How Can You Calculate Near-Peak Power Consumption for a DC Motor at High RPMs?

[robs314]

I'm a mechanical engineer selecting a DC motor. I have the values for rated continuous operation, but I would like to have a ball-park figure for "almost" peak power consumed (which can only be sustained for a few seconds).

I realize that absolute Peak Torque is when the motor is stalled. For my purpose, that is useless, and I would like to have a vague estimate for what the power consumed would be at, say, a few hundred rpm, at almost peak torque

I have a peak current of 400A, a Peak Power of 25.38 kW, a DC armature resistance of 17.5mOhms and a Speed Constant of 50rpm/V

Are there any ways of working this out simply? Or any general rule-of thumbs?

Many thanks for reading this

 

[edgepflow]

I would consider a few possibilities:

(1) Motor inrush current when starting motor. Ask manufacturer for curve.
(2) Locked rotor current at nameplate voltage.
(2) Energy required to get motor spinning at rated rpm in certain time.

Some of these formulas may help:

http://www.reliance.com/mtr/flaclcmn.htm

 

[I_am_learning]

Torque varies almost linerarly with speed
T = T(max)(1 - N / (Nmax))

Power consumption also varies roughly linearly with speed
P = P(max) (1 - N/(Nmax) )

Nmax is the maximum possible RPM, that is no load RPM.

 

[robs314]

thankyou both very much for your replies

@edgepflow, those options would give me a good estimate, but I have no more available data. That reliance website is good, I've come across it before.

@thecritic, after looking at some other manufacturers' performance graphs, I can see the linear relationship now. I think those equations should help me have a ball-park figure.

Many thanks :)

 

[I_am_learning]

And to Add one more relation,

Pout = (Nmax-N)*N

So maximum output power (i.e. Shaft power, not torque) occurs at roughly Nmax/2.

This equation can however be derived from above two equations by remembering that
Pout = Troque*Speed.
When the rotor has no Load, then Although speed = Nmax, torque = 0. So effectively Pout = 0.
When the rotor is fully jamed, Although Troque = Tmax, Speed = 0. So again effectively Pout = 0.
So, its not hard to realize the maximum output power occurs near the mid-point as the relations are linear.

 

[skeptic2]

I disagree with thecritic in posts #3 and #5. Torque varies with the current through the motor and the speed varies with the voltage across the motor minus the IR drop of the motor. The IR drop refers to the voltage developed by the current through the internal resistance of the motor. So the speed of the motor will be proportional to the voltage across the motor minus the current through the motor times the internal resistance. The power of the motor will be close to the effective voltage across the motor times the current through it. I say close because there still are some unaccounted for losses such as friction and eddy currents.

With most DC motor controllers, the voltage to the motor is increased as current is increased in order to compensate for the IR drop and maintain a constant effective voltage across the motor. The effect is to maintain a constant speed under varying loads. They also have a current limiter so that the maximum current spec. for the motor is not exceeded. This means that yes, you can have maximum power at maximum speed.

 

[I_am_learning]

I disagree with thecritic in posts #3 and #5.

But I don't 'totally' disagree with you. What you said are 'almost' equivalent to what I already told.

Torque varies with the current through the motor .

. Yes, Troque varies with the current through the motor. But how does current varies with speed? It varies Linearly with speed just like the Torque. So, inefect we get what I posted in #3.

and the speed varies with the voltage across the motor minus the IR drop of the motor. The IR drop refers to the voltage developed by the current through the internal resistance of the motor. So the speed of the motor will be proportional to the voltage across the motor minus the current through the motor times the internal resistance. The power of the motor will be close to the effective voltage across the motor times the current through it .

So, true. Input Power of the Motor = Voltage * Current. But, I simply removed the current and replaced it with its relation with speed in Post #3. Also Removed the voltage and modeled it with the variable Pmax.

In short, the difference is that, you are trying to express outputs (Torque, Power and Speeds) in terms of electrical mesarables such as Current and Voltage, However I was trying to express Torqe, Power input, and Power Output in terms of the Speed of the motor.

Which method is better depends on what is your control parameter, speed or Current/Voltage.

With most DC motor controllers, the voltage to the motor is increased as current is increased in order to compensate for the IR drop and maintain a constant effective voltage across the motor. The effect is to maintain a [BOLD]constant speed under varying loads.[BOLD] They also have a current limiter so that the maximum current spec. for the motor is not exceeded. [BOLD]This means that yes, you can have maximum power at maximum speed.[/Bold]

If the controller maintains constand speed, what do you call the maximum speed?

By using smart uP controllers we can even have it set such that, maximum power is obtained at only say x rpm, where x is any desired value you choose. The point I am trying to make is by using smart controllers which varry voltage, you can get all sorts of relations.
I was talking about fixed voltage DC motor without any controller. Sorry for not mentioning that earlier.

 

[skeptic2]

robs314, does this motor have a shunt wound or series wound field?

 

[I_am_learning]

Again I was assuming Shunt wound field. Sorry for asuming too many things.

 

[kubing]

400A..what a big project for?

 

[david90]

I'm a mechanical engineer selecting a DC motor. I have the values for rated continuous operation, but I would like to have a ball-park figure for "almost" peak power consumed (which can only be sustained for a few seconds).

I realize that absolute Peak Torque is when the motor is stalled. For my purpose, that is useless, and I would like to have a vague estimate for what the power consumed would be at, say, a few hundred rpm, at almost peak torque

I have a peak current of 400A, a Peak Power of 25.38 kW, a DC armature resistance of 17.5mOhms and a Speed Constant of 50rpm/V

Are there any ways of working this out simply? Or any general rule-of thumbs?

Many thanks for reading this

Basically you just want to know how much overloading your motor can handle?
If so, look into the "service factor" of your motor. Take a look here:
http://www.engineeringtoolbox.com/service-factor-d_735.html

btw, how did you determine that your machine requires a 25.38 kW motor? For example, if you designed an extruder machine, how do you determine the amount of torque required in order to turn the screw?