GB2133591A - Motor control circuit - Google Patents

Abstract

In an induction motor control circuit, a slip-frequency signal ???s is derived from a torque reference signal, and a stator M.M.F space angle signal ???' is derived from a combination of rotor angle ???, accrued slip-angle ???, and the argument of a complex function of the slip frequency signal, the latter providing enhanced stability particularly at low speed and being formed in a function generator circuit 33. The demand signals I*, ???' are used together with the measured rotor angle ??? to control stator current. <IMAGE>

Description

SPECIFICATION Motor control circuit This invention relates to motor control circuits and particularly to motor control circuits which are required to control the magnitude and rotation of a magnetic field.

One application for such a control circuit is to the control of an induction motor where it is necessary to provide a stator magneto-motive-force (M.M.F.) of controlled magnitude and rotary position (angle) in space in order to obtain speed, or more directly, torque control. One such motor control system is disclosed in U.K.

Patent Specification No. 2104692. In that patent specification, direct and quadrature components of the stator MMF are obtained in a frame which is locked to the rotor. Since the stator MMF moves relative to the rotor at the slip frequency these direct and quadrature components vary at the slip frequency.

The required stator MMF may be defined as a function of the required slip-frequency for, either, constant stator flux linkage or constant rotor flux linkage. The following analysis assumes the former. The stator current magnitude is therefore derived directly from the appropriate frequency/M.M.F. characteristic and the M.M.F. angular position is derived by integration of the slip frequency. Such an arrangement is very satisfactory in many circumstances by may have a tendency to instability at very low speeds.

Apart from such a particular arrangement there would be a more generally applicable advantage in providing a magnitude and angle control system for rotary magnetic fields, which did not suffer any tendency to instability in certain speed conditions.

An object of the present invention is therefore to provide a motor control system feature which avoids or at least inhibits a particular tendency to instability.

According to one aspect of the present invention, a motor control circuit for providing magnitude and phase angle signals to control a rotating magnetic field in response to a control frequency, has a control function which derives the magnitude and phase of the M.M.F. associated with said rotating magnetic field as a function of said control frequency.

According to another aspect of the invention, an induction motor control circuit comprises means providing a required-slip-frequency signal, control function means for deriving stator M.M.F. magnitude and phase signals as the modulus and argument respectively of a complex function of said required-slipfrequency, and means for incorporating the phase signal in a space angle signal determining the space angle of said stator M.M.F.

An induction motor control circuit as aforesaid may include means for deriving from the required-slipfrequency signal an accrued slip-angle signal to which is added the phase signal to give a required angle relation between the rotor position and the stator M.M.F.

According to a further aspect of the invention, an induction motor control circuit comprises means providing a required-torque reference signal, means converting the required-torque reference signal to a required-slip-frequency control signal and means as aforesaid for providing magnitude and space-angle determining signals for the stator M.M.F. in dependence upon the required-slip-frequency control signal.

A motor control circuit in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a block diagram of an induction motor control circuit; and Figure 2 is a block diagram of part of the circuit of Figure 1 in expanded form.

Referring to Figure 1, a cage induction motor 1 is supplied by a transistor inverter 3 from the output of a three-phase bridge rectifier 5. The inverter 3 has base drive circuits 7 supplied from pulse generators 9 at controlled intervals. Feedback of signals proportional to the actual motor phase currents is provided to summing circuits 11. The main inputs to the summing circuits 11 are derived from an M.M.F. control circuit 13 which is supplied with a stator M.M.F. demand signal I, giving the required magnitude of the stator M.M.F., a signal 4 giving the accrued slip angle, and a signal 6 which gives the rotor angular position from some arbitrary fixed datum.This latter signal 6 if added to the accrued slip angle gives the absolute rotational position, i.e. the space angle, of the stator M.M.F. continuously.

The transformation of I, + and 6 into stator phase current signals i5, ib and i, is explained in the above patent application but will he described here for completeness.

In an induction motor the torque is determined by by the amplitude of the stator and rotor M.M.F.s and the phase angle between them. The only control available is the amplitude and rotational, i.e. angular, position (on a continuous basis) of the stator M.M.F., since both stator and rotor M.M.F.'s rotate at so-called synchronous speed and the rotor itself slips back through the rotating field at the slip frequency (o5.

The stator M.M.F. may be resolved along 'direct' and 'quadrature' axes which rotate with, i.e. are locked to, the rotor. The direct and quadrature components ids and iq5 and given by ids = Is cos + iqs=lssinCt) where 15 is the resultant stator M.M.F and + is equal to w5t the accrued slip-angle between the stator M.M.F.

and the rotor.

The direct and quadrature components are first derived as above in the M.M.F. control 13 and then the individual stator phase current signals are derived, also in the M.M.F. control 13, by the following transformation (given in abbreviated form).

cos 6 COS H -sin 6 ids ibs = #2/3 cos (#-2#/3) -sin(#-2#/3) ics cos (#+2#/3) -sin(6+2#/3) iqs Thus ias = idScosO -iqssin0) and similarly for ibs and ics.

So, by means of this transformation employing the accrued slip angle + and the absolute rotor angle #, ibs and ics are obtained, relative to the arbitrary datum.

The present invention is, in the present embodiment, concerned with the derivation of demand values of l* and 4 as applied to the M.M.F. control 13, and irrespective of arbitrary rotor datum positions. These demand values are derived in a torque control circuit 15 shown in more detail in Figure 2.

Feedback values of the stator winding currents are derived from transformers 17 (shown as plain connections for simplicity). On the assumption that the system is balanced and that therefore there are no zero sequence components, it is only necessary to extract two of the three phase currents (i55 and ics), since the third can be obtained as the negative sum of these two.

Also applied to the torque control circuit 15 is a torque reference signal and the rotor position signal 6 giving giving the rotor angular position with reference to the arbitrary datum. The rotor angle 6 and a speed indication are obtained from a shaft encoder 21. Speed control may be obtained by comparing the actual rotor slip speed (difference between actual speed and synchronous speed) with a demand slip speed. A speed error is thus directly available as a slip frequency error.

In a torque control system the actual torque is derived and compared with a torque reference. In a system operating under the constraint of constant stator flux linkages, the torque is linear with the slip frequency.

Thus the torque error corresponds directly to a slip frequency error.

Referring to Figure 2, in calculating the actual torque, the first requirement is to derive the quadrature components of the stator M.M.F. i.e.- lds and iq5. An inverse version of the transform used in the M.M.F.

control 13 is employed in block 23, again as described in the above patent application. In abbreviated form, and taking account of the constraint that the zero sequence component is zero, the inverse transform becomes

sin(6-/3) sin6 i55 idq = - cos(6-ir/3) cos 6 cs In orderto calculate the torque, the rotor M.M.F. components are required, i.e. idrand iqr referred to the same rotor axes as the stator components. The resolution of the M.M.F.'s on a rotor locked reference frame provides the following very simple relationship between the stator and rotor components: idr= Kp.

1 + Trp. ids and = Kp.

iqr 1 + Trp. iqs where K is a constant, Tr is the rotor time constant

Lr Rr and p. is the time derivative operator d/dt. This calculation of the rotor M.M.F. components is performed in block 25, the resultant stator and rotor components being applied to a block 27 which makes the torque calculation as follows T = K (i,, dsq,) This value of the actual torque is subtracted from a torque reference signal in a differencing circuit 29 to produce a torque error signal.

With the constraint that the stator flux linkages are held to a fixed value the torque is a linear function of the slip frequency (o5. The torque error ouput from circuit 29 thus corresponds directly to a slip frequency error and the transfer is effected by a linear transfer circuit 30 which also provides stabilisation of the control loop. The resulting slip frequency signal is then applied to control function operator circuits 31,33 and 35.

In a reference frame locked to the stator M.M.F. the rotor equation can be shown to be: O = M(p.+ j#s)is + (Rr + Lr(p. + j#s))ir (1) where M is the mutual inductance between rotor and stator, p. is the time derivative, o, the slip frequency, and Rrand Lithe resistance and inductance of the rotor. With the above constant flux linkages assumption L5i5+ Mir = t where t is the fixed value of flux linkage and L5 is the stator inductance.

Hence ir = -L5i5 M which, substituted in the above equation (1) gives

where a = 1 - M2 LsLr Equation (2) thus gives the required control function of o, is = tf((sss) It can be seen that the function f (X5) contains both the magnitude (subject to the constant flux linkage t) and a phase referenceforthe statorcurrent/M.M.F.

The error slip frequency (Os derived from transfer circuit 30 is in fact applied to three function generator circuits. One such function circuit, 31 produces the modulus of the function f(o)5), i.e.,

The output of the function circuit 31 is applied to a multiplier 37 to which is also applied the flux linkage signal Ir, the result then being the demand stator current magnitude signal i*=li5 I which is applied to the M.M.F. control 13 in Figure 1.

The third function circuit 35, by integration of the slip frequency signal w5 gives the accrued slip angle signal, i.e. the angular position of the stator field relative to the rotor from some arbitrary datum on the rotor.

The precise value of this signal can then be in error by the resulting arbitrary phase. However, the function circuit 33 produces the 'argument' of the function f(5) i.e. a phase reference angle a of the current is relative to the same arbitrary datum.

Thus

This phase reference signal is added to the accrued slip angle + in summing circuit 39 to produce a slightly modified signal +' and correct this uncertainty.

Transformation of the l* and +' components is then performed in the M.M.F. control circuit 13, employing the rotor angle 6 as explained above, to give the individual phase drive signals in a space reference frame.

In the patent application referred to above, a similar system is disclosed but with the omission of any phase function generator such as that, 33, in Figure 2. This omission tends to produce instability in certain conditions, particularly at very low speeds.

It will be apparent that the motor control circuit described above and illustrated in Figure 1 is only one example of a control circuit in which the invention finds useful application. It would provide a significant advantage in any system of rotary magnetic fields where the angular position of a field relative to an arbitrary datum is derived by frequency integration.

It should be noted that in the above analysis 15, the stator M.M.F., was defined as a complex function of the variable oJ5 (the slip frequency) under the restraint that the stator flux linkages are held constant. 15 can equally, and in some circumstances, preferably, be defined as a complex function of (o5 under the alternative restraint of constant rotor flux linkages.

Claims (9)

1. A motor control circuit comprising means for providing magnitude and phase signals to control the magnitude and phase of a magnetomotive force associated with a rotating magnetic field, said means for providing a magnitude signal comprising a control function generator circuit providing the modulus of a complex control function of a control frequency signal, and said means for providing a phase signal comprising a control function generator circuit providing the argument of said complex control function.

2. A control circuit according to Claim 1 for an induction motor, wherein said control function is a function of the impedances of the rotor and stator conductors and of the slip frequency constituted by said control frequency.

3. An induction motor control circuit comprising means providing a required-slip-frequency signal, control function generator means for deriving stator M.M.F. magnitude and phase reference signals as the modulus and argument respectively of a complex function of said required-slip-frequency, and means for combining the phase reference signal with an accrued slip angle signal to determine the rotational position of the stator M.M.F. with respect to a reference.

4. An induction motor control circuit, according to Claim 3, wherein said complex function is a function of the rotor and stator impedances at the said required-slip-frequency.

5. An induction motor control circuit according to Claim 3, wherein the M.M.F. magnitude signal incorporates a factor ('it) dependent upon a predetermined value of the stator-rotor flux linkage.

6. An induction motor control circuit according to Claim 3, wherein said control function is constituted by the following expression for the stator M.M.F.:

where is is the stator M.M.F.

RI is the stator-rotorflux lingkage Rr is the rotor conductor resistance Lr is the rotor conductor resistance L5 is the stator conductor inductance N1 # is equal to 1 - M LrLs M is the mutual inductance 5 is the rotor slip frequency

7. An induction motor control circuit according to Claim 3 comprising means providing a required-torque reference signal and means for converting the required-torque reference signal to said required-slipfrequency control signal.

8. An induction motor control circuit according to Claim 7, including means for deriving an actual-torque signal representative of the actual motor torque, from stator winding currents, and means for incorporating said actual-torque signal as a negative-feedback signal in said required-torque reference signal.

9. An induction motor control circuit substantially as hereinbefore described, with reference to the accompanying drawings.