Restricting Motor Current in a Lube Oil Pump

It neither makes sense to expect to run a motor under power supply conditions it was not designed for save you use an inverter.

You need to decide which makes more sense; the cost of the inverter or the cost of a properly specified motor.

j.

The motor manufacturer specifies a high voltage "Star" rating and a low voltage "Delta" rating. The motor was selected based on that.

The question is - what does the inverter do that makes the motor draw reduced current at the same voltage and frequency level? and unfortunately,I do not know enough electronics to answer that.

If one calculates the current as per the specification for the Delta connection, it is 1.8 amps and with an inverter it draws exactly that. On the other hand, if connected directly, it draws 4 amps.

Hi Babu,

The answer to your question is simply that you are not connecting the motor the same in both conditions.

If the motor has 230V/60Hz delta from both your main supply and from a VFD then it will behave the same.

there are two possibilities:-

a) the frequency and voltage on the VFD is different to the main supply

OR b) you have the motor connected differently.

we need a bit more feedback;-

what is your mains supply?

do you have access to an oscilloscope to look at the waveform of your supply? if so are the waveforms reasonably similar? (V & I)

Have you tried this several times and always had the same result?

have you checked that all phases are present (obvious but easily overlooked)?

were both tests conducted with the same pump load?

Regards Chas

What kind of pump is it? If its not a positive displacement pump try restricting the discharge.

IF this is a synchronous motor,it will try to turn faster,due to the higher frequency.

(RPM= 120XFrequency/Number of poles).

This will create more load, and more rotor lag,consequently, more amps will be used.

Inverters control not only frequency, but also limit current internally.

If it is designed for dual voltage/Frequency, then double check your connections to make sure they are correct.

The motor does not know where it's supply is sourced, so it has to be a variation in supply voltage, or the internal current limits of the VFD.

You have a situation that appears somewhat complex to the lay person, but makes perfect sense to EEs.

First let's discuss voltage only. When you connect the motor n Delta to run on 230V, t WILL ALWAYS pull more current by a factor of the square-root of 3, or 1.732! As an example and taking power factor out of it for a moment to keep it simplistic, W = V*A, so A = W/V. Therefore 370W / 415V = .89A, but 370 / 230 = 1.61A, almost twice as much. So that is PART of the issue here.

Now for frequency. Motor shaft power (kW) is a function of speed and torque. Torque is created by current but dependent on voltage, speed is a factor of frequency. If you supply higher frequency and the same voltage, your motor is going to be capable of more shaft power. If the nature of your load is such that at higher speed it will perform more, the motor will deliver that higher power and consequently draw more current at the same voltage. Pumps are like that; the load is dependent on flow, flow is dependent on speed, so at a higher speed, you will get more flow and the motor will deliver more power, drawing more amps.

So in your case you changed voltage, which would have almost doubled the current draw to start with, then you changed frequency on a flow load, which increased the load on the motor by the cube of the speed change. So at 60Hz, your motor is going to attempt to deliver 120%³ power, or 172% x .37kW = .63kW, almost twice the power! So combine the motor attempting to deliver twice the power at a voltage that causes it to pull almost twice the current, and you have a 1A motor attempting to draw 4A!

So what to do? Best method would be to use the inverter, setting the maximum output frequency to 50Hz since the motor is designed that way. BUT, it WILL draw 1.732 times more current at 230V, there is no way around that. When you say your inverter was only making it draw 1.8A, think about it: 1A x 1.732? Just about 1.8A within a margin of error and a little efficiency loss in the inverter. If you do use the inverter, set it up for 220V output and it will not really care much because if the line is higher, i.e. 240V, the output will still remain at the programmed 220V level (the motor will have a 10% tolerance). If the input is lower though, the VFD cannot create voltage that is not there.

2nd less expensive method, but more dangerous: As was mentioned above by others, use a flow restriction valve on your lube oil system. Go ahead and run the pump at 230V 60Hz, but restrict the lube flow with the valve while monitoring the motor current and lower the current draw down to where the motor is only drawing 1.73A. Keep in mind that you will need to change out the motor protective overload device if you have it set for 1A now, otherwise it will trip.

The downside to this is threefold: 1) you may not know if this is enough oil flow to adequately lube your machine. 2) 230V is a "nominal" voltage for the 60Hz world. We have 230V, 220V, 240V, 208V etc etc. ANY deviation from the 230V that you set it up at will affect the motor performance and may result in motor damage or lube loss. 3) the motor V/Hz ratio is not going to be the same as it was designed for, which in your case is 230/50 or 4.6V/Hz. In order to maintain that ratio at 60Hz, your voltage would need to be 276V. That means your motor is going to be incorrectly excited and will heat up disproportionately to the applied load, meaning it will run hotter doing the same work as it would have at 50Hz. That will reduce the motor life.

Unless I have not understood the poster,he has had on overload condition,running the pump across the line or with an inverter. The motor is doing more work than designed for.2 choices larger motor to meet demand or lower demand to meet the motors hp rating.Restricting pump output will work if as stated lube for machine would be enough,and if its a type you can restrict. Have seen this condition time after time with pumps and blowers.

[KISS] keep it simple stupid

Babu,..

I would suggest a motor rating ( 60hz) and voltage in line with available power supply.

More frequency= more speed = more load = more current.

Less voltage = more current..

cheers.

Hi all,

I have read and reread Babu's posting and he is (or believes he is) giving the pump the same 230V delta input of 60Hz either from the vfd or from mains but the mains draws more current. Is everyone assuming Babu has made a typographical error and he really means that the VFD has 50 Hz out?

regards

Chas