NZ522230A - Speed controller for an induction motor - Google Patents

Abstract

A controller to manage the speed of an induction motor and a method of programming the controller are disclosed. A user selects an input speed 210 and the speed controller uses a lookup table to select a corresponding number of cycles for which AC voltage is supplied to the motor and then cut off. The controller detects zero crossings in the supply voltage to count the number of cycles and applies and disconnects the supply to the motor depending on the speed set by the user. This method eliminates RF interference and gives improved low speed efficiency and low noise compared with known controllers.

Description

522230 NEW ZEALAND PATENTS ACT, 1953 No: 522230/522311 Date: 24 October 2002 30 October 2002 Intellectual Prooc Office of HZ -i ly < Ji-M't RECE*V COMPLETE SPECIFICATION SPEED CONTROLLER FOR AN INDUCTION MOTOR I, CURTIS HENRY DOBBIE, 30 Faulder Avenue, Westmere, Auckland, New Zealand, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: FIELD OF INVENTION The present invention relates to a power supply or motor controller capable of controlling the speed of an induction motor.

BACKGROUND There are numerous methods known in the art for controlling the speed of induction motors. Induction motors work on the principal of a magnetic field generated from the stator which rotates in space. Typically the rotor is some form of conductor either in the form of a short circuited conductor or a wound rotor connected to variable resistors.

Induction motors are designed so that the speed of the rotor will be slightly less than the speed of the supply frequency. This is the most efficient operating condition and for example operating in a half speed condition will often be highly inefficient.

The torque of an induction motor varies with voltage squared. Therefore by adjusting the amplitude of the supply voltage the running value of the speed can be adjusted. Unfortunately as already mentioned this causes extra current to flow and the machine looses efficiency and in some cases over heat. Such voltage control might also result in extra noise and RFI depending on the technique used to reduce the voltage.

The over heating can be over come by using a wound rotor machine and controlling the speed by varying the value of external resistances. There is some extra resistance losses in the rotor but usually not sufficient to be a problem. The motor will still be inefficient at low speeds however.

A further improvement on efficiency is to replace the external resisters with a bridge rectifier connected to a DC motor. The DC motor is mounted on the same shaft as the induction motor and the DC field control is used to control the speed of the machine set.

A more efficient method of controlling the speed of the induction is to adjust the frequency of the rotating magnetic flux generating by the stator. This can be done by one of two methods. Firstly the number of poles can be varied either by connection of the physical poles in different configurations or reversing the connections between selected poles according to a modulating function. A more versatile alternative is to vary the supply frequency such that for any desired speed the rotor is able to rotate at just below the supply frequency such that efficiency is always maintained at a high level.

Thus in applications where a full range of speed control is required the above systems to greater or lesser extent suffer from high cost, complexity, inefficiency and or poor speed regulation.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a motor controller which goes some way to overcoming the above mentioned disadvantages or which will at least provide the public with a useful choice.

Accordingly in a first aspect the present invention consists in a motor controller for an induction motor comprising: user input for receiving a desired speed setting, energising means for selectively applying or disconnecting an AC supply voltage to said induction motor, and a controller which activates said energising means to continuously apply said supply voltage for a first number of cycles and remove said supply voltage for a second number of cycles such that on reapplication of said voltage the back EMF is not significantly different from said supply voltage.

Preferably said first number and said second number are predetermined based on said desired speed setting.

Preferably further comprising a back EMF sensor, wherein said first number and said second number are based on the difference between said supply voltage and back EMF being below a threshold.

Preferably said threshold is a phase or slope threshold.

Preferably said threshold is a magnitude threshold.

Preferably said controller configured to calculate said first number and said second number according to parameters related to said motor and/or its loading and said desired speed setting.

Preferably said user input further configured to receive said parameters and/or a selection of predetermined configurations.

Preferably said motor is coupled to a fan.

Preferably said controller is configured based on stored instructions to ensure that the polarity of the slope of said supply voltage matches that of the back EMF of said motor prior to application.

Preferably the polarity of said supply voltage is compared to said back EMF and is only applied once it matches.

In a second aspect the present invention consists in a method of programming a motor controller according to any of the above clauses comprising the steps of: selecting a desired speed selecting a desired number of cycles on selecting a desired number of cycles off wherein said supply voltage is not significantly out of phase with said back EMF on reapplication and in the steady state said motor is rotating substantially at said desired speed.

To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.

The invention consists in the forgoing and also envisages constructions of which he following gives examples.

BRIEF DESCRIPTION OF THE DRAWINGS One preferred form of the present invention will now be described with reference to the accompanying drawings in which; Figure 1 is a block diagram of the system; Figure 2 is a schematic of the induction motion; Figure 3 is a flow diagram of the switching algorithm; and Figure 4 is a waveform diagram showing the supply voltage and back EMF.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 1 the present invention is shown driving a fan 206. The user inputs a speed setting via panel 200 to which the controller 202 responds by applying the appropriate voltage to the motor 204. Other applications with a significant system inertia or low sensitivity to small speed fluctuations are equally appropriate systems including a fly wheel.

The back-EMF in an induction motor varies at mains frequency when it is connected to the mains supply but if the supply were to be cut then the back EMF would be at shaft frequency. In essence there is a magnetic field trapped in the rotor which naturally rotates at shaft speed but the present of the mains keeps dragging if forward and maintains it. This dragging causes the machine to always have a lagging power factor. If the supply were cut then the rotor becomes a rotating magnet and the frequency would drop slightly and then continue to fall as the machine slowed down. Also the amplitude would then start to fall off with small machines the rotor time constant may be 100ms, with larger machines it is bigger for example - 0.5 seconds. So in a short time all the rotor magnetism would be gone. Connected to the supply the magnetism in the rotor is maintained by the magnetising current.

Consequently by trying to achieve a half speed conditions with one full cycle with the supply on and one cycle with the supply switched off, the large difference between back EMF and supply each time it is reintroduced causes large currents to flow with over heating or inefficiency the result. Investigations by the applicant have indicated that the back EMF during the off cycles has to be matched in polarity to the applied AC voltage when the mains supply is reapplied.

It is therefore preferable that for any speed change, however small, must have enough cycles missed to re-align the direction of the induced back EMF to the incoming AC voltage. This switch on point must be triggered at or as near to zero-crossing as possible. Also the induced back EMF will be a lower frequency than the applied mains due to the induction slip angle and the reduction in speed while the supply is disconnected.

In the typical application shown in Figure 1 a single phase motor 204 with capacitive start/run winding would have its rotor 100 connected to the fan blades 206. The rotor is preferably a short circuit type squirrel cage rotor. The controller 202 applies the appropriate voltage 108 across the main winding 102 and the phase shift caused by capacitor 104 results in a rotating magnetic field. It will be appreciated that the system is equally applicable to 3 phase systems.

The speed controller 202 might for example include a microprocessor such as a Pic 12F67S from Microchip. It detects zero crossings in the supply voltage and applies and disconnects the voltage according to the algorithm in Figure 3. The user inputs the speed 210 and the Cycleson and Cycles0ff are set 208 according to a look up table for that speed. As given below the AC voltage is applied 212 for a number of cycles up to Cycleson 214. It is then disconnected 216 up to the Cycles0ff 218. The algorithm runs continuously resulting in a RFI free, low noise, variable speed controller that operates at improved low speed efficiency. Example values for the cycles are given below for a small single phase motor, as shown in Figure 2.

SPEED FULL 12 11 9 8 7 6 4 3 2 1 CYCLES ON Max 11 7 3 2 2 1 1 1 1 1 CYCLES OFF 0 4 4 4 3 3 4 6 8 SPEED (RPM) 2450 2210 2170 2000 1865 1666 1400 1260 1090 980 860 790 670 The ratios in this table have been chosen by experiment to exhibit the best steady operation of the motor under test consistent with a proportional reasonable drop from full speed 2500 RPM down to 500 RPM (at Speed 1). More or less steps can easily be added.

Referring to Figure 4 the waveforms are shown during application and disconnection of the mains or supply voltage. The supply voltage 404 is disconnected at t0ff 400 for four 50Hz cycles i.e. 80ms until time ton 406. During this period the back EMF 402 decays in amplitude and frequency. It is clear that only at t0ff 406 is the back EMF 402 a phase i.e. both in polarity and magnitude with the supply voltage 404. Again preferably the switching instant is a zero crossing 408 although not necessarily required in all embodiments. For example it might also be applicable to simply switch the supply voltage back in when the slope and magnitude of the back EMF (or at the very least the magnitude) match that or is in substantial proximity to the supply voltage. For example a range or threshold could be applied to the output of a comparator connected to the back EMF and supply voltage.

Another embodiment of the invention is to arrange the programme in the microprocessor to select the approximate number of off cycles for the desired speed. The actual trigger is calculated on the run with the use of a comparator to determine the in phase switching point for the mains, compared to the back EMF generated by the motor. This embodiment has the advantage that different induction motor types, 2 pole, 4 pole, etc. will be automatically adjusted for. However this increases the complexity and the price. There will be applications that suit both the simpler and the more complex option, depending on requirements.

Another further alternative would be to calculate the turn on turn off points for a given system. For example the system could be preprogrammed for different configurations or could allow entry of parameters to allow the correct switching instants to be calculated. By knowing approximately the expected back EMF waveform for the given configuration and desired speed steps, an appropriate switching regime can be calculated either on installation or in realtime.

Claims (13)

WHAT I CLAIM IS:

1. A motor controller for an induction motor comprising: user input for receiving a desired speed setting, energising means for selectively applying or disconnecting an AC supply voltage to said induction motor, and a controller which activates said energising means to continuously apply said supply voltage for a first number of cycles and remove said supply voltage for a second number of cycles such that on reapplication of said voltage the back EMF is not significantly different from said supply voltage.

2. A device as claimed in claim 1 wherein said first number and said second number are predetermined based on said desired speed setting.

3. A device as claimed in claim 1 further comprising a back EMF sensor, wherein said first number and said second number are based on the difference between said supply voltage and back EMF being below a threshold.

4. A device as claimed in claim 3 wherein said threshold is a phase or slope threshold.

5. A device as claimed in claim 3 wherein said threshold is a magnitude threshold.

6. A device as claimed in claim 1 wherein said controller configured to calculate said first number and said second number according to parameters related to said motor and/or its loading and said desired speed setting.

7. A device as claimed in claim 6 wherein said user input further configured to receive said parameters and/or a selection of predetermined configurations.

8. A device as claimed in claim 1 wherein said motor is coupled to a fan. -9-

9. A device as claimed in claim 3 wherein said controller is configured based on stored instructions to ensure that the polarity of the slope of said supply voltage matches that of the back EMF of said motor prior to application.

10. A device as claimed in claim 9 wherein the polarity of said supply voltage is compared to said back EMF and is only applied once it matches.

11. A method of programming a motor controller according to any of claims 1 to 10 comprising the steps of: selecting a desired speed selecting a desired number of cycles on selecting a desired number of cycles off wherein said supply voltage is not significantly out of phase with said back EMF on reapplication and in the steady state said motor is rotating substantially at said desired speed.

12. A motor controller substantially as herein described with reference to and as illustrated by the accompanying drawings.

13. A method of programming a motor controller substantially as herein described. DATED THIS ,l+ DAY OF OfVwoe-S aoqm- AJ Park PER C,—r,_-^ . AGENTS FOR THE APPLICANT