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.

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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