[PSUBS-MAILIST] PWM Controllers

[Subscuba@aol.com]

John,

Take a look on the PSUBS site. I have a copy of my schematic for my motor controller and it switches, for my motor about 55 amperes. The cost of the FETs are about $10 each. Since that time the cost has come down. It is feasible to switch currents up to at least 100 amperes. The PWM switching technique is very efficient and exceeds 95%.

I found buying a motor controller, which in most cases makes sense, expensive for me and made my own. You might do a search on the Internet for Curtis motor controllers.

As an example battery powered automobiles use the switching technique. As the battery bank voltages rise above 24 to 48 volts alternate devices such as IGBTs become cheaper. SCRs are risky devices to use due to their high voltage drop and a tendency to stay conducting. At the lower voltages used in most battery powered Psubs power FETs appear to be the way to go.

Most problems in doing your own PWM controller revolves around controlling the voltage spikes you get from switching these high currents through inevitable parasitic inductances. If you look at the schematic on the website there is a unique voltage clamp used to control voltage spikes across the switching FETs.

Use of relays to switch battery banks results in inefficient use of the battery bank capacity since you would draw different charges from the battery banks. Some motors have different windings to accept voltage changes but I don't know anything about those.

If you want to take the time review the PSUBS archives on motor controllers.

Have fun,

Ken Martindale