Motor power difference 120v Vs 240

[Jpflex]

I was wandering why when I wired 3 rock crusher motors rated at 120/240v at first 120 volts (parallel) the rocks became stuck in machine and seized it due to lack of power torque.

However, when I wired them at 240 volts the torque power appeared to increase enough to fully crush the rocks and stop motor lock up.

Since using a higher voltage yields lower current, while wiring to lower voltage yields to higher motor current draw but both yield the same/ similar power or watts, then how can it make a difference in power output? With WATTS being the same in either situation? Thanks

 

[Jpflex]

Also what can you do in other situations when motor nameplates are not readable, how would you know as a general rule on which leads to apply line or field?

 

[LarryFine]

Half the current = half the voltage drop. The lower-voltage connection requires much larger wire.

I'll bet the voltage at the motor terminals dropped by a higher percentage on 120v than on 240v.

 

[wwhitney]

Half the current = half the voltage drop. The lower-voltage connection requires much larger wire.

So does that apply only to the external circuit (assuming fixed wire size)? Because the internal motor wiring is either series or parallel, so the current is the same in either configuration?

Cheers, Wayne

 

[texie]

Half the current = half the voltage drop. The lower-voltage connection requires much larger wire.

I'll bet the voltage at the motor terminals dropped by a higher percentage on 120v than on 240v.

That is what I'm thinking as well. Especially given the application where it is probably loaded past the SF value at times.

 

[winnie]

As you note, if the motor is properly supplied with 120 or 240V matching the motor connection, then it should operate the same. But you note the reality of different operation.

There are two possibilities.

1) The voltage drop in the supply circuit is making the difference. The motor draws more current in the 120V configuration so I would expect the 120V supply to have more voltage drop. So when the motor was connected 120V it was getting a lower % of rated supply voltage when under load. This will drop the available torque, by the square of the actual available voltage.

2) I've noticed with some small single phase motor that the 'start' winding is always configured for 240V, even when the run winding is wired for 120V. This means that the motor characteristics are actually different for 120V vs 240V.

-Jon