I intend to run a 48V DC water pump which uses this motor (http://www.leeson.com/leeson/searchproduct.do?invoke=viewProductDetails&motorNo=098382.00&productType=0) through a solid-state relay like this one (http://www.crydom.com/en/products/catalog/power-plus-dc-series-100-dc-panel-mount.pdf - DC100D20C) and need help selecting a flyback diode.
The pump motor is rated at 1/2 hp, and at its full load/RPMs (1800) can apparently draw 11.0 amps. But according to the pump retailer, with the head height I am dealing with it will probably be at around 170 watts or around 3.5A @ 48V. To build in a little margin for safety let’s say we are dealing with a 10A load, even though I am pretty sure it will never reach that level (even at the maximum head height supported by the pump, the retailer shows only 314 watts / 6.5A).
From what I’ve read I understand that the flyback diode needs to be able to handle the exact same current that was flowing through the motor the moment it was switched-off (via the relay) since the inductor will want to continue flowing that same current (even after switched-off) through the flyback diode until that stored energy has been fully dissipated.
So I know I need a 10A+ diode. But what about some of the other attributes:
- Breakdown Voltage: As I understand it, this is the voltage at which the diode will allow current to flow in the reverse direction. I don't think this should ever happen (right?), so the breakdown voltage should be at the very least higher than the expected battery bank voltage range. How much higher? Any sort of back EMF voltage spiking that might occur would be positive with respect to the flyback diode, right (and therefore the breakdown voltage is not applicable in that scenario)? There shouldn't (in theory) be any voltage spikes on the back/blocking-side of the diode. Although I guess if there were a voltage spike up-stream from the pump/motor I guess it would be better for that to flow through the diode (backwards) rather than the pump.
- Working Voltage: Does this need to be at or above the voltage range of the battery bank (i.e. 44V-52V)? Or does it need to be higher so as to accommodate voltage spiking? Or is it that with the flyback diode there is no voltage spiking (i.e. if the voltage is 48V with the pump switched on, then immediately after it is switched off it slowly decays from 48V down to zero via the diode loop)?
- Maximum Reverse Standoff Voltage - "the voltage below which no significant conduction occurs" ... from another S.E. post: "breakdown voltage is usually 10 % above the reverse standoff voltage" ... so it sounds like this is related to Breakdown Voltage above and as long as it is sufficiently high it shouldn't matter.
- Clamping Voltage: "the voltage at which the device will conduct its fully rated current" ... again, should this be low-ish? so that the full <10A can be flowed immediately with no restrictions? Or does this need to be 48V to ensure that current will only be cycled back through the motor at that voltage (and not at some other voltage that might damage the motor?)?
Thanks in advance for your help! There are a bajillion different diodes out there to choose from, and I'm just looking for a little guidance on how to select the right one in order to prevent voltage spikes from damaging the pump/motor, solid-state relay, and/or other components in the system.
Update: How about the Vishay Semiconductor VS-T40HF10 (https://www.mouser.com/ds/2/427/vst40hfseries-50776.pdf)? Rated for an average forward current of 40A, a reverse voltage of 100V, and a surge current of 600A. Relatively high forward voltage of 1.3V, and probably way overkill for my needs all around, but this would be installed in a remote/rugged location (outdoors, but protected) and I like that it is screw mountable and has screw terminals. I know I could get something that would work for like $0.30, but I also don't mind spending $20 for a more robust design that will stand up to abuse. Its classification as a "Power Rectifiers Diode" has me questioning its viability, but as long as it behaves as a diode and only allows current to flow in one direction then it should be fine. I'm not using PWM or switching this circuit frequently; probably on and off only once or twice a day.