Control.com - Nerds in Control

I have a 20kW induction motor running as a generator. The prime mover would be supplying approximately 5kW of power with the generator off line at runaway speed (~1.8 x rated). If I then put the generator on line, would the the torque and current be similar to DOL as a motor, with torque in opposite direction?

Hydro...

1) If operated off-line as an induction-generator, what is the size of the capacitor connected to the motor's terminals?

2) DO NOT connect the motor to the line while running it at such a high rate of speed!

Regards, Phil Corso

When off-line, it would not be generating, there would be no load or capacitor connected.

I am looking for a way to bring the unit online without mechanically throttling the speed. Maybe a regenerative VFD? however I was hoping to keep it simpler than that.

Hydro... what does "5 kW, off-line at runaway speed (~ 1.8 x rated)" represent?

Phil

Hydro... further to my earlier post:

Runaway speed can be far greater than your IG's nominal speed rating. And, it is expected when there is a sudden loss of the connection between the IG and its network!

However, sudden connection of the IG to its network should never be done if the IG is rotating much faster than it would at nominal output.

For example, let's say you have a 2-pole motor, rated 2,850 rpm when connected to a 50 Hz supply. Its rated slip is then 100x[(3,000-2,850)/3,000} or 5%. Then its speed as an IG should be around 5% above 3,000 or 3,150rpm!

However, if its speed when connected to the network is much greater than 5%, it will act as a brake... resulting in a snapped shaft or much worse... torn off its foundation!

Phil

Thanks Phil,

I understand that there is going to be high torque, but as it is quite usual to start a 5kW motor from standstill (DOL). I am trying to understand if this breaking torque is any worse than DOL torque.

Hydro... consider the induction motor used in the example above! Its rated sub-synchronous speed as a motor is 2,850 rpm, while as a generator its hyper-synchronous speed is 3,150 rpm.

Its speed-torque curve operated as a motor, suitable for DOL-Starting, is similar to its torque-speed-curve as an induction-generator! For motor-action 'slip' is positive while for generator-action, 'slip' is negative.

The motor's rotor experiences a reverse torque as its speed transitions from sub-synchronous to hyper-synchronous-speed. Rated electrical power-output is reached when speed is about 5% above line-frequency. As speed increases beyond that, electrical power-output continues to increase until pull-out torque is reached! Then power-output will decrease! So, you are correct that the stress is similar to that experienced for DOL-starting!

Now consider the situation you described... what happens if an non-excited machine is suddenly connected to the network at a runaway speed, that is, say double rated speed? The above paragraph stated that the torque-speed curves are 'similar', but there is a notable difference.

Maximum torque varies with 'slip'! And slip is positive for sub-synchronous, but negative for hyper-synchronous speeds. Thus, pull-out torque magnitude is greater for hyper-synchronous speed. This characteristic 'could' severely damage the machine!

Please note, I said 'could' because whether or not the machine will experience damage is very, very, dependent on its electrical parameters! In other words, it is machine-specific

Regards, Phil

Hydro... the influence a motor's parameters has on its generator-action torque can be seen in the following thread:

http://www.control.com/thread/1026155353

The thread covers derivation of a motor's maximum torque, Tm, when operating sub-synchronously!

Note the denominator for Tmax. The last term, R1, is positive for sub-synchronous operation. For hyper-synchronous operation that term becomes negative, resulting in a higher (than design) Tmax!

Now you can see why Tmax is motor-specific.

Phil

Thanks Phil,

Most speed-torque curves I have seen show the curve mirrored/inverted about the synchronous speed/0 torque axes.

Are you saying that that is not totally correct, that the R1 term makes the magnitude on the generator side of the curve slightly larger? And since R1 is machine specific, the difference in magnitude is also machine specific?

Hydro... in a nutshell, yes!

I want to clarify my statement that "... stress is similar to that experienced for DOL-Starting!"

At starting a motor produces a starting-torque (also called locked-rotor or breakaway) which is about 150% of its full-load or rated-torque. As motor speed increases the torque reaches a maximum value of 200-250%, called its pullout or breakdown-torque. Then, torque decreases until it reaches full-load torque at design slip.

You have noted that for hyper-synchronous operation torque is negative. And, its generator-action value occurs long before the pullout magnitude is reached. That value, because of the R1 being negative, can be 300-400% of full-load value!

Phil