Control Strategies and Important Parameters

34.10.5 Control Strategies and Important Parameters

sin 2δ (34.103) The dq model captured in Eqs. (34.99)–(34.101) can be used

d −L q

to explain a number of control strategies for the Syncrel. It is beyond the scope of this section to present the derivation

It is obvious that this expression is maximized for a given value of i if δ

= 4 . Therefore, one should control the currents

The 3/2 conversion factor is required if the transformations are power-

so that δ stays at this angle, if maximum torque per ampere is

variant transformations, as opposed to the power-invariant transformations.

desired.

The power-variant transformations are the most common ones used because the single-phase machine parameters can be used directly in the resultant

Another control objective for the Syncrel is to maximize the

models, and the two-phase voltages and currents are identical in magnitude

power factor for the machine. This is important to minimize

to their three-phase counterparts.

the kVA for the inverter. It can be shown that the current angle

34 Motor Drives 989 to maximize the power factor is [4]

and maximum rate of change of torque are not affected as much. In order to get the correct current angle for maximum

δ = tan −1 ξ (34.104) torque per ampere, a lookup table of the saturation character- istic of the machine must be stored in the controller, which is

where consulted in order to calculate the desired current angle [7]. It has been found that iron losses in the stator and the rotor

ξ =L d /L q (the inductance ratio). also affect the optimal current angles. However, usually satu-

ration effects dominate, and the effects of iron losses can be

Remark

ignored.

Equation (34.104) indicates that ξ is the important parameter in relation to power factor. In order to obtain a power factor of 0.8,

34.10.7 A Syncrel Drive System

one requires an inductance ratio of approximately 10. The basic structure of a variable-speed drive system based on

Finally, we shall consider another control objective – max- using the Syncrel is shown in Fig. 34.114. Many components imize the rate of change of torque with a fixed-current-angle of this drive are very similar to those found in an induction control strategy. In effect, this means that one is maximizing machine drive system. One notable exception is the L d lookup the rate of change of the currents in the machine for a given table block and the current reference generator. The L d lookup

voltage applied to it. The analysis of this requirement results stores the current vs d-axis inductance table for the machine, in [4]

thereby allowing the inductance to be determined for various

current levels. This table is also used to generate the incremen-

δ = tan ξ

(34.105) tal d-axis inductance. The inductance values generated from this table are used in the state feedback block and the torque

Remark

estimator.

As with the maximum-power-factor case, ξ is the most important The state feedback block effectively generates an offset parameter in relation to the rate of change of torque. Because voltage to the PWM generator so that the voltage it pro-

this control effectively optimizes the current into the machine for duces is at least enough to counter the back-emf. This

a given voltage and angular velocity, this angle also corresponds technique effectively eliminates the back-emf disturbance from to that required to maximize the field-weakening range of the the current-control loops. machine.

The current reference generator takes the desired torque as an input and generates the required d- and q-axis currents at

Other control strategies for the machine can be devised, as the output. This block uses a lookup-table technique together well as the current angles required to obtain the maximum with an inverse of the torque equation to generate these cur- power from the machine during field-weakening operation.

rents and takes into account the saturation characteristics of the machine.

The three-to-two-phase block converts the currents from a If one carries out a thorough analysis of all the control properties three-phase stationary frame to a two-phase rotating frame.

Remark

of the Syncrel, then it emerges that all performance measures for This is a standard block in induction machine drives, and as the machine are enhanced by a large value of the ξ ratio.

with induction machine drives, this means that the Syncrel control algorithm is implemented in a rotating reference frame. The conversion from this frame back to the stationary frame

occurs implicitly in the space vector PWM generator. The Syncrel control algorithm is essentially a simplified The control strategies discussed in the previous section were all vector controller, and consequently the computational require- derived assuming that the machine does not exhibit saturation ments are not high. This means that a Syncrel controller can be and there are no iron losses. The q-axis of the machine does not implemented on a modest microprocessor. As far as input and have any saturation, as the flux path on this axis is dominated output hardware is concerned, the requirements are basically by air. However, the d-axis of the machine does exhibit sub- the same as those for an induction machine system – i.e. sam- stantial saturation under operational flux levels, and this effect ple two of the phase currents, the link voltage, and the rotor must be accounted for to optimize the drive performance.

34.10.6 Practical Considerations

position.

The effect that saturation has on the ideal current angles is to increase them. This increase is most pronounced for the