Electric Motor Size for Hydraulic Pump Drive

An electric motor can be overloaded for short periods during the cycle provided the average horsepower is no greater than its nameplate rating plus service factor where this applies.

The amount of intermittent overloading is up to the user, but we suggest the overload be no more than 25% above its nameplate current rating sustained no longer than about 10% of the time required for a complete cycle.

Operation on Incorrect Frequency (Hz)

Most A-C 60 Hz motors can be operated on a 50 Hz line and vice versa, but adjustments will have to be made in the current, HP, and speed ratings. The important thing to remember is that it is the current which causes heating. The HP which can be produced will be related to its current draw, and may be more or less than its nameplate rating.

*Voltage adjustment is to maintain current at rated value, to produce rated shaft torque. Current is always a limiting factor on a variation in rated Hz (frequency) or voltage.

Effects of Low Voltage

Nameplate HP is based on full voltage being available. HP output is a combination of voltage times current. If voltage is too low, then to produce rated HP the current must be too high, and this overheats the motor. Motors can usually accommodate as low as 90% of rated voltage and still produce nameplate HP although temperature rise in the windings will be greater than rated rise. For permanent operation on a voltage source known to be low, the HP load should be limited, and reduced by the same percentage that the voltage is low.

Effects of High Voltage

If motor load does not exceed nameplate HP rating, full load current will be lower than nameplate rating and the motor will run cooler than rating. However, its starting and breakdown current (at stall) will be higher than normal. The wiring, fusing, and thermal overload protection will have to be sized accordingly. Motor noise will increase.

Using an Oversize Motor

Using a 20 HP motor on a system which requires only 10 HP, for example, will give good results for running the pump but will consume more electricity than a 10 HP motor and will cause the power factor of the plant electric system to be poorer, especially during periods the motor is idling. Idling current of a 20 HP motor is about half the full load current of a 10 HP motor. This is an extra power waste during periods in the cycle when the pump is idling.

Using an Undersize Motor

Using a 20 HP motor on a system which requires 25 HP for brief periods is quite possible, but during overload periods the current of such a motor maybe-about twice the current of a 25 HP motor. There will be an extra waste of power during peak periods in the cycle. But the smaller motor could more than make this up during periods in the cycle when less than 20 HP is required.