Induction Motors

One possibility is that as the power supply systems that supply our power have greatly improved in capacity over the years the ability to support higher starting loads with much less overall voltage drops became possible.

That greater capacity plus more advanced local voltage control and regulation systems also help control localized voltage dips which further compensate for the much higher loads that direct starting larger motors creates.

<...why induction motor takes high starting current...>

Because the motor needs a good kicking to get its rotor up to speed. That kicking comes from the starting current.

An induction motor takes high starting current - because at the time of starting there is no back EMF. That's what the text books say!

Now, coming to the maximum size of motor that can be started Direct-on-Line, it is not 30kW as you have said, but about 5.5kW to 7.5kW only. Please note that this restriction applies only to those customers who receive a low volatge power supply from the grid. Because, if they start a higher capacity motor direct-on-line, the higher starting current would cause a higher voltage drop in the utility lines which would affect other customers connected to the same (LV) line. This, then, becomes a social issue. So, the authorities have fixed up a ceiling on the rating of motor that can be started direct-on-line.

Whereas, if you receive an MV or HV Power Supply, you will normally have a transformer, which would absorb the voltage dip. Also, in HV terms, the percentage dip may be so negligible as to affect your neighbouring (HV) customer. May be, some other smaller motors connected in the same MCC in your own plant might trip, but, now, that is YOUR problem and not a social problem. Now, you will decide upto what capacity of motor you can go for Direct-On-Line starting, duly considerting the effect of doing so on your other connected loads. That's why even 250kW motors can be started direct-on-line.

Hope it is clear.

Lets just break the induction motor down into its components.

All electric motors are also generators, this is what creates the back EMF. Induction motors are also transformers inducing the current in the rotor.

At standstill the rotor is a short circuit and the inrush current is largely determined by the turns ratio. The turns in the primary of at say 11kv is 26.5 times that of a 415v motor of the same power. A 250kw motor only has the same inrush as 9.5kw motor.

There are several more parameters to look at. Low voltage motors in the low KW range typically start under load. High powered motors that are started DOL usually have load applied once up to speed. Conveyor drives for instance would use a scoop coupling or a Controlled Start Transmission to name only two methods to accelerate the belt once the motor is at full rated speed and stabilised.

Now the final parameter, in industrial situations low voltage motors greater than 250kw are frequently dropped DOL because the stiffness of the supply allows this to happen especially if a coupling of some sort is used. On HV installations DOL starting of 4000kw motors with only inertial load is not unheard of, not exactly preferred but definitely do-able.

I can name lots of specific instances, but if you combine my technical industrial response with earlier posts about the social consequences of inrush in domestic situations, you will build an understanding. Primarily domestic and industrial are not similar, the supply to mine sites where I work is frequently 132kv.

Well, not all electric motors, unless you provide either a permanent magnet field, or a wound field + excitation current, or a magnetic rotor like in the steppers or AC brushless servos.

And in the case of the squirrel cage type, I've done the tests and even if you rotate the motor to rated speed you get depicable useless voltage at the windings, that won't even brake the motor if shorted out (thats why they sell DC injection brakes).

Its a whole different story when you supply current to two of the windings of a 3Ph squirrel cage motor; you could even make a rotating type 3rd. phase generator to run 3 Ph. loads when only single phase power is available.

Sorry, I was referring to your statement that all electric motors are also generators.

I work in AC traction and believe me all motors are also generators. That is how the back EMF is created causing the current to fall as target speed is approached.

When the speed of the rotor in an induction motor exceeds supply frequency the motor regenerates into the supply. This how retard is achieved in AC traction systems.

I definitely agree with you on this. Regenerative braking after all was used on subways, electric trains, elevators(lifts) and now is back in practice on hybrids(everything old is new again).

The "induction generator" is nothing more than a motor where a motor is driven beyond its' synchronous speed and starts generating current. It basically is speed controlled by the system that is receiving the current and will float at that speed as long as the mechanical power input in maintained within the necessary limit. Otherwise, it can self destruct through over current if driven with an excess of input power.

Regenerative braking is probably also used on long conveyor systems when there is a long downhill stretch.

And right on in that the induced EMF is why the motor starting current reduces down to load current or nameplate amps as the motor comes up to speed.

Spinco,

Right on on all fronts. The anti parallel diodes fitted to the half bridges in the inverter stage are there to return generated current to the DC link. In a normal industrial application this allows predictable ramp down of the motor speed and in traction applications (locos, trucks, winders, draglines (those with AC drives), long travel drives etc etc) the regenerated energy is used to retard the motion at a rate of at least 3 times the propulsion power.

Look up "Motor Starting Auto Transformers" for an explanation of reduced voltage motor starting and reducing starting current for direct on line motor starting.

Valid topics in their own right.

Auto trannies are not used real often any more with the cost of softstarters and VSD having come down so much.

I remember one system I worked on several years ago that was on the end of an 11kv line, it utilised twin wound motors and three stages of autotranny for the high speed winding. Several assumed it was for voltage drop purposes (30kw hanging off 11kv, yeah right). But the truth was, it was a pre VSD method of controlling water hammer in the 14 km long Asbestos cement water line. Stage timing had to be just right to prevent reflected peaks meeting pump peaks and blowing the line. The installation was new but a complete dog's breakfast, and I had to rewire the bugger to get reliable operation.

At a later date someone fiddled with the timers, and guess what.