Labeller infeed conveyor. SINAMICS G120C, 2.2kW, speed reference from an S7-1500 over PROFINET, telegram 1. The drive display shows 1450 rpm at 50Hz, a handheld tacho on the motor shaft says 1390, and the two of them have been arguing about it all week. Label pitch is 4 percent off, labels landing 4 mm late on every pack.
Nothing changed that I know of. The line ran fine for a year and then last month the operators started on about label position. So I put a second tacho on it, that one reads 1392, and I checked the gearbox ratio against the drawing, 5 to 1, which matches the nameplate on the box. Two days on it and still nothing.
Two tachos agree with each other. Is the display wrong, or is the gearbox not what the drawing says? Could someone explain which one I'm meant to believe?
What's the nameplate rpm on the motor? And is the drive in V/Hz or vector? Those two answer most of this.
Nameplate says 1440 rpm at 50Hz, 4 pole. So the motor can't even do 1450 at full load? Drive's in V/f linear, the default we shipped the G120 with. Nobody's been in it since.
The 1450 on the display is the setpoint, worked out from frequency and pole count. It isn't a measurement. Nothing is broken. In V/f the drive doesn't know anything about the shaft. An induction motor under load turns slower than synchronous: 1500 at 50Hz, nameplate 1440 at full load, that's your 4 percent slip. Your tachos are right. The display only tells you what it asked for.
Change the control mode to sensorless vector, p1300 = 20 if I remember right, and run the motor identification with the motor cold. The drive estimates the slip then and corrects the frequency. If the pitch has to be right to the millimetre you want an encoder and closed loop. The 4 mm was always there, something else drifted.
Switched to sensorless vector, motor ID ran about a minute, stationary. Tacho's on 1446 at 50Hz commanded. Labels land within 1 mm.
So it was slip the whole time. In V/f the drive shows what it asked for, not what the shaft is doing, and the motor slips 4 percent under load. Sensorless vector with the motor ID done is what fixed it.
Still 4 rpm out, and the HMI says 1450 because it reads the setpoint, so I'd like to get actual speed on there instead and I'm not sure telegram 1 carries it back. Thanks gunther, vinnie86.
Telegram 1 sends actual speed back in the second word. Scale it, show that.