We are running equipment at work that have Siemens MicroMaster drives. In some cases we need speed settings below 5% (below 3hz) and are having difficulty getting good torque and speed control. I noticed these drives have a setting to enable sensorless vector control. I was wondering if anyone here has experience with using sensorless vector control on drives and if you think it would give us good speed control at low speeds?
Sensorless vector gives better speed regulation than Volts-Hertz, but less than 3 Hz is too slow for good torque with an induction motor. Sounds like you need a servo motor or if this is continuously low speed consider a gear reducer.
My first drive was flux vector on a 50 Hp. drive (closed loop speed feedback). That gave pretty good low speed torque, which was required in the application (extruder), but I sure didn't operate all the time at low speed.
Sensorless vector is better than V/Hz, but at such low speeds, the motor model in the drive is not good enough to give good control.
You need tach feedback. A matched motor and drive can give full torque at zero speed because the drive knows what the motor is doing at all times. Most industrial motor manuf. offer a line of VFD rated motors with provision for mounting a tach or position encoder on the back end of the motor. One I have had experience with in the past is Marathon. I would be very surprised if Siemens can't provide a motor and drive that would meet your needs.
My CHNC needed encoder feedback on the 7.5 hp 3phase spindle for a VFD with encoder feedback. I mounted the standard 3phase motor on the mill table, turned it on reverse from the right angle adaptor in the spindle. bored a perfect on center hole just a scosh under size of the pin I drove in. Then mounted a USdigital encoder. Werks Grate.
To run that slow you'd also need to mount a cooling fan because the motor's fan will run too slow to help.
This is a machine our companies German owners crammed down our throat that was supposed to be running by the end of March. The first thing I thought of was changing the gear ratio and speeding up the motor but since it's not complete and not turned over to us, we have to get the stubborn Germans to agree to everything. I thought if it would do any good I could sneak and enable the sensorless vector control without them knowing, if it would do any good. I guess I thought if it could give 100% torque at zero rpm, it should give 100% torque at 1, 2, or 3rpm, I guess not.
This is for a weighing system that doses out chemical for rubber mixing and a dose could be from 0.07lbs to 20lbs all with an accuracy of 1% or 3g, whichever is greater. It has a coarse dosing and fine dosing, it's difficult to run slow enough to hit the target on smaller weights and be able to run fast enough to dose 20lbs in ~ 13seconds with 1% accuracy, all with a drive programmed for 60hz. But what do Americans know, they have "superior German Engineering". Also, we wanted Allen Bradley Controls but Siemens was crammed down our throat. Now we're busting our butts trying to help them get it to run...
They just discovered the low speed torque boost settings, and are going to show me how to enable it. They don't know that I already did it for some of the stations (35 in all).
Man, I feel your pain. I work for a multinational (these statements are my opinions, and mine alone, not those of my employer, etc.), and run into the Siemens-ABB mafia (EU is better) regularly. The only "user friendly" features of Siemens software usually got added because of howls of protest from US Siemens employees. However, I have enough experience, years, and rep to get listened to occasionally, so the two control projects I'm currently doing are Allen-Bradley.
You've got a speed range requirement of 285:1 if the charge time is held constant. That's a hell of a lot more than the normal 10:1 turndown to which I limit my stuff. If the coarse dosing and fine dosing have a built in 30:1 rate difference independent of speed, it ought to be doable, but just barely. Your minimum speed should be 6 Hz or higher.
Another gotcha is a low speed de-rating algorithm. On Allen-Bradley Powerflex drives, the default is to start de-rating the current capacity at 20% speed or below. After a couple minutes, the VFD will trip at normal loads. I usually disable this one since my small inverter duty motors are mostly TENV anyway.