To control the load of a three-phase electric motor is it better to measure the current I (Amperometrics) or the Power W (Wattmetrics)?
The MOTOR absorbs the active power Wa=1,73 V·I·cosφ from the mains, typically supplied at 400Vac.
The MOTOR transfers the power Wr = Wa x E (E=Efficiency) to its shaft.
A typical value of E can be 0,9.
Everything connected to the drive-shaft constitutes the LOAD of the motor.
We consider a motor with Wn=8kW Vn=400Vac.
The three-phase electric motor has the feature of absorbing the quantity of power Wa necessary to supply the corresponding Wr to the LOAD of the moment and this is valid for all values of Wr lower than Wn.
Example: in a continuous application, we suppose that Wr=7kW.
The motor, to supply Wr=7kW to the LOAD, will absorb from the mains
Wa = 7kW/0,9 = 7,77kW
The equality can be written as follows:
Wa = Wr/0,9 = 1,73 V·I·cosφ/0,9 (1)
in this equality we see that the Wa and Wr values depend on the values of the 3 quantities: V, I, cosφ.
The V is supplied by the electric energy distributor, and in first approximation it is considered constant, but in practice variations of 5-10% are possible.
In the equality (1) we see that, considering V=constant, the value Wa=7,77kW depend on I x cosφ, if I x cosφ doubles also Wa doubles, so Wa is linear with I x cosφ.
The amperometric control provides for the control of I (amplitude of the absorbed current) only.
Wr is proportional to I, but it is not linear with I.
To control a physical quantity means to measure the quantity and to compare it with a SET POINT, and there must be the possibility to adjust the SET POINT “close” to the quantity value in order to detect even a small variation of the quantity.
The Fig. 2 shows the case of a V voltage lowering, with Wr=constant, from the equality (1) it follows that I must increase and if it overcomes the SET POINT, a false increase of Wr is signaled.
Therefore:
1°) The Wa (power absorbed by the motor) is proportional to the motor load so, controlling Wa, a direct control of the motor LOAD is carried out.
2°) The control of a motor with a Wattmetric (which controls all 3 quantities: V, I, cosφ) is more complete.
REPLY: The Wattmetric control of the LOAD of a three-phase electric motor is more precise than the Amperometric control which can produce false alarms or failed triggerings.
The choice depends on the type of function required to the motor.
