Question about dc motors

Thread Starter

amadeok

Joined Mar 7, 2020
9
Hi,
I'm trying to choose a motor for a electric bicycle and i was going to choose this: http://www.aslong.net/en/h-pd-64.html#_pp=123_392
but the table there says like 20A at all times? that would kill the battery. Is that table wrong?
Also in this one( ttp://www.aslong.net/en/h-pd-101.html#_pp=123_392), it says 35W both at 12v and 24v, how is that possible?
 

MaxHeadRoom

Joined Jul 18, 2013
20,472
Don't trust Chinese spec sheets, not sure if it is because of ignorance or something lost in translation.
Also you do not need a regulated supply for a DC brushed motor.
Max.
 

MisterBill2

Joined Jan 23, 2018
5,806
Like Max states, some Chinese data sheets may not be entirely right by the time they are translated. And in addition a few of them are simply incorrect.
A motor claiming the same wattage at two different voltages may have a complex dual voltage option, or the specs may actually be describing two different versions available.
The link did not work for me and so that is the best I can offer.
 

Alec_t

Joined Sep 17, 2013
11,172
the table there says like 20A at all times? that would kill the battery. Is that table wrong?
I interpret the table as being for a whole range of models with different reduction ratio gearsets but with the same motor driving the gears. Hence the same stall current (20A) for all models.
 

MisterBill2

Joined Jan 23, 2018
5,806
I interpret the table as being for a whole range of models with different reduction ratio gearsets but with the same motor driving the gears. Hence the same stall current (20A) for all models.
Stall current is ALWAYS much greater than running current at full load. That would be the same as "locked rotor" current often specified for compressors. It is the starting current at the first instant of power applied. Normal running current is far less than that.
 

MaxHeadRoom

Joined Jul 18, 2013
20,472
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.
 

Thread Starter

amadeok

Joined Mar 7, 2020
9
Hi,
Sorry so it won't matter if it is working at half its rated speed due the the load it is under? So i understand that it's not the same as a completely stalled motor
 

MaxHeadRoom

Joined Jul 18, 2013
20,472
If it is powered with the full rated voltage, and it is actually running at half the rated rpm due to load, then most likely the continuous current rating of the motor will be exceeded. Which may result in the destruction of the motor if prolonged.
Max.
 

Thread Starter

amadeok

Joined Mar 7, 2020
9
If it is powered with the full rated voltage, and it is actually running at half the rated rpm due to load, then most likely the continuous current rating of the motor will be exceeded. Which may result in the destruction of the motor if prolonged.
Max.
I understand. You see this is for a diy electric bike. I could regulate the voltage of the motor according to the speed of the bike using an arduino and a accelerometer, but then i would lose torque as well right?
 

MaxHeadRoom

Joined Jul 18, 2013
20,472
As load increases you increase torque (current) to the motor, this can be increased up to the maximum continuous torque (current rating ) of the motor.
Max.
 

MisterBill2

Joined Jan 23, 2018
5,806
I understand. You see this is for a diy electric bike. I could regulate the voltage of the motor according to the speed of the bike using an arduino and a accelerometer, but then i would lose torque as well right?
FAR BETTER than using a voltage control for the motor would be to use a hand-throttle type of control to vary the duty cycle output of the driver circuit. That would be similar to a real motorcycle. The benefit comes from no power wasted as heat at less than 100% drive, and the option of bypassing the PWM circuit at full drive. And certainly using an over-running mechanism such as a free wheel will be more efficient than a friction drive, and not wear out the tires. But there will need to be an additional reduction stage to get the RPM down to a suitable speed to directly drive a bike wheel. I drove a motorbike like that 55 years ago, using a gear hub and a large 52 tooth sprocket on the hub and a 7-tooth sprocket on the engine. It worked fairly well , but the engine had a lower max RPM than 3000
 
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