The rectifier, intermediate circuit, inverter, and control unit are the key components of a VSD unit, as shown in the diagram below. The rectifier converts alternating current (AC) to direct current (DC). In the intermediate circuit, the rectified DC power is typically regulated by a combination of inductors and capacitors. The inverter converts the rectified and regulated DC power back into AC power with variable frequency and voltage. This is usually achieved by generating high-frequency pulse-width modulated signals with different frequencies and effective voltages. The control unit oversees the entire operation of the VSD; it monitors and controls the rectifier and intermediate circuit.
VSDs connect to sensors such as pressure or flow sensors and are programmed to maintain a certain value (setpoint). They can interface with multiple sensors to enable interlocking and other control functions, and connect to a current computer network that provides real-time operational data.
The energy-saving potential of a VSD depends on the characteristics of the load it drives. Loads are categorized into three types: variable torque, constant torque, and constant power. Variable torque loads are common in centrifugal fans and pumps and offer the greatest energy-saving potential. This is because torque varies with the square of the speed (H1/H2=(N1/N2)2), while power varies with the square of the speed (P1/P2=(N1/N2)3). Furthermore, flow rate varies with the speed (Q1/Q2=(N1/N2)).
So, are variable speed drives harmful to motors?
How would PWM-based variable voltage/variable frequency excitation from a typical inverter-based VSD affect a typical induction motor?
A VSD (Variable Voltage Detector) is used to change the speed of an induction motor. Induction motors have traditionally been constant-speed devices. They operate at a speed synchronized with (mostly) the frequency of the applied current, although the phase relationship lags. To change the speed, you need a power supply (potentially high power) whose frequency (and possibly voltage) can be varied. Doing so gives you a VSD. For a VSD controller, the applied voltage level typically adjusts downwards as the frequency decreases: because the back electromotive force decreases at lower speeds, a lower applied voltage is needed (otherwise the motor current would rise excessively).
So far, it's been very simple. But how do we get a variable frequency? We only have 50Hz or 60Hz available. Very simple. Or perhaps as simple as π radians. Take the AC line voltage, rectify it full-wave, filter it, and then apply that bus voltage to an H-bridge (or a three-phase H-bridge) made of power FETs or IGBT devices. As mentioned before, this is the core of the inverter. So far, so good.
The H-bridge drive signal is controlled using a PWM signal with appropriate timing and phase arrangement. By changing the pulse width/duty cycle, the H-bridge output will generate current in the motor windings, which, on average, mainly exhibits a sinusoidal curve. The inductance of the motor windings and their inertia tend to smooth things out. But this is where things can get a little tricky.
Those pesky PWM waveforms are okay on average—but the fact that they switch so rapidly means they contain a lot of high-frequency components. This can cause the motor windings to overheat. High di/dt also means high voltage transients on the motor windings. If the motor wasn't designed to work with an inverter, it could be damaged.
Damage can occur in places you might expect – the insulation on the motor windings may deteriorate or be completely destroyed. Smoke and electric arcs are obvious signs.
Damage can also occur in places you might not expect—bearings. Again, the high-frequency component of the PWM current is the culprit. It can couple current through the bearings to the motor housing via capacitors. This can cause damage that looks like pitting or corrosion—bearings are designed to carry mechanical loads, not current.
Another issue is EMI radiating from the connection between the inverter and the motor, or being conducted back through the input AC power lines. Adding an LC filter between the inverter and the motor helps address the aforementioned heating and insulation problems. Sometimes, heat is simply moved due to the circulating current from the motor to the filter element, so this should also be considered. Application engineering assistance must be obtained from the inverter and motor suppliers when addressing this issue.
