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Problems with VFD Flying Start at low speeds 1

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rockman7892

Electrical
Apr 7, 2008
1,178

I have a 350Hp motor spinning a seperator (type of fan application) that is controlled with an AB Powerflex 700S 480V VFD. We had an issue where a fan upstream of this seperator was pulling a draft which was causing this seperator to freespin. When this seperator was freespinning the drive was given a command to start and when started appeard to bring the motor to a stop at 0Hz before it began its ramp. This sudden stop with the seperator spinning caused mechanical damage of the coupling between the motor and gearbox.

I went ahead and enabled the flying start feature of the drive so that when the drive was given a start command, it would start at the same frequency at which the motor was already spinning. To test this we ran the drive up to full speed (60Hz output) and gave the drive a stop command which stopped the output to the motor and allowed the motor to coast to a stop. While the motor was coasting we attempted to restart the drive, and we saw that indeed the drive recognized the speed at which the motor was spinning and matched that speed when it began its ouput to the motor. After syncing the speed with the spinning motor the drive began its ramp up to setpoint. We repeated this several times and saw each time that the drive synced up to speed with the spinning motor.

When we attempted to restart the drive with the motor spinning at low speeds around 300-400rmp (10-15Hz) we noticed that the drive would not sync and start to the speed of the spinning motor, but would rather bring the motor to a stop before it began its ramp. We tried this several times at low speeds and saw that each time, it would not match the low speed of the spinning motor but would bring it to a suddent stop. This sudden stop at low speeds could have casued the mechanical coupling damage I mentioned earlier.

In speaking to Rockwell they mentioned that at low speeds the current ouput tha the drive uses to find the spinning frequency may be enough to bring the motor to a stop, where it will not for higher speeds. They recommended disabling the "fast flux ramp" paramater in the drive however this made no change, and the drive is still stopping the motor each time at low speeds. My understanding is that the fast flux is used to build the flux field in the rotor which may be causing the drive to stop?

Does anyone have any ideas why I am having problems with this flying start feature at low speeds?

I should mention that during these tests the motor was uncoupled from the load.
 
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Do you have a "minimum frequency" setting in the VFD? maybe this is set higher than 15Hz and when the VFD is 'looking' for the frequency/speed below the min freq then it is stopped in software and cannot go below that limit.

Flying start at low frequencies or even when the motor is stopped is notoriously unstable, for most VFD's. (the VFD does not know if the motor is spinning [if flying start is activated] until it tries to 'look' for the motor-it typically sends out a series of pulses to measure current and determine what it 'thinks' is the correct frequency').
There is some decent information in here:
(one thing A-B are very good at is providing concise and readable information!)
 
Ozmosis

Thanks for the information. The document mentions that an overcurrent trip may occur when the drive attempts to start the motor at 0Hz with the motor already spinning. Is this because when the drive applies a 0Hz frequency the motor sees a LRC?

The minimum frequency that we have set in the drive is 0Hz.

I'd like to figure out exactly how the drive detects the speed of the spinning motor. My understanding has always been that the drive outputed small voltage and that it monitored the current the motor drew. When the drive saw the current reverse direction (from the motor going from motoring to generating when frequency dropped below spinning frequency) This document explains that the drive searches for a motor voltage that corrosponds with the excitation current that is applied to the motor. I'm not sure if I understand how this technique works. Does the drive output a excitation current in order to produce a field in the motor and then monitor the regen voltage?

What is the function of the flux up time described in this document?
 

The drive is currently is a V/Hz control mode. The other control mode avaliable in this drive is a "Field Oriented Control Mode" (FOC). Will this FOC control mode lead to better results at lower speeds? We do not have an encoder on the drive so I was wondering if even without an encoder the FOC control mode would perform better at these lower speeds.
 
I have been researching this a bit more and it seems that if we install an encoder with the drive in the FOC mode we might have better success of detecting these low speeds and picking up the flying starts at these low speeds.

Does anyone have any experience with using an encoder for flying starts at these low speeds I mention?
 
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