On Fri, 27 Dec 2013 09:49:15 -0800,
et...@whidbey.com wrote:
>On Fri, 27 Dec 2013 09:23:30 -0500, Bob Engelhardt
><
BobEng...@comcast.net> wrote:
>
>>On 12/26/2013 9:43 PM, Gunner Asch wrote:
>> You lose the horsepower if you match the VFD to the motor. If you
>>> DOUBLE the size of the VFD..you loose very little if any power.
>>
>>I don't think so ... horsepower is torque x rpm; torque is proportional
>>to current; current is limited by motor windings.
>>
>>Bob
>What Bob says is true. Induction motors are designed to work at a
>certain frequency and voltage. If the frequency is increased then the
>voltage should be increased too in order for the motor to have the
>same torque. This results in the motor developing more horsepower as
>the speed increases. However, most VFDs will not increase the voltage
>along with the frequency.
All VFDs adjust voltage along with frequency. In fact, the simplest
open loop VFDs are often referred to as V/Hz (volts per hertz) drives
to reflect the fact that they maintain a constant ratio between
voltage and frequency, as opposed to the more complicated schemes used
by closed loop drives.
>This means that when the motor is running
>faster it will have less torque and instead will have a constant power
>output.
Generally speaking, with a VFD, torque is constant from close to zero
up to base speed. HP is constant above base speed.
>The VFD will also not reduce the voltage when it lowers the
>frequency because this would result in a loss of torque. So as the
>frequency drops the voltage still stays the same.
The winding current will rise and the motor will overheat if the
voltage is not reduced along with frequency.
>This results in
>motors operating slower than the nameplate rating becoming constant
>torque machines and motors operating above the nameplate rating
>becoming constant HP machines.
Bob was right when he said the torque is a function of current. The
voltage is reduced to control the current, and thus the heating, of
the motor. The constant torque is a consequence of the need to limit
current.
--
Ned Simmons