motor load versus resistive load

[tim89s]

Let's say you have a 120 volt 60 watt light bulb and apply 60 volts to it. The bulb will glow dim due to ohms law V=A*R, right? Take the same light bulb and put 240 volts to it. The light bulb will go poof. Again due to Ohm's law.
Now lets say you take a 120 volt motor, lets say a skil saw, and apply 60 volts to it. Eventually the motor will burn up. But it is not due to Ohm's law, is it? My guess is the saw is still trying to generate the same wattage, therefore, when volts go down the amperage will go up. But, not lets put 240 volt across the same motor. The motor again will burn up. If the motor is trying to generate the same wattage and the voltage goes up shouldn't the amperage go down causing less of a draw? I realize I am not considering impedance into the situation. If someone could explain this I would greatly appreciate it
Thank you
Tim

 

[charlie]

Re: motor load versus resistive load

Tim, the light bulb will dim due to Ohm's law but consider the resistance of the filament will also change. Therefore, applying 60 volts will not cut the amperes exactly in half. Doubling the voltage will just about double the wattage and burn up the filament.

The motor is transformer where the secondary (rotor) is mounted on bearings so it is permitted to rotate. A squirrel cage motor has the secondary winding (rotor) shorted out. OK, apply less voltage to an AC motor and the motor will try to stay up with the load because it is driven by the applied frequency. As the voltage goes down, the ampere draw goes up as the motor tries to maintain the same RPM. The ampere draw will increase enough at lower voltages to burn up the motor if it is not protected properly. Applying a higher voltage to the motor will reduce the ampere draw by the motor until the turn to turn voltage is high enough to break down the winding insulation and the motor burns up. The common 208/230 rated motors are wound with wire large enough to handle the ampere draw at 208 and have heavy enough insulation for 240 volt applications.

 

[dereckbc]

Re: motor load versus resistive load

Originally posted by charlie:
Doubling the voltage will just about double the wattage and burn up the filament.

How about 4 X

 

[charlie]

Re: motor load versus resistive load

OOOOOOOOOOOOOOPS

 

[tim89s]

Re: motor load versus resistive load

Thank you for the info. I am not clear why the winding insulation will breakdown with higher voltage and less amperage. I thought the heat was generated in the amps.

 

[hurk27]

Re: motor load versus resistive load

The break down is not from the heat it is from what is called the flashover point it is the point when an insulator can no longer prevent electrons from flowing to the opposite poll. like when the insulator gets dirty on a high voltage powerline and cause's a arc to form accross it.
By the way a 120 volt bulb on 60 volts will use about 15 watts of energy. And last for a long time.

 

[hurk27]

Re: motor load versus resistive load

Also a motor with brush's (drill,saws etc...) wont have this problem as they can be ran eather AC or DC and voltage change will only change the speed of the motor. until you again go over the breakdown point of the insulation in the windings or overheat the windings as with these motors they will try to produce more HP at higher voltages and this will cause more heat to have to be disapated in the windings.

voltage/hpx746=amps=heat(energy)+ motion(rpm's)

 

[dereckbc]

Re: motor load versus resistive load

Originally posted by tim89s:
I am not clear why the winding insulation will breakdown with higher voltage and less amperage. I thought the heat was generated in the amps.

Heat is the product of E*I, I^2R, or E^2/R not amps alone. Insulation can break down from either heat or pressure. In this case pressure is voltage.

 

[pasha]

Re: motor load versus resistive load

Failure of insulation is due to the failure of dielectric strength of the material being used for insulating the winding of the motor,Even for the same rated output power of the motor the insulation of the winding of the motor has to be increased if voltage increases. The thumb rule says that with increase in current the conductor size has to be increased while increase in voltage the insulation has to be increased. When higher than the rated voltage is applied to the motor the dielectric strength of insulation is no more able to sustain the voltage and ruptures the insulation causing a flash over in the winding Ultimately leading to shortcircuiting the windingof the motor.