15 Essential Variable Frequency Drive (VFD) Facts for Electrical Engineers
1. What is a frequency converter?
A frequency converter is a control device that uses the switching action of power semiconductor devices to convert power frequency power into electrical energy of another frequency. It can realize functions such as soft starting of AC asynchronous motor, variable frequency speed regulation, improved operating accuracy, power factor change, and overcurrent/overvoltage/overload protection.
2. Differences between PWM and PAM
PWM stands for Pulse Width Modulation, a modulation method that adjusts the output quantity and waveform by changing the pulse width of a pulse train according to a certain rule. PAM stands for Pulse Amplitude Modulation, a modulation method that adjusts the output value and waveform by changing the pulse amplitude of a pulse train according to a certain rule.
3. Why does the voltage of a frequency converter change proportionally to the frequency?
The electromagnetic torque of any motor is the result of the interaction between current and magnetic flux. The current must not exceed the rated value, otherwise the motor will overheat. Therefore, if the magnetic flux decreases, the electromagnetic torque will also decrease, leading to a reduction in load capacity.
From the formula E=4.44*K*F*N*Φ, it can be seen that during variable frequency speed control, the magnetic circuit of the motor changes within a considerable range with the operating frequency fX. This can easily cause severe saturation of the motor's magnetic circuit, resulting in severe distortion of the excitation current waveform and the generation of very high peak current spikes.
Therefore, the frequency and voltage must be changed proportionally. That is, while changing the frequency, the output voltage of the frequency converter is controlled to keep the magnetic flux of the motor constant and avoid the occurrence of weak magnetic field and magnetic saturation. This control method is mostly used in energy-saving frequency converters for fans and pumps.
4. How does the motor torque change when V and f are changed proportionally?
When the frequency decreases, the voltage is reduced proportionally. Since the AC impedance decreases while the DC resistance remains unchanged, the torque produced at low speeds tends to decrease. Therefore, at low frequencies, given a V/f ratio, the output voltage needs to be increased slightly to obtain a certain starting torque. This compensation is called enhanced starting. Various methods can be used, including automatic methods, selecting the V/f mode, or adjusting the potentiometer.
5. The manual states a speed range of 60–6Hz (10:1). Does this mean there is no output power below 6Hz?
Power can still be output below 6Hz, but based on factors such as motor temperature rise and starting torque, the minimum operating frequency is around 6Hz. At this frequency, the motor can output rated torque without causing serious overheating problems. The actual output frequency (starting frequency) of the inverter ranges from 0.5–3Hz depending on the model.
6. What does "open loop" mean?
A closed-loop control system uses a speed detector (PG) to feed back the actual speed to the control unit. A system operating without a PG is called an open-loop system. Most general-purpose frequency converters are open-loop, although some models offer PG feedback as an option. Sensorless closed-loop control calculates the actual motor speed based on a mathematical model and magnetic flux, essentially using a virtual speed sensor to create a closed-loop control.
7. What if the actual speed deviates from the given speed?
In open-loop operation, even if the inverter outputs a given frequency, the motor speed will vary within the rated slip range (1%–5%) when the motor is under load. For applications requiring high speed control accuracy, where operation must be close to the given speed even with load variations, an inverter with PG feedback function (optional) can be used.
8. If a motor with PG is used, will the speed accuracy improve after feedback?
Inverters with PG feedback function do improve accuracy. However, the speed accuracy value depends on the accuracy of the PG itself and the resolution of the inverter's output frequency.
9. What does stall prevention function mean?
If the given acceleration time is too short, the change in the inverter's output frequency will far exceed the change in speed (electrical angular frequency), causing the inverter to trip due to overcurrent and stop operation. This is called stall. To prevent stall and allow the motor to continue running, the current magnitude must be detected for frequency control. When the acceleration current is too large, the acceleration rate should be appropriately slowed down. The same applies during deceleration. The combination of the two constitutes the stall function.
10. What is the significance of models where acceleration and deceleration times can be set separately, and models where acceleration and deceleration times can be set together?
Models where acceleration and deceleration can be set separately are suitable for applications requiring short-duration acceleration and slow deceleration, or for small machine tools where strict production cycle time is required. However, for applications like fan drives, where acceleration and deceleration times are longer, both acceleration and deceleration times can be set together.
11. What protection functions does a frequency converter have?
Protection functions can be divided into the following two categories:
(1) Automatically correcting abnormal conditions, such as overcurrent stall prevention and regenerative overvoltage stall prevention.
(2) Blocking the PWM control signal of the power semiconductor device after detecting an abnormality, causing the motor to stop automatically. Examples include overcurrent cutoff, regenerative overvoltage cutoff, semiconductor cooling fan overheating, and instantaneous power outage protection.
12. Why do the frequency converter's protection functions activate when the load is connected using a clutch?
When a clutch is used to connect a load, at the moment of connection, the motor changes drastically from an unloaded state to a region with high slip. The large current flowing through it causes the inverter to trip due to overcurrent and become unable to operate.
13. Why does the inverter stop running when large motors in the same factory start together?
When a motor starts, a starting current corresponding to its capacity flows. This causes a voltage drop in the transformer on the motor's stator side. This voltage drop has a greater impact when the motor capacity is large. Inverters connected to the same transformer will make a judgment of undervoltage or momentary shutdown, thus sometimes triggering the protection function (IPE) and causing the motor to stop.
14. Is it possible to directly connect a motor to an inverter of a fixed frequency without using soft start?
It is possible at very low frequencies. However, if the given frequency is high, the conditions are similar to directly starting from a mains frequency power supply. A large starting current (6-7 times the rated current) will flow through, and because the inverter cuts off the overcurrent, the motor cannot start.
15. Can a frequency converter be used to drive a single-phase motor? Can it use a single-phase power supply?
Generally, no. For single-phase motors with speed controller switch-start, the auxiliary winding will burn out when operating below the operating point; for capacitor-start or capacitor-run motors, it will induce capacitor explosion. Frequency converters are usually three-phase, but some small-capacity models can operate on a single-phase power supply.
