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Smart Infeed idle current, parameters scaling

Created by: Marcin19811 at: 11/15/2020 10:06 AM (1 Replies)

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11/15/2020 10:06 AM	Rate (0)
Marcin19811 Posts: 3 Rating: (0)	1. Two Sinamics Smart Infeeds (920A rated power each) in parallel connection. Is that normal that current showed in r27 (read using Starter) never drops below about 100Arms? Even when there is no load. (all inverters on DC bus are off) I think (if there is really such current flow) it must be some reactive current. But why? 2. Im a bit confused with sclaing and reference values in Sinamics. For example in smart line modules i mentioned at point 1 reference current (p2002) is set to 920Arms. Absolute current (r27) and some other parameters are transfered to PLC via Profibus. And it looks that in PLC reference value for scaling is 3000A (ca. 3000A is maximum current of these two units in parallel connection) . In result currents displayed on HMI is about three times larger than that showed in r27(read using Starter) Wonder why somone put maximum current as reference value for scaling in PLC. The question is: Is r27 (when i read it using Starter) is _always_ real current in Ampers (RMS) _regardless_ what reference value is set for example for current in p2002, and scaling in PLC is simply wrong? Marcin
Marcin19811 Posts: 3 Rating: (0)	Last edited by: Marcin19811 at: 11/15/2020 10:25:28 Last edited by: Marcin19811 at: 11/15/2020 10:26:30 Last edited by: Marcin19811 at: 11/15/2020 10:28:10 Last edited by: Marcin19811 at: 11/15/2020 10:29:43
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11/16/2020 7:51 AM	Rate (1)
Technology Team Platinum Expert Joined: 10/11/2006 Last visit: 12/12/2024 Posts: 5969 Rating: (820)	Hello, 1. the reason for these reactive currents is described in the engineering manual chapter 1.6.2: The IGBTs for regenerative operation of the Smart Infeed are always switched on at the natural firing point and switched off after 120º (electr.) independently from the direction of the energy flow. As a result of this, a current resp. energy flow from the supply to the DC link or vice-versa is possible at any time. The direction of the actual current resp. energy flow is only determined by the voltage ratios between the supply and the DC link. In steady-state motor operation, the DC link voltage during the possible current-flow phase is always smaller than the supply voltage, so that the current flows from the supply to the DC link via the diodes. In steady-state regenerative operation, the DC link voltage during the possible current-flow phase is always larger than the supply voltage, so that the current flows from the DC link to the supply via the IGBTs. This control principle offers the advantage that the Smart Infeed can react relatively fast to load variations and can also change the direction of the current resp. energy flow at any time. However, a characteristic of the control principle described is that a harmonic reactive current flows on the line side in no-load operation. The cause is the sinusoidal supply voltage on the one hand, and an almost perfectly smoothed DC voltage in the DC link during no-load operation at the other hand. Consequently, directly after the firing of the IGBTs, a short-term current flows from the DC link to the supply because, at this time, the supply voltage is slightly lower than the DC link voltage. If the supply voltage then reaches its peak value, the voltage ratios are reversed and thus the current direction is also reversed. This reactive current decreases as the load on the Smart Infeed increases and disappears completely under full load. The reactive current under no load can be prevented by a regenerative operation disable command (parameter p3533: Infeed, inhibit generator mode). It can also be helpful to inhibit regenerative operation of the Infeed to obtain greater stability in operation when the unit is operating in motor mode if one or more of the following conditions apply: Operation of the Smart Infeed on a weak, unstable supply system with frequent line voltage dips. Operation of the Smart Infeed within a DC configuration with very high DC link capacitance C_DCLink. 12-pulse operation of the Smart Infeed on a three-winding transformer. Mixed Smart Infeed / Basic Infeed operation. Regenerative operation can be inhibited and enabled by the higher-level automation system, for example, if the process steps in which regenerative operation can occur are clearly defined in the process control system. 2. the currents shown in r27 and other parameters are always real values, independent from the reference values. Only when transferring them to PLC or to a DCC chart, they are normalized according to r2003 into a percentage value. So you must make sure, that the reference used in the PLC is equal to r2003.
	With best regards, Technology Team Application consulting and pre sales support for SINAMICS, SIMATIC Technology, SIMOTION, SIMOTICS and MOTION CONNECT
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