60 vs 400 hertz advise needed 1

[subsearobot]

Hello, I am doing some background work for a colleague (we're a very small company). The caviot here is that I am mechanical, so please forgive any ignorance expressed here!

First, our typical systems consist of powering equipment at 3000V, 60 hz through several thousand meters of conductor (we design remotely operated vehicles for subsea remediation and inspection). The main load is a hydraulic power unit, which operates on this high voltage, while system loads are converted to DC, typically 24V. Our present system has a client-supplied cable, which limits our flexibility.

We are looking into whether a 400Hz system would benefit our design. From a cursory look, I see that power-density of the HPU motor could be greatly increased (vehicle weight), while line-losses would also be increased with a 400 hz system (vehicle power).

I would appreciate any insight into this quandary, including but not limited to opinion, references, and direct experience with predicting line losses of a 400hz system (we are ok with calcs based on a 60 hz system).

thanks in advance.

 

[subsearobot]

forgot to mention, the present system will be about 20 hP.

 

[dpc]

Long conductors would not work well at 400 Hz due to inductive reactance increase and subsequent increased voltage drop. Resistive losses increase somewhat due to skin effect, but probably not a big factor at 400 Hz.

Ideally, you'd send the power down the line at dc then convert to 400 Hz or higher at the motor via a VFD if reducing motor weight is critical.

Otherwise, 50 or 60 Hz is probably the most cost-effective at 20 hp. Or do it all at dc with a brushless dc motor.

 

[Skogsgurra]

DC. As d(p)c says

Capacitive load is a problem with AC and several thousand meters of cable.

No such problems with DC. I would select the highest DC voltage for which there are inverters available for the hydraulic pump motor and then go from, say, 400 Hz to the various auxiliary voltages needed in the vehicle.

There are standard inverters for 690 V grids. They have 1 kV internal DC link and can be fed directly with 1 kV DC. The voltage drop in the cable will not change very much and the inverter has a broad input voltage range. Shouldn't be a problem.

I would probably also look for a PM motor for the hydraulic pump. And I would consider the optimum choice of frequency regarding motor size/weight, transmission and pump. 400 Hz isn't the only choice, it just happens to be common in aircrafts.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[cranky108]

The air plane industry uses 400 Hz because it makes all the motors, and transformers smaller. And because they don't go long distances, the losses are not much of a concern to them.

View the frequency/equipment size as a tradeoff. 60Hz or 50Hz would be the cheepest because there is so much already made for it. The hybred way is to use an inverter at the equipment end to convert to what you want, from what you transmit.

Consider what type of speed control scheme you have, and want. This might affect your choise.

 

[subsearobot]

cool. thanks for the input everyone.
DC is seldom used in ROVs because of the requirement of big capacitance at the load. caps are large, and typically are not pressure tolerant. this leads to a requirement for large, heavy and expensive pressure housings.

sounds like 60hz will be our continued choice due to line-loss. also, this allows us to not use gear-box at the hydraulic pump, as would be required with a 400 hz motor.

 

[Skogsgurra]

Capacitors and pressure is a new insight for me. More parameters than you think. As always.

BTW: Aren't there electrolytic capacitors in everything electronic? Are some capacitors more tolerant than others?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.