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Running motor at nearly hafl its normalspeed

amadeok

New Member
Hi,
Would it be ok to use a motor running nearly at half its rated speed because of the load? It like to use a 895 DC 3000-6000rpm motor with a 80T sprocket connected to a 12T sprocket to reduce the stall effect
 

MaxHeadRoom78

Well-Known Member
A typical DC motor can be ran at any rpm usually, you just need to observe the wattage rating.
Gearing is the more efficient/preferred way.
When ran a really low RPM, cooling may be needed also.
Max.
 

alec_t

Well-Known Member
Most Helpful Member
Running it at half speed under load will draw a current somewhere between the stall speed current and the rated full speed cuurent.
 

shortbus=

Well-Known Member
Most Helpful Member
Hi,
Would it be ok to use a motor running nearly at half its rated speed because of the load? It like to use a 895 DC 3000-6000rpm motor with a 80T sprocket connected to a 12T sprocket to reduce the stall effect
Why? Did you figure out what the ratio would be?
12 :80 = 6.66 : 1 3000RPM divided by 6.66 = 450.45 RPM
 

amadeok

New Member
A typical DC motor can be ran at any rpm usually, you just need to observe the wattage rating.
Gearing is the more efficient/preferred way.
When ran a really low RPM, cooling may be needed also.
Max.
So you see this is for an diy electric bike. I want to use a 895 dc motor that runs at 12v 3000rpm 5kg/cm torque and at 24 6000rpm 10kg/cm torque . I can gear it down with a ratio of 8 to 1, so the rpm would become 375 at 12v and 750rpm at 24v. The bike will go at to maximum of 25km/h, after that the motors will stop working (because of the law). but even just 12v 375rpm is about twice as much as the rpm that the wheels will have at 25km/h. So i'm worried this will not be efficient/ effective at all? I mean will it even be able to exert some torque in this scenario?
 

Les Jones

Well-Known Member
Most Helpful Member
For a permanent magnet motor the speed (RPM) is proportional to the voltage. This is not absolutely accurate for a real motor but is a good approximation.
The torque is proportional to the current. (Again a good approximation.) The torque is not related to the voltage unless you are talking about stalled torque. This is because when the motor is stalled the current through the windings (And thus the torque.) is only limited by the winding resistance so the current is then proportional to the voltage. You would better off designing the gearing so that the maximum speed of the motor corresponds to the maximum road speed..

Les.
 

Pommie

Well-Known Member
Most Helpful Member
Does the specification of the motor include a most efficient RPM?

Mike.
Edit, those probably are the most efficient speeds.
 
Last edited:

dr pepper

Well-Known Member
Most Helpful Member
There are 2 considerations, 1 the motor cannot run for long if its running at more than its rated power, 2 if the motor has a fan then running 1/2 speed or less you'll need to provide it with its own fan.
 

MaxHeadRoom78

Well-Known Member
Knowing the OP's application from other forums and the referred to motor, I very much doubt the performance curve for the continuous current and peak current area are available.
Max.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
So you see this is for an diy electric bike. I want to use a 895 dc motor that runs at 12v 3000rpm 5kg/cm torque and at 24 6000rpm 10kg/cm torque . I can gear it down with a ratio of 8 to 1, so the rpm would become 375 at 12v and 750rpm at 24v. The bike will go at to maximum of 25km/h, after that the motors will stop working (because of the law). but even just 12v 375rpm is about twice as much as the rpm that the wheels will have at 25km/h. So i'm worried this will not be efficient/ effective at all? I mean will it even be able to exert some torque in this scenario?
Gear it down more (16:1 ?), then run the motor at full power - far more efficient, and you'll get much better performance as well.
 

audioguru

Well-Known Member
Most Helpful Member
In one of these many forums, he said he will use a sprocket and chain system instead of the silly wooden roller wearing out the wobbling tire.
I do not know any product that uses a cheap brushed DC motor running for an hour or more when heavily loaded. Won't the brushes quickly wear out or burn out soon in this bike application? Don't real electric bikes use brushless motors that last forever?
 

amadeok

New Member
In one of these many forums, he said he will use a sprocket and chain system instead of the silly wooden roller wearing out the wobbling tire.
I do not know any product that uses a cheap brushed DC motor running for an hour or more when heavily loaded. Won't the brushes quickly wear out or burn out soon in this bike application? Don't real electric bikes use brushless motors that last forever?
Yes i'll make it with a chain and a freewheel. the motor i'll use is this: https://www.ebay.com/itm/254457889235?ul_noapp=true , which with the wheel size being 27 inches should give me 25km/h at 250w 2000rpm , which is the legal limit.
Now as far as the motor controller goes my idea was to use this: https://it.aliexpress.com/item/4000191062334.html?
I've been told to use one rated at least for 60A because of the high initial current draw of the motor?

It has to shutdown the motor after 25 km/h, because of the law, so i need to control this controller with an Arduino and a MPU6050 (or other accelerometer). That motor controller i chose however does not have a PWM signal input, just a potentiometer. I was thinking, would it work if i replace the potentiometer input with a 5v DAC input from the Arduino?
 

alec_t

Well-Known Member
Most Helpful Member
I was thinking, would it work if i replace the potentiometer input with a 5v DAC input from the Arduino?
That depends on how the controller has been designed.
 

audioguru

Well-Known Member
Most Helpful Member
AliExpress does not know anything about electronics. At the top of the ad they say the 60V speed controller is 60A by mistake and do not say that it is for a 2-wires brushed motor, not for the 3-wires brushless motor you are looking at. You need a brushless motor speed controller.
 

gophert

Well-Known Member
Most Helpful Member
It has to shutdown the motor after 25 km/h,
it would be easy to make a spoke counter and measure the time between spokes interrupting an optical beam, or, better option would be spokes reflecting an optical beam. you'll have to cut out the motor if spokes are passing the semsor faster than once per (about) 18.5mSec with a typical 27" tire.
 

shortbus=

Well-Known Member
Most Helpful Member
which with the wheel size being 27 inches should give me 25km/h at 250w 2000rpm
Is 27 inches the actual tire diameter, or is it the rim size? either way you need to do some more math. You need to figure out the tire circumference to find out the RPM to make your 25km/h and then find what gear ratio is needed to get that speed.
 

audioguru

Well-Known Member
Most Helpful Member
I lIke this video much better than the other video because this one uses welding instead of packing tape to hold the heavy batteries.
Both videos seem to be on the same You Tube channel in India.
 

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