Voltage Drops on Dimming Lines
Symptoms/Issue
We got a call from a customer were they were concerned that there were seeing different voltage from different module types, specifically 100A and 50A modules. The were seeing a 11v drop on some circuits when added a load, and the were lower on other Circuits.
Dimmer # 20 (50 AMP) - No Load = 116.5 VAC - Load = 105.5 VAC. (11v Drop)
Dimmer #13 (100 AMP) No Load = 121.4, Load = 110.0 VAC. (11.4v drop)
Dimmer # 8 (100 Amp) No load = 121.5 - Load = 112.0. (9.5v drop)
Dimmer # 45 (50 Amp) - No load = 116.0, Load = 108.1 VAC. (7.5v drop)
This is a normal occurrence, The Voltage drop occurs because there are no perfect conductors and there is some loss inherent in the system. The amount the voltage drops depends on the total loop impedance of the supply. That includes the supply wiring from the transformer to the rack, the choke in the module, and the resistance of the wiring to the load (including diameter, type (solid or stranded) and length). All the variables add up and contribute to the impedance of that circuit.
This 8-11v is not terrible unusual and for the human eye it would be barley delectable. For cameras this type of drop could be seen and cause a problem. It can also cause a problem for lamp life.
There is no way to “fix” this problem, but there are ways to improve the situation with several options:
* Increase the cross sectional area of the cable to the load. That will reduce voltage drop in the cable
* Reduce the length of cable to the load. This will reduce voltage drop in the cable
* Boost the voltage into the rack and use regulation. This is exactly what the voltage regulation in Sensor is designed for. It is common for TV studios to tap their transformers high (130-140V input to the rack) and use the voltage regulation features of Sensor to get a perfect 120V at the output of the circuit
* They *might* have external supply impedance issues although this should be fairly uncommon. The way to check for that is to measure the phase voltages at the rack with and without the load applied. If the voltage at the rack drops when the load is applied, they have a supply impedance problem which would mean rewiring the input to the rack to reduce cable impedance from the transformer to the rack.