DC Motor Current Goes DOWN When Stalling?

Hey guys,

I've got a DC motor inside a cordless 18v Black & Decker leaf blower. I measured the current of the motor using a volt meter and I get 5.15 Amps. Now, when I cover the blower, the motor should stall and the current should go up. (right?) Measuring the current of the covered leaf blower yields roughly 3 Amps. What's going on here? I thought current goes up when stalling/putting a load on a motor?

Wrong.

If you cover the blower, a vacuum will be created.
There will be no more air that gets pushed.
Effectively you will take away most of the load.
That's why you see the current go down.

The motor is not stalling.
It would if you'd find a way to block the fan in a safe way (do not try that, seriously !).

Leaf blowers are open loop pumps if it pushes or pulls the air and the air has know where to or come from it can slide past the blades to prevent large pressure build up.

I don't think it actually reduces the load, I would think it should be about equal but definitely more then when its not blocked.

The blades spin regardless, this creates a push/pull action, if the intake is blocked it try's to pull air, if there is none to take it creates a suction zone between the blades and the blower wall. This suction zone almost certainly is smaller area then the blower intake, thus restricting the motors ability to work compared to before the blockage. Motor work = more current.

As to the reduction, could be any number of things, really dependent on the design and the battery of the blower. My bet is on the battery with no other information.

Does that explain that he didn't see the current going down, but that it went up ?

OP: didn't you hear the blower rev up by any chance ?

Ever tried what happens if you block the hose of a running vacuum cleaner ?

When you spin a fan, one side is a positive pressure and the other is a negative pressure. If you block one side that amplifies the pressure region differential. Obviously when you have 2 area's of apposing pressure they want to equalize, on a blocked vent the only possible way for this to happen is for air to flow in the reverse direction past the blades; how much extra work on the motor is dependent on the area between the blades to allow this reverse flow. At some point a equilibrium is reached.

Sealed motor pumps will stall when you block a vent because air cannot flow in reverse and the pressure or vacuum at some point will exceed the torque of the motor. Common vacuum cleaners are likely very tightly fit fans, this allows them to have enough strength to pick up small amounts of water and light objects, blocking the end in my experience makes them scream at you (kind of like how a whistle works, air flowing through a small area very quickly) but not much else.

The fact that its a battery powered device strongly suggests the reduced current is the result of power consumption. Something is overheating and current limiting, or the battery is being over strained. It clearly has a control board for the charging, I wouldn't be surprised if they also have current limiting to protect the device and battery from damage.

I'm afraid harddrive is wrong with his argument.

A centrifugal blower is the pneumatic version of a centrifugal water pump.

The least loaded condition on this type of device is when there is no fluid being pumped - whether it be air or liquid.

Circulatory fluid within the pump body does produce some load but certainly nowhere near the pump's rated capacity.

The stonewall effect that occurs in large centrifugal compressors when the through-flow is restricted, which results in potentially destructive surge, does not apply to "toy" sized pumps such as leaf blowers - the manufacturing clearances are so wide that pump efficiencies are too low to experience such an effect.

When starting large centrifugal liquid pumps the outlet valve must be closed during the run-up phase, then, when the motor is up to full speed the outlet valve is gradually opened. Suction valves are not used for this process as the loss of inlet head to the pump would induce cavitation resulting in damage to the impeller blades.

Therefore when you blocked your leaf blower, the load flow through the "pump' reduced, resulting in less load on the motor, a corresponding increase in speed and a reduction in motor current.

Regardless of the tolerances between the blades and the walls load on the motor can do nothing but stay equal or increase when part of the system is blocked. The blades are fix design, spinning them will create a differential pressure zone.

You block the intake, and this creates a vacuum. A vacuum on the intake is in direct opposition to the motors load. The vacuum is trying to pull air one way and the motor is trying to pull air the other way, as the vacuum increases the motor has to work harder to pull the air. Same on blocking output, motor pushes the air and because it has know where to go it pushes back against the motor blades. On a open design like a leaf blower there is a lot of room for air to do a reverse flow, thus the amount of extra work the motor has to do won’t be that much larger.

Thus if the current decreased there is something about this system that we don’t know. Ex. If you over draw most batteries they start to shutdown.

Such shutting down would go with a rapid heating up of those batteries.
Is there some heating of these batteries, Reenforcements ?

Also, measure voltage across your motor.
If you block the inlet, what happens with that voltage ?

harddrive123:
Leaf blowers are open loop pumps if it pushes or pulls the air and the air has know where to or come from it can slide past the blades to prevent large pressure build up.

I don't think it actually reduces the load, I would think it should be about equal but definitely more then when its not blocked.

No, blocking the airflow to a turbine both reduces the load and speeds up the turbine.
You can hear the note rise in pitch if you do this with a vacuum cleaner very easily.

This is because its doing less work spinning the same air round and round than
it would be doing taking new air each time and accelerating it. The same old air is
just spinning with the fan and a vortex is formed.

Right MarkT, that's exactly what i was trying to tell.

I think one of the issues in this thread is two different uses of "stall". In aerodynamics
a stall is a disruption of normal airflow (usually catastrophic), whereas for a motor
a stall is when the load torque exceeds the motor's available torque and the thing
stops spinning (or drops to a very low speed). A motor driven fan has two ways of
stalling!