Current draw of a motor, under different situations

[jim hardy]

Simplified equivalent circuit is armature resistance in series with ideal voltage source Ec, counter-emf.

If external voltage < Ec current flows out and it's a generator, if > Ec current flows in and it's a motor.

If Ec = external voltage no current flows, hence no torque.

Any help, guys?

 

[FOIWATER]

yes, ha

much easier way of thinking logically about it, Jim.

 

[saskcat]

hi again. in regard to overloading:
this is known as the breakaway torque. the ac squirrel cage induction motor has markings on it that represent the characteristics of the unit. let's say 100hp 440v 60hz 1745rpm 1.15sf - just for argument sake. i don't know the actual current draw at full load - but not important for this. let's assume it is rated for 125amps. at rated load -torque- the motor will draw 125amps. it will be running at 1745 rpm. with less load, it spin run faster. there r 4 poles developed in that unit and at 60hz the magnetic field rotates around the stator at ~1800rpm.
the rotor can not reach that speed as it needs to cut the magnetic field in order to induce an voltage -field- about itself. now I am not sure whether it is a push pull effect but the rotor chases the field around in a circle. due to the strength of the field built under no load, less field less current. as load increases, rotor slows down, cutting more field, increasing in magnitude and producing more torque. this happens until motor design. full load, full torque, full amps, rated voltage and power factor. after that-if we apply more load, the motor will draw more than its designed rating - it will continue to draw more current - building more field being cut by more flux until it gets saturated and breaks away from the field and stalls. at that point up to 6x rated current is drawn and of coarse-melt down.
there are numbers to represent each motor as each are different by manufacturer, design, size, ect.
im just a sparky but that is the nuts n bolts of it. cheers

 

[Windadct]

The Starting current is high and a High Efficiency motor - is worse, up to 10 X. IN addition to soft starting - a drive will typically pay for itself quickly - since each application does not need the exact nameplate rating of the motor ( for example a 4 hp application needs a 5 HP motor - because 4 HP motors are not common). In this case the motor still draws nearly full current, with a poor power factor - by using a drive it regulates the voltage to make the motor operate at the 4 HP point, better PF, easier starting etc..

Starting a larger 3 PH PM motor - is pretty much impossible without a drive - I was just working with a customer on this : 61KW, 360V, I max 250 A - BUT the Rk(windings) is 0.036 Ohms!
Typical drives would start a PM motor at 50% duty cycle ( like a DC motor) - on this I think we will need 5 -10% duty, at least until we see some back EMF, and then ramp the Duty Cycle up until we see a 1 or 2 rpm then transition the drive the 2 PH AC (Sine) mode. Add to this that in the application the start could be under no load or locked rotor ( un-startable).