VFD & Energy Saving-2: Voltage to Frequency Ratio Selection

Where did you get your measurements?

Thanks for your Interest

In ACS 800 there are many options to be displayed on the panel (Torque, Out Put Voltage, DC Link Voltage/Current, Internal Temperature, Incoming Voltage etc). From there I select Output Voltage, Frequency, RPM and Power to be displayed. From Output Voltage and Frequency I compute V/F ratio.

I think Load (Power %) & Torque are also the factors contributing to V/F ratio (Below and Above 8). (Here in mention values we are not observing Torque)

According to my observation

If Load is less i-e Less than 40 %,V/F ratio goes above eight

If Load is greater than (Say 50 %),V/F ratio comes below eight

but here is another example a bit different (Less torque is required in this application and speed is impotant)

Motor-F: 232 KW, 400 V, 50 Hz, 399 A, 1495 RPM (Fan Type Application)

Power (%), Speed (RPM), Freq (Hz),V-Out , V/F Ratio

6.05 %, 1511.8 RPM, 50.40 Hz, 384.5 V, V/F = 7.62

57.1 %, 1496.5 RPM, 49.83 Hz, 370 V, V/F =7.425

48.5 %, 1476.1 RPM, 49.28 Hz, 368 V, V/F = 7.46

54.0 %, 1470.0 RPM, 49.11 Hz, 362 V, V/F = 7.37

OK, I just wanted to make sure there was no measurement error from using external meters, which rarely depict PWM output values accurately.

In general, the output V/F ratio is kept the same as the motor design parameter, in your case 8:1. But in a Vector Drive, or in this case a DTC drive which is similar, the V/F ration is "tweaked" and updated on a constant basis in order to maintain accuracy of the desired output, be it speed or torque or both. So the base V/F ratio will be a starting point, but these types of drives are intelligent enough and powerful enough to decide, on the fly, how to optimize the motor performance at any given moment. The algorithm to do that is highly proprietary within each manufacturer, but follow basic general principals that are readily available on the web. For the ACS800, they use a concept called Direct Torque Control and if you search that term, you can see several detailed white papers that go into the math involved if you are interested.

V/F is correction on changing the RPM. If the controller read RPM drop then It will make correction with increasing V/F, and also the opposite.

Volts per Hertz A ratio exists between voltage and frequency. This ratio is referred to as volts per hertz (V/Hz). A typical AC motor manufactured for use in the United States is rated for 460 VAC and 60 Hz. The ratio is 7.67 volts per hertz. Not every motor has a 7.67 V/Hz ratio. A 230 Volt, 60 Hz motor, for example, has a 3.8 V/Hz ratio.

Flux (Φ), magnetizing current (IM), and torque are all dependent on this ratio. Increasing frequency (F) without increasing voltage (E), for example, will cause a corresponding increase in speed. Flux, however, will decrease causing motor torque to decrease. Magnetizing current (IM) will also decrease. A decrease in magnetizing current will cause a corresponding decrease in stator or line (IS) current. These decreases are all related and greatly affect the motor's ability to handle a given load.

Constant Torque AC motors running on an AC line operate with a constant flux (Φ) because voltage and frequency are constant. Motors operated with constant flux are said to have constant torque. Actual torque produced, however, is determined by the demand of the load.

T = K Φ Iw

An AC drive is capable of operating a motor with constant flux (Φ) from approximately zero (0) to the motor's rated nameplate frequency (typically 60 Hz). This is the constant torque range. As long as a constant volts per hertz ratio is maintained the motor will have constant torque characteristics. AC drives change frequency to vary the speed of a motor and voltage proportionately to maintain constant flux.

To operate the 460 volt motor at 50% speed with the correct ratio, the applied voltage and frequency would be 230 volts, 30 Hz. To operate the 230 volt motor at 50% speed with the correct ratio, the applied voltage and frequency would be 115 volts, 30 Hz. The voltage and frequency ratio can be maintained for any speed up to 60 Hz. This usually defines the upper limits of the constant torque range