VFD Fundamental's

 

A Variable Frequency Drive (VFD) is an electronic device that vary the frequency and voltage of the power supplied to an electric motor to alter the speed and torque of the electric motor. 

The primary purpose of a most VFDs is to adjust the speed of the motor to match process requirements. For example, in a gas processing plant there are various aftercooler fans driven by VFD-fed motors, the VFD increases or decreases the speed on the cooler fan in response to the process variable setpoint.  In this case, the temperature of the gas to be cooled is the process variable.

HOW A VFD WORKS

A VFD works in three stages namely, Rectifier Stage, DC bus and Inverter stage.

In the rectifier stage, AC supplied to the VFD is converted to DC using diodes. The output from the rectifier is a pulsating DC. The Pulsating DC is smoothened out in the DC bus stage using capacitors and/or inductors. The Filtered DC from the DC Bus supplies electrical energy to the inverter stage of the VFD. The inverter is responsible for converting DC voltage into AC voltage at a variable frequency. 

The inverter would chop up voltage supplied from the DC bus by using insulated-gate bipolar transistors (IGBTs) to switch the DC voltage on and off rapidly thereby creating a pulse-width modulated (PWM) waveform.  By adjusting the frequency and width of these pulses, the inverter can control the effective output frequency and voltage supplied to the motor, thus regulating its speed and torque. 

Every Inverter has a Pusle Generator. The Pulse Generator is the control unit that decides how long an IGBT (electronic switch) stays open. If an IGBT stays open for the entire duration of the positive half cycle and goes off, the resulting wave form will be a square wave. To generate a wave form that closely mimics a sinusoidal wave, the Pulse generator sends signal to the IGBT to rapidly open and close in a pulsating pattern multiple times in a cycle. Each pulse in a cycle is known as a segment and each segment will vary in width to allow a certain amount of current flow per segment. The more segments we have in a cycle, the closer it will mimic a sine wave. In the real world this can be varied by changing the switching frequency of the VFD.

The output voltage from the inverter is determined by controlling how long the IGBTs stay on in a cycle. By increasing the on-time relative to the off-time within each cycle, the average voltage seen by the motor increases. This on-off ratio is also known as Duty Cycle. 
For example, if the duty cycle is 50%, the effective output voltage is 50% of the DC bus voltage. If the duty cycle is 80%, the effective output voltage is 80% of the DC bus voltage.

The Frequency from an inverter is determined by controlling the rate at which one cycle is repeated. If the PWM pattern is repeated faster, the resulting output waveform has a higher frequency. Conversely, if the pattern is repeated more slowly, the output frequency is lower.

VOLTAGE AND FREQENCY RELATIONSHIP

It is almost impossible to control the frequency without changing the voltage. This coordinated control of frequency and voltage is essential because most AC motors are designed to operate with a specific V/f (voltage to frequency) ratio. 

When the frequency applied to an induction motor is reduced, the applied voltage must also be reduced to limit the current drawn by the motor at reduced frequencies. The inductive reactance of an AC magnetic circuit is directly proportional to the frequency as shown by the formula 

XL = 2π f L. 

Where:

• XL = Inductive reactance in ohms
• π = 3.14,
• f = frequency in hertz
• L= inductance in Henrys 

 Simple math will show that at low frequencies, the inductive reactance is relatively small. This means the inductor offers less opposition to the AC current, allowing more current to pass through the circuit. More current would cause the drive to trip or the motor windings to heat up and burn out. As a consequence of this, VFDs maintain a constant volts/hertz relationship. To calculate this ratio, we divide the motor nameplate voltage(V) by the nameplate freq(Hz).

For a 415V motor designed to run at 50Hz motor this ratio is 8.3 volts/Hz

VFDs can cause spikes in voltage that damage motor windings. 

VFD RELATED MOTOR FAILURE.

(1) DC Bus Overvoltage.
Under certain circumstances, DC bus voltage on a VFD can be pumped up if the motor decelerates more quickly than it wants to coast down. The rotor momentarily runs faster than the synchronous speed (VFD freq). The motor now temporarily becomes a generator and the electrical energy it generates is rectified by the flyback diodes in the IGBTs this voltage then pumps up the DC bus voltage level. 

Eventually, the rotor will decelerate to a speed lower than the VFD's synchronous speed. During this process, the inverter may convert the temporarily high DC voltage on the DC bus back to AC, causing the motor to receive a voltage higher than intended. This voltage spike can lead to various types of failures in the motor. In most cases, the overvoltage protection function of the VFD (Variable-frequency Drive) will operate to stop the operation. 

Flyback diodes are used in the inverter circuit because when current flows through an inductive load, energy is stored in the magnetic field of the inductor. When the switching device (IGBT) is turned off, the collapsing magnetic field causes a sudden change in current. According to Faraday's Law of Induction, this change induces a high voltage in the opposite direction, known as back EMF. The flyback diodes are used to protect the IGBT from this voltage spike because they conduct during the IGBT off state and dissipate stored energy.

Motors with insulation class F and H have enough protection to mitigate against voltage spikes from the VFD.

 (2) Induced AC Motor Shaft Current and Bearing Damage

Using a VFD can cause damage to the bearings. Induced voltage in the shaft can reach levels high enough to overcome the dielectric strength of the bearing grease.  The voltage potential induced into the rotor will discharge through bearings and the arc would cause pitting in the rollers of the bearing. 

Shaft Grounding Rings are installed to protect VFD-fed motors from bearing damage by discharging shaft voltage. Insulated (Ceramic) bearings ai also an excellent alternative.

This voltage buildup occurs because of the high-frequency switching used to control the motor speed. We mentioned earlier on this article that the output from a VFD is not a true sinewave, but it closely mimics a sine wave by using rapid switching to output a series of positive and negative pulses.  This creates sharp voltage transients and high-frequency components. 

These high frequencies are more likely to result in capacitive coupling between the motor windings and the shaft. As we know. The capacitive reactance decreases with increasing frequency. Due to the high frequency switching pulses of a VFD, which can be in the range of 2 to 20 kHz or higher, there is little to no capacitive reactance and capacitive coupling occurs between the rotor and the winding.

Capacitive coupling is a phenomenon that occurs when two conductors are placed near each other, allowing an electric field to develop between them. When a voltage is applied to one conductor, it creates an electric field. If a second conductor is nearby, this electric field can induce a voltage in the second conductor, even though there is no direct electrical connection. The dielectric is the air gap between them, effectively creating a capacitor.

(3) Overheating

VFDs can shorten a motor service life by increasing the heat stress on the stator windings. It does this in two ways. The most common being when the motor is run below the rated minimum speed for prolonged periods. For TEFC motors, the internal and external fans run slower, and this can lead to overheating. 

The second is harmonics on the load side introduced the inverter switching. These harmonics cause increased copper losses (I²R losses) in the windings and increased iron losses (hysteresis and eddy current losses) in the stator core. This leads to overheating and reduced efficiency.