Operating Induction Machines (S300-S700)

To enable the servo amplifier to also operate induction machines with the exact speed, the control structure had to be extended. For the additional elements, you must adjust the corresponding parameters.

In all sensorless modes, the output frequency is limited to 599 Hz. If your application requires a higher output frequency, please contact our support.
This page explains, how the asynchronous machine is parametrized

Picture 1: Control structure for the induction machine

Description

To control induction machines the existing control structure of the S300/S700 essentially has been extended by four elements (picture 1, grey background):

1. flow estimation
2. flow control
3. slip calculation, and
4. weakening of the field

These elements are parametrized by the following new, or changed ASCII - commands.

New and changed ASCII - commands

MTYPE	induction machine = 3
MIMR	Current setpoint in D direction (A)
GF	proportional gain flow control
GFTN	time constant flow control (ms)
MTR	time constant rotor (ms)
MVR	nominal speed for the weakening of field (rpm)
MCFW	parameter for the weakening of field
GU	proportional gain voltage controller (weakening of field)
GUTN	time constant voltage controller (ms) (weakening of field)
UMAX	maximum output voltage (only active in weakening of field)

Base speed range

In the base speed range (up to the nominal speed), the field forming current (D current) remains at its nominal value (MIMR). These are the basic adjustments for the operation of the induction machine. If you plan to operate the machine within the range of weakening of field, you should nevertheless adjust all parameters in the base speed range (MVR 0) first. The control structure is simplified to picture 2. Afterwards you should activate the weakening of field (MVR = nominal speed). (picture 1)

Picture 2: control structure for the induction machine in the base speed range

MTYPE

In addition to the alternatives synchronous machine PM (MTYPE = 1), and linear motor (MTYPE = 2), it is possible to choose the induction machine (MTYPE = 3).
This adjustment activates the flow estimation, the flow control, and the slip calculation.

MIMR

Desired current (without weakening of field) in rotary direction.

For most of the induction machines , this means .Thus, can be considered as standard value.

With this value you should be able to obtain the nominal speed at nominal voltage without further weakening of field. Otherwise, the value for MIMR must be reduced even further. In general, to control the MIMR value, you can let idle the machine at nominal speed (weakening of field turned off, see below). Then the MIMR value is gradually increased until the speed cannot be kept any more, and diminishes by itself. The optimum MIMR value is at 5 - 10% lower.

GF and GFTN

The default values for GF respectively for GFTN are 1 respectively 100ms. Normally, they can be left at these values, as the rotor's flow only reacts very slowly.

MTR

If the values from the T - equivalent circuit diagram (picture 3) are available, MTR is calculated as follows:

Picture 3: T- equivalent circuit diagram of the induction machine

MTR = with
;

Alternatively, you can estimate the value for as follows:
Disregarding the following equation applies :
;MIMR

with

nominal power and ;
(synchronous speed) and (nominal torque)

The default value for MTR is 100ms. Within this range you will find the rotors' time constants of small machines.

Example

Induction machine 90L4 (manufacturer: Getriebebau NORD) at 230V

data from the rating plate:
U_R = 230V
f_R = 50Hz
I_mR = 0,48*IR = 2,95A
P_R = 1500W
I_R = 6,15A
p = 2
N_R = 1395rpm

Field Weakening

The necessary tension is proportional to the speed. Generally, the available tension is completely used if the nominal speed is reached. To further increase the speed, it is necessary to diminish the field in order to diminish the induced countervoltage. Indeed, this is done at the cost of maximum possible torque (picture 4).

Picture 4: Course of torque, of magnetic flow, output voltage, and power as function of speed.

In the servo amplifier, there are two possibilities to realize the weakening of field:

1. control in dependence on the speed ()
2. via a superimposed control of the output voltage

Both these variations are fundamentally equal, whereby the second alternative brings additional instability to the system as a further back - coupling is installed into the system. However, the processes are slow enough so that normally this is not to be expected.

Furthermore, you must check first, whether the driven mechanics (including the machine) is designed for high speed.

Weakening of field controlled dependent on the speed ()

(Default: configuration : DRVCNFG2 Bit 28 is not set (&0x08000000=false))
This is the "normal" way to weaken the field in a way that the desired speed can still be achieved (picture 1).

MVR

Nominal speed in rpm. From this speed on the weakening of field starts:

For MVR = 0 the weakening of field is turned off (default). Then the flow stays at its nominal value, and the machine cannot rotate faster than the nominal speed.

MCFW

In some cases, the weakening of field directly above the nominal speed does not suffice to provide enough tension. Then, you can either diminish MVR, in order to earlier start the weakening of field, or, alternatively, at the beginning of the weakening of field, you can diminish the flow using . MCFW establishes the value as a multiple of the nominal speed, until which the D current shall fall in squares.

E.g.. MCFW = 1,3 : Above the nominal speed, and until 1.3 * nominal speed, the D current decreases reciprocally in squares ( ) with the speed; above this value, it simply decreases reciprocally ()
default: 1 (turned off, within the whole speed range above the nominal speed)

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