AC Induction motor start with inverter
AC Induction motor start with inverter
(OP)
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?
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?





RE: AC Induction motor start with inverter
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: AC Induction motor start with inverter
RE: AC Induction motor start with inverter
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
RE: AC Induction motor start with inverter
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
RE: AC Induction motor start with inverter
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
RE: AC Induction motor start with inverter
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
RE: AC Induction motor start with inverter
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.
RE: AC Induction motor start with inverter
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
RE: AC Induction motor start with inverter
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
RE: AC Induction motor start with inverter
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
RE: AC Induction motor start with inverter
Eng-Tips: Help for your job, not for your homework Read FAQ731-376