Correct VFD and cables?

[Hollowpoint]

On Hollowpoint Said:

I don’t know all the maths behind the low speed low torque debate, but my inverter has some sort of low speed torque boost or compensation.

It doesn’t properly compensate for the loss of power at low speeds.

Let’s assume that an induction motor running at 50Hz has a torque of X Nm. If we now run the motor at 10Hz, ie, a speed reduction of five, the VFD will drive currents in the windings to produce the same torque, X, but since the motor is only running at one fifth of the speed the power will be down by a factor of five too.

Mechanical drive systems, belts or gears, get around this by multiplying the torque. So in the example above, at one fifth of the speed, the torque would be 5X. The torque has increased by 5, the speed has decreased by a factor of 5, so the power stays constant.

I am not going to delve into the details of motor control theory but suffice it to say that, for an induction motor, torque produced is primarily dictated by the winding current. The compensation mentioned simply means that the VFD can overdrive the phase currents, and create more torque, for a short period to get over a short term overload. A usual increase is in the range 150%/200%. It is a short term solution, an increase of 200%, ie, double the current, leads to four times the dissipation in the windings. Consquently the motor will overheat fairly quickly.

Julie

[Hollowpoint]

On Hollowpoint Said:

I don’t know all the maths behind the low speed low torque debate, but my inverter has some sort of low speed torque boost or compensation. Even at crawling speed it would rip your arm off if you were stupid enough to try to stop it. And that’s only a 550w motor.

I have the Invertek e3 by the way. Well Ive had 3 of them since they have proven reliable. I am actually fitting one to a Boxford right now!

There is no reduction in motor torque at lower than rated speed using a VFD. There is a reduction in power because power is proportional to torque and speed. Using a gearbox maintains power (less losses) so torque must increase. So it’s not a loss of torque with a VFD at low speed it’s the increase with a geared (or belt) reduction that is the difference.

If your machine has gearing you can continue to use it if you need to increase torque for some tasks. So it a bit of a non-issue.

In the UK and EU you get a “free” increase of 20% in speed and power by running the motor at 60Hz rather than the 50Hz of the mains. This does not overload the motor as they are almost all desigined for both frequencies.
Most modern motors can be run well above their nominal 50/60Hz speed. For my ML7 VFD conversion I chose a slower 6 pole (915 RPM @50Hz) motor and reduced the belt reduction slightly by fitting a larger pulley on the motor. This is also kinder on the belt. The motor is rated to more than 1640 RPM (90Hz, as it uses the same bearings, fan etc as the 4 pole version) at slightly increased power. The reason the power does not increase significantly is because the the output voltage of the VFD is not high enough to drive the required current (torque is proportional to current) against the higher back EMF of the motor at higher speeds.
This arrangement gives the best of both worlds, increased torque at low speeds. Retained midrange power and torque with lower gearing, and higher speed if required, keeping bearing limitations in mind. And being 6 pole it improves vibration, noise and ripple even more than a 4 pole 3 phase conversion.

Robert.

EDIT Julie beat me to it while I was typing.

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