Breakdown Torque Of Motor

Hi,

I am looking at a spec sheet of a motor which has a starting torque of 140% and a breakdown torque of 241%.

Now, if the load that is attached to the motor is small ( say 50% at the rated speed and even less than that at speeds below it..), does the motor still produce 241% of breakdown torque. ie, as the motor accelerates from 0 speed to rated speed, at the speed at which breakdown torque occurs in the torque speed curve, does the motor practically produce the 241% torque inspite of the motor load being small?

Thanks in advance...

Dol Starting Of Motors Affects Transformer Sizing

Hi All,

The quality of the content I get from this forum is just fantastic. Thanks for all the info.

I am in a process of specifying the transformer capacity for starting three 1250HP motors. I looked into a few websites for the transformer sizing guidelines and this is what I have come up with.

I assume that it is going to be a 5.7% impedance transformer. I would require the two motors running and the third motor to be started, with a maximum of 10 % voltage drop at the motor terminals.

Motor nameplate details:
1250 HP
3300V
Locked rotor current: 1254 A
Full load current: 198 A
Full load power factor: 86%
Code Letter: G


Calculation of motor starting kVA:

For Code G motor, starting kVA = 6.29 times Horsepower

So,

Motor starting kVA = 6.29 x 1250
= 7.862 MVA

Transformer sizing:

Total transformer kVA size required = ( Running kVA rating of two 1250 HP motors) + ( Transformer rating required to start the 3rd motor with 10 % Volt drop)

Running kVA Rating of 1250 HP motors = 1.732 * ( Rated Voltage) * ( Rated FL Current) / Assumed average power factor of 85%
=1.732 * 3300 * 198 / 0.85
= 1332 kVA
So, Running kVA of two 1250 HP Motors = 2 * 1332 kVA
= 2.664 MVA

Generally, Volt drop due to motor starting in % = Motor starting kVA / ( Motor starting kVA + Transformer short circuit kVA)

So, 10% = (7.862MVA) / ( 7.862 MVA + Transformer short circuit capacity)
So, calcualted transformer short circuit capacity = 70.76 MVA

Transformer Short circuit capacity = Transformer kVA rating / (Transformer impedance)
So, transformer kVA Rating = Transformer short circuit capacity * Transformer impedance
So, the transformer rating required to start the third motor with 10 % volt drop = 70.76 MVA * .057
transformer rating required to start the third motor with 10 % volt drop = 4.033 MVA

Recollect
Total transformer kVA size required = ( Running kVA rating of two 1250 HP motors) + ( Transformer rating required to start the 3rd motor with 10 % Volt drop)
= 2.664 MVA + 4.033 MVA
= 6.697 MVA

Does this approach seem right? Or am I going wrong somewhere?

I would actually be using softstarters in this case, but I need to prove that 5MVA transformer is not sufficient for DOL starting of third 1250 HP motor, with two other 1250 HP motors running.

Thanks in advance

Cheers

Torque-speed Curve Of Motor

Hi All,

I have got a question regarding the practical and theoretical speed-torque curve.

I have a motor datasheet which lists all the resistances, reactances of the equicalent circuit. I also have the speed torque curve for the motor. But when I use the values and calculate the speed torque curve from the equation, the curve is not having the similar values/shape as the curve they have provided. Is this because of the dynamic nature of the motor?

Is it possible to plot the speed torque curve from the motor parameters( R1, R2, Rm, X1, X2, Xm etc)? Will this plot be the same as what is plotted from the experimental values?

Speed-torque Curve

Hi All,

I have got a question regarding the practical and theoretical speed-torque curve.

I have a motor datasheet which lists all the resistances, reactances of the equicalent circuit. I also have the speed torque curve for the motor. But when I use the values and calculate the speed torque curve from the equation, the curve is not having the similar values/shape as the curve they have provided. Is this because of the dynamic nature of the motor?

Is it possible to plot the speed torque curve from the motor parameters( R1, R2, Rm, X1, X2, Xm etc)? Will this plot be the same as what is plotted from the experimental values?