| Electronics > Projects, Designs, and Technical Stuff |
| Shielded cable for sensing 900V, 15 kHz PWM signal over 10 meters |
| (1/3) > >> |
| matbob:
Hello All, In one of my projects, I want to sense a three phase switching voltage coming from a PWM inverter connected to a motor. The DC-link voltage is about 900V, so the switching happens between 0V and 900V at about 15 kHz. The voltage measurement has to be done at the motor terminal. The measuring instrument (maybe an oscilloscope or power analyzer) is about 10 meters far from the motor terminal. I would like to run a cable over this 10 meter carrying these switching signals. I need a bandwidth of about 20 MHz for preserving the sharp edges in the signal. The signal being large amplitude will not be affected by noise, but I want to avoid capacitive coupling from this cable to nearby equipment, so I need shielding for this cable. The shield will be connected to ground. At the end of this cable, I will connect a differential probe for connecting to an oscilloscope. I tired searching in websites of oscilloscope probe manufactures, but I could not find anything beyond 2-3 meters. Can anyone suggest a cable assembly for this purpose? Thank you. |
| jmelson:
Make your own. Get 50 Ohm coax cable, and build an attenuator that will be at the motor end. You sure don't have to worry over capacitive loading when measuring this system! So, a 100:1 probe would need a 4950 Ohm resistor, shunted by a capacitor that is 1/100th the cable capacitance. You can look up the capacitance of various cables per foot (or m) online. You must be VERY careful to make sure the 50 Ohm termination on the scope is always available, so I would recommend using an external 50 Ohm terminator at the scope end. Actually, this will deliver 9 V to the scope end, so most internal terminators would not handle that, anyway. The 4950 Ohm resistor would need to have a rating of 163 Watts! OOPS, looks like a problem!!! Maybe a 1000:1 probe makes more sense, that would give you 900 mV when the input is at 900 V. The resistor would need to be 99950 Ohms, and power rating of 8 W. That might be doable with a bank of 16 paralleled 1/2 W resistors. Jon |
| djacobow:
I'd like to understand this type of thing better, as I have some similar needs. Why not 3M3 ohm and 50 ohm resistor voltage divider at the source connected to 50 ohm coax and then attached to scope input through feed-through terminator or a tee with a 50 ohm terminator on it? The divider's output impedance is 3300000 || 50 ~= 50, power dissipated in the divider is < 250 mW at 900V. You do get a voltage ratio of 66000:1, but is that tragic for this app? You can use a lesser ratio at the cost of needing a resistor with some power handling capability. Does this approach without capacitors seriously curtail the bandwidth? Naively I'm thinking that the scope input capacitance is going to be a large impedance relative to the source, and so it can be ignored for a 20 MHz signal. (Like, 20 pF * 50 ohm = 1ns) |
| Amper:
Couldnt you remote operate the oscilloscope? Put it next to the motor, connect as usual and then run a gpib or ethernet cable to a laptop. |
| jbb:
The ‘make your own’ probe is fine, but will be earth referenced. If you need to measure phase to phase, you’ll need to do 2 channels & subtract in ‘scope. Also watch out for ground loops. I recommend starting with a differential probe with coax output. |
| Navigation |
| Message Index |
| Next page |