Why single phase induction motors are used for fractional HP applications

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1. Mar 5, 2016

srinaath

why single phase induction motor is commonly used for low HP rather than integral HP like 1,2,3HP..... i understand that single phase gives low power....but say you have designed such that your motor delivers 10HP for rated voltage and current...why such single phase machines are not common???can anyone please explain and prove technically??

2. Mar 5, 2016

Hesch

10 HP ≈ 7360 W

Rated voltage = 115 V → Effective Current = 64 A
cos φ = 0.75 (assumed) → current = 85 A
Motor efficiency = 0.85 (assumed) → current = 100A
Starting current could be 4 times the nominal current (assumed) → Starting current = 400 A

Do you have fuses where you are staying = 400 A ?

The manufacturer of such a motor must have customers, thus 3 phases as for a 10 HP motor.

3. Mar 5, 2016

srinaath

sorry am not getting you..what i understood is that, you are saying that there is no 400A fuse for single phase? and how to relate this with three phase machine....please correct me if am wrong

4. Mar 5, 2016

Hesch

Of course there is a 400A fuse, also for a single phase. You can get a 1600A fuse, if you want.
But it's not quite "normal" to install a 400A fuse for a single phase. You will have to change the cables in your house before it is legal.
Using 3 phases you will only need 1/3 of the current per phase, and you can use a Y/Δ-starter to decrease the starting current.

5. Mar 5, 2016

srinaath

got it bro, thanks....but one more doubt...why efficiency of single phase machine is less than three phase machine??? moreover efficiency is directly proportional to power,why is that so? (PS please forgive for asking too much doubts)

6. Mar 5, 2016

Hesch

Because typically a 3-phase machine har more HP's than a 1-phase.
Now say that we have 2 machines: A) 1 HP , B) 10 HP.
The expenses building B will not be 10 times the expenses building A, but say 5 times. So the expenses as per HP will be less with B. Thus you have some money to invest in efficiency, thereby balancing the expenses for kW's and building expenses as for the lifetime of the motor:

It's a matter of economy (where to spend your money ).
Same reason: Minimizing total expenses as for the lifetime of the motor.

I don't think that the efficiency is "directly proportional" to power.

7. Mar 5, 2016

Staff: Mentor

A lot of the reason is cost and circuit breaker size/panel space.

There may also be a minimum wire size and fuze sizes, so you probably have issues properly protecting a 0.3A motor. (Not certain)

8. Mar 14, 2016

rbelli1

Also consider that consumer equipment tends to use small motors. You get enormous economy of scale for this size of motors if you use single phase since you can use the motor in a larger variety of devices.

BoB

9. Mar 14, 2016

srinaath

thanks rbelli1 for the input. i have a query what will be the size of three phase motor if power rating is 1HP in comparison with single phase motor? (2) i observed that efficiency of three phase motor is directly proportional to synchronous speed of the motor..how is that so?....(for example for 3600rpm-80% eff but 1200rpm-75% eff)

10. Mar 14, 2016