Grinder: 110/220?

Talking about AC drives here... Here's an interesting question. I have read in past discussion that when you purchase a VFD that you get one bigger than the rated HP of the motor you want. Meaning for example that if you have a 2hp motor then you want a 3hp rated VFD. I noticed that looking at those penta drives that unless you have three phase going into the drive, it is only rated at 2hp. You guys feel comfortable using that drive with a 2hp motor right at the VFD rating? I have never heard of any problems at all, just asking the question.

I am about to purchase another VFD for a new motor I got last week. Ironically the motor is a 2hp, and I am ready to buy a 3hp VFD.

http://dealerselectric.com/item.asp?...ID=132&PID=620
 
gixxer said:
Talking about AC drives here... Here's an interesting question. I have read in past discussion that when you purchase a VFD that you get one bigger than the rated HP of the motor you want. Meaning for example that if you have a 2hp motor then you want a 3hp rated VFD. I noticed that looking at those penta drives that unless you have three phase going into the drive, it is only rated at 2hp. You guys feel comfortable using that drive with a 2hp motor right at the VFD rating? I have never heard of any problems at all, just asking the question.

I am about to purchase another VFD for a new motor I got last week. Ironically the motor is a 2hp, and I am ready to buy a 3hp VFD.

http://dealerselectric.com/item.asp?...ID=132&PID=620
Click to expand...

I bumped this up because I've heard the same thing and would like to understand what it's about.

Also, from earlier in the thread, the mention of DC motors losing less torque when run at lower speeds reminded me of having come across AC VFD's of two types (I may not have this just right but the idea is there)... variable torque and constant torque. If you know about this, I'd like to learn about it.

Mike
 
Last edited:
What makes motors go is the interaction of 2 magnetic fields - those of stator and rotor.

In most DC motors the stator has permanent magnets. In 3 phase AC motor there are no perm magnets, instead, the magnetic field of the rotor is "induced" by the stator.

As a result, at lower working currents DC motor will have higher torque compared to AC motors.

Through some creating engineering they can improve lower range torque by "boosting" driving voltage coming off the VFD, but only in certain range. The lowest RPMs will still have very low torque. The rotation at low RPMs will also feel rather "jerky" - not nearly as smooth as that of DC motor.
 
You must log in or register to reply here.
Back
Top