electric motor power/weight ratio 2

[stouffeg]

Is there any formula or "rule-of-thumb" that can provide me with a quick estimate of the approximate size and weight of an electric motor if I know the size and weight of an existing one? For example I have a 10kw motor that weighs X kilos and is a given dia and length, can I determine the approximate size of a 50kw motor?

Thanks for any help you can give.

Gerry

 

[rlpuck]

NO, you also have to condiser the speed as lower rpm motors require more diameter, also must consider construction material etc.

 

[electricpete]

I have a book by "Electric Machinery" by Liwschitz-Garak dated 1946.

He has some analysis which shows that P/(V*N) is roughly constant where P is horsepower, V is volume, N is rpm.

From analysis of specific tangential force on the rotor, it can be shown that this relationship is roughly true when the air-gap field is constant and the amp-turns-per-inch of rotor is constant.

Needless to say, the further you extrapolate the bigger the error. Also of course even motors with identical ratings may have different power to weight ratio's due to difference in features (for example ventilation) and other factors.

 

[jbartos]

Suggestion: Reference:
S. Gordon Monk "Induction Motors," Blackie & Son Limited, 1939.
Co=Constant=HP/(N x LD^2)=Bav x q x EFF x PF / (4.1 x 10^11)
where
HP ..Horsepower
N ...Speed in RPM
L ...Length in cm or inches
D ...Diameter of air gap or rotor in cm or inches
Bav .Total pole flux F divided by the total area of the gap per pole. It can vary from 3000 to 5500 lines per sq cm or 19000 to 35000 lines per sq in
EFF .Efficiency
PF ..Power Factor
q ...The specific ampere-conductors per sq cm of periphery

 

[stouffeg]

Gentlemen,

Thank you all for your responses. The information was helpful.

Gerry

 

[ozmosis]

stouffeg. (The following is intended purely as a thought provoke rather than a black&white suggestion)

It depends on what you are going to use to 'control' the motor. Let me explain this general statement.
The standard induction motor has been around for over 100years and essentially has not changed in design too much. One of the reasons is that (standard)motors are designed to be able to withstand the energy required to start Direct on line (DOL) and therefore the mass of the motor is relatively high, the inertia of the rotor is high and therefore the acceleration/deceleration time is that much longer. The advent of motor/motion control technology should in theory change that. If all motors were designed to run on variable speed drives, for example, then the physical size of a motor would be considerably smaller due to the improved control of the current during start up. (typically a DOL motor demands 5 to 6 times the current to start due to the immediate appliance of full volts, whereas a variable speed will pull only 1 to 1.5 times). You can see this in the development of servo motor technology. In this business the acceleration times are critical, as can be seen with the following comparison of a 'standard' induction motor, a standard induction motor with force ventilation and a dedicated servo motor. EG: a motor rated 8NM at 2800rpm.
Std motor= Mass 29kg; Inertia 55 x 10(-4)kgm(sq); accel time would be 0.2secs
Std motor+force vent fan=Mass 22kg; Inertia 38 x 10(-4)kgm(sq); accel time would be 0.14 secs
Servo Motor=Mass 10.1kg; Inertia 3.4 x 10(-4)kgm(sq); accel time would be 0.013 secs
So, to effectively 'do the same job' the motor dimensions change considerably depending on a) what is going to control them and then b) based on this information you can design the motor accordingly.
Now this is a special case in that servo drive systems need to have the provision to accelerate quickly and therefore are designed this way. However, this technology is moving into the realms of 'standard' induction motors that are only suitable for use on VSD's. Companies such as ABB, Siemens and Baldor (to name a few) have such technology now to provide a motor with quite impressive power to mass ratios. I personally have used this method when trying to offer an AC variable speed drive solution as a replacement to a DC solution on the main drive for a plastic injection machine. The DC motor (225kW)had a power to mass considerably smaller than a 'standard' induction motor. Offering a motor that is only designed for VSD operation realised a motor that was both very compact, low rotor inertia and, as it was AC, virtually maintenance free compared to the DC motor.
Maybe not what you were looking for but I trust of use.

 

[EdDanzer]

One thing to consider also is starting torque, as well as HP. Some servo drives end up quite large because of starting torque requirements.