3ph machinery powered by an inverter - is power an issue?

[Dissolve]

Hello,

I'm running a fobco star drill using a 1/2HP 3phase motor and a .4KW IMO inverter. I love the performance of this setup, but recently when testing some larger forstener bits, I've found the motor slows quite easily when running on a slow speed.

The forstner bits don't cut well and were very cheap, so I'm sure this doesn't help matters but what I wanted to know is if running the Fobco on it's fastest pulley ratio (4260rpm) does using a 10/15hz frequency on the inverter relate to this lack of power? Or would it be the same if I had a single phase machine and changed the pulley ratio to the equivalent RPM's?

Does the 1/2HP motor always kick out 1/2HP regardless of the inverters operating frequency? Or should I only use the inverter to adjust the RPM's Slightly?

 

[wallace]

I don't think using the inverter will neccesarily affect the power. If you were using a static converter then yes it would reduce power by 1/3. I think your problem is size of the drill motor in relation to what you are doing 1/2hp is small and you are trying to use a forstner bit slowly, which will put a huge strain on that little motor.

 

[CHJ]

.....
Does the 1/2HP motor always kick out 1/2HP regardless of the inverters operating frequency?

No, even given that the invertor may be configured to increase the power level (wave form adjust) a fraction to provide some compensation if you are reducing the frequency to 15 cycles it is only supplying 1/3 of the power pulses, reducing to 10 cycles it is only supplying 1/5 of the power pulses.

For maximum torque at the drill from your small motor you need to have your pulleys set for slowest drill speed (largest pulley)/maximum motor speed (smallest pulley) and run the inverter at a higher frequency.

 

[Dissolve]

.....
Does the 1/2HP motor always kick out 1/2HP regardless of the inverters operating frequency?

No, even given that the invertor may be configured to increase the power level (wave form adjust) a fraction to provide some compensation if you are reducing the frequency to 15 cycles it is only supplying 1/3 of the power pulses, reducing to 10 cycles it is only supplying 1/5 of the power pulses.

For maximum torque at the drill from your small motor you need to have your pulleys set for slowest drill speed (largest pulley)/maximum motor speed (smallest pulley) and run the inverter at a higher frequency.

Thank you for taking the time to explain this. My inverter is set up at the moment to run the motor at 0-50hz and the motor is rated at 50hz.

I've read that inverters can run motors at double speed, would this what you're referring to?

Should I keep my inverter set up to operate 0-50hz and simply change the pulley ratio when wanting to drill at slow speeds such as forstner bits?

I'd obviously like to reduce belt changes as much as possible, so if the inverter is best to run the motor at a maximum of 50hz then just switch the pulley ratio when wanting slow speeds high torque!

 

[CHJ]

Slowing the drill speed down with the pulley ratio gives you maximum torque, (like first gear going up hill)

There is nothing wrong with increasing the speed by upping the frequency if you decide to keep the pulley ratio biased towards lower drill speed.

 

[Dissolve]

Brilliant, so if my inverter is capable of running the motor at 100hz/twice it's intended speed, it's probably better to do this and keep the drill pulley on a slower setting than the other way round?

 

[CHJ]

Basic principle, yes, but I would limit the amount of invertor increase to 70-80 Hz max (assuming your inverter will go up this far) remember your motor has to absorb the extra energy you are inputting and cope with the increased stresses as is the pulley system, the motor will loose some efficiency/power as the frequency increases.
Just use increased motor speed with a bit of restraint as and when needed and experiment which combination of Frequency adjustment/pulley combination works best for your day to day use.

 

[Dissolve]

Basic principle, yes, but I would limit the amount of invertor increase to 70-80 Hz max (assuming your inverter will go up this far) remember your motor has to absorb the extra energy you are inputting and cope with the increased stresses as is the pulley system, the motor will loose some efficiency/power as the frequency increases.
Just use increased motor speed with a bit of restraint as and when needed and experiment which combination of Frequency adjustment/pulley combination works best for your day to day use.

Brilliant, thanks. I'm sure i've read that it is capable of doubling the motor speed but I'll be careful. I think I'll try with 75hz to begin with and see what happens. Appreciate the help. It's allowed to understand this in a practical sense without delving into theory which would probably boggle my mind! :lol: