synchronous motors running at low speed 2

[cokeguy]

We use small synchronous motors (rated 345 VAC, 90 Hz, and with several sizes from 3 to 15 amps aprox) on some of our older machines, running from the same VFD to keep several mechanisms and conveyors in sync (newer machines have servos).

We run the VFDs in V/Hz mode, with a 3.833 V/Hz ratio, however at low speeds (below 20 Hz) some of the motors begin to loss sync because of the diminished torque.

Traditionally, the VFDs have been set to run at a higher V/Hz ratio at lower speeds and therefore torque improves, but motors begin to overheat. Different people here at the plant have different opinions about at what point and to what value the V/Hz have to be increased when working at lower speeds. Somebody knows any web-links or rules of thumb for these cases? Thanks....

 

[waross]

The overheating may be caused by reduced cooling. Try an external fan.
The V/F ratio may be saturating the stator. If so, the motor will overheat. Increase the ratio just a little bit more and the motor will smoke. Reduce the V/F ratio just a bit and you may get much less heating with little loss of torque. As the V/F rises you enter the knee of the saturation curve. At the knee of the curve the current increases rapidly with very little increase in torque. If this is your problem, reducing the V/F ratio slightly may solve your problem.
respectfully

 

[cokeguy]

Thanks waross, in your opinion and experience, at what frequency should we "bend" the V-Hz curve? Right now we start with aproximately a 4.4 V/Hz ratio at around 15 Hz, and at around 30Hz we have the "bend point" where the ratio is set at the nominal 3.833, and from there all the way to 90 Hz it remains at 3.833.

When we run into the saturation, can the motor be damaged even if rated current is not exceeded, and external cooling with airflow equivalent to the one generated by the motor at rated speed is applied? In other words, can we raise V/Hz ratio arbirarily at low speeds, as long as enugh cooling is applied and rated current is not exceeded, to overcome torque problems without damaging the motor?

Thanks for your reply, and forgive me, I am new to sync motors.

 

[itsmoked]

I can't answer the V/F question but if you stay within the motor ratings(I) then likely you are NOT in saturation as things will go bad very quickly in saturation. You will quickly start exceeding the running current when you're in saturation.

You need to remember the fan's cooling drops with the square of its speed. So if you get slow and don't have aux cooling or are not dropping the HP load at the same sort of rate you will find your motor in 'hot water' pretty quickly.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hello cokeguy;
An indication of saturation is a disproportionately large increase in current for a small increase in voltage (or V/F ratio). You are at about 15% over voltage. Older motors will withstand +15%, but newer motors are rated for +10% only.
Try a V/F of 4.2
Is this a PM rotor or a wound rotor. If it is a wound rotor you may check the circuit to determine how the magnetizing current is derived. If the magnetizing supply is dropping with the supply voltage that may explain both the overeating and the slipping.
respectfully

 

[ScottyUK]

On these little synchronous motors, do you have independent control of the field current?

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If we learn from our mistakes I'm getting a great education!

 

[cokeguy]

Thanks for your enlightening comments on saturation, now a get it a little better and can understand the load current behaviour we have seen. and Scotty, we have no control on field/rotor current, I suppose these are PM motors.

 

[ScottyUK]

Glad you're on the right track. The reason for the question was that a weak field reduces the ability of the motor to develop torque before it breaks from synchronism.


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If we learn from our mistakes I'm getting a great education!

 

[gepman]

I know this isn't the major topic of the thread, however I would like to ask itsmoked about his statement that the cooling of the motor falls with the square of the speed. Airflow is usually linear with fan speed. Cooling involves the convective heat transfer coefficient which in many cases is proportional to speed to the .8 power. Power used by a variable torque load varies with the cube of the speed. Much of the heat generated by a motor (copper loss) is proportional to the square of the current which can remain constant on constant torque loads. This is why there normally isn't any cooling problems on centrifugal fans and pumps with VFDs but there can easily be cooling problems on conveyors, PD pumps, and other constant torque loads. If itsmoked has some information specifically regarding motor cooling fans I would like to see it in order to avoid any problem with my designs.

 

[cswilson]

The back EMF of the motor without any load is directly proportional to voltage, so in a synchronous motor, directly proportional to frequency. That is, the unloaded motor has a V/Hz ratio.

Any current you want in the motor to drive a load is on top of this, producing a voltage through the resistive drop of the windings that must be added to the back EMF. This voltage is not proportional to frequency.

If your drive is producing waveforms with a strict V/Hz ratio, you must set this V/Hz ratio higher than the motor's own "V/Hz ratio" (that is, its back EMF constant). The problem is, if this is the case, that the additional voltage headroom for torque-producing current goes down as speed goes down.

I don't have time to crank any numbers now, but this effect could be quite subtle, and it strikes me that it could be easy to overcorrect for this by bumping up the drive V/Hz ratio too much at lower speeds. It could well be worth your while to spend some time in paper analysis of these effects. You would need someone who understands vector analysis of synchronous motors, because with open-loop control of synchronous motors, the current-producing current is not in phase with the back EMF, and the angle between the two varies with the load (the key stabilizing mechanism of this type of control).

Curt Wilson
Delta Tau Data Systems