inverter and motor

[richerdick]

I wish to replace motor gearbox combination that is driven via a dc drive, with an ac motor and new gearbox fed from an inverter.
the system is designed to run at two speeds at its output low speed is 8rpm high speed 45rpm
the current gearbox output is rated at 50nm.
I am aware of the difference in starting charichteristics of ac and dc motors and aware that inverter technology limits the torque of a motor at low speeds
how do i go about sizing the inverter motor and gearbox for this application, i am tempeted to look at the KW rating of the existing motor and just oversize everything and trim it in with the inverter and ircomp facility but really i would like a more informed approch can anyone provide me with some guidance, formulae or poibt me in the right direction

 

[itsmoked]

Lets start your education.

Inverter technology is better stated as a "Variable Frequency Drive" or VFD. This is because an inverter can be many different things. A VFD is an inverter but it is much, much, more being variable and outputting complex waveforms not generally ascribed to an "inverter". A more classic inverter would be the ones in your computer's UPS (Uninterruptible Power Supply) which takes DC and inverts it into a fixed non-complex AC waveform to closely simulate your local AC power.

Next; a VFD can provide the motor's normal full torque right down to a fraction of an RPM. This will impact favorably, the solution you are seeking.

Also keep in mind that a 3ph motor is much more efficient than your existing DC motor so I am not seeing a need to 'up' your power requirements.

I will get off now and let others who live this particular subject to give you the other details you seek.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[DickDV]

richerdick, replace the DC motor with an inverter-duty AC motor of the same base speed and kw.

Select a drive with sensorless vector capability and also encoder feedback capability as a safety net if you end up needing it. Do not buy an encoder for the motor unless sensorless vector proves inadequate.

You will find, if properly commissioned and tuned, that the AC system will match the DC performance usually without the encoder option being used. If you, for example, have to develop full motor torque at zero speed and hold the shaft perfectly still while doing it, you may need to add the encoder. Otherwise, probably not.

The days of oversizing everything when switching from DC to AC are over.

Good luck.

 

[jraef]

I agree. It appears as though your notion of AC VFD performance may be a little outdated. While your statement about low speed starting torque limitations of AC inverter fed motors was true a few years ago, "Vector" technology, particularly "Sensorless Vector" technology has improved so much and costs so little now that all but the absolute cheapest VFDs have it and most of them are well executed. DickDV's suggestions are right on target. Here is some additional information for you.
faq237-1062 (click on this to be redirected)

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[CJCPE]

Although modern VFDs can provide full torque over a wide speed range, the motor may be limited. Motors are cooled by an internal or external fan on the motor shaft. When the motor is operating at low speed, the fan may not move enough air. You need a VFD motor rated for the required full-torque speed range (5.6:1 in your case). You may also need to make sure that the gear is rated for the same speed range. It is possible that some minimum input speed is required for proper lubrication.

Since you only need 50nm at 45 RPM, the output power is less than 1/4 kw. The gear’s input power requirement should be stated by the manufacturer or the gear should come mounted to an adequate motor. The required motor may be 1/2 kw or more, but you may still find a limited VFD offering at such a low power rating. The VFD should be a sensorless vector type rated for constant torque duty.

If the existing DC drive is a simple design without a tachometer, it is unlikely that the VFD will need encoder feedback.