Starting surge on a DC well pump

King-01

All,

I've read some conflicting info on starting surges on DC pumps. Some say no surge, some say yes but not as much as an AC pump. Question is - what would be a safe factor of multiplication on the surge (if there is one)? Thanks!

Dave Angelini

You have to state the pump make. That is probably why you are getting different answers.

King-01

Shurflo  9300 24V and 4008 24V to be exact. Shurflo "customer service" was less than helpful. Sent me the operating manual

vtmaps

I don't know the answer.   I have not heard of any surge issues... usually if your power source can handle the running load, it can handle the surge.  What is your source of 24 volts?

--vtMaps

Dave Angelini

No problem with that pump with a reasonable battery capacity and state of charge as vtmaps stated. The store here sells some very mighty DC pumps and some need electronics options to keep the surge down (flickering lighting).

King-01

vtmaps said:

I don't know the answer.   I have not heard of any surge issues... usually if your power source can handle the running load, it can handle the surge.  What is your source of 24 volts?

--vtMaps

I'm helping plan out a friend's system. He'll have plenty of battery, working on the wiring. Both pumps have a listed "max" amp draw, which is what I went by when I installed my system and it seems to work just fine. Was just wondering if anyone knew a better way to calculate surge draw (if there is a surge) rather than just rely on manufacturers "max" ratings?

BB.

The pumps that often give starting surge current "issues" are standard 120/240 VAC induction motor types on AC inverters. A standard induction motor may draw upwards of 5x rated current when starting (and a "good" AC inverter can support ~2x rated output power).

Put a 1.5 HP (1,200 Watt or larger) AC induction motor well pump on an AC system--And you are looking at needing a 3-4kWatt minimum AC inverter and a 400 AH @ 48 volts or 800 AH @ 24 volt battery bank minimum.

The Induction motors with an external starting capacitor can have lower starting surge current--Using a VFD (variable frequency drive) can reduce starting surge, or going with a good quality Electronically controlled PM (permanent magnet) motor (or brushed motors, which are less expensive, but have shorter life because of brush wear) will help too.

And going with a smaller pump (slow pumping to cistern/pressure tank)--And if you still need higher volume (say fire fighting), you can still pump directly from the tank (don't have the lift of a deep well to fight too).

-Bill

King-01

BB,

I'm aware of the surge current issues with A/C pumps. Was specifically looking for input on if the same surge current can be expected on DC pumps. Thanks.

inetdog

King-01 said:

BB,

I'm aware of the surge current issues with A/C pumps. Was specifically looking for input on if the same surge current can be expected on DC pumps. Thanks.

Basically, there will be a starting surge for a DC pump too unless it is a brushless electronically-comutated motor.

The surge may not be as great since there is no inductive reactance effectively paralleling the winding resistance, But they pump will not settle down to normal running current until the motor has gotten up to speed and the counter EMF from the motor rotation is reducing the effective voltage applied to the windings.

A brushless motor may have some soft start features built into its pulse controller, in which case the surge would be lower. Enough current has to flow to provide the starting torque, but that is low for an impeller type pump.

VB

King-01

inetdog,

Thank you for the detailed reply. In your opinion, would it be overkill to base the starting surge on that of an A/C motor(4x or 5X running amperage) or go with a figure more like 2x running amperage, based on a brushed motor. Thanks.

vtmaps

King-01 said:

Thank you for the detailed reply. In your opinion, would it be overkill to base the starting surge on that of an A/C motor(4x or 5X running amperage) or go with a figure more like 2x running amperage, based on a brushed motor. Thanks.

You might be overthinking this...  On those particular shurflo pumps you do not need to worry about surge when you design your system.  If the system is designed so that your battery and wiring can run the pump, then they will also be able to start the pump.

--vtMaps

zoneblue

agreed. it would only be an issue if you were trying to run it off a power brick or dc converter. personally ive never noticed any dramatic spikes on my monitering from the 2088, at least not as big as the danfoss fridge compressor.

King-01

vtmaps said:

King-01 said:

Thank you for the detailed reply. In your opinion, would it be overkill to base the starting surge on that of an A/C motor(4x or 5X running amperage) or go with a figure more like 2x running amperage, based on a brushed motor. Thanks.

You might be overthinking this...  On those particular shurflo pumps you do not need to worry about surge when you design your system.  If the system is designed so that your battery and wiring can run the pump, then they will also be able to start the pump.

--vtMaps

I've been accused from time to time of overthinking things....

Thank you for your input.

King-01

zoneblue said:

agreed. it would only be an issue if you were trying to run it off a power brick or dc converter. personally ive never noticed any dramatic spikes on my monitering from the 2088, at least not as big as the danfoss fridge compressor.

Good to know, thank you.