By Steve Meyer
Steve Meyer is the President, Solid Technologies (Lakewood, CO)
What’s wrong with specifying horsepower for motors, particularly in motion control systems? Plenty. First, horsepower is a unit of measure for doing work, the rate at which power is consumed. If you were to measure power in a typical machine during normal operation, you would see it vary considerably, especially when starting and stopping loads. Second, horsepower is a time-dependent measure that can mask certain operating conditions, which may be critical to the successful operation of a control system, especially a mechatronic implementation.
More often than not, specifying electrical equipment in terms of horsepower can pose a serious problem. For example, an engineer had a machine nameplate containing detailed information concerning a 2-hp motor that ran a pump. He wanted to use a 2-hp variable frequency drive to run it, but the drive he selected would not start the motor. He then tried a 5-hp drive to handle a larger starting current. He discovered that the 5-hp drive was not much better, but he said it would start the motor if he “coaxed” it.
He finally connected an ammeter to the motor and made a strip chart recording of the starting current, which turned out to be ten times the running load. The results are not so surprising if you consider that the pump is cast iron with a rubber impeller and intended to be started when dry. Because of the high dry friction, the torque required was huge, but as soon as liquid hit the pump cavity, the torque requirement dropped to about 3 A, well within the 2-hp rating. Eventually, a 10-hp drive solved the problem.
Contrary to popular belief, James Watt did not invent the steam engine, though he is credited with many inventions. It was Thomas Newcomen’s version of the steam powered water pump that was in widespread use throughout England for almost 100 years when Watt was asked to make repairs on one in 1764. It was Watt’s improvement of the Newcomen pump that made the steam engine the prime mover of the Industrial Revolution. The speed governor, the first planetary gear reducer, and many other patents bear his name, as well as the unit of energy we use to measure power.
Among Watt’s many inventions, however, was something even more significant. When Watt and his partner were trying to sell their improved steam engine, they came up with the term, horsepower, which would help people better understand how much time and effort the steam engine would save.
Although the precise definition changed slightly over time, it is accepted to mean the ability of a horse to lift a 550 lb weight one foot in one second. That is:
746 W = 1 hp
1 hp = Torque (ft-lb) X speed (rpm)/5250
Electric motors can be compared to one another in terms of horsepower only when they run at the same speed. In both motors and engines, horsepower is a speed-dependent unit of measure of work. It is possible to manipulate the required motor size for a given task by increasing the speed. Often, within certain limits, simply selecting a motor with a higher speed increases the horsepower rating. This lets a smaller motor at higher rated speed accomplish the same amount of work as a larger motor at a lower speed. Most constant-speed motors are designed to operate at 1800 rpm, so this provides a baseline to compare ac motors and variable speed drives in terms of horsepower ratings. When discussing the hp rating of equipment, ensure that everyone agrees that 1800 rpm is their reference speed.
Torque is force
Consider torque, the force acting at a distance (radius) which causes a shaft to rotate. In contrast to hp, torque does not require a unit of time. But we frequently refer to power, torque, and horsepower interchangeably, which gives rise to more confusion.
It is essential to understand the concept of work when applying electric motors to any machine. Electric motors can be designed to operate at low speeds for direct drive applications that run from zero to 500 rpm, or for high-speed applications that run at 20,000 rpm or more. Dental drills, for example, traditionally used 100,000 rpm air motors that tended to run noticeably slower under load. By comparison, new dental drive systems run at 50,000 rpm and use miniature brushless dc drives and closed-loop control circuits to compensate the speed droop common to the air-driven tools.
Electrical to mechanical energy
The basic goal of an ac motor connected directly to line power is to produce a rotating load. The process of converting electrical power in the motor to mechanical power in the load generates heat, and the maximum amount that can be dissipated determines the realistic limit of life and reliability. The motor can draw 10 times the continuously rated current provided it does not exceed its insulation temperature rating. Various NEMA motor ratings include specifications for starting torque. For example, the spec allows starting torque to be as much as 225% of running torque, and provides ratings for the allowable number of starts per hour. This is when the motor insulation experiences the most stress.
But this does not hold true for a motor connected to a solid-state starter, variable-frequency drive, or servo drive. This is due to the limited current carrying ability of IGBT and FET solid-state switching devices. Peak current capabilities are typically 200 to 300% of continuous and are further limited by a time-based ramp (di/dt) that must be followed or the device will fail. When a motor is connected to a drive, solid-state starter, or servo amplifier, the motor’s torque and current are limited to the current rating (di/dt) of that solid state device.
Gasoline vs. electric motors
The same horsepower-rating problem shows up when accelerating a car from zero speed. A 3246-lb Corvette may require 424 ft-lb (430 hp at 5900 rpm) to reach 60 mph in 3.91 seconds, but it may require only two to five hp to keep it running at 65 mph on level ground. That’s why some newer SUVs have a provision to shut down the operation of some of their cylinders in cruise mode, dramatically reducing fuel consumption.
Compare this to an electric car— a mobile, mechatronic system. What other kind of motor and drive system can provide low speed at high torque and high speed at low torque from the same set of hardware?
Understanding the work to be performed helps us understand the demands on the power source as well. The ideal power source must release high current for starting torque to accelerate a car. Lead-acid batteries do not fare well, even when disregarding the 1800 lb of battery packs needed to move the car. When compared to the amount of energy stored in a gallon of gasoline, it is easy to understand why we are still using it.
Solutions do exist
One solution to the drive problem is to control the motor’s field winding. Field-oriented control in an ac inverter is similar to setting the ignition timing on an internal combustion engine. By advancing and retarding the switching time of the power control transistors, it is possible to double the torque of the ac motor for a time. In an electric car, this is sometimes referred to as an “electronic, two-speed transmission” and eliminates the need for a mechanical transmission.
When sizing a motor application, consider the complete cycle of the operation it is intended to perform. Consider the duty cycle (on-time vs. off-time), the starting and stopping conditions of the application – especially high-inertia and high frictional loads – as part of the work done by the motor. A smaller, higher rpm motor may turn out to be more cost effective in many applications. These factors also help guide the technology selection, that is, what type of motor is best suited for the application?
Filed Under: Motion control • motor controls, Mechatronics