It has maximum torque @ mid stroke and maximum pressure (stalled).
Electric motors have a Full Load Amps (FLA) rating which is locked rotor amps.
When you reach FLA (stall) you have maximum torque.
Horsepower is a derived unit. It is just a measurement of power.
A unit of work done per a unit of time is a measurement of power.
An average horse was observed to be able to do 550pound-feet
of work per second, or 33000lb-ft/min, or 746 watts.
1 watt = 1 joule/second. 1 watt-hour = 3600 joules.
Watt is a unit of power. Watt-Hour is a unit of energy.
Horsepower is a unit of power. Joule is a unit of energy
Take the horse out of it and it makes the math a lot simpler.
Pooh Bear
Pooh Bear,
LRA does not equal FLA.
Max torque is frequently not at locked rotor, but later in the rpm range, frequently around 80% rpm.
Please look at torque charts for various Nema classes A through D if you get a chance.
Everyone, it is important to always remember that motor hp is at the shaft, not at the wire. Also, Pelec is not equal to Pmech. Looking at the Pmech equation at the locked rotor state
Pmech = Tau (torque in n-meters) * W (omega the radian velocity of the shaft.) We know that power is work per unit time, and that work is force through a distance, so we know in the locked rotor state that
0 = Tau * 0 therefore we cannot use the power mechanical equation all the way down to locked rotor. Instead, we have to rely on the motor class information to know the mechanical torque, or we must measure it directly.
We cannot substitute Pelec for Pmech without remembering that we know for a fact that Pelec and Pmech cannot be the same since Pmech is known to be zero because the shaft is not turning, and we know that Pelec is likely NOT zero if the motor is powered up. Therefore substituting one for the other is not helpful. And even if we were to try to do so, using amps is not a good substitute, since Pmech is REAL power, and Pelec is meant to be real electric power, but most folks only have access to apparent power measurements, not real. We know for a fact that amps is not an indication of real power in an AC system, and the power factor must be considered, and that efficiency must be considered too. Efficiency is off the charts bad at the locked rotor state, and that is what ruins the motor...it is being turned into a resistance and induction heater rather than a motor.
I find in practice that I must always use real power monitoring to have a clue about the applied mechanical power because up to 50% load, the power factor shifts severely more toward unity, making correlating amps to power (and therefore approximately torque as well) does not work very well. The entire lower torque range is obscured by power factor shift. But when I use real power monitoring it all falls into line...and I can set limits in the recipe to make the machine applies known amounts of mechanical power to the product being made.
I hope this is received well, and I hope I have not made any mistakes. I know what I am talking about
I think, but sometimes I may not always be saying what I mean.
It is interesting to think of induction motors as rotary transformers. I used to design substation transformers, and I used to try to think of them as translational motors. They will not rotate in any case, but they would dearly love to translate. A 10 MVA transformer is pretty much like a 10MVA translational motor, but massive end frames and weldments are put into place to try to make the transformer NOT "run."* If you short the secondary, and ramp up the voltage in the primary until the secondary is carrying full load current, you have found the "impedance voltage" and comparing that voltage to full rated, you have the "% impedance voltage" or the %Z. In an induction motor, the primary would be the stator, and the secondary would be the rotor. You are providing far more voltage to the motor primary than is required to have rated amps in the rotor, and that is why there is a resistance and inductive heater being made out of a motor.
*Clearly, as a designer, I had no Nema class information to tell me how to design the bracing to keep the transformer intact. Instead, we had to go back to first principles. But as a former electric machine designer, I cannot go back to first principles on a motor of which I don't know all the design details. But folks using amps to indicate torque in a locked rotor state are attempting to do just that, when they ought to rely instead on Nema class and motor manufacturer data expositions.
