Power Electronics And 3Phase Drives

3 Phase motor drives and DC drives dominate the industry in most applications from low to high power. (Single phase drives usually take care of the low power end.)

Basic 3Phase motors are:

• 3Phase induction cage rotor motor
• 3Phase induction wound rotor motor
• 3Phase synchronous motor

3Phase induction motors are used widely to serve general purpose applications, both adjustable speed and servo drives. 3Phase synchronous motor is found in special applications, mostly as servo drives. Some very large power adjustable speed drives also prefer synchronous motors because of the possibility of using low cost load-commutated-inverters (LCI) built from thyrestors.

Single Phase Drives - Servo Control Mode

Servo control use current control for rapid adjustment of motor torque. Voltage control will not be good for servo applications due to inherent delays before the control passes to adjust current. In PWM it is a delay in the motors electrical time constant L/R; in square wave control it is a sequence of delays at the capacitor of DC-link, electric time constant L/R of motor etc.

To obtain current control we use, so called, "current controlled PWM". There too, we have two options;

(a). Hysteresis current control mode

(b). Fixed frequency current control mode

(a). Hysteresis current control mode

This PWM acts to constrain the motor current I to a specified shape and amplitude, as suggested by the outer loops (e.g. Speed loop) of the closed loop control system. This requires motor current feedback as an input to the PWM modulator. Desired current is the other input.Switching principle is,

Single Phase Drives - Low Speed Control Mode

Power circuit for single phase drive - low speed control mode

At low speeds, motor voltage V should not have lower-order harmonics. An ideal would be a pure sinusoidal voltage but a compromise is acceptable.

The square wave voltage used in the high speed mode contains lower order harmonics of order 3,5,7,9...etc. So we con not use it for low speed operations. If attempted we will get some wobbling speed perturbations at low speeds.

We use switching strategy known as PWM (Pulse Width Modulation) to deliver near sinusoidal voltage for the motor. We have two operations of PWM.

(a). Bipolar PWM

(b). Unipolar PWM

Single Phase Drives - High Speed Control Mode

This mode of control is suitable when the speed is to be controlled at higher values; for example above base speed.

At higher speed the motor will not response to torque harmonics (because of filtering due to electromagnetic time constant) and hence we can apply simple square wave voltage input. The motor then responses to the fundamental component of voltage and (virtually) ignores harmonic voltages.

Switching pattern for squarewave voltage output

Ts - Switching cycle time.

Switching signals for squarewave voltage output

Power Electronics and Single Phase Drives

We consider drives built around single phase induction and single phase synchronous motors in this section.

Motor Diagram

Use of a variable speed drive instead of a fixed speed motor plus external mechanical control yields many advantages. It saves energy significantly and enhances the performance of the drive system (i.e. the load). Energy saving can add up to some MW (Mega watt) level savings in a country due to heavy proliferation of single phase motors in any country. (Note: The largest share of power consumption in most countries attributed to single phase motor).

Motor control requires its input voltage V and frequency f be adjustable with appropriate resolutions. There are 3 alternative control modes.

1. High speed control mode
2. Low speed control mode
3. Serve control mode

The drive system take power from an unregulated DC input. This is the norm for all drive systems, not only for 1 phase drives. This unregulated DC in obtained using a diode rectifier (1 phase or 3 phase rectifiers) on the utility AC system.

Note: There is a reason for using diode rectifiers on the AC utility as the first stage of a power electronic converter.

These are;

1. To minimize supply side distortions and disturbances
2. To make the subsequent conversions easier

Power Circuit of 1 Phase Drive

Q1, Q2, Q3, Q4 - Power switches
D1, D2, D3, D4 - Fast recovery diodes (freewheeling diodes)
C1 - DC-link capacitor
S1, S2, S3, S4 - Switching signals (Binary signals)

Switching signal S1, S2, S3, and S4 are generated by the control unit according to the mode of control used.

Read More:

Power Converter Technology

Apart from power switching devices, the power-converter technology has also gone through significant developments in the recent past. Resonant converters specially the multi resonant converters are critical examples. High-phase-number converters is another area of development. (where the motors are no longer restricted to 3 phases).

Motor technology also went through a similar phase of developments, mainly after 1980. In 1985, the development of high energy permanent magnet, known as Nd-Fe-B (Nyoaliminium Iron Boron) resulted in a set of new motors such as brussless DC motors, switch reluctant motors, hybrid stepper motor, permanent magnet linear motor ...etc. The brussless DC motor is a good alternative to conventional (brushed) DC motors, it delivers the same performance without problems associated with a mechanical commutator.

Note: High energy permanent magnet materials especially Sm-Co (Samarian Cobolt) was in use prior to developing Nd-Fe-B but the high cost of Sm-Co and not allow its usage in industrial grade motors. It was used expensively in aerospace applications.

Recent Developments

Motion control industry went through a revolutionary phase after 1980 following the remarkable development took place in the fields of power switching devices, high energy permanent magnets and microelectronics.

Advanced and sophisticated power switching devices such as power MOSFET and IGBT came into being during that time. This made a significant change in the trend of power electronic control, producing inverters and converters capable of switching at extremely high frequencies well into MHz range. (The old Thyristor converters could not do more that about 1 KHz at that time.) This level of very high switching frequency is essentials to realize complex control functions.

Circuit symbol of IGBT

Structure of Drive System

Figure 1: Structure of drive system

Load is the most important component. All other units are arranged to serve the requirement of the load. Any design begins with a list of specifications for the load. This depends on the type of the load and its environment. We need to answer several questions before preparing specifications.

Eg:

• Whether one, two or four quadrant operation is necessary
• Whether servo or adjustable speed action is necessary
• Whether breaking energy is to be dissipated or returned back to the utility
• Whether a soft or hard start is suitable
• What are the range of speed and the resolution of the speed control?
• What are maximum torque?
• Is there a space restriction?
• Is there a noise level restriction?
• Is there some starting current restriction?
• Is there any EMI (Electro Magnetic Interference) restriction?
• Is there supply end restrictions? (Eg: Total Harmonic Distrotion)?
• Whether the energy efficiency is a prime consideration

and many more.

The components in the drive system are independent. The load to some extent can suggest the type of motor required. The motor selection it self is a challenging task that requires good understanding and skill about the performance of different motors. This difficulty arise due to the availability of a large count of competitive motor types. The power electronic converters is often specified by the motor selected. For DC motors we need one type of converter but for AC motors we need a different type converter. A stepper motor needs a totally different converter and so on.

Eg:

Figure 2: Table of Choices of power electronic converter

Additional stages may also be necessary on top of the basic converter circuit to accommodate such features as returning breaking energy back to the supply network etc. In all the power converters listed in the table DC was used as input source. This is not because the primary power available from the power system is DC but the common practice of a diode rectifiers unit to interface the power system (This is done to minimize the distortions at the supply end).

Figure 3: AC to DC Converter

Microelectronic control unit is central to all other components in the system and it co-ordinates the activities of all. It receives feedback from the load and motor and dictates terms to the power electronic converter according to the user input. The microelectronic control unit is a computer. It can be a micro-controller or is some cases a PLC. Discretely assembled microelectronic control units too are not common.

Read More:

Overview of Technology State

Role of power electronics and drivers

• Power controlling using electronics consider here
• Signal controlling is consider about controlling information

Power electronics is about control of power electronically. Range of power can extend from some fraction of watt to several mega watts (MW). Drives are about the control of mechanical motion. It can be a linear motion, rotary motion or their combination. There is a greater compatibility between power electronics and drives as the former can provide smart control of power for the drives. This is evident from the varieties of fascinating motion control systems we find today.

There are several terms used in the context f motion control.

• Motion control systems
• Mechatronic systems
• Drive systems
• Adjustable speed drive systems
• Servo drive systems

Motion control system is the name used in west (Europe) to represent the low power drives (Computer pheripherals, home appliances...etc) upto several KW. Mechatronics system is the name used in the east (Japan) for the same, that is the low power drives. Drive system is the name that represent drives of the entire sprectum of power. Adjustable speed control system is the name reserved for the sub category whose steady-state performance only is emphasize. Servo drives in the name reserved for another sub category whose both the transient and the steady-state performance are empasized.

Automation is a multidisiplinary technology that integrates the knowledge of diverse areas such as instrumentation, remote sensing, advance control, data processing, computer technology, microelectronics, mechanics process system, motion control and many more. Motion control is one of the thrust areas that propells automation industry hard.

Read More: