I have a small water pump with a brushed D.C. motor that is designed to operate off a 12 volt battery. With no load on the pump the current drawn from the battery is around 3 amperes. If the motor RPM continues to rise until the back-EMF nearly equals the supply voltage, why is the pump drawing such a large current? The mechanical losses are from brush and bearing friction and windage . I suppose that under no load the iron losses will greatly exceed the copper losses . It seems out-of-place for the power going to the motor to be ~36 watts. What is going on?
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\$\begingroup\$ What type of pump is this? \$\endgroup\$– Peter GreenDec 30, 2018 at 5:48
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\$\begingroup\$ It is a bilge pump for a boat. Rule-Mate 1500 gallon-per-hour (with zero water head). The pumping head is centrifugal....no impeller with rubber vanes. The pump seems to be operating very nicely. The motor is cooled by the water that it pumps. \$\endgroup\$– whitegreg56Dec 30, 2018 at 6:12
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\$\begingroup\$ That is a strong pump. 1500 gallons is equal to 30 drums of 50 gallons each. That means it can fill up a 50 gallon barrel in 2 minutes. Plenty strong and worthy of 36 watts. \$\endgroup\$– John CanonDec 30, 2018 at 7:16
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\$\begingroup\$ What is the motor full load power/amperage? \$\endgroup\$– Andy akaDec 30, 2018 at 11:09
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\$\begingroup\$ @John It really kicks out a lot of water! The 36 watts is with zero water...the pump is sitting on the floor...completely dry. \$\endgroup\$– whitegreg56Dec 30, 2018 at 17:56
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2 Answers
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There is not enough information available to do more than guess at how the motor and pump are performing. The pump manual indicates the pump can pump at 20 gallons per minute with a 3.45 ft. (1 m) static head and 11.33 GPM with a 6.7 ft. (2 m) head. That is 12.6 watts and 14.3 watts actually used in lifting the water.
The motor is designed to cycle on and off frequently. Every 2.5 minutes the pump is supposed to run for a short time to detect the water level. When the motor is first turned on, the only thing limiting the current is the winding resistance. If the motor has a low winding resistance the current would be quite high and frequent starting would be hard on the commutator and brushes. It seems likely that the motor has a high winding resistance to limit the starting current. That means it is quite inefficient and doesn't draw much more current at maximum load than at minimum load.
Since there is probably some water in the pump most of the time, there will be some loss in stirring the water when there is no discharge. For all operating conditions, more than half of the input power is probably dissipated in the winding resistance.
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\$\begingroup\$ There are a bunch of different models of 1500 GPH pumps. Mine does NOT have water sensing technology or electrical load check every 2.5 minutes. It has a straight (integral) float switch. The pump is being run completely dry. I think you may be correct about it being designed to do this...... \$\endgroup\$ Dec 30, 2018 at 18:04
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\$\begingroup\$ Still unanswered is.....why is the motor designed to do this? It may be interesting to see what the resistance of the motor is....I'll check that out, soon. \$\endgroup\$ Jan 1, 2019 at 1:40
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\$\begingroup\$ The resistance comes in at 0.5~0.6 ohms. \$\endgroup\$ Jan 1, 2019 at 3:53
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\$\begingroup\$ The float switch can also switch the motor on quite frequently. If the motor resistance is about 0.55 ohms, the initial current would be 20 amps neglecting the internal resistance of the source and the resistance of wiring, fuse, switch and connections. That would drop rather rapidly as the motor accelerates, but it would not take much no-load torque for the no-load current to be 1/3 or 1/2 the rated current. Small, inexpensive motors are never very efficient. \$\endgroup\$– user80875Jan 1, 2019 at 4:35
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\$\begingroup\$ I think your last sentence sums up what it is....theses things are EXTREMELY price/cost sensitive! It is a bunch of factors that makes it the way it is......the effects of all the frictions plus poor magnetics. Just the same, it is remarkable how much water it pumps for a rock bottom price. \$\endgroup\$ Jan 1, 2019 at 5:34
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DC motors have a commutator that switches the magnetizing current in one (or more) windings. Those windings (electromagnets) take current regardless of mechanical power draw, maximum current when the motor is stalled. When rotating, the motor takes less current, but you still need to remagnetize the iron parts in different polarities throughout the rotation cycle, and the iron parts will warm up as the commutator does so. Power draw can be minimized by controller trickery, but a bilge pump probably works most reliably when the design is kept simple.
Iron losses could be minimized by exotic materials, but when the pump has a real load, those losses wouldn't be important anyhow.
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\$\begingroup\$ I think you are correct that the iron losses are significant. I'm certain that they are not using any exotic materials...these things are extremely sensitive to cost/price. It sure pumps...I know someone that pumped out a basement using one of these! \$\endgroup\$ Jan 1, 2019 at 5:47