If Ia exceeds Ix,even by a small amount,a large output signal is produced by the threshold circuit,the current control overrides the speed control,and the speed error is corrected essentially at a constant current equal to the maximum permissible value.
如果IA超出Ix，哪0怕一点点，阈值电路也会产生一个大的输出信号，电流环代替速度环起主要调节作用，电枢电流等于电机所允许的最大电流，并在此恒定电流下对速度误差进行调整。
When the speed reaches close to the desired valued.When the
speed reaches close to the desired value,Ia falls below Ix,the current control goes out of action and speed control takes over.
Thus in this scheme,at any given time the operation of the drive is mainly controlled either by the speed control loop or the current control loop,and hence it is also called parallel current control.
当速度接近于期望值时，IA下降到Ix以下，电流控制失效，速度控制接替。
因此，在此示意图中，在任意给定时间，传动装置主要由速度控制回路或电流控制回路控制，所以，也叫并联电流控制。
Another scheme of close-loop speed control is show in Fig.5B-2.
It employs an inner current control loop within an outer speed loop.The out of the speed controller e is applied to a current controller u adjusts the converter firing angle such that the actual speed is brought to a value set by speed commandω m*.
闭环速度控制的另一种示意图如图1-5B-2所示。
在外环速度回路中采用内环电流控制回路。
速度控制器的输出ec用于电流控制器，为内环电流控制回路设置电流参考值Ia*。
电流控制器的输出uc调整逆变器的触发角，以便使实际速度达到速度给定值ωm*所设置的值。
Any positive speed error,caused by either an increase in the speed command or an increase in the load torque,produce a higher
current reference I*.The motor accelerates due to an increase in Ia,to correct the speed error and finally settles at a new Ia* which makes the motor torque equal to the load torque and the speed error close to zero.
由速度给定或负载转矩的增加所引起的任何正的速度偏差，都会产生更大的参考电流值Ia*。
由于Ia增加，电机加速，以调整速度误差，最终停留在新的Ia*值上，使电机转矩与负载转矩相同，速度误差接近于零。
For any large positive speed error,the current limiter saturate and the current reference Ia*,is limited to a value Iam*,and the drive current not allowed to exceed the maximum permissible value.The speed error is corrected at the maximum permissible armature current until the speed error becomes small and the current limiter comes out of saturation.Now the speed error is corrected with Ia less than the permissible value.
对于任何大的正的速度误差，限流装置饱和，且电流参考值Ia*限制为Iam*，传动装置的电流不允许超过最大允许值。
在最大允许电枢电流下纠正速度误差，直到速度偏差减小且限流装置退出饱和状态。
现在，速度误差在Ia小于最大允许电流值的情况下进纠正。
A negative speed error will set the correct reference Ia* at a negative value.Since the motors current cannot reverse,a negative Ia* is of no use.It will however”charge”the PI controller.When the speed error becomes positive the“charge”PI controller will take a longer time to respond,causing unnecessary delay in the control action.The current limiter is therefore arranged to set a
zero-current reference for negative speed error.
负的速度误差将设置负的电流参考值Ia*。
因为电机电流不能反向，负的Ia*是没用的。
然而，它将“掌管”PI控制器。
当速度误差变正时，“被掌管”的PI控制器将花费较长时间响应，造成控制中不必要的延迟。
因此，对于负的速度误差，限流器的电流给定值被设为零。
Since the speed control loop and the current control loop are in cascade,the inner current control is also known an cascade control.
It is also called current guided control.It is more commonly used than the current –limit control because of the following advantages.
因为速度控制回路和电流控制回路串联，因此内环电流控制也被称为串联控制。
整体也被称为电流操纵控制。
由于具有如下优势，其应用比限流控制更为普遍：
1.It provides faster response to any supply voltage
disturbance.The can be explained by considering the response of two drives to a decrease in the supply voltage.A decrease in the supply voltage reduce the motor current and torque.In the
current-limiter control,the speed falls became the motor torque is less than the load torque that has not changed.The resulting speed error is brought to th
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