My motor with a power rating of 3.7kW typically draws around 5 amps when operating on Direct Online (DOL) starting, while running with a Variable Frequency Drive (VFD) at maximum speed causes it to draw 12.5 amps.
How do you measure motor amps? Most people use a standard clamp-on AMP meter for this purpose. While this is suitable for Direct-On-Line (DOL) readings and provides an accurate measurement of a pure sinusoidal waveform, the output of a Variable Frequency Drive (VFD) is not sinusoidal. Depending on the VFD, the output may be a pulsed square wave or a quasi-sinusoidal waveform, which cannot be accurately measured by a clamp-on meter. It is recommended to read the motor amps displayed on the VFD instead. When doing so, keep in mind that with DOL, you are reading full line voltage with the motor running at full rated speed (minus motor slip due to load), while with a VFD, you are reading the VFD output which can range from 6 Hz to 120Hz or more. Changes in speed will result in changes in amperage as well, depending on the load conditions. Some people have found success in reading the line current on the input to the VFD, as it provides a true measurement of the total load (motor load + VFD power + heat loss), although it may be higher than the motor load. Understanding these readings can help in determining the total power usage. This information can be valuable for optimizing motor performance and energy efficiency in industrial applications.
Expanding on Gary's response, it's important to run the VFD at maximum speed. However, it's crucial to ensure that the VFD is set at the same frequency as the DOL reading, especially if your power supply is 50 Hz. Additionally, monitor the output voltage and amperage on the VFD display to ensure optimal performance. - Paul
Thank you for responding. I am comparing the VFD display with the clamp meter reading for amps. There seems to be a discrepancy between the two, but the reading is approximately 12 amps for the VFD and around 5 amps for the DOL system according to the clamp meter.
Something seems to be amiss with the electrical setup in India. Typically, the standard line voltage in India is 220 Volts 3 Phase 50Hz. A 3.7 kW motor operating at full load should draw around 12 amps with a 220 volt 3-phase power supply. When measuring the motor's amperage while connected using Direct Online (DOL) method, it is important to check all three phases simultaneously. Ideally, the amperage should be consistent across all three leads, with each phase falling within the 4 to 6 amp range. To accurately assess the situation, it is recommended to use a clamp-on amp meter to measure the amperage of all three motor leads simultaneously. In a properly functioning system, the sum of the amperage in all three phases should be zero as they cancel each other out. However, variations in amperage readings could indicate issues such as ground faults in the motor or an ungrounded electrical system. By measuring the amperage on each motor lead and assessing the combined amperage of all three leads simultaneously, it may be possible to identify any anomalies. It is important to consider factors like ungrounded power systems or poorly grounded systems, which can affect amperage readings and lead to discrepancies in measurements between DOL and Variable Frequency Drive (VFD) setups. Addressing these discrepancies and ensuring proper grounding in the electrical system can help resolve any issues with amperage measurements. This methodical approach can provide insights into potential faults or issues that may be affecting the motor's performance. Happy Holidays to All!
If you are using an encoder with the VFD, there is a possibility of encountering high current issues if the tuning is too restrictive. A simple solution is to adjust the VFD parameters to utilize volts per hertz control instead of more complex control methods. By observing if the output current returns to normal levels, you can determine if further adjustments to parameter settings are necessary. This method can help troubleshoot and resolve high current problems associated with VFD usage.
This uptick in current draw when using a VFD is actually quite common and is due to the inefficiencies of the drive at max speed, which can cause a higher power demand. Remember, a direct online starter only has to take the motor up to its basic speed, but a VFD adjusts frequency and voltage to control motor speed. Hence, the additional power draw. Efficiency can be improved by not always running at maximum speed, but instead matching the speed to the load requirements.
That's an interesting observation. The reason behind this could be that VFDs control the speed of the motor by changing the frequency of the power supply, which can lead to an increase in current draw. Additionally, VFDs may cause some level of harmonic distortion, leading to a higher reading on your ammeter. Always optimize your VFD settings for your application, and consider carrying out a power quality analysis if your power consumption seems unreasonably high.
It's interesting how the power draw changes so much between DOL and VFD operation—this highlights the importance of proper motor control strategies. With the DOL starting, you're getting that initial torque without excessive current, but the VFD allows for more flexibility and efficiency, even if it does pull more amps at max speed. It might be worth looking into why that draw increases so significantly with the VFD; sometimes it can be due to how the load characteristics change or even the way the VFD is ramping up the speed. Have you noticed any performance differences under varying loads with the VFD?
That’s interesting! It’s pretty common for motors to draw significantly more current under a VFD, especially when they’re at maximum speed—this can be attributed to the adjusted frequency and voltage. Just be mindful of the thermal limits and any potential impacts on your electrical system; balancing performance with efficiency is key! Have you noticed any improvement in operational smoothness or energy savings when using the VFD compared to DOL?
That's interesting! It sounds like the difference in current draw between DOL and VFD operation could be influenced by how the VFD manages the start-up and speed control. While DOL can lead to a significant inrush current, the VFD may allow for smoother acceleration, but once at max speed, the higher amp draw suggests it's likely compensating for the torque requirements or providing more precise control. Have you noticed any other differences in performance or efficiency between the two methods?
✅ Work Order Management
✅ Asset Tracking
✅ Preventive Maintenance
✅ Inspection Report
We have received your information. We will share Schedule Demo details on your Mail Id.
Answer: - The difference in amp draw between DOL and VFD operation can be attributed to the way the motor is started and controlled. DOL starting draws a lower initial current surge compared to VFD operation, where the VFD controls the motor speed by adjusting the frequency of the power supplied.
Answer: - Yes, it is common for a motor to draw higher amperage when running on a VFD due to the nature of how VFDs control the motor speed. The increased amp draw on a VFD is often a result of the higher frequency and voltage needed to vary the motor speed.
Answer: - Various factors such as the motor load, speed control settings on the VFD, efficiency of the VFD, and the motor's design can all contribute to the differences in amp draw between DOL and VFD operation. Additionally, the type of load and application can also impact the motor's amp draw under different operating conditions.
Join hundreds of satisfied customers who have transformed their maintenance processes.
Sign up today and start optimizing your workflow.