Circuit to run seperately excited dc motor

[engineer1000]

I have a 90V 4.6 A Seperately excited dc motor . I am running it on two power supplies the armiture windings at 90V AND FIELD windings at about 30V.
I need desigb a control circuit t orun this motor with speed control . Direction is taken care of externally.
I have 120Vac supply to run it off. I can transform this ac down and rectifiy it for the field winding.
I want to vary the armiture voltage for speed control. I have looked at scrs and triacs but not sure on the firing circuit. I could use a singe triac and fire it using a resistor and cap, but this could only run at a mx speed of 50%
Anyone got any ideas???

 

[Dan Mills]

The usual approach is to vary the field current to change the speed (Less field current = faster), field power is almost always much smaller then armature power so it is easier to control this rather then the armature voltage.

I would probably be thinking in terms of producing 90V for the armature and using a PWM arrangement (using the field inductance as the inductor) to vary the field current with a couple of mosfets.

Interlock the armature supply with the field current and rig things so that the field will not be permitted to be reduced below the level that causes maximum armature current to be drawn (This way an attempt to throttle up too quickly will not cause an overload).

Note that on startup the field must be at full power before the armature is connected to the supply.

Regards, Dan.

 

[FvM]

A DC motor is usually designed for full field excitation current when operated at rated conditions. Field weakening involves reduced torque and motor power and is only meaningful with partial load. Many DC motor drives don't vary the field current at all.

 

[crutschow]

I second the idea of using PWM to control the field current.
What is the maximum field current?

 

[schmitt trigger]

I would not recommend field-weakening control to a novice. Too many things can go wrong, and at best the motor can be ruined, and at worst someone may be injured.

Additionally, field weakening only controls motor speed above base speed, not below. For instance, one can't go down to zero speed with field weakening.

Yes, armature control involves more power, but the motor's requirements are quite modest. This must be a 1/2Hp or smaller motor. This can be easily controlled with readily available SCRs.

 

[engineer1000]

I would not recommend field-weakening control to a novice. Too many things can go wrong, and at best the motor can be ruined, and at worst someone may be injured.

Additionally, field weakening only controls motor speed above base speed, not below. For instance, one can't go down to zero speed with field weakening.

Yes, armature control involves more power, but the motor's requirements are quite modest. This must be a 1/2Hp or smaller motor. This can be easily controlled with readily available SCRs.

The other problem I have is on the motor rating plate it says 90V 4.6A.I assume that this is for the armiture. How do I find out what voltage to apply to the field winding?

 

[schmitt trigger]

Take a good, well focused photo of the motor' plate.

The field voltage could be the same, but I've seen instances where it is also above or below the armature.

 

[FvM]

There will be a field winding current rating.