Why do amps decrease on turning motor

[Drew]

RGood day all

I understand that the amps on a motor change from start up until running.
they change from a resistive to a inductive load. Also mentioned is back emf?

While reading my electrical textbook ive noticed that back emf lowers the voltage within the motor which decreases the amps, but in another section the book mentions that due to the higher impedanace the amps drop? are both true? therefore a motors amps drop due to back emf and higher inductive impedance ?

the simple formulas weve been given to use do not use both values at once:

back emf : I = V {supply} - V {back emf}/R {winding}
no mention of impedance as the resistance used above is the motors stationary resistance {not impedance}

inductive impedance : I = V{supply}/Z{impedance}
no mention of back emf?

Does the lowering of the voltage or the raising of the impedance cause the motor to draw less amps?
So, therefore my question is does back emf or high impedance cause less current to be drawn once a motor starts?

[Drew]

i reasise i should try find an electrical forum to post this on but i am a friggie and electrician so i thought i would start here? any suggestions on an electrcal forum?

[Drew]

Asking the same question in a different way :

if if I locked the shaft of a motor what causes the amps to go up?

1. Is it the lack of back emf.

2. Or is it the changed inductive impedance of the motor increasing?

[seanf]

Might find answers in Electrical Machines, Drives, and Power Systems. T Wildi
https://ia600901.us.archive.org/9/items/ElectricalMachinesDrivesAndPowerSystems5ETheodoreWildi/Electrical%20Machines,%20Drives,%20and%20Power%20Systems%205E%20(Theodore%20Wildi)_text.pdf

Or in Electromagnetism for Engineers. P Hammond. But thats alot more maths.

[skarrs71]

Not sure I can be too much help, but higher start currents are due to the torque required to get the motor running if I understand it right, but I'm no electrical engineer! Once the motor is up and running the torque is reduced and therefore the current drops? I believe this is the main reason for using star/delta start on larger motors to reduce the load while motor gets up to speed.

[panqun]

ok, the back emf is relate with rotor speed. the speed is more high ,the back emf is more large.At the motor start, the speed is zero, so the back emf is zero.but is this time, motor start moving, it needs a high torque. so the amps is big. with the rotor start spining, the back emf is become high. but we needs the torque is less, so the amps is lower.

[Frikkie]

Just the resistance of the windings in ohms is low because it's only pure copper wire and the bigger the motor the thicker are the wires which means lower resistance. The resistance is so low that if it was only the hinderance to oppose the current flow then the current would be very high (see ohms law) and the motor would blow the supply fuse or circuit breaker.

The running motor is inductive which causes reactance in the windings which also hinders the current flow. When you add the reactance to the resistance you get the impedance which is the overall opposition to the current flow. The impedance constantly changes depending on rotational speed and load and various other factors but is always much higher than than just the resistance of the windings and only allows a smaller current to flow which is known as the running current.

When the rotational speed is very low or zero the reactance becomes far less and the value of the overall impedance becomes closer to the value of the resistance of the windings. This reduction in overall impedance allows a higher than desirable current to flow known as locked rotor current (LRA). If this locked rotor current is allowed to continue flowing for more than a few seconds if causes overheating and damage.