Is it safe to use 48v on a 40v motor?

[jeanpaulsc]

I need to power a 40V 5A DC motor for short (~30 sec) infrequent intervals. I have 48V DC available. I wouldn’t mind a bit higher RPM but don’t want to damage the motor. Is it safe to use 48V?

If not, and R = V / I. Then do I need 48V / 5A = 9.6 ohm resistance? If so should I assume it already has an 8 ohm resistance and requires only 1.6 additional ohms?

Am I safe if I have 6A available?

 

[NascentOxygen]

Adding that series resistance will give a major reduction to the motor's starting torque. What is the load your motor will be driving?

 

[jeanpaulsc]

The motor is a 40V leaf blower intended to start a small gas turbine.

 

[NascentOxygen]

DC motors, the type with brushes, get their high starting torque because they draw heavy currents at startup. So when you say you have "6A available" do you mean that your 48V supply cannot provide more than 6A? What will it do if the load tries to draw more than 6A?

 

[jeanpaulsc]

The 48V supply is from 4 12v lead acid batteries in series. Currently car batteries (plenty off amps) but I do intend to purchase 4 smaller deep cycle batteries to hold me over until I can upgrade to Lithium. My original question is safety of running the 40V on 48V. The "6A" reference was my attempt to apply Ohm's law. If I have plenty of amps is there any danger to the 40V motor? Would you recommend a capacitor for start up demand?

 

[NascentOxygen]

If the motor is of larger capability for the job than the load requires, probably the only consequence of 48V operation will be the higher speed. If the motor is closely matched to the load, then the higher starting current and higher running current may be a factor. If you are able to run it on 40V and measure RPM and casing or armature temperature every 10 minutes, then do the same next day on 48V with identical load, you can make the comparison. The IR at-a-distance electronic thermometer should make this easy.

If there is appreciable extra heating, you could consider adding a string of 4 or 5 automotive rectifiers in the 48V cable to discard some voltage, probably around 1.2V per diode. They'll need to be high current to safely carry the motor's starting surge.

 

[Tom.G]

If you don't need the full speed and power of the motor, running it on 36V may be a better option, even changing the gear ratio if needed to regain the 10% speed reduction. One advantage is fewer batteries needed.

 

[NascentOxygen]

The chosen lithium will probably be 36v or 42v.

 

[Rive]

The motor is a 40V leaf blower intended to start a small gas turbine.

In this case I would say it's not too big of a risk to apply 48V.

By the way what was the original power source of that leaf blower? I have a guess that it was just 'around' 40V.

Would you recommend a capacitor for start up demand?

Definitely NO. If the battery can't fully match the initial current then all what will happen is a bit lower voltage on the battery connectors for a short time. With an over-boosted motor, it's just fine.

 

[CWatters]

What's the load current at 40V? If the load causes the motor to draw well under the rated 5A on 40V then the higher voltage shouldn't be a problem. If the load already causes the motor to draw 5A at 40V and increases with voltage then you might have problems.

 

[jim hardy]

I need to power a 40V 5A DC motor for short (~30 sec) infrequent intervals. I have 48V DC available. I wouldn’t mind a bit higher RPM but don’t want to damage the motor. Is it safe to use 48V?

Wow . Lots of good discussion above.
Being completely practical...

Leaf blower is a centrifugal fan.
Look at Fan Laws at https://www.nyb.com/pdf/Catalog/Letters/EL-02.pdf to get some feel for blowers .You propose 20% overvoltage . I don't think that would hurt the motor for the short bursts you describe.
It would make the motor attempt to increase its speed by 20%
Since fan power is proportional to cube of speed, 20% overspeed would cause the power demand to rise by a factor of 1.78 .
That'll deposit heat in the motor conductors almost 1.5X the rate at normal operation.The motor should handle that for a few seconds.

I'd worry more about the plastic blower mounted to the motor shaft.
Centrifugal force goes up as square of speed.
1.2 squared is 1.44 .
Not knowing how much margin they built into that blower ,
and having actually witnessed fan blades coming apart from centrifugal force (it's violent)

I cannot tell you it's safe, .
You'll have to run a controlled test .

1. Block the air outlet completely, apply 40 volts and measure how fast it turns.
2. Place it somewhere that if it flies apart the pieces won't hit anything or anybody.
3. Rig up a remote power switch at least twenty feet away from it.
4. Block the air outlet completely. Standing twenty feet away and behind a car door, apply 48 volts. If it stays together measure how fast it turns.
5. Block the air outlet with an opening the same area as the turbine you plan to start, apply 48 volts and measure how fast it turns.

If result of step 5 is less than 90% of step 4 then i'd say you've demonstrated by test that it's safe.

old jim

 

[Rive]

I'd worry more about the plastic blower mounted to the motor shaft.

Wait a bit. I don't know much about starting gas turbines, but this thread was centered on the motor only. Should we really extend the question to the whole blower instead?

Because that's an entirely different question indeed...

 

[jim hardy]

Wait a bit. I don't know much about starting gas turbines, but this thread was centered on the motor only. Should we really extend the question to the whole blower instead?

Because that's an entirely different question indeed..

Hmmm. Well the original question was ambiguous as to scope.

Is it safe to use 48V?

Noblesse oblige ?
Sometimes we have to be a "Catcher in the Rye"

His job is to catch the children if, in their abandon, they come close to falling off the brink; to be, in effect, the "catcher in the rye". https://en.wikipedia.org/wiki/The_Catcher_in_the_Rye

I'd feel awful if i told him "Sure it is" and he lost an eye. That's all.

old jim

 

[Rive]

That's all.

'was quite an eye opener.

I've just looked up on youtube how a small model turbine started, and it is just by blowing from front and igniting when it is already spinning fast enough. So your comment was spot on, the question is most likely not about the motor, but about the blower.

Hats off

 

[CWatters]

Some used compressed air and others have a built in electric motor to get them up to the rpm at which they become self sustaining.

 

[jim hardy]

Thanks for the kind words, @Rive !

 

[NascentOxygen]

Since fan power is proportional to cube of speed, 20% overspeed would cause the power demand to rise by a factor of 1.78 .
That'll deposit heat in the motor conductors almost 1.5X the rate at normal operation.The motor should handle that for a few seconds.

I think the motor is already free of the leafblower. I understand that the motor has been repurposed to spin up a gas turbine [engine I presume]. So once it reaches the appropriate rotational speed for ignition won't the motor be disconnected, meaning powered overspeed operation shouldn't/needn't be encountered.

Your analysis for 48V operation of the leafblower sounds good, but may not be the situation. Though if I've misread, and OP really is intending operation of his leafblower using 4 car batteries, I'd invite him to post a photo.

 

[jim hardy]

I think the motor is already free of the leafblower. I understand that the motor has been repurposed to spin up a gas turbine [engine I presume].

Might well be, I couldn't tell from his description..

The motor is a 40V leaf blower intended to start a small gas turbine.

From that i guessed he's using the leaf blower as is to force air through the turbine to spin it up.. So i took the path of caution; perhaps over-caution? .
Better safe than sorry.

Anyhow it won't hurt him to learn about the fan laws.

Maybe he'll come back . .
I asked him to start a thread on his project. Apparently he's using a small automotive turbocharger to drive an alternator, hopefully through appropriate speed reduction. Sounds really interesting but i do not know his skill level.

old jim