DOL Motors and Long Cable Lengths

You are confusing everybody. Why DOL motors and why VFD?

The most important thing in this case, is let us know your motor capacity and starting method. Because with such a long distance, voltage drop would be the most critical to think about.

I don't have any VFDs' cable length application requirement in my hand right now. But, if the voltage drop is satified, having a look at the shielded cable would be appropriate. Do you know shielded cable?

EE65 made the other GA and the OP seems to be pretty much satisfied with the answer. It seemed to me that the OP has a very good "sense" in this kind of application so some of my words are just a bit of redundancy.

Just a little more adding on EE65 comment, that not only that the reflected waveform is applied to all of frequencies from 2Hz to 16kH (according to AB). The best way is to consulted with the supplier for the appropriate cable length since all of us know the distance shall be kept as short as possible or add up some reflected wave mitigation devices.

According to AB, the reflected waves do not to be considered for 240V drivers.

Thanks to the OP, I have picked up something from this.

Sorry, I mean that the reflected wave data is applied to all frenquencies from 2Hz to 16kHz.

Dear,

Ultimately, Yopu shall refer the VFD Out put volts in to Harmonics, og different frequencies. Active filters are available to suppress the High frequencies , 5th harmonics, up to 11th Harmonics. Further, You may use cables of higher size, to reduce the loop impedence , & restrict the loop impedence value within required limits as per the VFD manufacturer's requirements.

Definitely, With the above methods, I have commissioned 150 KW compressor motors, 230 KW compressor motors , without any problems.

This is valid not only for Motors, but for Any PLC Control Panels , Instrumentation panels also, where for reliability improvements, to reduce the card failures, same aspects are applicable.

I too am a little confused with your choice of words. A fan motor is a fan motor.. never heard of a DOL fan motor... or is it your way of telling us that this is a standard 3 ph 2 pole squirrel cage motor?

As for cable length 300meters IS NOT a long length of cable for a VSD. Yes, you have to make sure you have the correct CSA and calculate the VD per meter, however you state 50Hz is a low frequency??? Do you have a "high cycle" motor & what frequency are you expecting out of your VFD?? Most fan motors operate on either 50 or 60 Hz.

Am I right in saying that you are now trying to operate motors once controlled by a DOL with a VFD? What is the maximum frequency/motor RPM you require?

If you are worried about THD then I suggest you conduct a power quality survey, see if you have harmonics and fit a active filter to suit.

You might want to remember to keep your volts/frequency ratio constant.. if you're not sure what that is.. google it.

Let me clarify, I'm installing DOL motors with long supply cables without a VFD obviously. I know there are issues with long supply cables VFD driven motors. I was wondering if these problems apply to DOL driven motors without VFDs (as above).

The fans are propelled by .37kW motors on type D circuit breakers. The breakers have the same starting methodology as a DOL motor, except instead of a contactor and circuit breaker on upstream of the motor, you have just the circuit breaker.

In Order to respond to the OP, I would need to understand the meaning of the various TLAs ( three letter Acronyms) used in the correspondence. Can anyone assist please

Sleepy

and which TLS's in particular do you need explaining?

"As far as I know, the reason for this reflected waveform is the impedance mismatch that occurs between the motor terminals and the cable at high frequencies."

Yes, but in this case "high frequencies" is not as related to the pseudo-sine wave frequency output of the VFD to make the motor speed change, but the "switching frequency" of the high speed transistors used in the PWM output. In older VFDs, the switching / carrier frequency of the transistors, the "pulse rate" as it were, was around 1kHz and the standing wave phenomenon was virtually unheard of. But as the technology improved and the devices got smaller by making them switch faster, the 10-15kHz switching frequency started allowing the reflected waves to interact with the cable capacitance and build-up the pulse magnitude to where it now exceeds the insulation rating of the magnet wire. While technically a reflected wave can happen at low frequencies as well, the order of magnitude is so low as to be insignificant. I've seen it become a problem for things like Medium Voltage SCR based soft starters, only because the reflected wave can occasionally make the SCR self-commutate, but it was never an issue for the motor because the magnitude of the pulses was well below the insulation rating.

Bottom line, don't sweat it for a DOL controlled motor.