Sequence Of Operation; Rectifier Section - Danfoss VLT FC 100 Service Manual

Internal Adjustable Frequen... Service Manual
3 3
Figure 3.3 Typical Power Section

3.3 Sequence of Operation

3.3.1 Rectifier section

When power is first applied to the adjustable frequency drive, it enters through the input terminals (L1, L2, and L3) and on
to the disconnect and/or RFI filter option, depending on the configuration. If equipped with optional fuses, these fuses limit
damage caused by a short circuit in the power section. The input power is also connected to the inrush circuit. This circuit
supplies gate signals to the SCRs, with a high firing angle (near 180°) at first. The firing angle decreases with every
successive AC cycle until it reaches 0°. This process increases the DC voltage slowly over a period of several line cycles, thus
greatly reducing the current for charging the DC capacitors.
The low voltage power supplies are activated when the DC bus reaches approximately 50 V DC less than the alarm voltage
low for the DC bus. See chapter 1.9 Power-dependent Specifications. After a short delay, an inrush enable signal is sent from
the control card to the inrush card SCR gating circuit. The SCRs are automatically gated when forward biased, acting similar
to an uncontrolled rectifier as a result.
When the DC bus capacitors are fully charged, the voltage on the DC bus equals the peak voltage of the input AC line.
Theoretically, this figure can be calculated by multiplying the AC line value by 1.414 (V AC x 1.414). However, since AC
ripple voltage is present on the DC bus, the actual DC value is closer to V AC x 1.38 under unloaded conditions and can
drop to V AC x 1.32 while running under load. For example, an adjustable frequency drive connected to a nominal 460 V
line, while sitting idle, the DC bus voltage is approximately 635 V DC (460 x 1.38).
As long as power is applied to the adjustable frequency drive, this voltage is present in the intermediate and inverter
circuits. It is also fed to the switch mode power supply on the power card and is used for generating all other low voltage
supplies.
36 Danfoss A/S © Rev. 2014-02-10 All rights reserved. MG94A222