DC Shunt Motor Speed Control

The likely reason for the speed increase is that the field resistance increases as the field heats up. This reduces the field current, which weakens the field - causing speed increase [if the armature voltage remains constant]. This is normal behaviour for a DC motor and usually the designer has allowed for the expected effect and the pump and motor are not troubled by the expected speed increase. This seems likely, because the field obviously has a ballast resistor which should not increase as the motor warms up - which reduces the effect of field resistance increase.

But, since you have not specified..

1. Rated field voltage? And armature voltage?
2. Supply voltages for armature and field?
3. If armature or field voltages are constant [not contributing to speed change]?
4. Time for which you run motor without stopping?
5. If its load is rated current value or more/less than that?
6. If motor is continuous rated or a specified time?
7. How much speed increase you are seeing?
   

-- it is difficult to know what is happening - or if it matters. A jacking pump would not usually need to run for very long - perhaps you have run a long time for test, creating more speed (field temperature rise) than realistic.

See post #1.

If my calculations are correct, your rheostat should probably be rated => 825 Watts. The first one burned up and the 300 Watt unit is probably overheating. That would explain the speed drift.

I would prefer to use a switch mode device to PWM (Pulse-Width-Modulate) the field current. This would provide a more stable adjustment. Once you have electronic control of the field current, it is fairly easy to add a feedback loop which can continuously maintain a precise motor speed (RPM).

Thanks for data - as you can see from posts by MJB1962823, you have caused folk wasted effort by writing 415V when 220V DC applies!

1. Note that "Duty S1" means continuous rated. As a "rule of thumb", motor temperature would stop rising and reach steady operation after about 3 hours at rated current, for a continuous rated motor.
2. So your motor was probably still warming-up after 1 h 45 min.
3. But one cannot tell, because you have not written the value of actual motor armature current during your test!
4. Since the insulation class is "H" the temperature rise [as measured by the winding resistance change cold to hot] would be 125 Celsius maximum for field and armature windings. In the absence of any other value on rating plate, rated ambient would be 40 Celsius and up to 1000 metres above sea level, for rated shaft power and armature current.
5. The rated field voltage is 220 and the rated speed is 1500 rev/min and you seem to want 1500 rev/min. So why do you need any resistance added to the field??
6. Does the pump data sheet reveal anything about speed requirements?
   
7. You have not written the actual setting of "42 ohm" field circuit resistor. What was it??
8. While other posts have questioned adequacy of 300 watt resistor for field I feel it is OK, so I withhold judgment.
9. You have not told us the actual supply voltage - do not presume it is, or has to be, exactly what is written on the motor. In many cases, a "220 volt supply" is a "220 volt battery" (110 lead-acid cells?). A battery is usually permanently on charge and probably has a charger of over 100 Amp capacity (in this case). Consequently, unless you turned-off charger for test, you will get the float voltage of 250 volts or so delivered to the motor.

Regarding item 6 above, please note that - if you mean "an open resistance wire spiral with a sliding contact" when you talk of "42 ohm resistor" - these resistors come, in my experience in two materials, NiCr (which hardly changes resistance between ambient and red hot) and a cheaper iron alloy (with which resistance increases about 30% between cold and maximum rated temperature). So measure field current and resistor voltage - do not assume resistance is "textbook" constant. Remember that the real rating of variable resistors is by current - 300 watts would apply at 42 ohm setting and stay at 2.7 amps at lower resistance.

Please understand that I have had to worry about turbine lubricating oil pumps as a designer and I think your concern about getting an exact speed all the time is unnecessary [DC motor designs usually have to cope with variable speed and it is likely the motor you have could run up to 3000 rev/min and down to 1000 rev/min with adjustment to rated current]. Your reported range of 1355 to 1515 rev/min seems good to me [in the absence of any information about actual armature current and any requirements for operating time during start-up and shut-down of turbine].

Have you examined the control logic to see when and for how long this motor/pump will run?? Is your test running time of 1.75 hours consistent with actual requirements?

The critical task for a DC pump may be during a "black" rundown of the turbine [powered from battery, without AC power] during which it only has to give enough flow to stop bearing overheating and damage - meeting that may mean continuing flow after the turbine is stationary (due to heat soak from the hot turbine disc). The "jacking" task may just part of its duty.