# Measure the load of a three-phase asynchronous motor

Hello,
i want to measure the actual load of a 1.5 kW three-phase asynchronous motor.
The motor is driven by a frequency converter. This converter generates 0-10 V Signal of the actual load. That works good. In the most of time the motor has to drive a load. Sometimes the load is negative and the motor has to break the load to the desired speed and i want to know that. But the converter has no signal about the pos or neg load (1,2 quadrant).

How is it possible to measure that? Maybe is there a phase shift or … ?

Any ideas?

I would have thought the speed of the motor would be the best indicator . Assuming this is an induction motor the speed is dependant on the load , although it is only a small change .
There might be a power factor change , you’d need to google that.

What is the actual problem you’re trying to solve? Knowing when the motor is regenerating rather than motoring doesn’t help because drive is still going trip on an overvoltage fault.

The only way a standard two quadrant AC drive can handle a regenerative load is to have a brake chopper fitted to dissipate the braking energy into a resistor.

What is the actual problem you're trying to solve?
This: "Sometimes the load is negative and the motor has to break the load to the desired speed
and i want to know that. But the converter has no signal about the pos or neg load (1,2 quadrant)."

Differences in speed are to small to measure.
We use a brake chopper and resistor...

Maybe other converter can provide a signal about that, but this converter can't.
It is not possible to change the converters from all machines.

Rubbernose:
Hello,
i want to measure the actual load of a 1.5 kW three-phase asynchronous motor.
The motor is driven by a frequency converter. This converter generates 0-10 V Signal of the actual load. That works good. In the most of time the motor has to drive a load. Sometimes the load is negative and the motor has to break the load to the desired speed and i want to know that. But the converter has no signal about the pos or neg load (1,2 quadrant).

How is it possible to measure that? Maybe is there a phase shift or ... ?

Any ideas?

You need a three-phase power meter that supports bidirectional power flow (ie can display negative real power).
It also needs to support the range of frequencies that the VFD can produce, not just mains frequency.

Yes, i measured a rising voltage on the brake resistor/chopper. This voltage i can change in a 0-10 V signal, bit the difference is very weak.
A power meter is very expensive. We have to place it not in the power line but between motor and converter. The voltage and current is always changing, sometimes 0 V.

The best way to do this is change to a motor controller that monitors power flow in both directions. In fact a vector/field oriented motor controller has all the hardware for doing this as it needs current sensing anyway, and with field oriented control you can do much more with the motor anyway.

I don't see a cheap solution for aribtrary voltage/direction 3-phase power measurement.

Can you stick a temperature sensor on the braking resistor in the VFD?

"Can you stick a temperature sensor on the braking resistor in the VFD?" Yes, but that's to slow and depends on the wheather conditions.

What is the manufacturer and model number of the VFD?

Thats a sourcetronic ST9100. The manufacturer told me that that there are no parameters to measure

Rubbernose:
Thats a sourcetronic ST9100. The manufacturer told me that that there are no parameters to measure

That does do vector control, it ought to be able to tell you lots of info - perhaps you pay more for
the models that do that (same hardware, extra software, much more expensive?)

That’s a poor website, no operation manual.

Since the drive provides vector control, it understands torque in the motor. If the drive has any type of programming, compare the speed and torque values. When the signs of those values are not equal, the drive is regenerating.

If that cannot be done, any type of voltage detection on the braking resistor will indicate regeneration. The dc voltage will be mains rms times 1.414. Use a series string of voltage rated resistors connected to the led of a simple optocoupler. You’ll have to measure the on time which will provide a relative indication of the percentage of regenerative torque being absorbed by the resistor.

At the moment we try to use the analog output signal 0-10 V from the resistor voltage. That is very complex. It depends on the load, speed, temperature, .....
But it seems that it could work, without changing a lot of converters!
Thanks for all the ideas!

Temperature is not a factor and overall it is not complex, just difficult when you’re trying to measure a digital pulse width with an unknown bandwidth analog output.

If there is an adjustable filter time for the analog output, increase it to the maximum value and test again. The problem is that the resistor on time varies proportionally with the amount of regeneration. A low level of regeneration may not be detectable due to 1) losses in the drive and 2) the very short on time of the bus load resistor. Increasing the output filter time may allow you to detect the case of #2. Case #1 is not detectable with this approach as the resistor will never be energized when the drive losses are greater than the regeneration energy.