Sparkz
Member
The thing about those standard 'variable speed' controllers people often refer to, is that they don't vary speed directly, they only change the supplied voltage. They come in different shapes like variacs or electronic controllers with triacs or even IGBTs. And this works great with blowers, fans, in fact, it works great with nearly all single phase motors (with running capacitor a.k.a. PSC) driving exponentially changing loads, so even pumping liquids will work over a very moderate range.
For all other load applications it's a no go! If speed has to be set way below 40Hz without significant loss of torque, or if it has to be increased beyond 50Hz, an inverter is all that's capable of doing it.
At www.invertek.co.uk (hit 'Products' - Optidrive E1') you'll find single phase motor inverters, ready to go. Those inverters all work according the same principle: The running capacitor is removed and the starter winding is fed with a (smaller) voltage that has the correct phase shift (90 electrical degrees or PI/2) at all frequencies.
I just tried to figure out a way of using standard inverters. At that time I used Hitachi a lot, so I used one of their low cost 230V types to do some experimenting.
Like I said before, removing the running capacitor (or PSC) has all to do about torque. If you connect a PSC motor to 2 outputs of a 3-ph VFD, like you normally would connect it to the mains, the motor simply wouldn't start under approx 30Hz.
If you were to connect the starter winding to the third phase but still leave the running capacitor in place, then first of all the starter winding current would not be sufficient enough, and second it would never have the right phase shift at the lower frequencies. Thus, also resulting in poor (starting) torque.
Best results were achieved by removing the PSC and feeding the starter winding directly with the third phase of the VFD. The starter winding current does not exactly have the right phase shift (120 electrical degrees in stead of 90) but at least the current level is more than sufficient to make the motor deliver maximum torque at 10Hz and up. Unfortunately this can only be done up till the frequency where starter winding current reaches its maximum permissable value. Since it's impossible to set the V/F characteristic of one output differently from the other two, I had to figure out a way to bypass the overcurrent problem. That's where I came up with the solid state relay, to disconnect the starter winding above a certain speed.
One would think that a PSC motor MUST have the capacitor (thus current through the starter winding) available at all times, but guess what: IT DOES NOT. Once the motor is running at 20Hz or more, one can easily disconnect the starter winding circuit. Under no load conditions you will notice a substantial increase in motor current (due to the power factor being worse now), but other than that the motor will still be able to deliver the rated power.
Now, I'm pretty sure I used several terms like 'way too much trouble', 'quite a hassle' and 'not worth your time', so please bear with me if someone else has some experimental genes like I do. ;o)
Never thought this would become such an issue, but then again I get the same disbelieve amongst engineers with the 'infamous 87Hz technology'. You know, using a 230V-50Hz 3-ph motor on a 400V inverter and setting the base frequency to 87Hz (50 x 1.73), thus increasing the motor's power and torque by 1.73. Lots of people have heard of it, but are too afraid of implementing it.
BTW Keith, I also have to agree that there might be VFDs not capable of handling a 2-phase load or would trip in the event of.
For all other load applications it's a no go! If speed has to be set way below 40Hz without significant loss of torque, or if it has to be increased beyond 50Hz, an inverter is all that's capable of doing it.
At www.invertek.co.uk (hit 'Products' - Optidrive E1') you'll find single phase motor inverters, ready to go. Those inverters all work according the same principle: The running capacitor is removed and the starter winding is fed with a (smaller) voltage that has the correct phase shift (90 electrical degrees or PI/2) at all frequencies.
I just tried to figure out a way of using standard inverters. At that time I used Hitachi a lot, so I used one of their low cost 230V types to do some experimenting.
Like I said before, removing the running capacitor (or PSC) has all to do about torque. If you connect a PSC motor to 2 outputs of a 3-ph VFD, like you normally would connect it to the mains, the motor simply wouldn't start under approx 30Hz.
If you were to connect the starter winding to the third phase but still leave the running capacitor in place, then first of all the starter winding current would not be sufficient enough, and second it would never have the right phase shift at the lower frequencies. Thus, also resulting in poor (starting) torque.
Best results were achieved by removing the PSC and feeding the starter winding directly with the third phase of the VFD. The starter winding current does not exactly have the right phase shift (120 electrical degrees in stead of 90) but at least the current level is more than sufficient to make the motor deliver maximum torque at 10Hz and up. Unfortunately this can only be done up till the frequency where starter winding current reaches its maximum permissable value. Since it's impossible to set the V/F characteristic of one output differently from the other two, I had to figure out a way to bypass the overcurrent problem. That's where I came up with the solid state relay, to disconnect the starter winding above a certain speed.
One would think that a PSC motor MUST have the capacitor (thus current through the starter winding) available at all times, but guess what: IT DOES NOT. Once the motor is running at 20Hz or more, one can easily disconnect the starter winding circuit. Under no load conditions you will notice a substantial increase in motor current (due to the power factor being worse now), but other than that the motor will still be able to deliver the rated power.
Now, I'm pretty sure I used several terms like 'way too much trouble', 'quite a hassle' and 'not worth your time', so please bear with me if someone else has some experimental genes like I do. ;o)
Never thought this would become such an issue, but then again I get the same disbelieve amongst engineers with the 'infamous 87Hz technology'. You know, using a 230V-50Hz 3-ph motor on a 400V inverter and setting the base frequency to 87Hz (50 x 1.73), thus increasing the motor's power and torque by 1.73. Lots of people have heard of it, but are too afraid of implementing it.
BTW Keith, I also have to agree that there might be VFDs not capable of handling a 2-phase load or would trip in the event of.
