synchronous motors running at low speed
synchronous motors running at low speed
(OP)
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....
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....





RE: synchronous motors running at low speed
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
RE: synchronous motors running at low speed
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.
RE: synchronous motors running at low speed
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
RE: synchronous motors running at low speed
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
RE: synchronous motors running at low speed
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If we learn from our mistakes I'm getting a great education!
RE: synchronous motors running at low speed
RE: synchronous motors running at low speed
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: synchronous motors running at low speed
RE: synchronous motors running at low speed
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