3
\$\begingroup\$

A 6 pole 3-phase AC induction motor is delta-connected and has the following rated values on its nameplate:

  • Frequency: 50 Hz
  • Power: 22 kW   30 hp
  • RPM: 970
  • \$\cos \phi\$ (power factor): 0.80
  • Primer voltage (\$V_{\text{line}}\$): 380 V
  • Primer current (\$I_{\text{line}}\$): 47 A

Since the total power of a 3-phase AC induction motor when delta-connected is

$$ P_t = \sqrt{3} \cdot V_{\text{line}} \cdot I_{\text{line}} \cdot \cos \phi $$

I calculate the electrical power as

$$ P_t = 24.75 \;\mathrm{kW} $$

but on the nameplate there are two powers, written as 22 kW and 30 hp (30 hp is 22.37 kW); are they the same thing?

Why am I calculating the power \$P_t\$ differently? Are the power ratings on the nameplate mechanical power or electrical power?

\$\endgroup\$

1 Answer 1

Reset to default
3
\$\begingroup\$

The power that can be read on the name space is the mechanical power you can take from the shaft of the motor. What you calculated is the electrical power that you put in the motor. There has to be a difference due to mechanical/electrical losses (friction, heat etc.) in the motor.

Concerning the 30 hp: I think 1 hp = 736 W, so 30 hp =22080 Watts which is close enough to 22 kW.

EDIT

According to their formula: $$ HP = \frac{1,73* E *I * Eff*PF}{746} = \frac{1,73 * 380 V * 47 A * (\frac {22 kW}{24,75 kW}) * 0,8}{746} = 29,5 hp $$ That should be close enough to 30 hp.

\$\endgroup\$
5
  • \$\begingroup\$ i thought so, but please check out this link from a very trustable manufacturer: ab.com/support/abdrives/documentation/fb/1018.pdf if you look at carfully you see that kW and hp are diffrent things(efficiency makes the difference). thats why i was confused. \$\endgroup\$
    – user16307
    Commented Sep 14, 2015 at 17:02
  • \$\begingroup\$ 736 W is "metric horsepower". I never met a metric horse, though. It's 746 W otherwise...go figure. \$\endgroup\$
    – gbarry
    Commented Sep 14, 2015 at 18:24
  • \$\begingroup\$ Technically, both hp and kW measure power. In Europe, they often specify mechanical power in W and kW rather than horsepower. The electrical power input to a motor is always higher than the mechanical power output due to efficiency being less than 100%. Basically, that whole entire sheet is telling you that power out = power in * efficiency. There is some other stuff dealing with power factor, also. \$\endgroup\$
    – user57037
    Commented Sep 14, 2015 at 18:52
  • \$\begingroup\$ If you actually look at that link, you will also see that the difference is between INput power and OUTput power. The efficiency is applied to the OUTput power. \$\endgroup\$
    – R Drast
    Commented Sep 15, 2015 at 12:13
  • 1
    \$\begingroup\$ Answer is correct. A 22kW motor produces 22kW of mechanical shaft power. Calculating volts × amps gives the electrical input power in kVA, which includes efficiency and power factor. </ Practicing electrical engineer>. \$\endgroup\$
    – Li-aung Yip
    Commented Sep 15, 2015 at 14:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.