AB PowerFlex VFD's

[alaskanassassin]

I am curious about what kind of motors you can install VFD's on, I know there must be certain types of motors you can install them on. These units would be for 480VAC three phase applications. Thanks Doug

 

[Jraef]

As a general rule, any new applications for VFDs should have what are commonly referred to as "inverter duty" motors. VFDs use a PWM output that is like AC, but is really a series of DC pulses. These pulses can cause voltage spikes that will damage older motors, especially at voltages over 230V. This is because under the right circumstances (and Murphy's Law will apply), the spikes will exceed the insulation rating of older motor windings and cause small phase-to-phase shorts, but which rapidly lead to overall failure. Inverter Duty motors use much higher insulation values in the magnet wire used for the windings so that they can survive that onslaught. There are also cooling issues because if you are slowing down the motor with the VFD (its primary purpose), then you are also slowing down any cooling fans the motor may have been relying on. So inverter motors often have externally powered constant speed fans on them.

If you have existing motors, and they are ones that do not need external cooling fans, i.e. TENV, then you can often use load filters to mitigate the voltage spike problems, rather than replace the motors. But there are a lot of other issues to consider as well, there is no "one size fits all" answer.

 

[lakee911]

So inverter motors often have externally powered constant speed fans on them.

I'm not sure that I've seen many of these, but I typically deal with centrifugal pumps and generally we're as high as maybe 40-some Hz before we can't pump any less and that's generally still fast enough for cooling.

I've seen some PD pumps, though, without externally powered cooling fans and the manufacturer only allowed us to turn them as slow as 6Hz. I don't know if it was because of cooling though.

A lot of times, people will take their chances with old existing motors until they burn up and then they will replace them. They really don't build them like they used to and some of those old motors are actually much beefer than something you can buy today. Sometimes they work just fine...for like ever...dude.


Jason

 

[weressl]

As a general rule, any new applications for VFDs should have what are commonly referred to as "inverter duty" motors. VFDs use a PWM output that is like AC, but is really a series of DC pulses. These pulses can cause voltage spikes that will damage older motors, especially at voltages over 230V. This is because under the right circumstances (and Murphy's Law will apply), the spikes will exceed the insulation rating of older motor windings and cause small phase-to-phase shorts, but which rapidly lead to overall failure. Inverter Duty motors use much higher insulation values in the magnet wire used for the windings so that they can survive that onslaught. There are also cooling issues because if you are slowing down the motor with the VFD (its primary purpose), then you are also slowing down any cooling fans the motor may have been relying on. So inverter motors often have externally powered constant speed fans on them.

If you have existing motors, and they are ones that do not need external cooling fans, i.e. TENV, then you can often use load filters to mitigate the voltage spike problems, rather than replace the motors. But there are a lot of other issues to consider as well, there is no "one size fits all" answer.

I have specified hundreds of motors for ASD service, I yet to have an application that would require an external control fan. Variable torque loads - 95% of the applications - do not require additional, external cooling. Perhaps the only exception to this would be open fan's in hurricane alley wher you would inject current to hold the fan blades in place against the wind.

 

[robbietan]

As a general rule, any new applications for VFDs should have what are commonly referred to as "inverter duty" motors. VFDs use a PWM output that is like AC, but is really a series of DC pulses. These pulses can cause voltage spikes that will damage older motors, especially at voltages over 230V. This is because under the right circumstances (and Murphy's Law will apply), the spikes will exceed the insulation rating of older motor windings and cause small phase-to-phase shorts, but which rapidly lead to overall failure. Inverter Duty motors use much higher insulation values in the magnet wire used for the windings so that they can survive that onslaught... There are also cooling issues because if you are slowing down the motor with the VFD (its primary purpose), then you are also slowing down any cooling fans the motor may have been relying on. So inverter motors often have externally powered constant speed fans on them...
.

I have seen some motors overheat as the motors are placed in a hot environment and the slowing of their cooling fans have contributed to the deterioration of the motor.

we have also recommended line/load reactors to motors with VFD, to protect them from VFD harmonics, among other things

 

[Jraef]

Variable torque loads - 95% of the applications - do not require additional, external cooling.

I'm not sure that I've seen many of these, but I typically deal with centrifugal pumps and generally we're as high as maybe 40-some Hz before we can't pump any less and that's generally still fast enough for cooling.

I don't disagree in either case, but the OP did not state the nature of the connected load.