Motor bogging down

The application of the motor is on a hydraulic lift.
Originally the unit had a 15hp motor. Line voltage is 485v 3 phase.
Someone at our company, decided to order a new pump unit, and ordered a unit with a 30HP 3phase motor. The motor runs fine when the lift is empty, or close to full load. When it gets up near full load the motor leads get hot, the amperage starts going off scale, the line voltage drops about 10 to 15 volts. The motor bogs down until the overload trips.
The warranty department/field engineering has told them to replace the motor, pump, valve, none have which have made a difference. They said the motor being larger wouldn't cause this. I guess they think the bigger HP motor is not being asked to do more work than the old 15hp motor, and should have no problems.
The crew doing the work, asked me. My reaction was the motor is to big, install a 15hp or 20hp motor. My reason being is the wire going to equipment is to small, and is causing the voltage drops at higher current.
I'm asking for more opinions on this problem, and some insight on what is happening to this 30Hp motor.
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The oversized motor will draw a higher no-load current- and this can be close to the full load current of the original motor (rough estimate of no-load current is about 1/2 the full load current) so, even at no load, the motor may be drawing nearly as much as the old motor at full load. Under load, the total current may be excessive-possibly by nearly 50%. Suck the voltage down and the current will be higher yet. Your gut feeling may be right on target. \
Putting in a 30HP motor to handle a 15HP motor load is overkill and inefficient use of the motor and money. Have you any measurements of current with the old motor and the new one?? That might be interesting.
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Don Kelly snipped-for-privacy@shawcross.ca
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UPNDOWN wrote:

Is it a dual voltage motor? If so, check whether you've got the windings configured for the correct voltage....
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If voltage drops to 10-15 volts (!), the motor is too big for the circuit. Your on target.
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For 3-phase 460 volts: 15hp FLA = 21A requires No. 10 30hp FLA = 40A requires No. 8 For varying duty larger wire sizes may be required unless motor is rated for other than continuous duty. Voltage drop for No. 10 with 40A for 100 feet is about 10 volts for 3- phase. However, your line voltage is a little high and more than compensates for the voltage drop problem, but you did not give all the nameplate data. The entire Healy Power Plant in Healy, Alaska was designed using 480 volt supply but specified 460 volt motors to overcome most voltage drop problems. However, you shouldn't install a 30 hp motor in place of a 15 hp motor without upsizing the conductors. Ref: http://www.electrician2.com/calculators/motor_ver_1.html
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wrote:

I should qualify the statement that there is no voltage drop problem. This statement is not entirely correct. There is no voltage drop problem if the conductor temperature does not rise above 75 degrees C. However, when a No. 10 conductor is loaded with 40 amperes under normal conditions (30 degrees C ambient and no more than three current carrying conductors in a raceway or cable) the conductor temperature will rise to above 75 degrees C. As the conductor temperature rises the voltage drop will increase. When selecting conductor sizes two independent decisions have to be made, the first requires the correct ampacity so the conductor's insulation temperature is not exceeded and the second requires the voltage drop calculation so there is enough volts for the load. Additionally, if you replace a 15 hp motor with a 30 hp motor the disconnect, the conductors, the motor overload protection, and the short circuit and ground fault protective devices have to be resized to conform to the NEC rules.
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If you are running the pump on a circuit rated for fifteen HP then I'm sure that is your problem, like you suspected.
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30 HP is 22kW. ;-(
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From KT24 - in "Leafy Surrey"

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A lot of folks are addressing the use of a larger motor.
I'd like to ask why the pump being driven by this motor is requiring more than 15 hp? In a hydraulic system the pump is almost always a positive displacement type. So about the only way you can get more load on it is to either a) raise the speed of the pump or b) raise the discharge pressure. Neither of these are being addressed.
If the original system was setup that it only needed 15 hp to run the pump, then it shouldn't require more than 15 hp from the new motor (motors only develop the hp required of them by the shaft load).
Is the new motor running at the same speed as the original or is it double the RPM? That would explain a lot.
Or, as one other poster asked, is it drawing a lot of current because it's connected for lower voltage in a dual-voltage motor. Then it would draw too much current before developing the full 30 hp.
While a bit wasteful, installing a 30 hp motor where a 15 hp load is to be driven doesn't automatically make the motor develop the full 30 hp.
daestrom
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----------------------------

True, but no load current will be considerably higher than that of a 15HP motor. Using a rough estimate that no-load current is 25-30% of full load current, doubling the motor size, with speed and voltage the same, would result in a no load current roughly equivalent to 30-60% of the rated current of the original motor. Now consider that the load component of current is nearly the same for both motors- the full load current will be about 10% - 15% over the rated current of the original motor- ignoring any voltage drops. The pf will be lower due to the high exciting current so ignoring the decrease in efficiency at half vs full load, there will be a somewhat higher voltage drop, further worsening the situation. Considering this and starting problems, a larger motor might require re-wiring- which may have not been done even if breakers were changed. Could it be that the owners got a bargain basement (or had an older one) 60HP machine and simply used it rather than a motor properly sized for the load?
Bigger is better is not necessarily true.
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I should clarify a few things. The load is staying the same. While the new motor is 3600 rpm versus 1800 rpm of the old motor, the pump was changed, and the gallons per minute are the same. In other words, the load is the same and the speed of the lift is the same.
The new motor is wound for single voltage only(480v).
I wish I could give more information, but I am not directly involved. The mechanic involved in the work is a city away and called me for help. As soon as he gets back to the job, I try to get him to get me more information, and I'll respond to the group.
Thanks
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Mechanic? That explains it. Try hiring a qualified electrician.
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----------------------------
wrote:

If the pump was changed and the gtallons per minute are the same at the new speed, and assuming the same pressure (any info ?) then the motor should still be a 15HP motor. The larger motor has a higher "no-load" current which will affect the total current at all loads and could lead to overheating under load -somewhere between 5 and 10% at normal voltage. Voltage drops in the wiring will worsen this. Oversized motor , undersized wiring and possibly undersized protection limits.
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Don Kelly snipped-for-privacy@shawcross.ca
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UPNDOWN wrote:

Was the motor branch circuit updated to handle the 30 HP unit? This includes conductors, overcurrent protection, and overload protection.
If not, then (as others have pointed out) you could have a low voltage, high motor current situation affecting the new motor.
Why did the 15 HP unit quit? If there is a hydraulics problem external to the new pump, the new unit could be overloaded even if the wiring was updated correctly.
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