Erratic behavior from VFD

[rabler]

I run one of my Bridgeport mills with the VFD output connected to the drum switch. I do not change the drum switch while the VFD is energized.

Why? Does your VFD not provide a reverse function?

 

[jwmelvin]

Why? Does your VFD not provide a reverse function?

I do that to select the high/low windings (2- vs 4-poles) and switch the VFD parameters simultaneously.

 

[rabler]

I do that to select the high/low windings (2- vs 4-poles) and switch the VFD parameters simultaneously.

Makes sense, didn’t think of two speed motors.

 

[Ischgl99]

I do that to select the high/low windings (2- vs 4-poles) and switch the VFD parameters simultaneously.

Are you using an input in the VFD to tell it whether you are on the 2 or 4 pole side of the switch? My VFD does not have the option to change the motor parameters in the drive by use of a switch, you need to go into the programming mode and manually make the change.

The problem with using a two speed motor is the HP on the lower speed windings is usually significantly lower than the full speed option, so you are not gaining anything by switching to a lower speed instead of using a speed potentiometer and slowing the motor down with the VFD, and you are actually worse off. Most two speed motors actually produce less HP than simply dialing the speed down in the VFD since they are meant for air handling or pumping applications that are variable torque instead of constant torque like machine tools. A typical Baldor motor rated for 1 HP at full speed 2 pole operation only produces 0.25HP on the 4 pole setting, if using a VFD and changing the speed from 100% to 50%, you would have 0.5 HP, or double the two speed motor HP.

 

[jwmelvin]

Are you using an input in the VFD to tell it whether you are on the 2 or 4 pole side of the switch? My VFD does not have the option to change the motor parameters in the drive by use of a switch, you need to go into the programming mode and manually make the change.

The problem with using a two speed motor is the HP on the lower speed windings is usually significantly lower than the full speed option, so you are not gaining anything by switching to a lower speed instead of using a speed potentiometer and slowing the motor down with the VFD, and you are actually worse off.

My VFD, an Automation Direct GS20, does have the option to select motors with a digital input. I use an extra set of contacts on the H/L drum switch to do so, making the switchover a sub-second affair.

I believe my motor has the same rated power for low and high speeds:

 

[Ischgl99]

Deleted, wrong info

 

[mksj]

Not much reason for going that route, the motor is a constant Hp, so wiring it for 4P, 1700 RPM, 60Hz, 5.4A and then setting the VFD to go to 120 Hz (3400 RPM) will give the same performance with a lot less complexity.

 

[strantor]

What you are describing is what happens if you use the wrong motor voltage or speed settings in the VFD.

+1

(Assuming you're using low voltage digital input through the drum switch rather than reversing phases on the high voltage VFD output): If it were the contacts in your drum switch, then it would either work or not, or work intermittently. The drifting in reverse and grumbling noise from the motor are symptoms most would associate with a 3ph motor having single phase applied, but it can happen any time the wrong volts:hertz ratio is applied. Verify that the motor nameplate data is entered correctly in the VFD.

 

[jwmelvin]

Not much reason for going that route, the motor is a constant Hp, so wiring it for 4P, 1700 RPM, 60Hz, 5.4A and then setting the VFD to go to 120 Hz (3400 RPM) will give the same performance with a lot less complexity.

I thought there was reason not to push an older motor like this past 90 Hz or so?

Mostly I did it because it was easier than removing the H/L switch and I thought it would be an enjoyable experiment to see the difference myself.

With 4-poles vs. 2-poles, isn’t there a torque difference for the same current?

 

[Ischgl99]

Not much reason for going that route, the motor is a constant Hp, so wiring it for 4P, 1700 RPM, 60Hz, 5.4A and then setting the VFD to go to 120 Hz (3400 RPM) will give the same performance with a lot less complexity.

Not exactly, he'll have constant torque and variable HP up to 60 hz, then constant HP above 60 hz, but the torque will decrease as the speed increases to maintain the constant HP as well as overcoming the inefficiencies of the worsening V/f ratio. Running above nameplate speed could be useful for small cutters that need high speeds and low cutting forces, but not very good if you want to hog out material.

This is a good article on it. http://pumped101.com/hp torque vfd.pdf