Motor Slip using VFD

Hi all
I recently worked on an inverter fed 6pole 1.1kw 900rpm 50hz 240v 1.9A motor drivig a roller via a gearbox with a 60/1 ratio. The inverter output was limited to 100hz and the motor seemed to drive ok at that frequency.
The roller was not reaching the required linear speed of 7M/min so I lifted the frequency limit to 125hz as a temporary 'fix' while some new chain drive sprockets were ordered correct the drive ratio.
However when the frequency fed to the motor reached about 115 hz the motor would slow down even though the frequency of the inverter output remained at 115hz. the only load was the gearbox and drive to the roller.
At first I thought the motor had run out of torque so far above the base frequency but the current reading was less than 1 amp ( I did not check the voltage). Then I wondered if it had something to do with it being a 6pole motor? I have seen similar sized 4 pole motors run at higher frequencies.

thanks for any enlightenment,

ian

This is going to be personal opinion. If the application requires the speed/torque to exceed what the motor can deliver at the "normal" frequency then you arent using the right motor. As you know the 6 pole(s) is what makes it a 900 rpm motor (1000-slip). A 4 pole motor would give you 1500 rpm ...ie approximately 50% increase in speed without loss of torque. I would change the motor to a 4 pole inverter duty motor. I also attempt to limit exceeding 90HZ if possible but know there are applications that need to exceed that.

Inverters are great for variable speed applications but I still believe in matching motors and gearing as close as possible to max rpm and torque values. Also note that some gear boxes have issues with exceeding a certain rpm input.

May want to verify the inverter is setup to allow FLA (full load amps) in this case 1.9A.

Torque is a product of HP x 5250/rpm
RPM is a product of 120 x frequency/noumber of poles
Current isnt always a good indicator, as you can see the higher the rpm the lower the torque.

When running an induction motor over base frequency, you first enter a range where the torque falls off in the same proportion as the overspeed resulting in constant horsepower. As the frequency increases, the motor eventually cannot sustain the constant horsepower characteristic and the torque begins falling faster than the increase in speed resulting in falling horsepower.

In the constant horsepower range, the motor amps hold nearly constant due to the magnetizing amps dropping and the torque producing amps increasing by essentially the same amount. In the falling hp range, the field amps continue to fall while the torque producing amps increase at a smaller rate resulting in motor lead amps dropping off.

Since you have a 50hz design motor, surely you have entered the falling hp range by 100hz (double speed) and are far into it at 115 and 125hz. I would expect the input amps and kw to fall off just like you describe since the output kw is falling too. Otherwise, the motor would have to vent the extra kw as heat and they don't normally do that at such high speeds.

The most important thing to get right at higher rotational velecity, is the number of poles the motor has. 4 is not always the right answer! Neither is 6! It is quite refreshing to find a true 6 pole out there. Only seen it a couple of times and some claimed 6 but they weren't a true 6! Give yourself some try time and set the poles back to 4, see if under no load you experience the same loss of rotational velocity.

4 is not always the right answer! Neither is 6! It is quite refreshing to find a true 6 pole out there. Only seen it a couple of times and some claimed 6 but they weren't a true 6! Give yourself some try time and set the poles back to 4, see if under no load you experience the same loss of rotational velocity.

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Could you clarify this statement?
Number of poles is directly related to rpm.
rpm=120xfrequency/number of poles
I normally work with 2, 4, 6, 8, and 10 pole motors on a regular basis, never knew there to be anything uncommon about them.

Give yourself some try time and set the poles back to 4, see if under no load you experience the same loss of rotational velocity

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This is the part that has me really confused....how do you set the poles back to 4?

I didnt think about this before but your motor may be OK for the job but the 60:1 ratio on the gearbox may be to high. With 900rpm input that would put your output at 15rpm. I dont know enough details to transform that to your 7M/min at the roller.