When a DC motor (say 12V, 150W) regenerates, what would happen if no load is connected across its terminals ?
It would produce tens of thousands of volts (right?)
Now suddenly, a power relay connects a car battery to terminals of motor like this:
RELAY _/ .---------------+--o/ o-------. | | | | | | | || | 12V | ||-+ MOTOR --- ||
No- only at the instant load is removed, depending on how much parasitic capacitance and resistance are in the circuit. It rapidly falls to the open circuit voltage at that rpm which for a normal low voltage motor is still a reasonably low voltage.
Rises?
No. As circuit is opened it jumps to a high value, but decays exponentially with a time constant determined by circuit elements. This is true opening ANY inductive circuit that is carrying current. A motor has a 'kick' just like any inductor. The decay is quick, usually milliseconds, and it decays to the normal open circuit generator output as the generator "flywheels" to a stop. If it has a decent moment of intertia, and low friction, it will coast slowly to a stop if the circuit is open, but stop rapidly if a load is switched in.
I design equipment for automotive use, so I get to deal with this. There's usually something connected across the battery leads, but there are times most equipment is removed from the alternator (such as engine crank). This event is known as load dump (the load is dumped from the alternator which then generates a nice high voltage due to the change in current in the windings). The voltages that appear can go quite high. I have a 40V TVS designed for this sort of thing (SM8S40A) that I use quite a bit, and has always worked fine.
There is a spec for this stuff : ISO7367-1 (12V vehicles) and ISO7367-2 (24V vehicles). Test pulse 5 is the relevant test, where the load dump transient is specified as 70V - 200V (-2) or 26.5V to 86.5V (-1). Note the pulses are voltages *on top* of the normal battery voltage.
The spec goes into the reasons for choosing these levels.
Cheers
PeteS
Now I have an idea for building a pseudo Greinacher cascade out of a bunch of cheap motors...
Load dump occurs when output load is suddenly removed; especially the battery. During cranking, that is only possible if the alternator is excited and if the charge lead from it to the battery breaks. But why load up the alternator when cranking the engine that's driving the alternator??
The total energy in a load dump is quite low; of the order of a few Joule. That's not significiant to electric motors, etc but certainly to unprotected microelectronics.
-- /"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia \ / ASCII ribbon campaign | German words of the year 2005: X against HTML mail | Bundeskanzlerin Tsunami Gammelfleisch / \ and postings | Sadly, in that order.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.