Control systems for AC Motor VSDs

Most modern AC variable speed drives (VSDs) are of modular construction. Some of the technical details of the main components, such as the input rectifier, DC link, output inverter and the connected motor have already been covered in the previous sections.

This section covers the control system, embodied in the control circuits.

++++ Main components of an AC variable speed drive

Although the main function of the control system for modern PWM-type AC VVVF converters is to control the semiconductor switches of the PWM inverter, there are a number of other important functions, which need to be controlled. The overall control system can be divided into 4 main areas:

• Inverter control system

• Speed feedback and control system

• Current feedback and control system

• External interface, which includes the following:

- Parameter settings by the user

- Operator information and fault diagnostics

- Digital and analog inputs to receive control signals (start, stop, etc)

- Digital and analog outputs to pass on status information (running, faulted, etc)

With the rapid advances is digital electronics over the last decade, modern VSD control systems are based on one or more microprocessors. The control system must be designed to achieve the following main objectives:

• High level of reliability

• High inverter performance to ensure that the output current waveform provides sufficient motor torque, at selected speed, with minimum of motor losses

• Inverter losses should be minimized

• It must be possible to integrate the control system into the overall process control system, with facilities for external control and communications interfaces

• High tolerance to power supply fluctuations and EMI

Power Supply to Control System

For reliable operation of a VSD, it’s essential that a reliable power supply is available to provide power to the control circuits of the AC converter, even under abnormal situations, such as a power dip, high levels of interference, etc. The general requirements for power in a modern VSD are set out in the table below.

++++ General requirements for power in a PWM variable speed drive

The simplest method of providing auxiliary power to the converter control circuits is from an auxiliary transformer connected to the mains. Multiple secondary windings are necessary to provide isolation for the control circuits and the device drivers. The major problem with this approach arises when there is an interruption of the mains power.

Control of the inverter is lost and the VSD would have to be stopped, even for short dips in the supply. In many drive applications, there is a requirement for VSDs to 'ride through' voltage dips of short duration.

Consequently, most modern AC converters use switched mode power supplies (SMPS), fed directly from the DC link, to provide the auxiliary power to the control system. These are essentially DC-DC converter. The main advantage of this approach is that control power can be maintained right up to the time that the motor stops, irrespective of the condition of the mains supply. When the mains power fails, auxiliary power is maintained initially from the large capacitors connected across the DC link and later from the inertia motor itself. When mains power is interrupted, most AC converters are programmed to reduce frequency and retrieve power from the motor, which behaves as an AC induction generator when the frequency is reduced. There are many types of switched mode power supplies, including fly-back converters, forward converters and bridge converters. They can be isolated or non-isolated and have single or multiple outputs. Since they operate at high frequency (10 kHz to 100 kHz), they are physically much smaller than conventional mains frequency transformer based power supplies and despite the added complexity of SMPSs, they are of comparable cost.

Due to the modular nature of modern drives, it’s common to have multiple auxiliary power supplies, each of which is dedicated to a single module of the VSD, such as the control module, the pulse amplifier driver stage, the cooling fans, etc. These different SMPSs may operate independently from the DC link or from a central SMPS that converts the DC link voltage to a single isolated low voltage supply, such as 24 V DC. Each module may then take its power requirements from this 24 V DC power supply.

As shown, the device driver power supplies need to be provided with 4 or 6 isolated power outputs. These need to be isolated because the three power electronic switches connected to the positive terminal of the DC link have their emitter (IGBT and BJT), source (MOSFET) or cathode (GTO) terminals connected to the output phases to the motor. This terminal is the reference terminal for the driver stage, while the base or gate terminal must be driven positive to turn on or negative to turn off. The power supply reference point for each of these three devices is at a different potential, therefore requiring isolation.

The three power electronic switches connected to the negative terminal of the DC link all have their emitter, source or cathode terminals connected to the negative bus, and so a single power supply could be used for all three device driver circuits, hence the minimum of 4 isolated power supplies shown in the table. However, it’s more common to use 6 identical power supplies to operate the device driver stages, as there are benefits in terms of modularity and commonality of wiring.

There are two main methods for deriving these device driver power supplies. The first is to provide the six isolated supplies from either a mains frequency transformer or a SMPS, in the same way all other control power is produced. An alternative is to provide a single high frequency square wave supply, which is coupled directly into the six driver circuits through dedicated high frequency (usually toroidal) transformers that are part of each driver circuit. Separate rectifier and regulation circuits then provide the necessary plus and minus supplies for each driver stage.

The cooling fans for the converter heat-sinks can be powered from the SMPS or directly from the mains, whichever is a cheaper solution. The major drawback of the mains supply is the inability to deal with the different mains voltages and frequencies which are found throughout the world. This can usually be solved by supplying the fan through an auxiliary transformer with several primary connections to match the most common voltage options.

DC bus charging control system

++++ Example of a DC bus pre-charging circuit

A modern PWM-type AC drive operates with a fixed voltage DC bus. The fixed DC bus voltage is normally obtained from via a 6-pulse diode rectifier bridge from a 3-phase power supply. This voltage is usually 415 volts, 3-phase, 50 Hz while in some countries, the voltage is 380 volts, 3-phase, 50 Hz.

When the mains power is first connected to the input terminals of the AC drive, very high inrush currents would occur as the bank of filter capacitors across the DC bus charge. While the diodes in the rectifier module and the capacitors may be able to withstand these high currents, it’s quite possible that upstream fuses or circuit breakers would operate to trip out the VSD. Therefore, some provision needs to be made to limit this inrush current. The DC bus pre-charge circuit is normally provided for this purpose.

There are two main approaches to solving the problem of inrush current:

1. •Pre-charge resistors, with a bypass contactor, either on AC side or DC side of the AC/DC rectifier bridge
2. The AC/DC rectifier can be a controlled rectifier bridge instead of an uncontrolled diode bridge

The first method is the most common method. Charging resistors are inserted between the input supply and the capacitor bank to limit the current when power is first applied. Once the capacitors are charged, these resistors would introduce additional losses in the VSD and therefore need to be bypassed during normal operation. A relay (small VSDs) or contactor (large VSDs) is used to bypass the charging resistors and carry the full rated current of the drive.

The control of the relay may be either via a simple timer circuit with a fixed time delay between power being applied and the inverter stage being enabled. A better method is to monitor the DC bus voltage and the bypass relay is closed after a certain voltage level has been attained. In the better quality VSDs, feedback may be provided from each of the power supplies in the central controller to verify their status.

Some form of interlock needs to be provided to ensure that the relay is closed before allowing the inverter stage to operate. If not, the high load current through the VSD will heat up and burn out the charging resistors. In addition, it’s critical that all power supplies have had the opportunity to stabilize and establish regulation. As a result, most VSDs have a start-up lock-out circuit that delays starting for a short period after the VSD is powered up.

There are many variations on this theme, e.g. the resistors and relay can be either in the DC link or the 3-phase supply lines. There may be a single set of large resistors and one large relay or there may be multiple sets of smaller resistors and relays.

Other variations of this technique include the use of semi-conductor bypass switches.

The main advantages of this method are:

• Simplicity of the control circuit

• Cheap and easy to implement

The main disadvantages of this method are:

• The losses associated with the relay contacts and coils

• The physical size of these components

• The reliability of these electromechanical devices, particularly when the motor control system requires a high number of energization and de-energizations

The second, less common, approach is to replace the normal diode rectifier with a phase-controlled rectifier bridge. This allows the capacitor voltage to be increased gradually, by controlling the firing angle, and thereby controlling the inrush current. This method is most often used on VSDs with larger power ratings above about 22 kW. The main advantages of this method are:

• Conduction losses are lower

• Physical size is reduced by not having the relay

The main disadvantages of this method are:

• Power thyristors are more expensive than power diodes

• The control circuit is more complex in comparison with the relay circuit

• There is potential for false triggering of the phase control circuit due to notching and other disturbances on the mains

• Overall reactive power requirements are slightly higher

++++ DC bus charging using a phase-controlled thyristor bridge.