Determine shunt motor field current maximum.

[junkhound]

Very seldom do any dc motor work.

Designing motor drive for 1.2 kW 24V 71A armature forklift motor with a blown Curtis drive. This is for a food bank on volunteer basis, no $$ for a pricey Curtis drive. I can tack on a few high power surplus FETs to a low cost Chinese controller and make all forklift functions work.

Dc motor specs (at least those available online) do not seem to provide a field current set of plots.

I put the motor on dynamometer and ran at load with variable field current drive and determined that 0.8 to 1.3A field current gave optimum efficiency.

Is there an easier way to determine field current? ( I do not have a similar machine -Toyoota forklift-to measure the field current on or would have done that)

 

[gar]

230620-0859 EDT

Junkhound:

If you have a shunt wound DC motor the field coil is probably designed to run continuously at full rated voltage of the motor. At this maximum shunt field current the flux level is at its highest density. Thus, maximum full motor speed is at its lowest, and maximum torque is at its highest.

Reduce field excitation level, and maximum speed increases for a fixed armature voltage level, and maximum torque is reduced.

....

 

[junkhound]

Field dc resistance is only 0.7 ohms, so is designed for separately excited field. Not designed for continuous motor voltage application.
As you say, motor no load speed is maximum with just residual field

 

[gar]

230620-1112 EDT

junkhound:

0.7 ohms at 24 V is 34 A. Ballpark 1 A at 0.7 ohms is 0.7 V. Something looks fishy.

Where would 0.7 V come from? And why would it be designed this way. In some ways it looks like a series type motor. But that does correlate either.:

.Motor power is possibly 1.7 kW at 24 V and 71 A. As a wild guess I might expect that field power to be about 200 W.
200 W at 24 V is about 10 A. 200 W at 71 A is about 2.8 V. So a series connection would not be a major problem, and would provide high starting torque.

Can you in any way figure out how the field was powered?

.

 

[junkhound]

Field was powered externally and reversible.

The burntout Curtis controller output to the motor field is 2 small 'fast on' connectors with 20 AWG wires going to the field pigtail terminals on the Toyota forklift (also 20 AWG).

The Curtis armature controller design is/was a PWM buck regulator in the return line, motor reversal was by reversing the field polarity. I have not taken the trouble 'yet' to reverse engineer the Curtis controller (Curtis 1243 gen2) as to the field control specifics, spec shows 1.6A field current minimum.

When I tested on dyno, supplied the field with constant current power supply thru 20 ohm resistor to get the constant 0.8 to 1.3A. 1.6A at constant torque speed drooped slightly.

 

[junkhound]

PS: Motor is a Thrige 214999. Thrige now Titan T-T after corporate buyouts, etc....

The Toyota forklift and the Curtis supply have some FPGA interfaces to program field current and other parameters, no idea what those proprietary setting are, at least could not find any literature with details online.

 

[gar]

230620-1723 EDT

A DC motor with separate control of armature voltage and field excitation has the following characteristics ---

(1) With fixed field excitation ( meaning a constant field intensity ) a DC motor will have an output speed that is proportional to armature excitation voltage minus an internal voltage drop of armature resistance times load current. This means you get a linear speed drop form no load to full load.

(2) With variable field excitation and constant armature excitation voltage you get a speed drop from the characteristics of (1) above. Now if you change excitation voltage to the armature without changing field excitation, then you get speed proportional to armature excitation voltage. This results from the motor armature counter EMF. Counter EMF is proportional to RPM.

Thus, very easy to analyze a DC motor.

. .

 

[junkhound]

Opened up the Curtis, the field supply is an H bridge driven by a FPGr. Reverse polarity at the very least, data sheet says the field current can be programmed with appropriate additional hardware.

I will add a simple variable resistor on a transistor base to be able to adjust field once entire forklift again running, then permanently set for maximum allowable OSHA speed limit for walk behind forklift operation.

Guess I could always open up the motor, count the turns, measure core and the air gap and calculate field saturation current, not too excited to have to do that.

 

[gar]

230620-2100 EDT

You could use a Hall device magnetic field probe to monitor field flux density as you increase excitation to the field to deterime the approximate region of saturation.

.

 

[gar]

230620-2214 EDT

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/restmp.html copper and aluminum are the same.

To determine maximum allowable field current you make some assumptions.---

(1) The maximum temperature that you judge safe.

(2) Let your motor rest long enough that internally it becomes a constant temperature.

(3) Field temperature rise, when it becomes stable, can provide provide an estimate of average field temperature. Then usually 10 C is added as an estimate of maximum temperature.

(4) This testing should be done at full armature load.

Any loss of field current should present no power dissipation problem. However, note that as field excitation is reduced the RPM increases. DC motors are known to runaway on loss of field excitation, and blowup. Thus, one needs loss of field excitation detection, and this should be used to remove armature power. In our DC motor labs one student's job was to cut armature power in the case of loss of field excitation.

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