrical power via an auxiliary source to stator mounted primary transformer windings and then to the rotor mounted secondary transformer windings through the air gaps between the stator mounted primary transformer windings and the rotor mounted secondary transformer windings.

The electrical power is then passed via a bridge rectifier which supplies DC power to the rotor winding 16.

Figure 5 shows transformer 20 which has a transformer rotor 28 and a transformer stator 30.

The transformer stator 30 has primary copper windings which in the prototype were 2mm enamel coated copper wire and the transformer stator 30 had 275 turns. The transformer rotor 28 in the prototype had secondary copper windings which in the prototype was 60 turns.

These are based on a 45mm x 0.3mm thick copper sheet with 75 micron thick insulation material between each winding.

The rotary transformer design has been selected to operate at mains frequency (50-60 Hz) because the low volume use of transformer core material (NGO Electrical steel) is cheaper than high frequency core material such as compressed ferrite powder cores.

The transformer rotor 28 and motor rotor assembly 16 are mechanically fixed to the motor shell 18 and all rotate together about the stationary fixed stator shaft 26. Electrical power from the transformer rotor 28 is supplied to the motor rotor coil assembly 16 via a bridge rectifier

Electrically, the transformer stator 30 is connected to an electrical power supply which in the prototype of the present invention is an auxiliary 220V single phase 50Hz supply.

Electrical power induced at the transformer secondary is nominally rated at 50VAC and 15A. The motor rotor coil 16 operates at a nominal current of 10A DC.

The motor stator 14 windings are connected to an electrical power supply which in the prototype of the present invention is supplied via a 380V, 3 phase, 50Hz, 1 1 kW frequency supply.

In the prototype embodiment, a 16 pole configuration in the 320mm diameter roll operating at full mains frequency 50Hz and 380 V 3 phase AC was selected.

This enables optimum operation of a 4kW motor driving a conveyor belt at 7 m/s belt speeds for use in heavy duty conveyor applications. Roll diameters smaller than this would present problems with power transmission from the drum to the belt due to the relatively small contact area available at the belt-roll interface.

If smaller roll diameters are used then belt-roll slippage could only be prevented by the application of suitable lagging materials such as rubber or ceramic paste to the drum.

The rotor DC field windings are isolated from the auxiliary power supply and kept open or short circuit during start-up to minimise cogging effects. DC power is only suppled to the motor rotor field coil 16 once the rotor speed is typically within 80-90% of synchronous speed. This control can be done via the frequency inverter drive auxiliary contacts. Speed or time delay set-points can be programmed within the inverter to specify when power must be supplied to the rotary transformer stator winding 30.

Starting torque can be increased by shorting the DC field winding during start-up.

It will be appreciated that the design philosophy of the present invention is to use synchronous AC induction motors which are cost effective, suited to mass production, are reliable, do not require external cooling and can be connected to standard electronic variable speed drives that can be sourced from multiple vendors.

Permanent magnets were not considered for the motor design due to the volatile cost of magnetic material.

Most synchronous motors usually use slip rings to provide power from the stator to the rotor because it is easier and cheaper to implement than a rotary transformer, but the slip rings do require maintenance to ensure reliable operation. The rotary transformer does not require any maintenance.

The synchronous motor configuration was selected to improve the motor power factor in order to reduce heat production and increase motor efficiency. Motor power factor and efficiency inherently decreases with increasing number of motor poles and reduced motor dimensions.

The synchronous motor must also have as high a starting torque as possible in order to start fully loaded conveyor belts.

A 16 pole motor was designed so that the motor could produce the necessary starting and full load operating torques.

In addition the DC rotor field windings were chosen to improve the inherent poor power factor caused by the large number of poles and small motor footprint and thus reduce heat.

The copper field windings were chosen instead of permanent magnets because of the relatively high ambient operating temperatures. Permanent magnets would also increase the drive complexity and reduces the number of frequency drives that can be sourced.

Exciters are not suitable as rotor field power sources because they extract mechanical power from the synchronous motor, which is undesirable in this application as the rotor fields require an excitation of at least 0.6 to 1 kVA to achieve rotor magnetic field strengths required for maximum motor efficiency.

Claims

CLAIMS:

1 . A synchronous induction motor including: a stationary three-phase AC powered stator including stator windings; a DC powered rotor located external to the stator and arranged to rotate around the stator, the rotor including rotor windings; a rotary transformer to provide DC power to the rotor.

2. A motor according to claim 1 wherein the rotary transformer includes a transformer rotor and a transformer stator.

3. A motor according to claim 2 wherein the transformer stator is located inside the transformer rotor.

4. A motor according to claim 2 or claim 3 wherein electrical power is supplied from the transformer rotor to the motor rotor via a bridge rectifier.

5. A motor according to any one of claims 2 to 4 wherein the transformer stator is connected to an electrical power supply.

6. A motor according to claim 5 wherein the electrical power supply is an auxiliary 220V single phase 50Hz supply.

7. A motor according to any preceding claim wherein the motor rotor operates at a nominal current of 10A DC.

8. A motor according to any preceding claim wherein the stator windings are connected to an electrical power supply.

9. A motor according to claim 8 wherein the electrical power supply is 380V, 3 phase, 50Hz, 1 1 kW frequency supply.

10. A motor according to any preceding claim further including a motor shell that accommodates therein the stator, rotor and transformer.

1 1. A motor according to claim 10 further including a fixed stator shaft at the centre of the motor shell to which the stator and transformer stator are connected, wherein the transformer rotor and motor rotor are mechanically fixed to the motor shell and all rotate together about the fixed stator shaft which remains stationary in use.

12. A motor according to claim 10 or claim 1 1 wherein the motor shell is a conveyor idler roller which is adapted to drive conveyor belts.