GB2436874A

GB2436874A - Determining speed of a 3 phase AC motor

Info

Publication number: GB2436874A
Application number: GB0606995A
Authority: GB
Inventors: Robert Carter; Dieter Eckardt
Original assignee: Siemens PLC
Current assignee: Siemens PLC
Priority date: 2006-04-07
Filing date: 2006-04-07
Publication date: 2007-10-10
Anticipated expiration: 2026-04-07
Also published as: GB2436874B; GB0606995D0

Abstract

A method of determining the steed of a 3 phase AC motor, where said AC motor is controlled by a control circuit 2 outputting six switching element control lines to a motor inverter circuit 1, said six lines representing an upper and lower line for each of the three phases, characterised in generating a speed and/or direction feedback signal dependent on the output from at least three of said lines.

Description

2436874

5

3 Phase AC Motor Control

This invention relates to 3 phase AC drives (motors) and more specifically to method and apparatus to determine the speed and/or direction of an AC motor.

10 The speed and direction of an AC motor is determined by the speed and direction of the voltage space phasor applied to the motor; the motor's rotor attempts to follow the rotating magnetic field thus generated in the motor's airgap.

Figure 1 shows a schematic representation of a prior art method of determining motor 15 speed. The motor controller 1 (alternatively and hereinafter referred to as the inverter controller which is essentially a pulse width modulator/generator) typically controls an inverter 2 which feeds electrical power signals to the motor. The input to the inverter comprises six switching element control terminals (or lines), an upper set for each of the 3 phases (U, V, W) and a lower set for each phase, designated UU, VU, 20 WU, UL, VL and WL respectively.

In order to determine the speed of the motor it is known to include a sign discriminator in the form of a latch for example, connecting from any two of the either the upper or lower switching element control lines. A digital signal encoded as 25 the sign of the motor voltage or is fedback; i.e. it is a logic square wave at lx the motor frequency.

The problem with this is when the motor is going slowly speed feedback update is only available at 2x motor frequency i.e. every 500ms at 1Hz.

30

The direction of rotation of the motor can not be determined

It is an object of the invention to overcome these problems.

35 The invention comprises a method of determining the speed and direction of a 3 phase AC motor, where said AC motor is controlled by a control circuit outputting six

10

15

20

25

switching element control lines to a motor inverter circuit, said six lines representing an upper and lower line for each of the three phases, characterised in generating a speed feedback signals dependent on the output from at least three of said lines.

Preferably the speed feedback signal is generated from comparison of three pairs of lines.

In a practical embodiment each of said pairs is input to a sign discriminator and said outputs of said sign discriminator are summed. The sign discriminator comprises a latch or preferably an integrator.

The invention will now be described with reference to the following figures of which:

Figure 2 shows a diagram of a simple embodiment of the invention.

Figure 3 shows an embodiment of the same invention as in figure 2 with an embodiment of the invention which determines motor direction.

Figure 2 shows an embodiment of the invention showing an inverter 1 and inverter controller 2, which is essentially a pulse width modulator (generator). The three lower inverter switching element control lines UL, VL and WL are input into three sign discriminators 3, such that each sign discriminator each receives a pair of lines, said pair each having lines representing two different phases, and each pair having a different combination of phases.

The outputs from the sign discriminators are summed and the output is put through a Schmitt trigger 4 so as to add some hysteresis to give a more steady reading of the speed signal.

Combining the three circuits improves speed feedback update rate. In other words the three logic outputs are combined logically to generate a 3x frequency signal, by looking at gate drive signals U-V, V-W and W-U.

This improves the speed update rate for a 1Hz operation to an update every 167ms.

In the most simple implementation of this the three signals are mixed using resistors. This creates an average signals which switches between 1/3 Vdd and 2/3 Vdd. A 5 Schmidt detector generates a full amplitudes logic signal from this.

Direction feed back

10 Often the direction of a motor needs to be known; it is not particularly safe to know that a motor is driven at the right speed if it is going in the wrong direction. The use of all three different phases can again be useful to determine motor direction in another invention.

15 The invention further comprises a method of determining the direction of a 3 phase AC motor, where said AC motor is controlled by a control circuit outputting six switching element control lines to a motor inverter circuit, said six lines representing an upper and lower line for each of the three phases, characterised in generating a direction dependent signal dependent on the output from at least three of said lines.

By looking at all three lower or upper switching element control lines, a rotation direction signal may be derived.

Figure 3 shows an embodiment of how this is achieved. Again three lines are taken 25 from each of three different phases from either the upper or lower signals and three pairs are fed into sign discriminator circuits as in figure 2. Reference numerals are the same for figure 3 as in figure 2. It also includes general circuitry circuitry designated 5 where a direction feed back signal is obtained using logic elements and utilising cross-coupled NAND gates as a latch. Speed output is at 6 and direction output is at

A state change of any of the three phases allows a direction to be derived from the state of the other two phases. A programmable logic chip or low cost microcontroller can also be used to implement this method.

20

30 7.

Claims

1. A method of determining the speed of a 3 phase AC motor, where said AC motor is controlled by a control circuit outputting six switching element control lines to a motor inverter circuit, said six lines representing an upper and lower

10 line for each of the three phases, characterised in generating a speed feedback signal dependent on the output from at least three of said lines.

2. A method as claimed in claim 1 wherein the speed feedback signal is generated from comparison of at least two pairs of lines, each of said pair

15 comprising different phases.

3. A method as claimed in claim 1 wherein the speed feedback signal is generated from comparison of three pairs of lines.

20

4. A method as claimed in claims wherein each of said pairs is input to a sign discriminator and said outputs of said sign discriminator are summed

5. A method as claimed in claim wherein said sign discriminator comprises a latch or integrator.

25

6. A system to determine the speed of a 3 phase AC motor, where said AC motor is controlled by a control circuit comprising motor inverter circuit, the inverter circuit including six switching element control lines representing an upper and lower line for each of the three phases, and a speed determiner

30 connected to at least three of said lines.

7. A system as claimed in claim 1 wherein the speed determiner is connected to at least two pairs of said terminals, each of said pair comprising different phases.

8. A system as claimed in claim 1 wherein the speed determiner is connected to three pairs of said terminals.

9. A system as claimed in claims comprising sign discriminators each connected 5 to said pairs of terminals.

10. A system as claimed in claim wherein said sign discriminator comprises a latch or integrator.

10

11. A system as claimed in claim including hysteresis means.

12. A system as claimed in claim wherein said is a Schmidt trigger

13. A method of determining the direction of a 3 phase AC motor, where 15 said AC motor is controlled by a control circuit outputting six switching element control lines to a motor inverter circuit, said six lines representing an upper and lower line for each of the three phases, characterised in generated a direction dependent on the output from at least three of said lines.

20

14. A method as claimed in claim wherein the direction signal is generated from comparison of at least two pairs of lines, each of said pair comprising different phases.

15. A method as claimed in claim 1 wherein the direction is generated 25 from comparison of three pairs of lines.

16. A method as claimed in claims wherein each of said pairs is input to a sign discriminator and said outputs of said sign discriminator are introduced to a logic circuit, the output of which indicates direction..

30

17. A method as claimed in claim wherein said sign discriminator comprises a latch or integrator.

- $ -

18. A system to determine the direction of a 3 phase AC motor, where said AC motor is controlled by a control circuit comprising motor inverter circuit, the inverter circuit including six terminals lines representing an upper and lower line for each of the three phases, and a direction determiner connected to at least three

5 of said lines.

19. A system as claimed in claim 1 wherein the direction determiner is connected to at least two pairs of said terminals, each of said pair comprising different phases.

10

20. A system as claimed in claim 1 wherein the direction determiner is connected to three pairs of said terminals.

21. A system as claimed in claims wherein said direction determiner comprises 15 sign discriminators each connected to said pairs of terminals, and a logic circuit.

22. A system as claimed in claim wherein said sign discriminator comprises a latch or integrator.