AC Induction motor start with inverter

[slowwilly]

I am a mechanical engineer with very limited AC motor experience, so please be patient with me!

I would like to run a single phase AC induction motor off a DC system via an inverter, but I'm worried about motor start.

The inverter is capable of 8.7 Amps continuous, 18.8A surge. Motor running load is 6.6A. Motor LRA rating is 50 A. I don't have high speed measuring equipment. Is there any way to estimate start up current?

Also, the motor manufacturer has an optional start capacitor/potential relay. Would it help with start up to obtain these?

 

[itsmoked]

What is the motor's load going to be?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[slowwilly]

It's a compressor motor, about 1.5 kW. The motor is set by the compressor manufacturer, I'm just trying to determine if I can give the customer what they're asking for (ie: ability to start/run off the selected inverter).

 

[Marke]

Hello slowwilly
In order to start that motor, it is going to want to draw LRA irrespective of the connected load. The connected load will determine how long it takes to get to full speed and therefore the duration of the overload.

It is most likely that when you try to start this motor off the inverter, that the inverter will trip out on overload. It is possible that the inverter will current limit at 19 amps and the reduced voltage on the output may be sufficient to start the motor, but probably not, plus there would be a severe voltage reduction that would drop out other connected load.

It is also possible that the major overload presented by the motor could damage the inverter, depending on the design of the inverter.

I would suggest that you ask the inverter supplier what the effect on the inverter would be of connecting a 50 Amp load to it's output.

I expect that this is not a viable option based on the information given.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[waross]

Check with the Invertor Manufacturer/Vender. Some invertors are rated for motor starting. Invertors for the residential market often are rated for starting motors such as refrigerators. The starting current will start at or near the LRA and drop to the running current as the motor accelerates.
The optional capacitor sounds like a good idea.
I don't think that 18.8 amp surge capability will be sufficient to handle 50 amps locked rotor current.
You may be able to locate a similar size invertor with greater surge capability, or you may need a larger invertor.
Get the invertor manufacturers specs on motor starting.
yours

 

[Barry1961]

There used to be a company that made inverters for single phase motors but I thought they folded.

Are you going to feed DC into the bus of the inverter? If so what voltage DC do you have?

Can you replace the single phase motor with a three phase?

Barry1961

 

[jraef]

Assuming you get beyond the primary issue raised by Marke, a word of caution to back up what waross posted: starting caps can be troublesome for inverters, make sure that whatever inverter you select is capable of feeding induction motors, not just a 50A surge capacity. Often there is a caveat in the inverter specs that says it is only suitable for use on PSC and Shaded Pole 1 phase motors, and if your starting cap is optional, it does not appear to be either one.

Conversely, make sure the potential relay is compatible with having an inverter supply. If not, it may energize too quickly and take the cap out too soon to be of any use.

Barry makes a good point, there are many VFDs that can be fed with DC and provide 3 phase output, even if you don't need the variable speed capability. At 1.5kW however, your DC input will need to be around 300V because that is all that is made.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[slowwilly]

Thanks all for the excellent information. The inverter manufacturer was concerned about the locked rotor current. The compressor manufacturer said that the start up current was less than the locked rotor current. I was in the middle scratching my head.

12 VDC - We can get 3phase compressor motors, but the inverter we were looking at is single phase. Perhaps there are 3 phase manufacturers out there?

The problem is that having to size the inverter for LRA puts it out of the acceptable price range. And, other than start up, it runs at a fraction of it's rated power.

Thanks all for the excellent and rapid responses. They are greatly appreciated.

 

[Marke]

Hello slowwilly

You could use the inverter you propose along with a single phase input three phase output variable frequency drive and a three phase motor. The variable frequency drive converts single phase into three phase and will restrict the current drawn to what is required by the motor to run at speeds from zero to full speed. The motor is not operated under high slip conditions and therfore does not draw sever overload currents. You would need to confirm that the inverter would be OK with a speed controller on it's output.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[itsmoked]

Yo Marke; What about the OPs 12VDC? In the inverter solution... I haven't seen any 12VDC to 240Vav 3ph output VFDs.

Or are we talking 12Vdc to 120Vac(inverter) to 120Vac input-doubled inverter to motor? Won't a single phase voltage doubling input VFD thrash most inverters?

slowwilly; Is this a stationary system or some sales tool that has to be lugged by mortals?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

The problem is that having to size the inverter for LRA puts it out of the acceptable price range. And, other than start up, it runs at a fraction of it's rated power.

You may be right, but I wouldn't give up without first checking the specs on other inverters.
Is this an air compressor or a refrigeration system? Belt drive or sealed? For refrigeration there used to be 12 volt DC refrigerators and freezers available.
respectfully

 

[jraef]

Keith, I interpreted Marke's response that way, use the commercially available inverter to go 12VDC to 120VACm then use a 120VAC 1 phase VFD to start and run a 220V 3 phase motor. Cumbersome, but it gets around all the limitations.

Eng-Tips: Help for your job, not for your homework Read faq731-376