Pump enigma.

[Pommie]

I need to power an air pump. I measured the static resistance at 1.2Ω. It's a 12V pump that takes up to 35A. When the pump is powered (12V) under no load it draws 18A. I thought the static resistance would give me the startup/stall current but obviously not. What is wrong with this picture?

Thanks,

Mike.

 

[rjenkinsgb]

Probably a dirty or tarnished commutator adding to the resistance, or the meter is not good accurate enough at low ohms readings.

Edit - typo..

 

[danadak]

Post a pic of the pump name plate or datasheet or part number or characteristics.

Physically how big is the pump ? A pic....


Regards, Dana.

 

[ZipZapOuch]

I need to power an air pump. I measured the static resistance at 1.2Ω. It's a 12V pump that takes up to 35A. When the pump is powered (12V) under no load it draws 18A. I thought the static resistance would give me the startup/stall current but obviously not. What is wrong with this picture?

Thanks,

Mike.

Touch your test leads together and subtract out the resistance of your leads (there is no way a 12v potential can push 18 amps through 1.2 ohms unless you broke Ohms law).

 

[Tony Stewart]

The operating V/I can never be lower than the DCR unless the measurement was faulty.

 

[Diver300]

The operating V/I can never be lower than the DCR unless the measurement was faulty.

The contact resistance between the brushes and the commutator will not be constant. The movement when the motor runs can result in completely different effects than what happens when a low-current source in a multimeter is used.

 

[Nigel Goodwin]

The operating V/I can never be lower than the DCR unless the measurement was faulty.

As from back in post #2, a multimeter is no good for attempting such measurements as this.

 

[Tony Stewart]

As from back in post #2, a multimeter is no good for attempting such measurements as this.

With 0.1 Ohm resolution as stated in #4 the probe resistance can be subtracted from the lowest static armature reading. It will change with rotation angle.

There is no reason given for this type of DMM reading to be "no good" to read a 1 Ohm range result.

But it is possible there is an armature commutation short.

 

[Pommie]

I can't remember the make of the meter but I'm sure it was a good one.
We checked two different pumps(can't get picture) and the rating plate said 12V up to 35A.

Mike.

 

[ZipZapOuch]

I can't remember the make of the meter but I'm sure it was a good one.
We checked two different pumps(can't get picture) and the rating plate said 12V up to 35A.

Mike.

Even a "good meter " can have 0.4 to 0.8 ohms of resistance in the leads plus contact resistance of the sharp point contacts (or narrow dull, slightly oxidized tips.

 

[Pommie]

0.8Ω would tie in nicely with the results. That would make the static resistance 0.4Ω which would give a start/stall current in the region of 30A. I'll see if I can find a better way to measure it later this week. The 35A could be with the battery at ~14V. Actually, those figures tie in very nicely.

Mike.

 

[Nigel Goodwin]

You can get some quite nice low-ohm meters from AliExpress etc. in China for very little money.

I bought one a while back, and recently used it for finding a short on a PCB - it's goes low enough, and accurate enough, to move along the traces and locate where on the board the short is.

Mine has ranges from 2 kilo-ohm down to 20 milli-ohms FSD.

Edit:

Just looked it up

https://www.aliexpress.com/item/32705866146.html?spm=a2g0o.order_list.order_list_main.462.8b2e1802kQ8xQn

 

[ZipZapOuch]

I bought a 6-digit Fluke bench meter from a guy off Craigslist for under $100 after some good negotiation. I don't know where it came from or how he got a stack of 6 units but he seemed happy with the deal after all was said and done.

Anyhow, the 4-wire ohms measurement is quite good because the excitation current is in different wires than the measuring probes. Really good for low resistance measurements. You can build your own constant current source of about 200mA to 1Amp (not enough to turn the motor but enough to measure the IxR voltage drop with 0.4ohms voltage. You'll get about 80 mV range. Use more current to get a bigger voltage range.

 

[MaxHeadRoom78]

What is the pump technology?
For example a common Radial centrifugal type will show max load current when intake and output are open, and unconnected.
When output load resistance Increases. load current Decreases.