The Big 5 for Electric Motors

The Big 5 for Electric Motors

Five important points about specifying an electric motor to drive a hydraulic pump.

Driving the pump of a hydraulic power unit (HPU) is a relatively easy application for an electric motor. However, the relationship between the motor and its power unit is like the relationship between the spark plugs and a car’s engine. The plugs are small compared to the engine (and relatively small in cost as well), but the engine stops if the plugs don’t perform their function. Similarly, if the motor stops, the hydraulic system goes down. In fact, you rarely hear about electric motors in hydraulic systems — unless they stop running.

Standard 10 hp, 1800-rpm, 208-230/460-V, 3-phase electric motor in 215TC frame size with feet. This model is totally enclosed and fan cooled, with a NEMA F-1 mounting for the junction box.

The obvious point is that the electric motor is a critical part of any power unit. Therefore, understanding the basic parameters of electric motors is key when specifying one for a particular HPU application. Five basic factors must be considered:

1. Horsepower — the designation of power you need from your system — 5 hp, 10 hp, 20 hp, etc. Note that a service factor (1.15 on standard units) lets motors run above their rated horsepower for intermittent periods.
Locked-rotor torque, also called startup torque, is an issue when any motor must start under load. The common locked-rotor torque is 100% of full-load torque, but we recommend that you select a motor that is designed to provide a more generous 140% or greater locked-rotor torque.

2. Rotational speed (rpm) — how many complete revolutions the motor shaft makes in a 60-second period. Most ac motors run at nominal fixed speeds of 3,600, 1,800, 1,200, or 900 rpm. Most hydraulic pumps for industrial applications are designed to run at 1,800 or 1,200 rpm. These lower-speed motors are more efficient, and have fewer balance problems than those operating a higher speeds.

3. Voltage and phase — determined by the electric service that will power the motor. Most motors operate at 230 or 460 Vac, 3-phase. When specifying voltage for an electric motor, always cite the number of phases. Single-phase is normally associated with residential and commercial power, whereas 3-phase is associated with industrial power — and is most commonly used in power unit applications.

4. Frame size and configuration — the National Electrical Manufacturers Association (NEMA) has established detailed standard motor dimensions for interchangeability. The motors on most power units mount to the pumps via flanges and are supported by feet. However, some mountings require no feet. Make sure you specify feet (or base) or no feet (round body) when ordering. (Many designers select direct coupling to attach the motor to a pump because this arrangement saves space.)

Also, where is the junction box on the motor? When looking at the motor from the shaft end, the standard location for the junction box on the left hand side (designated an F-1 mount). If you need a motor with the junction box on the right-hand side, make sure that you specify an F-2 mount.

Finally, a short shaft (known as a TS frame) should be specified on all 1800- or 1200-rpm motors. This may require a modification of the basic motor. It’s important to note that most manufacturers do not stock special motors, so some extra lead time should be expected on F-2 mount, short shaft, or round body models.

5. Enclosure — protect the motor’s internal parts from moisture, while still allowing cooling. Open drip-proof motors (which are cooled by the outside atmosphere) can be used where ambient conditions are fairly friendly. The more-expensive alternative, totally enclosed fan-cooled motors afford maximum protection even in a hazardous or aggressive environment. After the five basics have been established, you can request several different documents from your motor vendor to help you finalize your selection — and provide information for the end user if he or she will be someone else.

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There are four pieces of information that are almost mandatory when dealing with your motor vendor:

A performance data sheet — This will list all specifications (electrical performance) you need to know about an electric motor.
•  Dimensional drawings — These can eliminate costly errors due to size differences between what your application needs and what the motor vendor provides.
•  List of features and benefits — This will explain the motor’s electrical and mechanical features, and help you to make a more accurate comparison between various products from different manufacturers.
•  Installation and maintenance manual — This will provide you with all necessary care-and-feeding instructions for the motor. The information should cover factors such as storage, transport, mounting, balancing, re-greasing the bearings; operating sound levels; and other technical information not found elsewhere.

Knowing what to ask for and what to expect from a motor vendor will help ensure that you get the right motor for your hydraulic power unit. If you are subsequently selling the power units, the ability to provide your customers with all necessary intelligence about the equipment they are buying adds real value to the transaction.

This information was provided by WorldWide Electric Corp., Pittsford, N.Y.; (800) 808-2131.

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