Control systems for AC Motor VSDs--part 2

PWM rectifier for AC converter

A conventional AC VVVF converter is made up of 5 main sub-assemblies:

• Ac/DC converter, usually comprising a diode rectifier, for converting the 3 phase AC voltage to a DC voltage of constant amplitude. In some cases a phase-controlled thyristor bridge is used for DC bus charging. Once full DC voltage is achieved, the thyristor bridge is controlled to behave as a diode bridge.

• The DC link, usually comprising a DC choke, DC capacitor and a DC bus, for maintaining a smooth fixed DC voltage for the inverter stage.

• The DC/AC inverter, comprising a semiconductor bridge, for converting the DC voltage to a variable frequency variable voltage AC output.

• The power supply modules, for providing power to the control circuits for the interface system and the inverter switches.

• The digital control system, comprising the sequence control, internal control loops, protection circuits and user interfaces.

The AC/DC rectifier at the front end of the converter supplies the DC bus and capacitor with voltage from the AC mains supply. Using a 6-pulse diode bridge rectifier for this purpose has two main disadvantages:

• The AC line current waveform is non-sinusoidal (refer to Section 4) and is the source of odd harmonics, such as the 5th, 7th, 11th, 13th, etc. This high level of interference can couple to other equipment and disturb their normal operation.

• The harmonic current distortion results in a distortion of the voltage at the point of common coupling (PCC) which, if high enough (large VS drives), can affect the performance of other electrical equipment connected to the power supply system.

• Full four-quadrant operation is difficult with the diode rectifier because electrical power can only be transferred in one direction (refer to Section 3), which makes regenerative braking impractical with a standard AC VVVF drive.

If a PWM-type controlled rectifier (also called an 'active front end') were used, it could provide a solution to many of these problems. The 6-pulse PWM bridge converter with IGBTs is electrically similar to a normal PWM inverter. As with the PWM inverter, it can transfer electrical energy in either direction, depending on the switching sequence of the IGBTs. For correct operation, it requires some minimum value of inductance in the line to avoid damage to the power semiconductor devices during switching. Line chokes may need to be added if the supply has a high fault level (low source impedance).

++++ The PWM rectifier for AC converters

One of the main problems of AC to DC power conversion using thyristor bridges, is the poor displacement factor due to phase-shifting of the current relative to the voltage and a bad power factor due to the distortion of the AC current waveform, which is non sinusoidal. The PWM converter is capable of correcting both of these problems by drawing nearly sinusoidal current from the mains at unity displacement factor.

The unity displacement factor is achieved by forcing the current waveforms to exactly follow the fundamental voltage waveform at fundamental frequency, usually 50 Hz. The in-phase component of current is controlled to maintain the capacitor voltage at a required level, while the out-of phase (quadrature) component of current can be made to be zero or leading to give a degree of power-factor correction for other loads.

Using pulse width modulation techniques, the current waveform can be made to be relatively undistorted (sinusoidal) and the reactive power requirement due to distortion will also be eliminated. This is assisted by the filtering effect of the line inductance.

VSD control loops

An AC frequency converter is designed to control both the voltage and frequency fed to the motor and is therefore often called a variable voltage variable frequency (VVVF) controller. The digital control system automates this process. E.g., when an operator selects a speed setting on a potentiometer, the VSD control system implements this selection by adjusting the output frequency and voltage to ensure that the motor runs at the set speed. The accuracy of the control system and its response to the operator's command is determined by the type of control system used on that particular VSD. The type of control used in VSD control systems follows an approach similar to that used in normal industrial process control. The level of control can be:

• • Simple open-loop control, no feedback from the process
• • Closed-loop control, feedback of a process variable
• • Cascade closed-loop control, feedback from more than one variable

Open-loop control

The purpose of an electrical VSD is to convert the electrical energy of the mains power supply into the mechanical energy of a load at variable speed and torque. In many applications, VSDs are simply required to control the speed of the load, based on a setpoint command provided by an operator or a process controller.

Conventional VVVF converters are voltage source devices, which control the magnitude and frequency of the output voltage. The current that flows depends on the motor conditions and load, these are not controlled by the AC converter, but are the result of the application of voltage. The only current control that is exercised is to limit the current when its magnitude reaches a high level, E.g. at 150% of full load current.

There is no provision made for feedback of speed information from the motor to check if it’s running at the required speed or if it’s running at all. If the load torque changes, and slip increases or decreases, the converter would not adjust its output to compensate for these changes in the process.

This method of open-loop control is adequate for controlling steady-state conditions and simple applications, such as centrifugal pumps and fans or conveyors, which allow a lot of time for speed changes from one level to another and where the consequences of the changes in the process are not severe.

Closed-loop control

In industry, there are also those more difficult applications, where speed and/or torque must be continuously and accurately controlled. The required accuracy of the control is important and can have a large influence on the choice of drive technology. For those drive applications that require tight dynamic control, closed-loop control is necessary. This type of performance can be achieved with closed loop vector control AC drives and standard DC drives.

Standard VVVF AC drives can be used in closed-loop control systems, such as pumping systems, which regulate pressure or flow, but in general these applications are not capable of high performance.

The typical configuration of a closed-loop VSD system and consists of the following main components:

• The motor, whose role is to convert the electrical energy of the supply into the mechanical energy necessary to affect the load

• A transducer for measuring the load quantity, which is to be controlled.

This is used as a feedback signal to the control system. Where speed is important, the transducer can be a tachometer (analog system) or an encoder (digital system).

Where position is important, the transducer is a resolver (analog system) or an absolute encoder (digital system). However, there are less expensive means for measuring speed and position, depending on the required accuracy. Where current is important, the transducer is a current transformer.

• A converter which controls the flow of electric power to the motor. This is achieved with a power electronic converter, involving solid-state devices switching at high frequency under the control of a digital circuit.

• A controller, which compares the desired value of speed or position, called the set-point (SP), with the measured value, called the process variable (PV) and then gives a control output which adjusts the speed and torque to reduce the error (SP-PV) to zero. Previously, controllers were implemented by analog circuits, using operational amplifiers (Op-Amps). Modern controllers are implemented using microprocessors and digital circuits.

++++ Schematic of a closed-loop VSD control system

The desired value of the load, such as the speed, can be set manually by an operator turning a potentiometer (analog system) or by dialing up a value on a keypad (digital system). If the VSD is part of a complex control system, the desired value can be passed down from the process control system (PLC or DCS), either by means of a 4-20 mA signal (analog system) or by means of a serial data link (digital system). If each quantity in the control loop was directly proportional to the quantity before it, simple open-loop control of speed would be adequate, without the need for feedback of the process variable (PV). In AC drives, if accurate speed control is required, then feedback of the torque and speed variables is necessary. In particular, the motor current responds to an increase in motor frequency with a rise time dependent on its leakage inductance. On the other hand, the motor speed follows the torque with a rise time dependent on its inertia. While these inaccuracies may be acceptable in simple applications, such as pump speed control, it may not be acceptable for other difficult applications, such as the variable speed drives in a paper machine, where several drives operate in tandem. In these difficult applications, improved performance can be obtained with the use of several closed-loop control systems working together, known as multi loop or cascade control.

This type of closed-loop control system has been redrawn to emphasize the most important control aspects. The term closed-loop feedback control emphasizes the nature of the control system, where feedback is provided from the output back to the input of the controller.

++++ A closed-loop feedback control system

In a closed-loop variable speed drive, the following takes place:

• Measurement of the process variable using an encoder

• Comparison of the process variable (measured speed) with the set point (desired speed) to give an error signal. SP - PV = error signal

• This error signal is then processed by the controller to adjust the output signal to the process, in this case, the AC converter, motor and speed transducer.

++++ could be misleading because, in practice, the error point is usually part of the controller

Cascaded closed-loop controller

For the difficult applications, which require very close speed and torque control, with a fast response to changes in the process, a single-loop controller may not be adequate to anticipate all the delays in the process. These make the controller difficult to design and difficult to setup during commissioning. Fortunately, a technique that deals with the problem in several smaller steps has evolved from past experience with DC drives. The solution consists of two cascaded closed-loop controllers. The basic setpoint is the speed setpoint, which is set by an operator via a potentiometer or from a PLC. Rather than attempting to calculate the desired inverter frequency directly to meet these speed requirements, a DC drive achieves this in two stages.

• The first speed control loop uses the speed error to calculate the desired torque setpoint to either increase speed (accelerate) or decrease speed (decelerate). The speed control loop only has to allow for one of the time delays in the system, which is the delay between the torque and the measured speed. This compensates for the mechanical transients in the system, mainly load inertia.

• The second torque control loop compares this set torque, the output of the speed controller, with the actual measured value and calculates the desired output frequency. The measured process variable in this case is the measured motor current, which is proportional to the motor torque. Therefore, this control loop is often called the current loop.

A vector control drive uses a similar strategy. In the design of the torque control loop, it’s assumed that the rate of change of current is much faster than the rate of change of speed. This is equivalent to assuming that the motor is running at a constant speed.

Consequently, the current loop only has to allow for the time delay between the output frequency and the current. As well as giving the desired inverter output frequency, it also gives the desired inverter voltage since the two are related. Both quantities are passed to the PWM switching logic, which controls the inverter switching sequence and speeds.

The current control loop compensates for the electrical transients, mainly the winding inductance and resistance.

The block diagram of the cascaded loop controller comprises:

• An outer (major) speed control loop

• An inner (minor) torque control loop

++++ Cascade controller for speed and torque

A major advantage of the cascaded controller is that it’s possible to impose a current limit on the drive output by placing a limit on the input to the current/torque loop. This is usually set to prevent the speed control loop from asking for any more than about 150% of rated current from the current loop.

The current loop can respond quickly, in less than 10ms. The speed loop responds more slowly because motor and load inertia are usually substantial. A response time of about 100ms is typical for the speed control loop.

The response of the amplifier for the speed or current controller to a step change. The simplest type of controller amplifier is one whose output is proportional to the input, called a proportional amplifier or P-control. P-control is not used in speed and current control loops because it does not respond well to the requirements for high accuracy and a fast dynamic response. It’s more common in high performance VSDs to use proportional-integral control, or PI-control. The step response for PI-control consists of a combination of the step output of the P-control and a ramp due to the integral control.

++++ Response of P and PI controllers to a step input

(a) Step input control change (b) Proportional controller output (c) Proportional/Integral controller output.