hi,

why is there insufficient torque at low speed for an ac induction motor? say at 10 hz through vfd.

why should the motor be not operated at very low frequency? why is there a torque problem?

and what physical quantity is proportional to the magnetizing current in the ac motor?

 

I will be far less rigorous than say, Phil:^)

Many factors go into the design of of a motor.

I believe the whole induction thing is why induction motors fall off at lower frequencies. You are inducing current in the rotor and it's like a transformer. Inductive power transfer occurs with a changing current. Frequency, or angular velocity, determines the rate of change, so the current induced in the rotor falls off as the frequency is lowered. The stator current increases because it's impedance, dominated by inductive reactance at line frequency moves towards the winding resistance at zero frequency or DC. These together limit what you can do to boost torque at the lower frequencies.

A motor designed for line frequencies doesn't have the inductance to limit current at 10 Hz. And the coupling to the rotor is inefficient. Driven to provide reasonable torque it will heat badly.

Induction motors for low frequencies would need more copper and/or iron, which is why you don't see (m)any.

Magnetizing current is primarily inductive and so is proportional to inductance and frequency.

In short, induction motors are useful AC motors. As you move towards DC they stop working.

I've been working on my technical writing, is that reasonably clear? There are a lot of other factors but I think that covers the basics without the math.

Regards
cww

 

To add to Curt's comments -

Torque is proportional to magnetic flux and current - for the induction motor, it's the rotor current we are interested in. The maximum flux is effectively limited to a fixed value depending on the saturation of the core, and the current is limited by the ability of the rotor to get rid of the heat produced.But cooling air flow is roughly proportional to speed so at 10 Hz will be 20 % of that at 50 Hz. Rotor current must generally be reduced to compensate.

The current induced in the rotor depends on the slip between the rotor and stator field, measured in Hz or RPM - not % of supply. So a 4-pole motor which normally operates at 1450 rpm from a 50 Hz supply will develop the same torque at 250 rpm from a 10 Hz supply - all other things being equal.

And, as Curt mentioned, the magnetising current in the stator will be inversely proportional to the VFD output frequency. Info on induction motor behaviour with variable supply frequency is hard to come by so there are probably a few assumptions and approximations in there - but it tallies with what I've seen in operation.

 

ccw... actually you did quite well!
Phil

 

thank you cww & bruce for such a plain language explanation, still i am confused with

> " a 4-pole motor which normally operates at 1450 rpm from
> a 50 Hz supply will develop the same torque at 250 rpm from a 10 Hz supply -
> all other things being equal." &

> "induction motors are useful AC motors, as you move towards DC, they stop working".

these 2 statements seem to disagree (with my current understanding, i might be wrong and its possible that i have not understood them properly). i believe, Bruce meant 1250 rpm with 50 hz and 250 rpm with 10 hz. so its simple speed control and torque is constant. this doesn't match with the other statement. the performance of the motor is not in any way affected at low speed or low frequency that is when we head towards more DC character. this needs to be discussed.

Bruce, in this example of frequency and speed relationship, which type of motor control did you take into account, was feedback used for speed control, is it vector control? when 250 rpm is mentioned, is it the steady state value only after some time has passed to the step change?

again i am saying, i might be wrong in comparing these two statements, blame it on the rain

 

A typical 4-pole induction motor will run at about 1450 rpm from a 50 Hz drive output (synchronous speed 1500 rpm) at its rated power. The power and delivered torque is proportional to the slip, which is the difference between the synchronous speed and actual rotor speed - it is this frequency difference that produces an EMF in the rotor, and the rotor current has a corresponding frequency. If the connected mechanical load is reduced, the slip will reduce more or less in proportion. In my example, the induced rotor current will have a frequency of (50 - 48.333) = 1.666 Hz

If the drive output frequency is lowered, the synchronous speed will be lowered in proportion. The rotor speed is offset from this be a fixed amount which depends on the torque. So at 25 Hz drive output frequency, the synchronous speed will be 750 rpm. For an induced frequency in the rotor of 1.666 Hz, the rotor speed must be 700 rpm. With a 10 Hz drive output frequency, the synchronous speed is 300 rpm, and the rotor speed will be 250 rpm for 1.666 Hz rotor frequency.

This what the motor does with a given applied supply frequency - it has nothing to do with any control mechanism. There are other factors that limit what you can do with the motor at low frequencies. One of the most important of these is the need to reduce supply voltage as frequency is lowered to maintain a set level of airgap flux to avoid saturation - the V/f ratio is held constant. Some control approaches will lower supply voltage at lower loads on the motor regardless of frequency, but this can also be used in fixed-speed operation to increase power factor.

The torque vs speed curves for an induction motor hold their shape and scale as the supply frequency is lowered - the curve gets shifted to the left but is otherwise unchanged. If the voltage is reduced as well to hold a set V/f ratio, then the peak torque will reduce in proportion to the supply frequency.

But probably the main limiting effect at low speed is the cooling. If magnetising current is held constant, the rotor power loss as a fraction of the total power supplied to the rotor is (slip in Hz)/(synchronous speed). This fraction will increase, and the losses will also increase as drive output frequency falls. However, the cooling ability of a fan will fall as the speed drops, with the cooling air flow being roughly proportional to rotor speed. The limit depends on the driven load and the mechanical power needed top drive it at different speeds - for a centrifugal fan or pump, this falls in proportion to the cube of speed and therefore the required rotor power input at half speed is about 1/8 that at full speed. This is a relatively easy case but even with a centrifugal load load the low end of a realistic speed range is usually considered to be 50 %.

 

A bit more rigorous and involved but correct. The general gist is that there is a lot going on and as you start approaching DC, things change for the worse and it's not practical or possible to compensate for all of then. Some types of drives can do quite a bit. And some motors are designed for this type of service with more iron, more copper and auxiliary cooling fans that will cool at even zero speed. But for very low speed/frequency, PM or externally excited rotors would be more efficient. But these are usually only seen on packaged motor/drive systems. And these are generally in servo territory.

Regards
cww

 

I've been working oh communicating with folks from other disciplines. I thought the other changing fields from slip and pole shaping, etc. would produce eye glazing:^)

Regards
cww

 

a little more needed,

when talking about the motor loadability curves, the torque vs speed graph in the range of 0-50 hz shows torque as an inclined line (y = mx +c curve) y is torque, x is speed, and so shall it should be due to the heating of the motor at low frequency, but some times it is mentioned as constant torque region (0-50 hz region). paradox.

 

It is possible to obtain 100% of an induction motor's torque at zero speed. We have applied induction motors to applications that require such performance continuously.

Doing so requires excitation of the stator to produce the required magnetization flux and torque producing flux.

One of the problems with obtaining constant torque at zero speed by merely controlling the Volts/ Hz ratio of the motor excitation is that motor losses consume a larger percentage of the excitation current and voltage at lower voltages. Both the magnetization flux and torque producing flux will fall off as the frequency and voltage approaches zero.

The losses of torque near zero speed can be offset by boosting the voltage at lower frequencies, but this is an inefficient method providing poor regulation of torque.

Obtaining adequate torque and speed regulation at low speeds requires use of a vector or DTC drive. Achieving the best performance requires feedback of motor speed to the drive, typically using a digital tachometer (encoder).

Concerning motor thermal performance at low speeds, since most industrial induction motors are Totally Enclosed Fan Cooled motors, cooled by rotor-driven fans, such motors will overheat if operated at speeds insufficient for the fans to provide the needed cooling. Several manufacturers offer induction motors that are rated to operate at constant torque at speeds a low as 1/2000 (constant torque turn-down ratio of 2000:1) of motor rated base speed. These motors may be of TENV, TEFC or TEAO construction.

AC motors not constructed specifically for constant torque application at very low speeds are often rated to provide continuous constant torque a speed ratio (turn down ratio) of 10:1 to 20:1 Some motors designed for fan duty will only provide a 2:1 speed ratio at constant torque.

When properly excited, all AC motors will deliver rated torque and greater at zero speed. The RMS speed/torque operation must be within the rated speed/torque ratio or the motor will overheat.

Obtaining the constant torque speed ratio or curves for standard induction motors. motors not specifically designed to provide high torques at low operating speeds, can be challenging. Curves showing constant torque performance as a function of speed will consist of multiple curves for different operating frequencies, instead of the typical single curve showing the speed/torque performance at 60 hz only.

 

salute

 

I think this may be caused by VFD. Your VFD may work bad at low frequency of 10 hz.

But frankly speaking, 10 hz is not too low. Some VFDs can even work well at 0 Hz.

 

thanks Richard,

as pointed out by you, 10 hz is/was not generally a problem for vfd.
in my current view and as discussed earlier, the type of drive (v/f, vector, dtc), the torque demand of the load and the motor being a motor, all contribute to it.

 

Yes, you can get motors that will give 100% torque at constant zero speed. SEW Eurodrive supply Torque Motors. Torque Motors can be considered as electric springs ie they will supply their full torque when stalled and do not burn out when in this state constantly.