# What determines the maximum weight an electric motor can lift?

1. Aug 7, 2017

### Tabaristiio

What determines the maximum amount of weight an electric motor can lift / move for its size? So assuming the electric motor weighs a specific amount, what would be the limiting factor to how much weight it can move / lift for the weight it weighs?

2. Aug 7, 2017

### BvU

Hi,
Bit of a funny question: in a lifting device there is usually a gearbox that introduces power loss.
If you want to lift something heavy with a small motor, you need a big gear ratio and the loss will be considerable.
Power of the motor minus power loss in the gearbox is what power $P$ you have left over to do the lifting.
(and there may be some friction in pulleys, etc).

The power $P$ needed to lift a mass $m$ with a speed $v$ is $P = mgv$. The smaller $P$, the smaller $v$.
In practice, you don't want to wait a few days to lift a refrigerator to the fifth floor.

3. Aug 7, 2017

Ideally a "motor" can lift any amount of weight

Maximum weight lifted "x" distance = an amount of ENERGY.

Motors are rated by POWER = Energy x Time. So with out talking about how fast you want to lift it, we can not size a motor. They are rated by KW or HP.

To compare size to power - is power to weight ratio.

4. Aug 7, 2017

### 256bits

I kind of agree.

Funny answer = the weight of the thing to which the motor is attached. no more than that.
If the motor is not attached to anything, it would be able to lift something only a little bit less than its own weight with, for example, a rope slung over a pulley, as long as the motor had the torque to do so and did not stall. Add a gearbox to increase torque, and then the weight that can be lifted would be a little less than the motor and gearbox weight. Need more weight lifted, increase torque and bolt down to a heavier base, such as a bed ( tow truck - could be an electrical motor driving a hydraulic system ) or even the earth. In that case the limiting factor besides the torque, would be the shear or tensile strength of the connection of the motor/gearbox to the base.

5. Aug 7, 2017

### Tabaristiio

Maybe I should've rephrased the question instead. What determines the maximum amount of power an electric motor can have, given its size?

6. Aug 7, 2017

### BvU

Much more interesting ! You did some googling already, I suppose , and found at least this table and maybe also this ?
Deep inside something like the Lorentz force is the originator, so conductivity of the coil material, permeability and saturation of the core material etc. will come in.
You need to keep the thing together, which requires a sturdy structure. And you need to carry off the heat generated internally, with a fan or something.

7. Aug 7, 2017

### RonL

The mechanics of an electric motor revolve around three things, copper mass, iron mass and heat discharge.
The arrangement of copper determines current draw and speed, iron determines the amount of magnetic field strength and to some extent how much heat can be absorbed and dissipated.
This applies to most motors in general, however there are ways to work around these things in very special cases.
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