Machine Riddle No.45 - Speed limit of dc motor
How much speed can be increased of a dc motor with the help of field control method?

#1
Sun, February 3rd, 2013 - 19:10
In addition of mechanical limitation , there are some electrical limitation for speed variaion in dc motor as following items:

Field Resistance Control

The lower the field current in a shunt (or separately excited) dc motor, the faster it turns: and the higher the field current, the slower it turns. Since an increase in field current causes decrease in speed, there is always a minimum achievable speed by field circuit control.  This minimum speed occurs when the motor’s field circuit has the maximum permissible current flowing through it.

If a motor is operating at its rated terminal voltage, power and field current, then it will be running at rated speed, also known as base speed.  Field resistance control can control the speed of the motor for speeds above base speed but not for speeds below base speed.  To achieve a speed slower than base speed by field circuit control would require excessive field current, possibly burning up the field windings.

Armature Voltage Control

The lower the armature voltage on a separately excited dc motor, the slower it turns, and the higher the armature voltage, the faster it turns.  Since an increase in armature voltage causes an increase in speed, there is always a maximum achievable speed by armature voltage control.  This maximum speed occurs when the motor’s armature voltage reaches its maximum permissible level.

If a motor is operating at its rated terminal voltage, power and field current, then it will be running at rated speed, also known as base speed. Armature voltage control can control the speed of the motor for speeds below base speed but not for speeds above base speed.  To achieve a speed faster than base speed by armature voltage control would require excessive armature voltage, possibly damaging the armature circuit.

-  These two techniques of speed control are obviously complementary.  Armature voltage control works well for speeds below base speed, and field resistance control works well for speeds above base speed.
There is a significant difference in the torque and power limits on the machine under these two types of speed control.  The limiting factor in either case is the heating of the armature conductors, which places an upper limit on the magnitude of the armature current IA.

For armature voltage control, the flux in the motor is constant, so the maximum torque in the motor is Tmax =KΦIAmax

This maximum torque is constant regardless of the speed of the rotation of the motor.  Since the power out of the motor is given by P=Tω, the maximum power is Pmax = Tmax ωThus, the max power out is directly proportional to its operating speed under armature voltage control.

On the other hand, when field resistance control is used, the flux does change.  In this form of control, a speed increase is caused by a decrease in the machine’s flux.  In order for the armature current limit is not exceeded, the induced torque limit must decrease as the speed of the motor increases.  Since the power out of the motor is given by P=Tωand the torque limit decreases as the speed of the motor increases, the max power out of a dc motor under field current control is constant, while the maximum torque varies as the reciprocal of the motor’s speed.

#2
Fri, March 8th, 2013 - 11:25
Dear Hamid,

Thank you and congratulations for your blog and your expertise !

I would like to say that the speed increase of a DC motor through field working weakening is also limited by the armature reaction flux that becomes very significant versus the inductor   decreased flux , distorting it  and leading to speed and current instability at high velocity (even on compensated machines). I faced this phenomenon on hoisting machines working at constant power between  2.5~3.5 times their base speed .

#3
Sat, March 9th, 2013 - 08:04
Dear Jean,

Thank you very much for your good comment.

Best Regards
Hamid