Vsd during running would result into some losses in terms of heat..my doubt is whether energy saving can be achieved at every range of reduced speed ( say @95%) considering those losses?
Friction and windage loss will reduced considerably which is less than 2% of total power consumed by motor, but for same power out put(H.P) as speed reduces the torque needed to be increased, hence more current will be drown, and will increased the copper loss, of both stator and rotor. Hence for same H.P out put the efficiency will drop considerably, for induction motor. For D.C motor(shunt), by reducing speed from rated to lower speed, as the torque remains constant, the input will be less, for less output at lower speed as the input also reduces, and as friction windage loss also reduces, the efficiency will increased at lower speed.
The amount of energy savings will partially be based on the application.
Example: A fan
For two 50 kW motors running at 100%, the motor with the VFD will require a little more power due to the losses associated with the VFD.
However, say the fan is to run at 75% flow, and due to the Fan Affinity laws, the 50 kW motor would draw approximately 22kW.
You'll need to understand your application clearly because if you don't get much turn down on your motor a VFD may not save you anything.
The energy saving definitely depends on the torque-speed characteristics of the load and type of cooling of motor. For a constant torque type of load, input current will be high even at low speed. Thus, copper losses will remain high. Now, if the motor has an integral cooling fan, its air flow will become reduced, causing inadequate cooling. Hence winding temperature will become higher, leading to still further increased copper loss ! For a fan or pump type of load, the torque demand at low speeds is reduced, leading to a better situation. In applications where torque becomes reduced at low speeds, some drives operate with reduced flux at such conditions (sensing low torque demand) to reduce iron loss so that efficiency can be improved through energy saving.
IMHO The question is ambiguous for the following reasons
1. Is it the invertor +motor unit alone that is being considered or plus
the application(fan?/ Pump ) .
2 The answer of Fred is appropriate for I+M+P . The variable speed
is efficient when the flow for the application needs to be regulated. As I understand
the old method was to use "Throttles " or "Valves" in process industries
because the standard induction motor was basically fixed speed.. this resulted in
Energy loss .. and the flow could be controlled only in steps..but with a Vsd it is
possible to set speeds smoothly. as only speed (frequency) of the motor is
varied. The alternative in old days would be voltage control/ slip ring motors
or dc motors .. all not really attractive..from various points..of view.. so as Fred has
stated Vsd for energy saving depends on application
3. If the Q refers only to I+M .. then the answers of Sujit and Bhupendra
are applicable.. roughly in Vsd we can expect efficiency to deteriorate
at lower speed because i^2r loss will be more or less constant + although iron
loss +windage decreases with frequency... while Pout will decrease..Here we should also mention the load at which we compute these figures..I think it is usual
to expect max eta at 75% of rated load..as I understand the motors for VSD
are specially designed...(including choice of bearings / insulated end shield
for shaft voltage etc..)
Cheers
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