Motor torque and speed basic

Apologizes in advance for a newbie question. In VFDs, there is speed input and torque input. What symptoms would I see on a 3 phase induction motor if I give speed command but not enough torque command? Would the motor stall? Would "slip" develop then position error?

Conceptually, I can understand that motor torque is not the function of speed, and vice versa. Torque is current flux and speed is IGBT switching. But doesn't motor speed require torque? What good is the IGBT switching if there is no flux strength to move to rotating phases.

Thanks in advance for education.

You generally can't command both. You might be able to use one input as a limit, but usually you either command the drive in velocity mode or torque mode, not both.

Motor speed does require torque - yes.

You can't command both at the same time, but it is not uncommon to command speed but have a torque limit. In the case that your load exceeds the torque limit, you will see the actual speed die off - possibly to zero speed (drive still running, command speed still the same, but actual speed zero or less than command speed)

Perhaps it would be better to explain why you even HAVE a "torque input" on a VFD.

The basic task that an AC induction motor performs is to provide torque at a given speed. The speed is based on the frequency applied, the torque is a function of the current drawn as a result of the LOAD put on the motor. More load = more current drawn = more torque produced, up to the limits of what the motor can do safely. You don't need a VFD to make that happen, that's inherent in how the motor works.

What the VFD does is to allow the motor to produce it's rated torque at any speed, by not just changing the frequency, but by changing it along WITH the voltage at a consistent rate. That's because the rated torque capability of the motor is based on maintaining a constant ratio of voltage and frequency, a V/Hz ratio. So for example a 460V 60Hz motor was designed around a V/Hz ratio of 7.67:1 so as long as you maintain that ratio, you get full torque capability of the motor. if I want to run at 60% speed, then 60% of 60Hz is 36Hz, so I need my voltage to be 36 x 7.67 = 276V RMS; that's what the VFD will do for me.

But if you get into ADVANCED use of VFDs, they can do what's called "Vector Control". This is based on the concept of the fact that during EACH sine wave going to the motor, FIRST, the power creates magnetic flux in the stator, THEN it produces torque. The V/Hz vector to produce current required to produce flux remains relatively constant regardless of speed, but the vector of V/Hz to produce TORQUE changes with loading. In a standard drive if I want more torque and just maintain that basic V/Hz ratio, I am over fluxing the motor at higher torque than necessary, so I am indirectly limiting the motor's ability to respond to changes in load without stressing it. So a Vector Drive is capable of separating those vectors in mid-cycle so that it only give is enough flux as it needs, leaving MORE available in the Torque current vector without adding unnecessary thermal stress. it also makes my motor respond faster and more accurately to changes in load.

So the last step is that now that I have that ability to control torque SEPARATELY from flux, it also gives me the ability to control torque regardless of speed. That's where the torque input come in to play. So lets say I have a winder application. If I am needing to have a constant tension in the payout and take up reels, I must maintain a VERY consistent torque on those reels. But at the same time, the speed is going to be changing on both sides, because the diameter will be changing. So by using a torque input, I can keep the torque perfectly controlled and pay no attention to the speed, because that is technically irrelevant to me now.

So bottom like, a torque input ONLY applies to doing torque control applications when the VFD is in Vector Control mode. If you are doing anything that is based on velocity, torque is consequential to loading and you don't need the input.

UNLESS... in your velocity control system, you have a MAXIMUM torque that your system can handle, then you MAY want to use that input as a to adjust the Torque Limit on the fly (you can usually set a fixed torque limit in programming, no need for the input).

You either have it in torque control mode, with speed limits, or speed control mode with torque limits.

If you are using a torque and speed reference at the same time, then you will have one set up as a limit and one as a setpoint.

Something important to remember, that when a VFD drives a motor above rated frequency (60HZ in US)the voltage stays constant at the rated voltage. This has the effect of decreasing available torque in motor. As Jraef said, below rated frequency the VFD will operate in V/HZ and maintain constant available torque up to the rated speed. Available torque and actual torque are two different things. Available toque is how much torque the motor is capable of producing safely. Actual torque is determined by the load,a motor will only produce enough torque to drive the load at the desired speed. In torque control mode though, the VFD will drive the motor to keep a constant torque, regardless of the speed. Generally speaking,the torque required by the load goes up as the speed is increased. In a constant torque application, the VFD will increase the motor speed until the torque of the load is equal to the required torque setting, and decrease the motor speed if the load torque is above the setting until they are equal.

T=P(5252)/n, where T is torque in lbft, P is HP, and n is speed in RPM.

A 100HP motor at 1750 rpm produces 300lbft of torque.

If the motor is running at a slower speed but with full voltage applied the the motor, then the torque would be higher than the rated torque of the motor. Since torque and current are proportional, current is higher and causes excessive heating in the motor. As Jraef alluded to, the way the VFD controls this is by lowering the voltage, and by ohms law the current.

Above the rated speed of the motor, the voltage is held constant and thus the torque drops off. So for the same 100HP motor running at 90hz, the rated torque produced would be only 200lbft. But if your load required 220lbft at that speed you will overload your motor and possibly even stall out.

Check out this web page for a chart and more formulas relating to motor power and torque:

http://www.engineeringtoolbox.com/electrical-motors-hp-torque-rpm-d_1503.html

Jraef:
very nice, complete answer.
I think its the hat.
;-)