AC vari drive motor

[cmiller]

Motor has mind of its own!! I have a Toshiba 3 phase induction motor, (480 VAC, 0.75 Kw, 4 pole, 1.5 Amps) being driven by a Toshiba VFA5 480V inverter, that randomly accelerates to max speed without any noticable change in current, voltage, frequency, etc. It is driving the customer and I silly! Does anyone know why this would happen?

Thanks,

Casey

 

[TheDOG]

What is the type of load? How and where are you checking the current, voltage, frequency?
There must be a change in the frequency for the motor to change speed (speed = 120 x Frequency / number of poles).
If the speed reference is from an analog signal, I would suggest that you check what the signal level is at the VSD.
Also, the VSD may be going into a "current limiting mode", where the VSD will try to limit the current to the motor. This is done when the current limit threshold is reached, the drive does not accelerate (and sometimes decelerates) until the current is below the current limit setting.

 

[tmahan]

Casey can you clarify what you mean by "randomly accelerates to max speed without any noticable change in current, voltage, frequency, etc."

Is the display changing the reported frequency and the motor is running at some constant speed or it the motor shaft rotation changing and the drives LED display remaining constant.

Voltage, frequency and current are all tied pretty close so if one is really changing at least one of the others should as well

 

[cmiller]

Thanks to "TheDog" and to "tmahan" for your replies.
This inverter and motor are used in a machine that polishes the surface of silicon wafers before any circuits are laid down. Wafers are arranged under 'heads' in a cloverleaf pattern with this particular motor driving a center hub that rotates the 'heads'. Each 'head' has an applied downforce of between 45kg to 150kg.

The machine has been installed and operating normally for more than one year before this problem arose. I have monitored the voltage at the outputs from the inverter, the voltage is steady at 480 VAC. I monitored the freq using a Fluke DMM, and never saw the frequency change. I monitored the current from the LED display on the inverter, and this also held steady.
By "randomaly accelerates to max speed etc." I mean that the motor takes off at full speed (98rpm) about once or twice during a 12 hr shift. I have measured the rpm using a digital photo tachometer. I have seen the motor 'take off' while monitoring voltage, current, frequency, and they all read normal, as if the motor is running normally. After shutting down the machine and re-starting everything is good for the next several hours. I know that a motor will usually slow down or stop if anything in the loop goes bad, but what would make a motor ramp up to full speed??

 

[buzzp]

The only thing I can think of is the number of poles is changing. Sounds ridiculous, I know. If a pole dropped out then the speed would go up. I am not a motor designer but I believe this could be possible. If the freq/volts is the same, then assuming there is not a load rotating the motor faster, then the only thing that can affect the speed is the number of poles. syn rpm=freq*120/# of poles. Good luck and please post your findings. Thanks Buzzp

 

[dydt]

hi..
you said this drive is fine for almost one year..
do you have any record of the parameters?
toshiba drives are quite reliable so why don't you
try to go over the parameters again..and
analyze them one by one...or compare them with
another identical machines..

dydt

 

I would suggest that you open the motor and examine the windings esp. the solder, due to the loose connection might show now and then, the circuit is open and the motor turns out to be 2 pole momentarily.

 

[cbarn24050]

hi, what youdescribe seems imposible, the motor speed is directly related to the frequency. The drive should have a frquency display, check that it reads the correct value.

 

[tmahan]

A couple of questions/suggestions for you:


Is the PID control set to the on position? This is what I suspect is causing your problem. With this drive if the feedback signal is lost it interpets the loss of signal as an infinte error and responds by immediately ramping to the programmed frequency limit and staying there. This most often occurs when the sensor providing feedback is powered from the drive's terminal block and has its own ground. (The common positions on the terminal block are floating and should never be connected to earth)

Other thoughts
Which state is the true torque control in. This is the "Open Loop vector control".

Have any long lead filters been added to the system or have any other sources of impedence been added to the output side.

Typically the A5/G3 performs best when the open loop vector is in the on position.

If you are unable solve the problem I would suggest contacing the Toshiba US Tech support. The A5 is an LED keypad version of the G3 sold in the US and operate more or less the same

 

[buzzp]

I like tmahan's possible origin of the problem. The only thing is, I thought the drive was showing the same frequency. Is this the case or was it just measured with the Fluke? His explanation seems more probable than loose internal connections within the motor. I would bet one of these theories is the cause of the unexpected motor acceleration.

 

[malinaw]

If you are using a tach. feedback you could be having a tach going out I have seen this many times.

 

[elhead]

What frequency is being displayed on the Fluke DMM? Could be the "carrier" frequncey of the PWM waveform. I think I would monitor the drive display frequncy during one of these episodes, as someone else suggested.

 

[tmahan]

I the A5 series would give a fault indication in the event of a loose connection. The drive would see the sudden change in impedence and fault out. This is assuming that he is open loop vector control.

CMiller, once the problem is solved be sure and post the solution. The suspense is killing me

tmahan