VFD Hz HP correlation

I heard that there is a dirrect correlation between hz and hp. Not sure if it is linear though.

When running a 220v 3phase motor on 220 single phase power,
and Motor rpm is 1750 @ 60hz,
when you drop the hz to 30hz rpm is half of 1750,
is hp/torque halved as well?

So if I have a piece of machinery that I power with a .5 hp 3 ph motor @1750rpm@60hz,
if I slow it down to 291 rpm I think the hz is at 10hz,
Does the effective hp/torque become .083 hp?

Jonathans said:

I heard that there is a dirrect correlation between hz and hp. Not sure if it is linear though.

When running a 220v 3phase motor on 220 single phase power,
and Motor rpm is 1750 @ 60hz,
when you drop the hz to 30hz rpm is half of 1750,
is hp/torque halved as well?

So if I have a piece of machinery that I power with a .5 hp 3 ph motor @1750rpm@60hz,
if I slow it down to 291 rpm I think the hz is at 10hz,
Does the effective hp/torque become .083 hp?

Click to expand...

Assuming the motor base speed is 1750rpm, then yes to both questions.

Before you ask 3rd question, no. HP remains constant above base speed.

Sent from my SM-G900V using Tapatalk

Thanks Mike,
Yes, that was my third question!

HP is an expression of torque and speed together, so if you keep the torque the same and reduce the speed, the HP value is reduced by the same percentage. But that is HP CAPABILITY of the motor, not the actual HP used by the load. The HP used by the load is whatever the load needs, UP TO AND INCLUDING the capability of the motor.

Jraef said:

HP is an expression of torque and speed together, so if you keep the torque the same and reduce the speed, the HP value is reduced by the same percentage. But that is HP CAPABILITY of the motor, not the actual HP used by the load. The HP used by the load is whatever the load needs, UP TO AND INCLUDING the capability of the motor.

Click to expand...

So.....whats the relationship between Hz and torque?

I'm pretty sure torque is constant below 60Hz, and falls linearly above 60Hz.

Jonathans said:

So.....whats the relationship between Hz and torque?

Click to expand...

NONE.

Hz is related to SPEED: RPM= 120* HZ / no_motor_poles

Torque = 3/2 *Zp * L^2m/Lr * Yrd * Isq
where:
Zp=Pole pairs
Lm=coupling inductance
Lr=rotor inductance
Yrd=rotor flux
Isq=torque producing current

You DID ask!

The whole point of a VFD is mainly to change (lower) speed WITHOUT LOSING TORQUE. So if your load requires a certain amount of torque to operate, but you want a different speed, the VFD allows you to slow the motor down without losing torque. If you were to lower the frequency alone, you will saturate the motor windings and they over heat without producing more torque and the motor burns up. If you change the voltage alone, you lose torque at the square of the voltage change and the motor stalls (and burns up). What the VFD does is MAINTAIN the same ratio of voltage and frequency that the motor requires to produce its rated torque, changing them at the same rate together. So at 50%Hz, the voltage is 50% of rated. At 25%Hz, the voltage is 25% of rated. Torque remains constant though.