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# Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/08/2014 6:06 AM

Hi;

Normally the rating of all kind of AC and DC motors quote in kilowatts but with Servo motor, its rating doesn't quote, just torque expressed in Newton meter. What is the technical reason behind it?

For example, how do I determine the kilowatts of servo motor which name plate image is given below?

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#1

### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/08/2014 6:16 AM

Power can be calculated from the torque and angular velocity (both of which are stated on the rating plate).

http://www.sensors.co.uk/torqsense/torque-calculator/

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#6
In reply to #1

### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/08/2014 2:27 PM

Trouble with that is torque x angular velocity = 18kW, but amps x volts as suggested by Rixter in #4 = ~ 9.5kW (assuming it's DC). So it clearly can't deliver Mo and Nn simultaneously.

Signode -is it just a motor, or a motor/gearbox? If the latter, at what point are the speed and torque specified? What exactly is meant by Veff and Aeff?

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#8
In reply to #6

### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/08/2014 5:45 PM

That occured to me too. Wonder if M0 is starting (or stall) torque. Maybe the motor manufacturer can help out, here.

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#2

### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/08/2014 7:05 AM

Because a motor is an electro-mechanical device. This company chose to rate their motor in mechanical units instead of electrical (energy) units.

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#3

### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/08/2014 7:13 AM

the second part of your question is already answered by John.

Speaking of rating in Nm.

IMO,

DC motor is just a single equipment, whereas servo motor is combination of DC motor, gear unit (speed variation), and sensors.

DC motors are usually preferred in higher speed applications, where torque need not be mentioned.so, only electrical input is specified.

since servo motor come with gear reduction unit and are used for heavier jobs, it is specified by torque rating.

please neglect, if you find it irrelevant.

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#4

### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/08/2014 9:21 AM

Just guessing, but wouldn't Un x In (350 Volts x 27 Amps) give you Watts?

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#5

### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/08/2014 10:58 AM

They spend most of their life acc/decelerating and positioning under precise control, not top speeding, so torque ratings (and other factors like current and inertia) are more important for their characterization, but who says they're not power rated? S.M.

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#7

### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/08/2014 4:24 PM

The reason that servo motors are rated in units of torque rather than power is because the designers of machines that use servos are much more interested in torque as this is a measure of the ability of the motor to move a given load in a required time.

The actual power consumed in operation is a secondary consideration, especially to the mechanical designers. They work out how much torque/force they need and leave it to the electrical designers to supply the power.

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#9
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### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/08/2014 10:23 PM

Exactly.

In most cases, the purpose of a servo motor is not even about power, although that may be a secondary consideration.

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#10
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### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/08/2014 10:23 PM

My point much more eloquently put. GA

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#12
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### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/12/2014 1:33 PM

I don't think this answer is correct.

Torque is like a force just in rotary motion. To move a given load in a required time needs power (e.g. in kW).

You can find power by a given torque and rpm by this calculation:

P (kW) = T (Nm) * n (rpm) / 9550, here: P (kW) = 115 * 1500 / 9550 = 18 kW

PS: it is also incorrect to say this: Because a motor is an electro-mechanical device. This company chose to rate their motor in mechanical units instead of electrical (energy) units.

You cannot say that Nm is mechanical and its electrical unit is kW. Both terms, kW and Nm, apply equally to mechanical or electrical devices.

Nm is a force, kW is power, P = W / t, where work W is Force F * distance d (in m working in metric).

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#13
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### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/12/2014 3:21 PM

Not quite.

Nm is not force. It's either work (= energy) or torque, depending whether, in vector terms, it's the dot-product or cross-product of force and distance.

Nm and kW are not different units for the same quantity. Nm is energy (when it's not torque ), kW is power (energy/time).

For the original post, I'm still curious about the discrepancy between power from torque*rpm and from volts*amps. I think OP should ask the supplier to explain, and define Veff and Aeff.

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#14
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### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/12/2014 5:04 PM

In this case of rotary motion Nm is Torque and is a force. It becomes work (energy) with distance meaning with rotation and it becomes power with rotation per time = rpm. In torque Nm has no distance, it is a force only. Only in linear motion Nm is work. Now, do I earn another Off topic for saying it right?

Did you give me the Off Topic negative vote? Why would my contribution be off topic escapes me. There are too many trigger happy people who vote not by substance but by liking an answer or not.

I agree with this sentence but I did not say that, I was criticizing it too. Nm and kW are not different units for the same quantity. Nm is energy (when it's not torque), kW is power (energy/time).

I also agree with your last statement.

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#15
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### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/12/2014 5:48 PM

I didn't off-topic you, not guilty!

You could say torque is a generalised force, with rotation the corresponding generalised displacement, but that doesn't mean torque is a force as normally defined. You say In torque Nm has no distance, it is a force only. Only in linear motion Nm is work. But the m in Nm is distance, the component at right angles to the force. Torque = force*distance*sin (angle between), work = force*distance*cos (angle).

I agree with this sentence but I did not say that, I was criticizing it too. Yes, OK, I misread your post, I thought you were saying that, apologies!

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#18
In reply to #15

### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/12/2014 6:53 PM

Thanks Codemaster, it must have been someone else. Accepted, also the apologies.

The m in Nm is distance alright but not distance traveled. That is the difference.

Nm * rotation (distance) is work, Nm on its own in torque is just a force. Although Nm in linear displacement and in Torque have the same definition, they are not the same.

You could replace the force on a lever with a weight and it would be the same as long as there is no movement. Once movement happens it is work without movement its just a force. It is a fine difference and I hope you see my point.

That is why for a car acceleration torque alone is meaningless unless coupled with rotation. One we have rpm (rotation per time) we have power which is needed to accelerate. Torque on its own is no measure of acceleration, meaning one cannot calculate acceleration having a torque value alone.

Trust you agree.

Greetings

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#19
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### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/12/2014 6:57 PM

That is still confused.

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#20
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### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/13/2014 5:55 AM

Hello Floram

Yes, I think we've pretty much covered it. You expressed differently what I was trying to say with dot-products and cross-products, and sines and cosines.

Cheers

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#17
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### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/12/2014 6:51 PM

You earn another OT from me for saying it wrong again; if anything, even more so than before.

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#16
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### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/12/2014 5:58 PM

... and Mo

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#21
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### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/13/2014 6:04 AM

That's a good point, looking at it again it's not clear what most of the suffixes on the nameplate actually mean, and how the figures relate. I think we're all agreed it can't do the torque and rpm simultaneously, so some explanation needed. We're not getting much feedback from the OP!

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#11

### Re: Why Does The Servo Motor Quotes With Newton Meter Instead Of Kilowatt?

05/12/2014 7:21 AM

This servo motor is for a rotary cut-off unit of a Corrugator machine which comes without gear box.

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