Q3Electric Drives and Their Control

Advantages of Variable Frequency Control and the Speed Control Scheme

Variable frequency control of induction motor drives offers substantial advantages over other speed control methods discussed elsewhere in this examination, including a much wider achievable speed control range (spanning from very low speeds up to and beyond synchronous speed, rather than the narrow range achievable with stator voltage or rotor resistance control), high efficiency across the entire speed range (since the motor can be operated near its optimal slip at every speed, avoiding the excessive rotor copper losses that rotor-resistance or stator-voltage control methods incur at reduced speed), and smooth, continuously variable speed control without the discrete steps associated with pole-changing motors.

The variable frequency speed control scheme, most commonly implemented via a Voltage Source Inverter (VSI, discussed further below) fed from a front-end rectifier, varies both the frequency and the magnitude of the applied stator voltage together, maintaining a constant V/f ratio below base frequency to keep the air-gap flux approximately constant across the controlled speed range, thereby maintaining approximately constant available torque capability throughout this constant-flux operating region. Above base frequency (where the inverter's maximum available voltage, limited by the DC link voltage, has already been reached), frequency continues to increase while voltage is held at its maximum value, causing the V/f ratio and hence flux to decrease - this field-weakening region extends the usable speed range beyond base speed at the cost of reduced maximum available torque, exactly mirroring the analogous field-weakening operation used in DC motor drives above base speed.

Voltage Source Inverter (VSI) Operation

A Voltage Source Inverter (VSI) converts a fixed-magnitude DC link voltage (established and held nearly constant by a large DC link filter capacitor) into a variable-frequency, variable-magnitude three-phase AC voltage waveform, using pulse-width modulation (PWM) switching of the inverter's six power semiconductor switches, most commonly controlled via sinusoidal PWM or space vector modulation to synthesize an output waveform whose fundamental component closely approximates a sinusoid at the desired frequency and magnitude, while shifting the unavoidable switching harmonics to a high frequency band that is comparatively easy to filter out using the motor's own leakage inductance. VSI-fed drives are by far the dominant modern approach for variable-speed AC motor drives across essentially the entire power range, given the maturity, reliability, and cost-effectiveness of modern IGBT-based inverter modules combined with sophisticated digital control implementations.

The variable frequency control scheme and the VSI hardware that typically implements it together form one of the most economically and technically important developments in modern industrial motor drive technology, since the combination of wide speed range, high efficiency, and smooth continuous control that variable frequency VSI drives provide has made them the dominant choice for essentially all new variable-speed induction motor drive installations across virtually every industrial sector, progressively displacing the older stator-voltage-control, rotor-resistance-control, and pole-changing methods discussed elsewhere in this examination wherever a wide, efficient speed range is required.

An important extension of variable-frequency control worth noting is that maintaining a constant V/f ratio, while adequate for preserving approximately constant flux and hence constant available torque across the normal operating speed range, breaks down at very low frequencies, where the fixed stator resistance voltage drop becomes a proportionally larger fraction of the reduced applied voltage, causing the actual air-gap flux to fall below its rated value and torque capability to reduce - this is compensated in practical V/f drives by a low-frequency voltage boost that adds a fixed offset voltage independent of frequency, restoring adequate flux and starting torque capability even at very low output frequencies including standstill.

A Voltage Source Inverter (VSI) is a power-electronic converter that converts a fixed or controlled DC input voltage into a three-phase (or single-phase) AC output voltage of controllable frequency and magnitude, by sequentially switching each output phase between the positive and negative DC bus rails using pairs of controllable semiconductor switches (IGBTs in most modern designs) arranged in three half-bridge legs for a three-phase output, with each leg's two switches operated in a complementary manner (one on while the other is off) to avoid short-circuiting the DC bus. The VSI is termed a voltage source inverter because the DC link voltage is held essentially constant (typically by a large DC link capacitor) and is largely independent of the load current drawn, in contrast to a current source inverter where the DC link current is held constant instead. Modern VSIs almost universally use pulse width modulation (PWM), in which each switch is turned on and off many times within each fundamental output cycle according to a modulation strategy (commonly sinusoidal PWM, comparing a sinusoidal reference wave against a high-frequency triangular carrier wave) so that the averaged, low-frequency component of the resulting pulse train approximates the desired sinusoidal output voltage waveform, while the high-frequency switching harmonics are pushed to frequencies well above the fundamental where they can be readily filtered by the motor's own leakage inductance, giving VSI-PWM drives excellent output waveform quality, fast dynamic response, and high efficiency, making them the dominant converter topology in essentially all modern variable-frequency AC motor drives.

It is useful to additionally note that variable frequency control, while enabling wide-range, smooth speed control of induction motors that was historically only achievable with DC drives, also introduces certain practical challenges of its own, notably the generation of torque pulsations and additional harmonic losses in the motor due to the non-sinusoidal (PWM-synthesized) nature of the inverter output voltage waveform, and the requirement for the motor's insulation system to withstand the repetitive high dv/dt voltage transitions produced by fast-switching modern IGBT-based inverters, which can in some cases lead to accelerated insulation aging or bearing currents caused by common-mode voltages unless appropriate mitigation measures (such as output filters or insulated bearings) are employed, particularly in retrofits where an older motor not originally designed for inverter duty is connected to a new variable-frequency drive.

In summary, variable frequency control together with the voltage source inverter that realizes it in practice form the cornerstone technology of modern induction motor speed control, having displaced older methods such as pole changing and rotor resistance control in the great majority of new variable-speed industrial drive installations due to their superior efficiency, wide speed range, and smooth control characteristics.