# motor speed and torque

#### wes

Joined Aug 24, 2007
242
Hi, i'm not sure if this is the right place but it said electronics related so i figured it was, well i was reading some stuff on electric motors but one thing i'm not sure about is, does voltage cause a motor to go faster and does the current create a higher amount of torque and if so then if you were to limit the current and then increase the voltage would it still spin faster?

I was going to test this later but i need about 15-20\$ for everything, so i'll have to wait a bit, but in the mean time any info would really help, later

#### beenthere

Joined Apr 20, 2004
15,819
If you are speaking of a DC brush motor, then increasing the voltage will increase the speed - up to a point. The voltage increase pushes more current into the armature. That will intensify the magnatic field and cause the rotation to increase. At some point, though, the current causes enough extra heating that the insulation fails and the motor burns up.

Increasing voltage while limiting current won't increase speed.

If it's an AC induction motor, the rotation is controlled by the AC frequency.

#### wes

Joined Aug 24, 2007
242
ok thanks but if you were to increase the amperage but still keep the same voltage then would this just cause rotor to speed up and then when you had a load on it this would have also increased the torque or would it just increase the amount of torque it can produced but not increase the speed and oh yea if you had a motor that had a huge current but a low voltage would this have alot of torque but low speed. I was asking this one because i saw a dc motor that had like 12 or 24 volt input at like 28 amps, but a speed of around 1000 rpm but it didnt say how much torque is was able to produce, it just said like 1/4 Hp.

#### beenthere

Joined Apr 20, 2004
15,819
You can't increase current unless you either increase voltage or decrease resistance. Since you have no control over motor resistance, the only way to push more current through a motor is to increase voltage.

The technique for varying a DC motor's speed while maintaining torque is pulse width modulation (PWM). There are numerous threads on this site that deal with the technique, plus a zillion hits on Google that talk about it.

#### thingmaker3

Joined May 16, 2005
5,084
I was asking this one because i saw a dc motor that had like 12 or 24 volt input at like 28 amps, but a speed of around 1000 rpm but it didnt say how much torque is was able to produce, it just said like 1/4 Hp.
hp = (FtLbs* RPM) / 5250

Therefore FtLbs = (hp*5250) / RPM

#### wes

Joined Aug 24, 2007
242
thanks thingmaker3, that really helps knowing that, so the lower the rpms the higher the torque and the opposite for higher rpms, thats a great formula to know. thanks to beenthere to.

later

#### beenthere

Joined Apr 20, 2004
15,819
That is a useful characteristic that steam engines and electric motors have in common - torque is maximum at stall.

#### h.d

Joined Oct 22, 2007
150
in dc-motors there is inverse linear relation between motor (speed , torque),(speed, current)
and linear proportional between terminal voltage and motor spped.

#### wes

Joined Aug 24, 2007
242
hey i have one more question, well it's a hypothetical question, it's one you have to kinda think about atleast i do. well you all know that in a DC or AC(atleast i'm pretty sure AC as well) motor the current is at maximum when the motor is at a stall and thus the torque is at maximum. Now when the rpms increase the current decreases and thus the torque to. Now all this is caused by the back emf that is created when the rotor spins causeing a back emf which limits the current( kinda like resistance) and the torque to, Now here's th hypothetical part. If you had a motor where there is no back emf and it was dc then would this cause the motor's torque to be at maximum at all times, not taking into account some minor losses or big ones? because if there is no back emf then there is no limit on the current besides resistance.

thanks for any feedback by the way i know it might be inefficent but maybe not with high loads and plus you could just use somethingto control the power input

#### thingmaker3

Joined May 16, 2005
5,084
If you had a motor where there is no back emf and it was dc then would this cause the motor's torque to be at maximum at all times, not taking into account some minor losses or big ones? because if there is no back emf then there is no limit on the current besides resistance.
This is why some DC motors have extra electronic protection in the event of a blocked rotor condition.

#### h.d

Joined Oct 22, 2007
150
if u disconnect the field in dc-separetly motor the motor will accelerate to infinty and dameged, thats occured too in seires dc-motor when the load is deactiavted,

#### wes

Joined Aug 24, 2007
242
To h.d, thats because there's no back emf to limit it right and the torque goes way up because the current goes way up and so it spins faster and faster......... and it damages itself because the current basicaly burns the motor up. was that correct, if not then please correct me and thanks for the info everyone

#### h.d

Joined Oct 22, 2007
150
the motor dameged because its spins so fast and the body &component of motor not designes to infinty speed .

#### thingmaker3

Joined May 16, 2005
5,084
"No load speed" is perhaps a better term than "infinity speed." Friction exists.

#### wes

Joined Aug 24, 2007
242
alright I got it now, well thanks everyone

#### h.d

Joined Oct 22, 2007
150
"No load speed" is perhaps a better term than "infinity speed." Friction exists.
yes, thanks for correction,
i mean that speed goes to infinity.

#### thingmaker3

Joined May 16, 2005
5,084
There seems to be a translation error. The word "infinity," in English, is a math term for a concept. The concept is "increases without bound" The concept does not actually exist in real motors. Friction of the shaft bearings, and friction of the air on the rotor, limit speed to some finite value. Even if there were no friction, an infinite amount of time would be needed to accelerate to an infinite speed.

#### h.d

Joined Oct 22, 2007
150
There seems to be a translation error. The word "infinity," in English, is a math term for a concept. The concept is "increases without bound" The concept does not actually exist in real motors. Friction of the shaft bearings, and friction of the air on the rotor, limit speed to some finite value. Even if there were no friction, an infinite amount of time would be needed to accelerate to an infinite speed.
ok, thats right
its proportional thing ,the 1km is relatively is infinty for 1mm
so the infinty is imaginary number its express avery large value for another one which is so small.