Electric Motor power

[masamichi]

Hi,
I am developing a undercarriage with electric motor with my partner. I am a engineer, but I don't know much about electric motor. Our goal is to achieve high torque with low speed (100rpm w/900N*m) and lower torque with high speed (300rpm w/ 300N*m). so we would like to use 10kw variable speed motor. Then, one question came to me. What is the difference between providing 10kw power to 10kw motor and 30kw motor. Will I get the same result? Is it OK to keep providing 10wk power to 30kw motor? Is it efficient?
I tried to use 10kw w/ 3000rpm and gearbox (1:10) to reduce the speed. A motor manufacture recommended not to run the motor less than 2000rpm. Then, I can't reach enough torque. So I thought If I have a large motor such as 30kw 40kw, I can have wide range of torque.
If anyone can give me an opinion, I would really appreciate it.

Best regards,

 

[itsmoked]

Depending on lots of things you can use a VFD to get the full torque at lower speeds. You will need to provide auxiliary cooling to the motor though. you can often specify a separately powered fan on "VFD" motors.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[racookpe1978]

If you are designing for an undercarriage, be aware that the very early electric motor inventors and designers faced tremendous problems with dirt, grease, oil, snow, water, and debris getting into the motors underneath the early vehicles: trolleys and trains were especially bad up north in the winter.

If you use an envelope case (waterproof seals and no openings in the envelope around the motor) be ready to provide some cooling mechanism for the motor itself to the outside in addition to the mentioned fans above. .

 

[Parchie]

Huh? If you're designing units for use up north during winter, I don't believe you'll need cooling! You'll only need to make sure your drive motor doesn't suck-in snow and get filled with melted snow-->water!

 

[waross]

I remember when a Southern based oil company aquired a large interest in an upgrader in Northern Canada. The southern engineers scoffed and ordered the removal of the winterization hoardings. When it was sugested that the hoardings would be needed in the coming winter, The response was;
"We get winter in Texas too."
At -40 degrees the plant came to a grinding halt. Trying to restart in the cold, things went worse and there was an expensive fire.
What's the point?
The corollary to winter in Texas may be summer in the north (Canada). While it may not be up to Texas standards, there are areas where summer temperatures of over 100 degrees F are not uncommon. I have experienced 110 degrees F in the Southern Okanagan Valley. The Lilloet area in the Frazer Canyon gets hotter. Some vehicles are designed for year round use.
Historically series wound DC motors were used for maximum low speed torque. A locomotive may have 8 motors connected in series to derive the maximum current from the diesel DC generator. When they motor speed came close to maximum for the voltage, a transposition was made and the motors were reconnected in two parallel strings of four motors. Less current but about twice the speed.
The next transition may be to four parallel strings of two motors. Faster yet.
Finally 8 motors in parallel. The least torque but the maximum speed.
That was then, this is now.
Many diesel electric locomotives are now driven by induction motors fed from VFDs as suggested by itsmoked.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Our goal is to achieve high torque with low speed (100rpm w/900N*m) and lower torque with high speed (300rpm w/ 300N*m). so we would like to use 10kw variable speed motor. Then, one question came to me. What is the difference between providing 10kw power to 10kw motor and 30kw motor. Will I get the same result? Is it OK to keep providing 10wk power to 30kw motor? Is it efficient?

The kW output of the motor will match the load requirement. If you have a 30kW motor and the load only needs 10kW, the motor responds by putting out 10kW. But you cannot really "force" the 30kW motor to only draw 10kW without limiting the load requirements. So if your supply source can only deliver 10kW, and the load demands 30kW, the motor would attempt to deliver it and overload your supply source. Conversely if the load requires 30kW, a 10kW motor cannot respond in kind.

Don't approach this as a kW problem, approach it as a torque problem. Decide on the gear ratio and torque that works for you first, then use that to select the motor size and base speed, understanding that the VFD will reduce that speed as necessary while maintaining the torque. That's what they are for. The change in kW capacity is consequential to that.

If your motor supplier recommended that you not run the motor below 2000RPM, you are talking to the wrong motor supplier. You can get motors and drives that will deliver full torque at zero speed. You just need to tell prospective suppliers what you expect of it and if they balk, move on.

But do keep in mind the extremes of ambient environments as suggested. Electric motors and drives are not suitable for every kind of extreme and precautions must be taken. You (OP) are not the one that brought it up, so we don't know your intent, but it is worthy of consideration and discussion.



"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[controlsdude]

If your worried about the vfd producing that torque under heavy load and you sized it right. I thought alot of vfd manufacturer rate there vfds for normal and heavy duty? So use the heavy duty size vfd for the motor size.

 

[sreid]

Motors are naturally high speed devices. I would run an induction motor at 3000 RPM and use a 10:1 step down gearbox. With standard motors and VFD's, a 300 VDC bus will be required.