Motor Speed

<...Manufacturer says if voltage will be less then speed will be more...>

Rubbish.

The manufacturer will be able to produce a set of performance curves relating impeller speed with pressure and flowrate. Superimposed upon them will be motor power required, and from that the motor can be selected to allow for the voltage and frequency at this facility.

I agree with everything said and would like to add it would be helpful to know the type of motor it is. Is the motor DC, AC induction type or something else? It is not uncommon (especially on small items) for things to be designed to plug into AC but the motors have there own power supply that turns it into DC (or something much like DC) to run the motor.

In general AC motors are more speed uniform than DC motors (yes there are exceptions and ways to regulate).

Motor speed(RPM) is in relation to the frequency in an AC motor.

Srpm = 120 x F
P

Srpm = synchronous revolutions per minute.
120 = constant
F = supply frequency (in cycles/sec)
P = number of motor winding poles

Reduction in voltage will effect the horsepower of the motor. Recommend that you acquire a pump with sufficient hp to handle the work you want it to do with the varying voltage.

Could lower voltage been mistaken for higher amps--not speed?

I believe it is not the spped but the amp will increase in case of low voltage. RPM of motor is 120 x f( frequency of volrage supply)/ Poles of motor.

Voltage has got no relation with speed

For Voltage variation of -20% there will not be any approciable drop in the RPM of motor. Current consumption will increase and the total Power rating of the motor will have to be derated (approx. 15 % deration of rated output). Consider this when calculating motor kW required.

Higher RPM (2 pole ~ 2900 RPM) is suitable incentrifugal pumps upto say 200 m3/hr for higher flows go in for 1450 RPM as wear & tear is higher.

For same flow/head at 1450 RPM (inlieu of 2900 RPM) pump size (& cost) increases. Motor power is normally not getting effected. But mantainace is lower / life of all wear parts is more (as RPM / Speed is lower).

You Decide.

First of all syn. speed is proportional directly to supply frequency and inversely to polarity of motors.The speed will not not be more unless frequency is is high.I also fail to understand the logic of the manufacturer that1450rpm will not be affected by low voltage while 2950 will be affected.The motor rpm is decided by the pump selection .You can have a pump running at the same conditions of head and discharge either at 1500,2900, depending upon various other parameters.

Universal motors which have separately-excited field windings may cause an increase in armature speed when the field current goes down due to decreased voltage. However, the speed of the armature is a function of the applied voltage on it. If the speed versus voltage curves are given for field and armature, then it may show that speed will increase, decrease, or stay the same with decreased voltage.

This is further to my earlier message.First of all mostly sq.cage induction motors are used for this application mostly.DC, and other types will not be considered unless there is a compelling reason.Now as regards the speed (assuming that the motor is sq.cage induction type )it will reduce with reduced voltage.The torque varies as sq. of voltage and hence will be lower than what it is at rated voltage.If u plot the torque Vs speed of the motor for the rated and reduced voltage we will find that under low voltage conditions the Torque Vs speed curve will be below the T-S curve at rated voltage.The operating point that is the point of intersection of the pump curve with motor curve for reduced voltage will be at a lower speed.This can be easily seen by plotting the curve and projecting the operating point on X axis (as normally speed is on X axis and torque is on Y axis)So the speed will drop.The acceleration torque will also be less and motor will take a longer time to reach the rated speed or the op.point.