To understand the differences in power characteristics between an electric motor and internal-combustion engine, we'll first examine characteristics of a standard 3-phase electric motor. Figure 1 shows the torque-speed relation-ship of a 20 hp, 1800 rpm, NEMA Design B motor. Upon receiving power, the motor develops an initial, locked-rotor torque, and the rotor turns. As the rotor accelerates, torque decreases slightly, then begins to increase as the rotor accelerates beyond about 400 rpm. This dip in the torque curve generally is referred to as the pull-up torque. Torque eventually reaches a maximum value at around 1500 rpm, which is the motor's breakdown torque. As rotor speed increases beyond this point, torque applied to the rotor decreases sharply. This is know as the running torque, which becomes the full-load torque when the motor is running at its rated full-load speed — usually 1725 or 1750 rpm.

The torque-speed curve for a 3600-rpm motor would look almost identical to that of the 1800-rpm motor. The difference would be that speed values would be doubled and torque values would be halved.

Common practice is to ensure that torque required from the motor will always be less than breakdown torque. Applying torque equal to or greater than breakdown torque will cause the motor's speed to drop suddenly and severely, which will tend to stall the motor and most likely burn it out. If the motor is already running, it is possible to momentarily load a motor to near its breakdown torque. But for simplicity of discussion, assume the electric motor is selected based on full-load torque.

Note that Figure 1 shows a temporary large torque excess that can provide additional muscle to drive the hydraulic pump through momentary load increases. These types of electric motors also can be run indefinitely at their rated hp plus an additional percentage based on their service factor — generally 1.15 to 1.25 (at altitudes to 3300 ft).

Catalog ratings for electric motors list their usable power at a rated speed. If the load increases, motor speed will decrease and torque will increase to a value higher than full-load torque (but less than break-down torque). So when operating the pump at 1800 rpm, the electric motor has more than enough torque in reserve to drive the pump.

Torque behavior of engines
A gasoline engine has a dramatically different torque-speed curve, Figure 2, than an electric motor does. This means a gasoline engine exhibits a much less variable torque output throughout its speed range. Depending on their design, diesel engines with the same power ratings may generate slightly higher or lower torque at lower speeds than gasoline engines do, but diesels exhibit a similar torque curve throughout their operating speed range.

Prior calculations determined that 58 lb-ft of torque is required to drive the pump at any speed. Referring to Figure 2, the 20-hp gasoline engine develops a maximum torque of only 31 lb-ft — clearly not enough to drive the pump. This is because its 20-hp rating is based on performance at 3600 rpm. Maximum torque occurs at speeds near 3000 rpm, but is still well below the 58 lb-ft required by the pump. Even if the engine produced enough torque at this speed, power would still be inadequate due to the lower speed.

This is where the 2 1 /2 sizing rule comes from. An HPU requiring a 20-hp electric motor to drive the pump at 1800 rpm would require a gas or diesel engine rated at about 50 hp. Moreover, these values are based on an engine operating at its maximum torque and power ratings. However, manufacturers recommend that gasoline and diesel engines only operate continuously at about 85% of their maximum rated values to prevent seriously shortening of their service lives. So referring again to Figure 2, a 20- hp gasoline engine would develop just over 26 lb-ft of maximum torque, and only 24 lb-ft at 3600 rpm.

It is also interesting to compare this performance with fuel consumption. The fuel consumption chart, Figure 3, shows that a 20-hp gasoline engine achieves greatest fuel efficiency at about 2400 rpm, where it consumes just over 8.2 lb/hr (0.41 lb/hp 20 hp). At 3600 rpm, the engine would be considerably less fuel efficient.