# Converted power range of three phase motor

• posted

Hi.

I have 11kw 3 phase motor controlled by variable frequency drive. The drive can be powered with one phase and it can give 3 phase output. So i was thinking to try this solution. I will connect one phase to the drive to see how much power can i get on motor side.

My question would be, how to calculate maximum horsepower or KW used by

3 phase motor, if he is be connected to 220 / 50 HZ - 8KW source ?

Regards !

• posted

Well, first you look at the nameplates of the VFD and the motor. Is the VFD designed to work off of single-phase, or does it just sorta-kinda work? What is the VFD rated power? What is the motor rated power? What is the VFD efficiency? Does the VFD list current draw?

If the VFD is well documented, the maximum power _available_ to the motor is either what the VFD is capable of, or what the circuit feeding it is capable of (and if your VFD draws more current than your circuit can supply, you'll be tripping breakers). But the amount of power that the motor will _actually_ draw depends on what the motor is, what it's driving, and (possibly) what the VFD is set to.

I'd suggest that you back off a bit, and tell us the story: what VFD do you have, what motor do you have, and (importantly), what is the motor hooked up to and what are you doing with it (i.e., lots of starts and stops, continuous run, etc.)

You may also want to consider cross-posting your response to rec.crafts.metalworking. If you can ignore the level of (American) political foo-faw on the group, there are some helpful folks there with this sort of question, who have a lot more practical experience with motors and VFDs than a crew of circuit design engineers.

--
Tim Wescott
Control system and signal processing consulting
• posted

sqrt(3).

Jamie

• posted

"Jamie" wrote in message news:kzJbs.240\$ snipped-for-privacy@newsfe02.iad...

Nope. Power factor goes from about 0.9 to 0.5. It's less than half, amp for amp.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
• posted

Ok, so where's the problem? 0.9/sqrt(3) = 0.52

If there is going to be a useless debate, I won't be joining in.

Jamie

• posted

"Jamie" wrote in message news:ETKbs.1980\$ snipped-for-privacy@newsfe13.iad...

No, I mean in addition to the sqrt(3). You get the sqrt(3) from having two wires instead of three. 10A at 240V 3ph is 4.16kVA, 10A at 240V 1ph is 2.4kVA. The power factor is additionally less, or more like 1.2kW for

10A 240V.

I should actually say it's more like a quarter, amp for amp.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
• posted

output.

the

source ?

I have a 1.5 kW VFD that will work on single phase, but larger drives typically need three phase for full power operation. If you can add enough capacitance on the DC link to maintain minimum peak voltage during zero crossing, you might get full rated power. Drive efficiency is typically

95%, so you could get about 7.5 kW from an 8 kW source. However, you need to consider power factor of the rectifier/capacitor circuit as well as that of the motor. So if your source is actually 8 kVA, and power factor is 60%, you might get only about 5 kW for the motor. If you use a PFC DC-DC converter to get the DC link voltage, you might get the full 7.5 kW for the motor, but it also depends on the PF of the motor and the power rating and losses of the VFD.

If you add capacitors to the DC link, you may need to use larger rectifiers than the 3 phase bridge in the drive. For 50 Hz you need a TC of at least 10 mSec, and the effective load is about 360V/36A = 10 ohms. Thus about

1000 uF might be sufficient.

Paul

• posted

output.

the

source

enough

zero

typically

need

factor

kW

power

TC

ohms.

========================= ========================= ======== (nescafe replied by email, 3 times):

Thank you very much for this answer. I got some additional answers on one other newsgroup but the idea is almost the same. 2/3 of the power should be reached.

I use siemens micromaster that is connected to 11kw motor so if i have

8kw input and if i calculate the 2/3 of the original power then 7,5 should be reachable. The 8kw is real 8kw input.( 220 - 40A )

I dont understand the dc-dc part. My micromaster has dc-dc input link. Do you think i have to add something to dc link. Sorry for this dump question but i didn't work with this kind of configuration till now.

========================= ========================= ======= My reply (which apparently was not received, perhaps because may be bogus?

Actually what I meant was an AC-DC power supply with PFC. If you just use AC through rectifiers and a capacitor, you will get a very low power factor and will draw about twice the current compared to a supply with PFC. And it might be difficult to find a DC supply with the proper specifications. (360V

36A). In fact, if you can get close to unity PF, you only need 22.5 amps for 8 kW at 360V. So you might be able to use seven 48VDC 25A 1200W power supplies to get the DC power you need.

You might be able to find a 10 kW single phase 240V UPS with PFC. It will probably have an internal 300VDC bus you could use for the DC link.

Paul

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