Things you want to know about VFD for pumps

The water pump frequency converter saves energy by reducing voltage under light loads. The greater the speed of the torque load it drives, the better the energy-saving effect. When applied to fans, if a lower air volume is needed, the motor reduces its speed. We know that a fan’s energy consumption is directly proportional to the 1.7 power of its speed, so the motor’s torque drops sharply, resulting in significant energy savings.

In other words, if speed control is required by the environment, the energy-saving effect of the water pump frequency converter becomes more apparent. In situations where speed control is not used, the energy-saving effect of the frequency converter is less noticeable, but it can still improve power factor or save on labor costs through automation.

Here are some common questions about water pump frequency converters:

1.Energy Saving with Frequency Converter at 50Hz:

Q: If two identical motors, one with a frequency converter and one without, operate at 50Hz, does the frequency converter save energy?
A: In this case, the main function of the frequency converter is to improve the power factor, not significantly saving electricity. Both motors are operating at their rated conditions, so the energy-saving effect of the frequency converter is not prominent.

2. Energy Savings in Non-Rated Torque Conditions:

Q: How much energy can be saved by using a frequency converter on a motor operating under non-rated torque conditions?
A: If the frequency converter’s automatic energy-saving function is used, it can save some electricity by lowering the voltage. However, the energy-saving effect is not very significant because the motor does not deviate substantially from its rated operating state.

3. Energy Savings in No-Load Conditions:

Q: How much electricity can be saved using a frequency converter in a no-load state, and which of the three states offers the most energy savings?
A: In a no-load condition, the frequency converter can significantly save electricity, possibly over 40%. However, if the pump unit operates at 50Hz for extended periods, not using a frequency converter might be more energy-efficient. Thus, the no-load condition shows the most significant energy-saving effect, but the actual application mode should be considered.

4.Motor Start-up Current and Torque with Frequency Converter:

Q: When using a water pump frequency converter, what are the motor’s start-up current and torque?
A: Using a frequency converter allows for a smooth start-up of the motor, limiting the start-up current to below 150% of the rated current, reducing mechanical and electrical impact. The start-up torque is generally between 70% and 120% of the rated torque, much lower than the high start-up current produced when starting with a mains frequency source.

5.Frequency Converter Shutdown during Large Motor Start-up:

Q: Why does the frequency converter stop operating when a large motor starts?
A: The start-up current of a large motor can cause a voltage drop in the transformer, especially when the motor’s capacity is large. Frequency converters connected to the same transformer may detect undervoltage or momentary stoppage, triggering protective functions and causing them to stop operating.

6.Installation Direction of Frequency Converters:

A: The interior and back design of the frequency converter consider cooling effects, so vertical installation is essential to ensure proper ventilation and cooling.

7.Using a Fixed Frequency Water Pump Frequency Converter Directly:

Q: Is it feasible to directly input a motor into a fixed frequency water pump frequency converter?
A: It’s feasible at very low frequencies. However, if a high frequency is given, similar to starting with a mains frequency source, it might cause a significant start-up current.

8.Running Motors Above 60Hz:

Q: What should be considered when operating a motor above 60Hz?
A:

• Ensure mechanical and device safety at high speeds.
• Be aware that the motor enters a constant power output range and that the output torque meets operational requirements.
• Consider bearing life.
• For medium to large capacity motors, especially 2-pole motors, running above 60Hz requires careful consultation with the manufacturer.

9.Using Frequency Converters with Gear Motors:

Q: Can a water pump frequency converter drive a gear motor?
A: It’s possible, but be mindful of the gearbox structure and lubrication method. Gear structures typically have a maximum limit of 70-80Hz, and continuous low-speed operation requires attention to gear lubrication and damage.

10.Driving Single-Phase Motors with Frequency Converters:

Q: Can a water pump frequency converter drive a single-phase motor? Can it use a single-phase power source?
A: It’s less common. Single-phase motors, especially those with speed controller switches, might damage auxiliary windings when operated below the working point; capacitor start or run methods could lead to capacitor explosions. Frequency converters typically use three-phase power, but small-capacity models might be able to use single-phase power.

11.Power Consumption of Frequency Converters:

Q: How much power does a water pump frequency converter consume by itself?
A: It depends on factors like the model, operational state, and frequency of use. Frequency converters below 60Hz have an efficiency of about 94%-96%, from which power loss can be estimated. However, those with built-in regenerative braking have higher power consumption considering the loss during braking.

12.Continuous Operation in the 6-60Hz Range:

Q: Why can’t continuous operation occur in the full 6-60Hz range?
A: General motors use external fans or rotor end ring blades for cooling. If the speed is reduced, cooling efficiency decreases, and the motor cannot handle the same heat as in high-speed operation. Therefore, in low-speed operations, load torque must be reduced, or larger capacity frequency converters and motor combinations or specialized motors should be used.

13.Using Motors with Brakes:

Q: What should be noted when using a motor with a brake?
A: The excitation circuit power for brakes should be drawn from the input side of the frequency converter. If the brake operates while the frequency converter outputs power, it might cause an overcurrent trip. Therefore, the brake should be activated after the frequency converter stops outputting.

14.Starting Frequency Converters with Motors with Power Factor Improvement Capacitors:

Q: Why can’t a water pump frequency converter start when using a motor with capacitors for power factor improvement?
A: The frequency converter’s current flowing into the capacitors for power factor improvement might trigger an overcurrent protection due to the charging current of the capacitors, preventing startup. The solution is to remove the capacitors before operation or connect an AC reactor at the input side of the frequency converter.

15.Lifespan of Frequency Converters:

Q: How long is the lifespan of a water pump frequency converter?
A: Although frequency converters are stationary devices, they have consumable components like filtering capacitors and cooling fans. With regular maintenance, they can last over 10 years.