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Using PTCs on Small Motors With VFDs

11/16/2009 5:29 AM

Is it typical for all motors powered by VFD to use PTCs for thermal protection...

OR

Is there a typical minimum size before requiring PTCs?

Thanks in advance

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#1

Re: Using PTCs on small motor with VFD

11/16/2009 5:56 AM

No, the motor will be protected by the vsd even without ptc's fitted to the motor.The motor details need to be programmed in the vfd for this to be affective however, ie motor voltage, running current, frequency, speed of motor and power factor of the motor.

PTC's fitted in motor windings is not mandatory and each manufacturer decides from what size motor to fit the ptc's. This is just additional protection for the motor.

In some cases the large end users have their own specifications requiring ptc's. They will also then specify from what size of motor.

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#2

Re: Using PTCs on Small Motor With VFD

11/16/2009 1:07 PM

The reason why it is a good idea to use PTCs however is because when the motor is turning very slowly, the current going to it may be very low and not show up on the thermal model in the drive as a problem. But most VFDs do not have the ability to alter their motor thermal model algorithm to adjust for the lack of cooling from the fact that the cooling fans may not work at low speeds. So the PTC thermistors will override any lack of motor thermal overload protection by looking at actual motor heat rather than calculated heat.

And that can happen with ANY size of VFD. The only time I tell people not to bother with PTCs is if the application involves something like a centrifugal pump where it will not be used if the speed drops much below 30%, because the pump will stop pumping anyway.

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#4
In reply to #2

Re: Using PTCs on Small Motor With VFD

11/17/2009 4:29 AM

Regards.

A GA for good answer.

I like to add:

Most of the motor drives especially Varieable speed may be used at low freqs for quite a long time to go to full speed, in those motors Over-temperature protection is a good idea.

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Power-User

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#5
In reply to #4

Re: Using PTCs on Small Motor With VFD

11/17/2009 4:37 AM

PTC is available as a factory fit on most industrial frame sizes but it is not such a good idea to retrofit if the motor winding is not already prepared for PTC. If you insert where no space exists this may cause PTC or wire strain leading to failure.

Some motors have space inserts placed which allow PTC to be fitted later but best is to specify at point of manufacturing.

VFD overload works on current so if the relationship is incorrect (v/f wrong or motor misconnected) you can see 'normal' currents at poor efficiency leading to failure.

Correctly set, VFD overload protection is very good (particularly coupled to output phase loss protection which some do not have), however, motor PTC is more complete protection all round, if it gets too hot, it activates well before insulation damage occurs.

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#7
In reply to #5

Re: Using PTCs on Small Motor With VFD

11/17/2009 12:47 PM

Quite OK

All VFDs has the V/HZ ratio which is normally called V/F ratio. As the frequency increases voltage increases, which means the Start-up voltage is very low as at lower freq current through a wound component [Transformer or Motor Widing].

PTCs or Over-Temp trips, Resettable fuses are fitted in the upper layer of Static windings and such components can be replaced and there is no danger of damage to those, originally fitted or fitted afterwards.

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#3

Re: Using PTCs on Small Motors With VFDs

11/16/2009 4:30 PM

thank you for the thoughts. Very much appreciated

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#6

Re: Using PTCs on Small Motors With VFDs

11/17/2009 5:28 AM

I disagree with every response, thus far. Let's start with the definition and the acronym:

Positive Temperature Coefficient; it senses (primarily) the temperature on the body of the device and may, or may not sense motor temperature. This is dependent on several facotrs i.e. is the PTC in contact with the motor body? It's main function IS NOT to sense motor temperature, but to vary capacitance as a function of AMBIENT temperature.

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#8
In reply to #6

Re: Using PTCs on Small Motors With VFDs

11/17/2009 5:21 PM

You are technically correct but practically off topic. In the given context of motor protection, PTCs are used to allow tripping of a device that senses that change in resistance when the motor heats up.

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#9
In reply to #6

Re: Using PTCs on Small Motors With VFDs

11/17/2009 8:26 PM

All the PTC's I know of are inserted an the end of the stator windings to sense (and then protect) from temperature related insulation failure.

I have also personal experience of PTC failure in aftermarket insertion due to mechanical constraints, in my opinion.

Furthermore, the temperature of the motor is a function of the power delivered and the motor efficiency, if the motor is severely over or underfluxed the potentail to overheat is greatly increased.

PTC is generally the best way to protect a motor from failure due to overtemperature for whatever reason and generally available down to Fractional HP sizes.

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#10
In reply to #6

Re: Using PTCs on Small Motors With VFDs

11/18/2009 7:14 AM

Post #9 is correct. Temperature of Stator is monitored & safety of stator is attained.

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#11

Re: Using PTCs on Small Motors With VFDs

11/24/2009 5:30 PM

Puting any (meaning ANY) control cirquitry between VFD and motor is a recipy for disaster and if motor already has any it must be removed or isolated. Modestly programming VFD values is the way to go. If it's critical to do something more put temp sensors on the motor to signal to the VFD it's time to stop at overheats. Remember VFD's are more expencive than motors yet.

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