Max Frequency for VFD

[PaulKim1003 3]

Hi guys,

When you program a vfd from scratch, how do you find your max frequency ?

I have a schneider avitar 320 vfd in our manufacturer and it has 75hz for the max frequency. I am wondering where the max frequency (75hz) came from..

Thanks

 

[panic mode 246]

everything has upper (and lower) limit.

in this case baseline would be based on mains frequency which is what motors would be running on without VFD. VFD just allows some adjustment (as the name suggests, VFD = variable frequency drive) and you probably want to stay reasonably close to that base or rated frequency. 

in most countries mains is at 50Hz, and in some places it is 60Hz. with VFD one can vary frequency but exceeding rated frequency is less efficient and it is usually capped at some value such as 50% or 75% above (or below) the norm. some drives may support 200Hz or higher but that does not mean it will be usable with standard motors. the other limit is minimum frequency. as the motors are operated at lower frequency, efficiency drops as well and motors have less cooling ability. in some cases motors may have separate small motor driving fan and in this case fan is operated at full speed, independent of motor speed. 

 

 

[ElectronGuru 81]

The motor will have a nameplate that shows the rated frequency, rated speed, and sometimes the max safe speed. If you have a motor rated for 1800RPM at 50Hz, and a max safe speed of 2300RPM, the max frequency you'd program in the drive would be about 64Hz.

As for the loss of motor efficiency at higher or lower frequencies that @panic mode referred to:

There is an inverse relationship between speed and torque in an AC motor, shown by the formula; T = (HP * 5252) / N

• T = torque
• HP = Horsepower (which is constant)
• 5252 is the torque constant for AC motors
• N = speed (which is the only variable)

So a 100HP motor running at 1800 RPM would produce just over 290 pounds of torque. If we speed the motor up to 2300RPM, torque would drop to just over 228 pounds. The faster you go, the more torque you lose.

 

As for the relationship between frequency and efficiency, at lower frequencies motor temperature is the enemy, as shown by this formula: Xl = 2πFL

• Xl = inductive reactance
• 2 & π are constants
• F = the frequency the motor is being given from the drive (and is the only variable)
• L = is the motor's inductance (which is constant)

So if frequency goes up or down, Xl follows. 'Probably helpful to know that Xl is measured in ohms. So yes, the motor's internal resistance is driven down with lower frequencies, which (if prolonged) raises the temperature of the motor, which reduces performance and motor life. Because of the higher temps at lower freqs, it's important to purchase motors that are rated for use with variable frequencies.

That was probably much more of an answer than you were looking for, but now you know the technical limitations and where they come from. As for why they specifically chose 75Hz for the max freq on that drive and motor, it's most likely to do with the motor name plate info, and the needs and limitations of your specific process.

[PaulKim1003 3]

@ElectronGuru @panic mode Thank you for your reply :)