flux regulation 1

[Lenz81]

Hi

Can any body explain me how does sensorless control mode regulate the flux?also why they say not to use a VFD with(Base Volt/Base Freq) > (Nominal Volt/ Nominal Freq )?

thanks in advance.

 

[DickDV]

Let me try to answer these:

First, regulating the flux is really regulating the motor shaft torque. In order to regulate the torque, you need to identify the torque. This is done by resolving the two vectors in the motor current. There is magnetizing amps, and at 90 degrees to that is torque-producing amps. The hypotynose of the triangle formed by these two vectors is total amps as measured in the motor leads.

The drive can measure the total amps and knows the magnetizing amps from the motor ID run done when the drive was originally commissioned. It can then solve for the torque-producing amps which, when compared to torque-producing amps at full load, gives the % torque loading. Knowing the % torque loading gives you the % flux intensity.

Second, I'm not exactly clear on your question but, if you were asking why the voltage cannot be increased above nameplate when the frequency is increased above nameplate, thus maintaining the available torque, the limiting factor is the voltage-withstand capability of the winding insulation. Raising the voltage above that level simply results in insulation failure and a damaged motor. If you question is why frequency cannot be raised above nameplate, you have been misinformed. Frequency can and often is increased above nameplate with the limit being the maximum mechanical safe speed of the rotor assembly. In practice, the maximum practical overspeed is the point where the torque is falling off faster than the speed is increasing. This is the maximum speed at which constant hp is maintained and is usually the upper practical limit of speed. As long as the system is designed to deal with the falling torque as the motor ranges above base speed, there is no problem and several advantages to doing that.

 

[waross]

All the windings in a 230/460V motor are insulated for 460V. These motors are occasionally wired for 240V but the drive is fed with 480V. These applications an maintain the V/Hz ratio up to 200% of rated frequency and voltage. Be sure to use an inverter rated motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rockman7892]

If flux density in a motor is equal to flux = Vm/wNcost wt

Then if the V/hz stays constant throughout the operating range of the drive and motor then wont the flux stay constant as well? For example if the drive is operating between 0-60hz and the V/hz is kept constant in propotrtion then wont flux density stay constant in motor througout this range? Except maybe in lower speed ranges whre thre will not be enough voltage?

 

[Lenz81]

thank you gentlemen ,AV300i VFD manual says it is not allowed to use VFD with (Base Volt/Base Freq) > (Nominal Volt/ Nominal Freq ) ,I decreased the base frequency from 60HZ to 50HZ and i did not get any alarm ,motor is running fine now , I just wanted to know that is it fine to use the VFD under this condition?can it cause any damage in future?

 

[DickDV]

If, by Nominal Voltage and Nominal Frequency you mean motor nameplate voltage and frequency, then reducing the base frequency only raises the V/Hz ratio and can easily overexcite or saturate the motor windings.

The result will be less torque and more wasted heat.

Your motor is going to run best with the base voltage and base frequency entered into the drive as shown on the motor nameplate.

You will not damage the drive. For any given shaft load, the amps will be higher than normal and overcurrent faulting and overload faulting can be expected.

 

[Lenz81]

I ran the motor with the base frequency equal to my nameplate frquency but i got very low output voltage and high current,how can i keep my voltage high and my current low if my base frequency is equal to nameplate one?

Thanks in Advance.

 

[Alij23]

hi lenz81

Try to regulate your flux with constant current not constant voltage.

 

[DickDV]

Usually, when you enter motor voltage and motor base frequency into a drive, that automatically sets the volts/hz ratio over the frequency range from zero to base frequency. That assumes, of course, the the voltage supplied to the drive input is equal or greater than the motor nameplate voltage.

There are some drives out there that do not set the V/hz ratio automatically as described above. In those, you have to set the maximum voltage and the frequency at which it occurs or some other way of customizing the v/hz ratio.

I think its time to study your instruction manual and see if this is maybe what you've got. Otherwise, if the drive is supplied with 480V and the motor nameplate data was entered as 480V base voltage and 60hz base frequency, you should see 480V at 60hz, 230V at 30hz, 115V at 15hz, etc.

If that's not what is happening, its time to launch a serious investigation.

One other thing, I assume you are reading voltage from the drive display. Do not try to measure it with a voltage meter on the output leads. No meter is capable of really accurate voltage readings due to the high frequency complex pulses on the leads.